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The pillars of land plants: new insights into stem development
Available online at www.sciencedirect.com
ScienceDirect The pillars of land plants: new insights into stem development Antonio Serrano-Mislata
1
and Robert Sablowski
In spite of its central importance in evolution, plant architecture and crop improvement, stem development remains poorly understood relative to other plant organs. Here, we summarise current knowledge of stem ontogenesis and its regulation, including insights from new image analysis and biophysical approaches. The stem initiates in the rib zone (RZ) of the shoot apical meristem, under transcriptional control by DELLA and BLH proteins. Links have emerged between these regulators and cell proliferation, patterning and oriented growth in the RZ. During subsequent internode elongation, cell wall properties and mechanics have been analysed in detail, revealing pectin modification as a prominent control point. Recent work has also highlighted signalling to coordinate growth of stem tissues with different mechanical properties. Addresses 1 Instituto de Biologı´a Molecular y Celular de Plantas, CSIC-UPV, 46022 Valencia, Spain 2 Department of Cell and Developmental Biology, John Innes Centre, Norwich Research Park, Norwich NR4 7UH, United Kingdom Corresponding authors: Serrano-Mislata, Antonio ([email protected]. es), Sablowski, Robert ([email protected])
Current Opinion in Plant Biology 2018, 45:11–17 This review comes from a themed issue on Cell signalling and gene regulation Edited by Jorge Casal and Javier Palatnik
https://doi.org/10.1016/j.pbi.2018.04.016 1369-5266/ã 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons. org/licenses/by/4.0/).
2
stem, is an ancient and fundamental aspect of plant development. The control of stem elongation is also of great importance in crop improvement. Apart from the direct use of stems in the production of fibres, yarns, paper, wood and biofuels [3,4], changes in stem height and architecture have been a key factor for increases in grain yields the last 50 years [5]. Semi-dwarf mutants played a key role in the ‘green revolution’ that averted a food security crisis in the 1960–70s and continue to be important in commercial varieties. The yield gain associated with shorter stems is attributed to an increase in harvest index (yield relative to total crop biomass) and to decreased lodging (falling over) caused by wind and rain [6]. Not all effects of these mutations, however, are advantageous for crop productivity. The regulatory genes affected control a large number of genes and processes and consequently their mutations have pleiotropic, undesired effects, for example, on seed size or seedling establishment [7,8]. More precise genetic tools for modifying stem growth and architecture are needed to increase crop yield. In spite of its basic and strategic importance, stem development has been relatively neglected, partly because ontogenesis of the stem is not as easily imaged as that of roots and lateral organ primordia, and perhaps because, at least externally, stems lack the attractive morphological complexity of leaves and flowers. Here, we review recent insights on stem ontogenesis and its regulation. We focus primarily on the initiation of stem tissues and on internode elongation, as secondary growth of the stem has been recently reviewed [9,10].
Initial stages of stem development: rib zone function Introduction Development of a vertical shoot axis capable of bearing organs above the ground was one of the key steps in the evolution of land plants. The shoot of modern land plants is thought to derive from the sporophyte of a bryophytelike ancestor, which acquired indeterminate growth maintained by apical initial cells [1,2]. These apical initials were the precursors of the shoot apical meristem (SAM), and the first organ they produced was likely the equivalent of the stem in seed plants. Indeterminate growth and branching of the shoot axis set the stage for the evolution not only of overall shoot architecture, but also of leaves and probably roots [1,2]. Therefore, development of the main axis of the shoot, represented in seed plants by the www.sciencedirect.com
Like all other post-embryonic shoot organs, the stem is initiated at the SAM. Lateral organs, such as leaves and floral buds, are initiated in the SAM peripheral zone (PZ), which is replenished by the descendants of stem cells present in the central zone (CZ) [11]. In seed plants, the stem originates from the sub-apical region of the SAM, named the rib zone (RZ) because of its characteristic pattern of oriented cell divisions (Figure 1). The RZ includes a central region, which produces the pith of the stem, and a peripheral region that is continuous with the overlying PZ and gives rise to the stem epidermis and cortex; the stem vasculature develops at the boundary between both regions [12]. The lower boundary of the RZ is not anatomically clear, but the zone of active cell Current Opinion in Plant Biology 2018, 45:11–17
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Figure 1
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(b)
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Current Opinion in Plant Biology
Early stages of inflorescence stem growth. (a) Three-dimensional (3D) view based on a stack of confocal images, showing the Arabidopsis inflorescence apex with most floral buds removed; SAM: shoot apical meristem; fb: position of floral buds; red arrows indicate the proliferating internode epidermis. (b) Longitudinal section through the image in (a), with the central zone (CZ), peripheral zone (PZ) and rib zone (RZ) indicated; the RZ is subdivided into central (cRZ) and peripheral (pRZ) regions; different stem tissues are indicated in yellow. (c) Close up of a SAM section similar to (b), with different SAM regions indicated as in (b); planes of recent cell divisions are coloured according to their orientation in 3D (the colour bar above the image shows angles between the stem main axis and the vector normal to a plane fitted to the new cell wall; red indicates divisions perpendicular to the main axis, green marks divisions at a low angle to the main axis). (d) Clonal analysis showing growth patterns in the SAM; each set of coloured dots indicates the position of cells descended from a single cell that had been genetically marked three days before imaging; clones were imaged in different apices, which were aligned and overlapped based on the position of floral buds; grey dots correspond to the position of epidermal cells in each of the overlapped apices. Bars: 50 mm. For details of methods used in (c) and (d), see [14].
division typically extends to at least 1–2 cm from the apex [12]. In monocotyledons, stem elongation is also promoted by intercalary meristems (IMs) located at the base of each internode. Although located differently, the IMs are functionally comparable to the RZ in dicots and are subject to similar regulation. For example, both the RZ and IMs are the main sites where stem growth is stimulated by gibberellin (GA) [12,13] and both are regulated by closely related BEL1-like (BLH) transcription factors, represented in Arabidopsis by REPLUMLESS (RPL) (also known as BELLRINGER, PENNYWISE, LARSON, VAAMANA and BLH9) and in maize by BLH12 and BLH14 [14,15]. Early work showed that cell division in the RZ is stimulated by GA [12,16]. This hormone acts by promoting the degradation of DELLA proteins [17], which modulate the activity of a wide set of proteins to integrate environmental and endogenous signals in plant growth. Recent work in both dicotyledons and grasses revealed the links between GA function and other hormones that regulate stem growth, particularly brassinosteroid (BR) and abscisic acid (ABA) [18–21]. DELLAs were known to control Current Opinion in Plant Biology 2018, 45:11–17
cell division in the stem by binding and inhibiting the activity of class I TCP (TEOSINTE BRANCHED 1, CYCLOIDEA, and PROLIFERATING CELL FACTOR) transcription factors, which activate genes controlling cell cycle progression [22]. More recently, a direct link emerged between DELLAs and cell proliferation in the RZ, through activation of the cell-cycle inhibitor KRP2 (CDK/Kip-Related Protein2). Surprisingly, this revealed that control of cell division by DELLAs in the RZ affects not only stem elongation, but also the size of the overlying SAM and consequently the rate of floral bud production [23]. KNOX-family transcription factors are central regulators of shoot meristem function in combination with BLH proteins [24]. Some of these, such as RPL in Arabidopsis, are particularly important for RZ development. RPL has been found to maintain RZ function by repressing the expression of organ boundary genes; in rpl mutants, ectopic expression of the LSH4 (LIGHT-DEPENDENT SHORT HYPOCOTYL 4) boundary gene disrupted the distinct patterns of oriented cell division that distinguish the central and peripheral regions of the RZ (Figure 1) and inhibited stem elongation [14]. Recent work highlighting the importance of oriented divisions in plant www.sciencedirect.com
Stem development Serrano-Mislata and Sablowski 13
Figure 2
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day 4 Current Opinion in Plant Biology
Internode elongation in Arabidopsis. The images show the same Arabidopsis inflorescence photographed at 1-day intervals; to facilitate growth tracking, inks marks were placed on part of the stem on day 1; yellow arrows on the right show the regions that continued to grow in subsequent time points; the bars on the left of each image track growth of different regions of the stem in day 1 (0–1 cm: white; 1–2 cm: pink; 2–3 cm: red). Note that the growth rapidly ceases below the proliferative region (white), which is responsible for most of the stem elongation seen in the following days.
development suggests that they could be important for RZ function [25]; alternatively, changes in oriented divisions and inhibited stem growth could be both consequences of changes in tissue patterning and hormone signalling downstream of RPL and LSH4 [14]. In addition to LSH4, RPL directly regulates numerous other organ boundary genes, several of which mediate the flowering and growth defects seen in rpl mutants [14,26,27]. A subset of these, including LSH4, LOB (LATERAL ORGAN BOUNDARIES) and CUC (CUPSHAPED COTYLEDONS) genes, are also directly targeted by DELLA proteins [23], suggesting that organ boundary genes are particularly important in the control of stem elongation. One reason for this could be the known role of organ boundary genes in antagonising hormone signals, exemplified by the role of LOB in activating BR catabolism [28,29]. Thus, boundary genes could regulate stem growth by restricting signals that control stem elongation, such as BR, which regulates GA synthesis in rice [21] and Arabidopsis [18] and whose role in stem elongation has been highlighted by dwarf mutants affected in BR synthesis in diverse plants, including barley [30], maize [20] and apple trees [31]. A prominent aspect of the early stages of stem development is the establishment of the vascular pattern. As in the rest of the plant, the path of new stem veins is established by self-reinforcing auxin flow from new organ primordia towards pre-existing veins [32], resulting in a vascular network that tracks the phyllotactic pattern www.sciencedirect.com
established in the SAM [33]. In dicotyledons, this vascular network forms at the boundary between the peripheral and central regions of the RZ, possibly because, as in the SAM, auxin signalling is inhibited in the central region [34]. Therefore, the role of RPL in maintaining the radial pattern of the RZ, as described above, may contribute to defects in the vascular pattern seen in rpl mutants [35,36]. However, RPL must also regulate vascular patterning through other mechanisms, because close homologs of RPL control the formation of vascular connections in the maize stem, where vascular bundles are dispersed instead of arranged around a central stem pith [15].
Later stages of stem development: internode elongation As for other plant organs, the initial internode growth based on cell proliferation is gradually replaced by growth based on cell expansion, after which cells complete their differentiation. The relation between cell proliferation and expansion in internode elongation has been measured in detail in the Arabidopsis inflorescence stem: relatively slow growth by cell proliferation near the apex is followed by rapid but brief cell expansion, which ceases a few centimetres from the meristem after a three-fold increase in cell length [37]. The slower but sustained proliferative growth near the apex stem is responsible for most of the increase in stem length over a period of several days [14] (Figure 2). Differential growth during the period of rapid cell expansion, however, is particularly important to adjust stem growth to changeable Current Opinion in Plant Biology 2018, 45:11–17
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conditions, as seen in tropisms [38], propriosensing [39] and in circadian tracking of the sun by sunflower heads [40]. Regardless whether tissues grow predominantly through cell proliferation or cell expansion, in both cases growth is limited by the mechanical properties of cell walls. Earlier work tracked changes in cell wall composition and in the expression of cell wall-related genes during stem elongation [41–43] and recent papers showed that regulators such as RPL and DELLA proteins directly interact with genes involved in cell wall function [14,23,36,44]. A detailed analysis of cell wall polysaccharide dynamics and mechanical properties in the apical region of the Arabidopsis inflorescence stem showed little change in cellulose composition, cell wall thickness or turgor pressure, and suggested that changes in the association between pectin and cellulose microfibers have a predominant role in the control of internode elongation [45]. The prominent role of pectin changes during internode growth is in line with earlier evidence that RPL promotes internode elongation by activating expression of the pectin methylesterase PME5 [46]. The studies above revealed correlations between growth and the average properties of cell walls. However, stems are made of heterogeneous cell types, with different geometry and cell wall properties, raising the question whether specific tissues are rate-limiting for growth. Considering that the epidermis imposes a mechanical restraint on plant growth [47], the SAM can be mechanically modelled as an inflated shell [48]. If, similarly, the growing stem behaved as a cylinder under pressure, epidermal cells would be under tension in transverse orientation. Tissue contraction after cutting and the orientation of cortical microtubules, however, showed that the stem epidermis is under tension along the main axis of the stem, suggesting that although the epidermis may limit growth rate, inner tissues determine the direction of axial growth [49]. Thus, maintaining stem structure requires coordinated growth of tissues with different mechanical properties. This has been strikingly illustrated in double mutants for CLAVATA3 and DE-ETIOLATED 3 (clv3-8 det3-1), in which pressure exerted by excess cells in the central tissues, combined with inability to accommodate extra growth by cell expansion in the outer tissues, leads to spontaneous rupture of the inflorescence stem [50]. Mechanical stress that builds up during growth must be sensed by cells to adjust their wall properties. The highly conserved DEK1 Ca++-dependent calpain has been implicated as a component of this mechanosensing mechanism [51]. Reflecting the importance of DEK1 in regulating cell wall properties, DEK1 knockdown reduced deposition of cellulose and pectin, resulting in weak stems [52]. Current Opinion in Plant Biology 2018, 45:11–17
Coordination between tissue layers is mediated not only through tissue mechanics, but also chemical signalling, as demonstrated for epidermis-produced BR [53] and very long chain fatty acids (VLCFA) [54]. Reinforcing the importance of epidermal signals in shoot growth, it has been shown that during hypocotyl elongation auxin has specific roles in the epidermis, including the control of BR synthesis and signalling [55]. It remains to be seen whether this also applies to the more complex growth of the post-embryonic stem. Conversely, internal tissues also produce signals that coordinate stem growth; for example, the LRR receptor ERECTA (ER) is required specifically in the phloem, where it receives peptide signals produced in the stem endodermis to regulate cell numbers across stem tissues [56,57]. Cell differentiation eventually leads to the cessation of growth, associated with formation of secondary walls. Lignified secondary walls in the vascular bundles have a predominant effect on the mechanical properties of mature stems [3], therefore, the timing of vascular differentiation could also be expected to have a role in the control of internode elongation. However, a careful analysis of cell wall changes showed that lignin levels were constant throughout the elongating region of the Arabidopsis inflorescence stem [45], suggesting that lignification mostly contributes to changes in stem mechanics during secondary growth, after elongation has ceased.
Conclusions Classic studies in the 1950–60s analysed in detail morphological and physiological aspects of early stem development. Since then, stem elongation has received relatively little attention, in contrast to stem secondary growth and vascular development. However, recent technical advances in quantitative imaging [14,23] and biophysical techniques [3,45,48] have allowed to improve our understanding of the cellular and mechanical bases of stem growth, and how these are influenced by regulatory genes. Stem development is not only a neglected but economically important process, but now also a convenient system to address plant developmental questions on its own right. Despite the recent progress, many essential questions remain unanswered. Clonal analysis suggests that ground tissues of the RZ descend from the same stem cell population that gives rise to lateral organs (Figure 1) [14]. This raises the question about how stem initiation is controlled separately from the emergence of other organs from the SAM. The initial stages of vascular development in the RZ are also poorly understood. Much more work is needed to fully understand how the growth of different tissues is coordinated during stem elongation and to identify the non-cell autonomous signals that mediate this coordination. Future research lines should also explore how environmental signals are integrated in www.sciencedirect.com
Stem development Serrano-Mislata and Sablowski 15
the shoot apex to modulate RZ activity and stem growth. State-of-the-art developmental genetics, bio-imaging and biophysical techniques applied to stem development will continue to advance our understanding of this central process in plant development and will reveal more precise ways to modify stem architecture for crop improvement.
Acknowledgements A.S.-M. was funded by the European Union’s Horizon 2020 Research and Innovation program under Marie Skłodowska-Curie grant agreement No. 746396. R.S. was supported by the Biotechnology and Biological Sciences Research Council (BBSRC) grants BB/M003825/1 and BB/P013511/1.
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45. Phyo P, Wang T, Kiemle SN, O’Neill H, Pingali SV, Hong M, Cosgrove DJ: Gradients in wall mechanics and polysaccharides along growing inflorescence stems. Plant Physiol 2017 http://dx.doi.org/10.1104/pp.17.01270. The authors measured growth rates and mechanical properties (elastic and plastic compliance) of the Arabidopsis inflorescence stem and correlated these with changes in cell wall properties along the apicalbasal axis. Cellulose content was constant and small-angle neutron scattering showed little change in the alignment of cellulose microfibrils, suggesting a role of fibril alignment in the orientation, but not in the rate of cell wall extension. In contrast, faster growth correlated with increased side branching, methylesterification and mobility of pectins, measured in intact cell walls using solid-state NMR spectroscopy. Overall, the data indicated that faster stem elongation was associated with weaker pectincellulose interactions, which strengthened as the stem matured and growth diminished.
32. Scarpella E: The logic of plant vascular patterning, polarity, continuity and plasticity in the formation of the veins and of their networks. Curr Opin Genet Dev 2017, 45:34-43. 33. Dengler NG: The shoot apical meristem and development of vascular architecture. Botany 2006, 84:1660-1671. 34. Vernoux T, Brunoud G, Farcot E, Morin V, Van den Daele H, Legrand J, Oliva M, Das P, Larrieu A, Wells D et al.: The auxin signalling network translates dynamic input into robust patterning at the shoot apex. Mol Syst Biol 2011, 7:508. 35. Smith HMS, Hake S: The interaction of two homeobox genes, BREVIPEDICELLUS and PENNYWISE, regulates internode patterning in the Arabidopsis inflorescence. Plant Cell 2003, 15:1717-1727. 36. Etchells JP, Moore L, Jiang WZ, Prescott H, Capper R, Saunders NJ, Bhatt AM, Dickinson HG: A role for BELLRINGER in cell wall development is supported by loss-of-function phenotypes. BMC Plant Biol 2012, 12:212. 37. Hall H, Ellis B: Developmentally equivalent tissue sampling based on growth kinematic profiling of Arabidopsis inflorescence stems. New Phytol 2012, 194:287-296. 38. a´dnı´kova´ P, Smet D, Zhu Q, Van der Straeten D, Benkova´ E: Strategies of seedlings to overcome their sessile nature: auxin in mobility control. Front Plant Sci 2015, 6:218. 39. Okamoto K, Ueda H, Shimada T, Tamura K, Kato T, Tasaka M, Morita MT, Hara-Nishimura I: Regulation of organ straightening and plant posture by an actin–myosin XI cytoskeleton. Nat Plants 2015, 1:15031. This paper shows that the plant stem has a myosin-dependent mechanism to correct its own posture. Bending of the shoot in response to gravity was followed by a straightening response when wild-type plants were rotated in a clinostat to eliminate a constant gravity vector. This straightening response was eliminated in the double mutant myosin xif myosin xik, whose stems showed no obvious anatomical or mechanical differences. Expression of myosins XIf and XIk overlapped in elongating internodes within in a thin band of immature fibre cells that surround the inner stem tissues (pith and xylem). Actin-dependent cytoplasmic streaming was inhibited in the mutant fibre cells, and disruption of the actin cytoskeleton also affected self-straightening. Based on these results, the authors propose that bending of fibre cells is sensed by the acto-myosin cytoskeleton to trigger a straightening response. Current Opinion in Plant Biology 2018, 45:11–17
46. Peaucelle A, Louvet R, Johansen JN, Salsac F, Morin H, Fournet F, Belcram K, Gillet F, Ho¨fte H, Laufs P et al.: The transcription factor BELLRINGER modulates phyllotaxis by regulating the expression of a pectin methylesterase in Arabidopsis. Development 2011, 138:4733-4741. 47. Galletti R, Verger S, Hamant O, Ingram GC: Developing a ‘thick skin’: a paradoxical role for mechanical tension in maintaining epidermal integrity? Development 2016, 143:3249-3258. 48. Beauzamy L, Louveaux M, Hamant O, Boudaoud A: Mechanically, the shoot apical meristem of Arabidopsis behaves like a shell inflated by a pressure of about 1 MPa. Front Plant Sci 2015, 6:1038. 49. Baskin TI, Jensen OE: On the role of stress anisotropy in the growth of stems. J Exp Bot 2013, 64:4697-4707. 50. Maeda S, Gunji S, Hanai K, Hirano T, Kazama Y, Ohbayashi I, Abe T, Sawa S, Tsukaya H, Ferjani A: The conflict between cell proliferation and expansion primarily affects stem organogenesis in Arabidopsis. Plant Cell Physiol 2014, 55:1994-2007. 51. Tran D, Galletti R, Neumann ED, Dubois A, Sharif-Naeini R, Geitmann A, Frachisse J-M, Hamant O, Ingram GC: A mechanosensitive Ca 2+ channel activity is dependent on the developmental regulator DEK1. Nat Commun 2017, 8:1009. DEK1 is a conserved plant gene encoding a membrane protein with a calpain Ca++ dependent protease domain, which can be released by proteolysis and has been associated with mechanosensing channels in animals. Patch-clamped protoplast membranes were used to investigate a role for DEK1 in mechanosensing, revealing the presence of a pressureactivated Ca++ channel, whose activity required the transmembrane domain of DEK1. The authors proposed that mechanical stimuli activate the DEK1 Ca++ channel, leading to increased Ca++ on the cytoplasmic www.sciencedirect.com
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side of the plasma membrane and cleavage of the calpain domain, which activates downstream responses of plant cells to mechanical stress. 52. Amanda D, Doblin MS, MacMillan CP, Galletti R, Golz JF, Bacic A, Ingram GC, Johnson KL: Arabidopsis DEFECTIVE KERNEL1 regulates cell wall composition and axial growth in the inflorescence stem. Plant Direct 2017, 1. 53. Savaldi-Goldstein S, Peto C, Chory J: The epidermis both drives and restricts plant shoot growth. Nature 2007, 446:199-202.
signaling (axr3-1) in specific tissues had different effects on hypocotyl elongation in light and shade, with a more pronounced effect in shade when axr3-1 was expressed in the epidermis. Auxin induced distinct responses in inner tissues and in the epidermis, which showed increased local synthesis of auxin and BR. In line with the effect on BR synthesis, auxin activated expression of the BR-responsive transcription factor BZR1 and exogenous BR partially rescued hypocotyl growth in plants expressing axr3-1 in the epidermis. The authors propose that auxin functions in the epidermis at least in part by activating BR signaling to control growth in other tissues.
54. Nobusawa T, Okushima Y, Nagata N, Kojima M, Sakakibara H, Umeda M: Synthesis of very-long-chain fatty acids in the epidermis controls plant organ growth by restricting cell proliferation. PLoS Biol 2013, 11:e1001531.
56. Tameshige T, Ikematsu S, Torii KU, Uchida N: Stem development through vascular tissues: EPFL–ERECTA family signaling that bounces in and out of phloem. J Exp Bot 2016, 68:45-53.
55. Procko C, Burko Y, Jaillais Y, Ljung K, Long JA, Chory J: The epidermis coordinates auxin-induced stem growth in response to shade. Genes Dev 2016, 30:1529-1541. Auxin induces hypocotyl elongation in the shade, but its exact site of action was not clear. Localised expression of a dominant inhibitor of auxin
57. Uchida N, Lee JS, Horst RJ, Lai H-H, Kajita R, Kakimoto T, Tasaka M, Torii KU: Regulation of inflorescence architecture by intertissue layer ligand–receptor communication between endodermis and phloem. Proc Natl Acad Sci U S A 2012, 109:6337-6342.
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Current Opinion in Plant Biology 2018, 45:11–17
ScienceDirect The pillars of land plants: new insights into stem development Antonio Serrano-Mislata
1
and Robert Sablowski
In spite of its central importance in evolution, plant architecture and crop improvement, stem development remains poorly understood relative to other plant organs. Here, we summarise current knowledge of stem ontogenesis and its regulation, including insights from new image analysis and biophysical approaches. The stem initiates in the rib zone (RZ) of the shoot apical meristem, under transcriptional control by DELLA and BLH proteins. Links have emerged between these regulators and cell proliferation, patterning and oriented growth in the RZ. During subsequent internode elongation, cell wall properties and mechanics have been analysed in detail, revealing pectin modification as a prominent control point. Recent work has also highlighted signalling to coordinate growth of stem tissues with different mechanical properties. Addresses 1 Instituto de Biologı´a Molecular y Celular de Plantas, CSIC-UPV, 46022 Valencia, Spain 2 Department of Cell and Developmental Biology, John Innes Centre, Norwich Research Park, Norwich NR4 7UH, United Kingdom Corresponding authors: Serrano-Mislata, Antonio ([email protected]. es), Sablowski, Robert ([email protected])
Current Opinion in Plant Biology 2018, 45:11–17 This review comes from a themed issue on Cell signalling and gene regulation Edited by Jorge Casal and Javier Palatnik
https://doi.org/10.1016/j.pbi.2018.04.016 1369-5266/ã 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons. org/licenses/by/4.0/).
2
stem, is an ancient and fundamental aspect of plant development. The control of stem elongation is also of great importance in crop improvement. Apart from the direct use of stems in the production of fibres, yarns, paper, wood and biofuels [3,4], changes in stem height and architecture have been a key factor for increases in grain yields the last 50 years [5]. Semi-dwarf mutants played a key role in the ‘green revolution’ that averted a food security crisis in the 1960–70s and continue to be important in commercial varieties. The yield gain associated with shorter stems is attributed to an increase in harvest index (yield relative to total crop biomass) and to decreased lodging (falling over) caused by wind and rain [6]. Not all effects of these mutations, however, are advantageous for crop productivity. The regulatory genes affected control a large number of genes and processes and consequently their mutations have pleiotropic, undesired effects, for example, on seed size or seedling establishment [7,8]. More precise genetic tools for modifying stem growth and architecture are needed to increase crop yield. In spite of its basic and strategic importance, stem development has been relatively neglected, partly because ontogenesis of the stem is not as easily imaged as that of roots and lateral organ primordia, and perhaps because, at least externally, stems lack the attractive morphological complexity of leaves and flowers. Here, we review recent insights on stem ontogenesis and its regulation. We focus primarily on the initiation of stem tissues and on internode elongation, as secondary growth of the stem has been recently reviewed [9,10].
Initial stages of stem development: rib zone function Introduction Development of a vertical shoot axis capable of bearing organs above the ground was one of the key steps in the evolution of land plants. The shoot of modern land plants is thought to derive from the sporophyte of a bryophytelike ancestor, which acquired indeterminate growth maintained by apical initial cells [1,2]. These apical initials were the precursors of the shoot apical meristem (SAM), and the first organ they produced was likely the equivalent of the stem in seed plants. Indeterminate growth and branching of the shoot axis set the stage for the evolution not only of overall shoot architecture, but also of leaves and probably roots [1,2]. Therefore, development of the main axis of the shoot, represented in seed plants by the www.sciencedirect.com
Like all other post-embryonic shoot organs, the stem is initiated at the SAM. Lateral organs, such as leaves and floral buds, are initiated in the SAM peripheral zone (PZ), which is replenished by the descendants of stem cells present in the central zone (CZ) [11]. In seed plants, the stem originates from the sub-apical region of the SAM, named the rib zone (RZ) because of its characteristic pattern of oriented cell divisions (Figure 1). The RZ includes a central region, which produces the pith of the stem, and a peripheral region that is continuous with the overlying PZ and gives rise to the stem epidermis and cortex; the stem vasculature develops at the boundary between both regions [12]. The lower boundary of the RZ is not anatomically clear, but the zone of active cell Current Opinion in Plant Biology 2018, 45:11–17
12 Cell signalling and gene regulation
Figure 1
(a)
(b)
(c)
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(d)
Current Opinion in Plant Biology
Early stages of inflorescence stem growth. (a) Three-dimensional (3D) view based on a stack of confocal images, showing the Arabidopsis inflorescence apex with most floral buds removed; SAM: shoot apical meristem; fb: position of floral buds; red arrows indicate the proliferating internode epidermis. (b) Longitudinal section through the image in (a), with the central zone (CZ), peripheral zone (PZ) and rib zone (RZ) indicated; the RZ is subdivided into central (cRZ) and peripheral (pRZ) regions; different stem tissues are indicated in yellow. (c) Close up of a SAM section similar to (b), with different SAM regions indicated as in (b); planes of recent cell divisions are coloured according to their orientation in 3D (the colour bar above the image shows angles between the stem main axis and the vector normal to a plane fitted to the new cell wall; red indicates divisions perpendicular to the main axis, green marks divisions at a low angle to the main axis). (d) Clonal analysis showing growth patterns in the SAM; each set of coloured dots indicates the position of cells descended from a single cell that had been genetically marked three days before imaging; clones were imaged in different apices, which were aligned and overlapped based on the position of floral buds; grey dots correspond to the position of epidermal cells in each of the overlapped apices. Bars: 50 mm. For details of methods used in (c) and (d), see [14].
division typically extends to at least 1–2 cm from the apex [12]. In monocotyledons, stem elongation is also promoted by intercalary meristems (IMs) located at the base of each internode. Although located differently, the IMs are functionally comparable to the RZ in dicots and are subject to similar regulation. For example, both the RZ and IMs are the main sites where stem growth is stimulated by gibberellin (GA) [12,13] and both are regulated by closely related BEL1-like (BLH) transcription factors, represented in Arabidopsis by REPLUMLESS (RPL) (also known as BELLRINGER, PENNYWISE, LARSON, VAAMANA and BLH9) and in maize by BLH12 and BLH14 [14,15]. Early work showed that cell division in the RZ is stimulated by GA [12,16]. This hormone acts by promoting the degradation of DELLA proteins [17], which modulate the activity of a wide set of proteins to integrate environmental and endogenous signals in plant growth. Recent work in both dicotyledons and grasses revealed the links between GA function and other hormones that regulate stem growth, particularly brassinosteroid (BR) and abscisic acid (ABA) [18–21]. DELLAs were known to control Current Opinion in Plant Biology 2018, 45:11–17
cell division in the stem by binding and inhibiting the activity of class I TCP (TEOSINTE BRANCHED 1, CYCLOIDEA, and PROLIFERATING CELL FACTOR) transcription factors, which activate genes controlling cell cycle progression [22]. More recently, a direct link emerged between DELLAs and cell proliferation in the RZ, through activation of the cell-cycle inhibitor KRP2 (CDK/Kip-Related Protein2). Surprisingly, this revealed that control of cell division by DELLAs in the RZ affects not only stem elongation, but also the size of the overlying SAM and consequently the rate of floral bud production [23]. KNOX-family transcription factors are central regulators of shoot meristem function in combination with BLH proteins [24]. Some of these, such as RPL in Arabidopsis, are particularly important for RZ development. RPL has been found to maintain RZ function by repressing the expression of organ boundary genes; in rpl mutants, ectopic expression of the LSH4 (LIGHT-DEPENDENT SHORT HYPOCOTYL 4) boundary gene disrupted the distinct patterns of oriented cell division that distinguish the central and peripheral regions of the RZ (Figure 1) and inhibited stem elongation [14]. Recent work highlighting the importance of oriented divisions in plant www.sciencedirect.com
Stem development Serrano-Mislata and Sablowski 13
Figure 2
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day 4 Current Opinion in Plant Biology
Internode elongation in Arabidopsis. The images show the same Arabidopsis inflorescence photographed at 1-day intervals; to facilitate growth tracking, inks marks were placed on part of the stem on day 1; yellow arrows on the right show the regions that continued to grow in subsequent time points; the bars on the left of each image track growth of different regions of the stem in day 1 (0–1 cm: white; 1–2 cm: pink; 2–3 cm: red). Note that the growth rapidly ceases below the proliferative region (white), which is responsible for most of the stem elongation seen in the following days.
development suggests that they could be important for RZ function [25]; alternatively, changes in oriented divisions and inhibited stem growth could be both consequences of changes in tissue patterning and hormone signalling downstream of RPL and LSH4 [14]. In addition to LSH4, RPL directly regulates numerous other organ boundary genes, several of which mediate the flowering and growth defects seen in rpl mutants [14,26,27]. A subset of these, including LSH4, LOB (LATERAL ORGAN BOUNDARIES) and CUC (CUPSHAPED COTYLEDONS) genes, are also directly targeted by DELLA proteins [23], suggesting that organ boundary genes are particularly important in the control of stem elongation. One reason for this could be the known role of organ boundary genes in antagonising hormone signals, exemplified by the role of LOB in activating BR catabolism [28,29]. Thus, boundary genes could regulate stem growth by restricting signals that control stem elongation, such as BR, which regulates GA synthesis in rice [21] and Arabidopsis [18] and whose role in stem elongation has been highlighted by dwarf mutants affected in BR synthesis in diverse plants, including barley [30], maize [20] and apple trees [31]. A prominent aspect of the early stages of stem development is the establishment of the vascular pattern. As in the rest of the plant, the path of new stem veins is established by self-reinforcing auxin flow from new organ primordia towards pre-existing veins [32], resulting in a vascular network that tracks the phyllotactic pattern www.sciencedirect.com
established in the SAM [33]. In dicotyledons, this vascular network forms at the boundary between the peripheral and central regions of the RZ, possibly because, as in the SAM, auxin signalling is inhibited in the central region [34]. Therefore, the role of RPL in maintaining the radial pattern of the RZ, as described above, may contribute to defects in the vascular pattern seen in rpl mutants [35,36]. However, RPL must also regulate vascular patterning through other mechanisms, because close homologs of RPL control the formation of vascular connections in the maize stem, where vascular bundles are dispersed instead of arranged around a central stem pith [15].
Later stages of stem development: internode elongation As for other plant organs, the initial internode growth based on cell proliferation is gradually replaced by growth based on cell expansion, after which cells complete their differentiation. The relation between cell proliferation and expansion in internode elongation has been measured in detail in the Arabidopsis inflorescence stem: relatively slow growth by cell proliferation near the apex is followed by rapid but brief cell expansion, which ceases a few centimetres from the meristem after a three-fold increase in cell length [37]. The slower but sustained proliferative growth near the apex stem is responsible for most of the increase in stem length over a period of several days [14] (Figure 2). Differential growth during the period of rapid cell expansion, however, is particularly important to adjust stem growth to changeable Current Opinion in Plant Biology 2018, 45:11–17
14 Cell signalling and gene regulation
conditions, as seen in tropisms [38], propriosensing [39] and in circadian tracking of the sun by sunflower heads [40]. Regardless whether tissues grow predominantly through cell proliferation or cell expansion, in both cases growth is limited by the mechanical properties of cell walls. Earlier work tracked changes in cell wall composition and in the expression of cell wall-related genes during stem elongation [41–43] and recent papers showed that regulators such as RPL and DELLA proteins directly interact with genes involved in cell wall function [14,23,36,44]. A detailed analysis of cell wall polysaccharide dynamics and mechanical properties in the apical region of the Arabidopsis inflorescence stem showed little change in cellulose composition, cell wall thickness or turgor pressure, and suggested that changes in the association between pectin and cellulose microfibers have a predominant role in the control of internode elongation [45]. The prominent role of pectin changes during internode growth is in line with earlier evidence that RPL promotes internode elongation by activating expression of the pectin methylesterase PME5 [46]. The studies above revealed correlations between growth and the average properties of cell walls. However, stems are made of heterogeneous cell types, with different geometry and cell wall properties, raising the question whether specific tissues are rate-limiting for growth. Considering that the epidermis imposes a mechanical restraint on plant growth [47], the SAM can be mechanically modelled as an inflated shell [48]. If, similarly, the growing stem behaved as a cylinder under pressure, epidermal cells would be under tension in transverse orientation. Tissue contraction after cutting and the orientation of cortical microtubules, however, showed that the stem epidermis is under tension along the main axis of the stem, suggesting that although the epidermis may limit growth rate, inner tissues determine the direction of axial growth [49]. Thus, maintaining stem structure requires coordinated growth of tissues with different mechanical properties. This has been strikingly illustrated in double mutants for CLAVATA3 and DE-ETIOLATED 3 (clv3-8 det3-1), in which pressure exerted by excess cells in the central tissues, combined with inability to accommodate extra growth by cell expansion in the outer tissues, leads to spontaneous rupture of the inflorescence stem [50]. Mechanical stress that builds up during growth must be sensed by cells to adjust their wall properties. The highly conserved DEK1 Ca++-dependent calpain has been implicated as a component of this mechanosensing mechanism [51]. Reflecting the importance of DEK1 in regulating cell wall properties, DEK1 knockdown reduced deposition of cellulose and pectin, resulting in weak stems [52]. Current Opinion in Plant Biology 2018, 45:11–17
Coordination between tissue layers is mediated not only through tissue mechanics, but also chemical signalling, as demonstrated for epidermis-produced BR [53] and very long chain fatty acids (VLCFA) [54]. Reinforcing the importance of epidermal signals in shoot growth, it has been shown that during hypocotyl elongation auxin has specific roles in the epidermis, including the control of BR synthesis and signalling [55]. It remains to be seen whether this also applies to the more complex growth of the post-embryonic stem. Conversely, internal tissues also produce signals that coordinate stem growth; for example, the LRR receptor ERECTA (ER) is required specifically in the phloem, where it receives peptide signals produced in the stem endodermis to regulate cell numbers across stem tissues [56,57]. Cell differentiation eventually leads to the cessation of growth, associated with formation of secondary walls. Lignified secondary walls in the vascular bundles have a predominant effect on the mechanical properties of mature stems [3], therefore, the timing of vascular differentiation could also be expected to have a role in the control of internode elongation. However, a careful analysis of cell wall changes showed that lignin levels were constant throughout the elongating region of the Arabidopsis inflorescence stem [45], suggesting that lignification mostly contributes to changes in stem mechanics during secondary growth, after elongation has ceased.
Conclusions Classic studies in the 1950–60s analysed in detail morphological and physiological aspects of early stem development. Since then, stem elongation has received relatively little attention, in contrast to stem secondary growth and vascular development. However, recent technical advances in quantitative imaging [14,23] and biophysical techniques [3,45,48] have allowed to improve our understanding of the cellular and mechanical bases of stem growth, and how these are influenced by regulatory genes. Stem development is not only a neglected but economically important process, but now also a convenient system to address plant developmental questions on its own right. Despite the recent progress, many essential questions remain unanswered. Clonal analysis suggests that ground tissues of the RZ descend from the same stem cell population that gives rise to lateral organs (Figure 1) [14]. This raises the question about how stem initiation is controlled separately from the emergence of other organs from the SAM. The initial stages of vascular development in the RZ are also poorly understood. Much more work is needed to fully understand how the growth of different tissues is coordinated during stem elongation and to identify the non-cell autonomous signals that mediate this coordination. Future research lines should also explore how environmental signals are integrated in www.sciencedirect.com
Stem development Serrano-Mislata and Sablowski 15
the shoot apex to modulate RZ activity and stem growth. State-of-the-art developmental genetics, bio-imaging and biophysical techniques applied to stem development will continue to advance our understanding of this central process in plant development and will reveal more precise ways to modify stem architecture for crop improvement.
Acknowledgements A.S.-M. was funded by the European Union’s Horizon 2020 Research and Innovation program under Marie Skłodowska-Curie grant agreement No. 746396. R.S. was supported by the Biotechnology and Biological Sciences Research Council (BBSRC) grants BB/M003825/1 and BB/P013511/1.
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number of organ boundary genes. One of these, LSH4 (LIGHT-DEPENDENT SHORT HYPOCOTYL 4), was repressed by RPL in the central region of the RZ and was required for the defects of rpl mutants in both oriented RZ growth and in stem elongation. 15. Tsuda K, Abraham-Juarez M-J, Maeno A, Dong Z, Aromdee D, Meeley R, Shiroishi T, Nonomura KI, Hake S: KNOTTED1 cofactors, BLH12 and BLH14, regulate internode patterning and vein anastomosis in maize. Plant Cell 2017, 29:1105-1118. This paper showed that the role of BLH proteins in stem development is conserved in monocots. The maize BLH12 and BHL14 proteins, which are close homologs of Arabidopsis RPL, accumulate in the shoot meristem and in provascular bundles. blh12; blh14 double mutants showed pleiotropic defects during inflorescence development, including lack of axillary branches, severely reduced tassels and short internodes caused by precocious cell differentiation. BLH12 and BHL14 also prevent premature anastomosis (fusion) of vasculature in the stem, and consequently blh12; blh14 mutant stems had fewer, thicker vascular strands. 16. Sachs RM, Bretz CF, Lang A: Shoot histogenesis – the early effects of gibberellin upon stem elongation in 2 rosette plants. Am J Bot 1959, 46:376-384. 17. Davie`re JM, Achard P: A pivotal role of DELLAs in regulating multiple hormone signals. Mol Plant 2016, 9:10-20. 18. Unterholzner SJ, Rozhon W, Papacek M, Ciomas J, Lange T, Kugler KG, Mayer KF, Sieberer T, Poppenberger B: Brassinosteroids are master regulators of gibberellin biosynthesis in Arabidopsis. Plant Cell 2015, 27:2261-2272. 19. Shu K, Chen Q, Wu Y, Liu R, Zhang H, Wang P, Li Y, Wang S, Tang S, Liu C: ABI4 mediates antagonistic effects of abscisic acid and gibberellins at transcript and protein levels. Plant J 2016, 85:348-361. 20. Best NB, Hartwig T, Budka J, Fujioka S, Johal G, Schulz B, Dilkes BP: nana plant2 encodes a maize ortholog of the Arabidopsis brassinosteroid biosynthesis gene DWARF1, identifying developmental interactions between brassinosteroids and gibberellins. Plant Physiol 2016, 171:2633-2647. 21. Tong H, Xiao Y, Liu D, Gao S, Liu L, Yin Y, Jin Y, Qian Q, Chu C: Brassinosteroid regulates cell elongation by modulating gibberellin metabolism in rice. Plant Cell 2014, 26:4376-4393. 22. Davie`re JM, Wild M, Regnault T, Baumberger N, Eisler H, Genschik P, Achard P: Class I TCP-DELLA interactions in inflorescence shoot apex determine plant height. Curr Biol 2014, 24:1923-1928. 23. Serrano-Mislata A, Bencivenga S, Bush M, Schiessl K, Boden S, Sablowski R: DELLA genes restrict inflorescence meristem function independently of plant height. Nat Plants 2017, 3:749. This work uncovered an unexpected role of DELLA proteins in the control of SAM size that was genetically separable from their role in repressing stem elongation. A combination of quantitative imaging and global analysis of DELLA target genes in the Arabidopsis inflorescence apex showed that DELLA proteins restrain cell proliferation in the RZ by activating expression of the cell-cycle inhibitor KRP2. Loss-of-function of krp2 restored meristem size, but not stem growth, and accelerated flower production in DELLA semi-dwarf gain-of-function backgrounds. The separable roles in meristem size and stem growth were also seen in barley, suggesting that correction of undesirable effects of DELLA genes on meristem size could be used to further improve semi-dwarf crop varieties. 24. Tsuda K, Hake S: Diverse functions of KNOX transcription factors in the diploid body plan of plants. Curr Opin Plant Biol 2015, 27:91-96. 25. Martinez P, Luo A, Sylvester A, Rasmussen CG: Proper division plane orientation and mitotic progression together allow normal growth of maize. Proc Natl Acad Sci U S A 2017, 114:2759. Mutants with aberrant planes of cell division, such as tangled in maize, have organ growth defects, suggesting that oriented growth is important for morphogenesis. However, tangled causes defects in both division orientation and cell cycle progression, either of which could be responsible for defective organ growth. To separate the two functions, the authors complemented the mutant with a construct expressing TANGLED containing a mitotic destruction box from CYCLIN1B (DTAN1-YFP). This construct restored proper orientation of cell divisions, but did not correct the cell cycle progression defects. The complemented Current Opinion in Plant Biology 2018, 45:11–17
16 Cell signalling and gene regulation
plants looked normal, indicating that oriented cell divisions were critical for organ growth. 26. Khan M, Ragni L, Tabb P, Salasini BC, Chatfield S, Datla R, Lock J, Kuai X, Despre´s C, Proveniers M et al.: Repression of lateral organ boundary genes by PENNYWISE and POUND-FOOLISH is essential for meristem maintenance and flowering in Arabidopsis. Plant Physiol 2015, 169:2166-2186. 27. Andre´s F, Romera-Branchat M, Martı´nez-Gallegos R, Patel V, Schneeberger K, Jang S, Altmuller J, Nurnberg P, Coupland G: Floral induction in Arabidopsis by FLOWERING LOCUS T requires direct repression of BLADE-ON-PETIOLE genes by the homeodomain protein PENNYWISE. Plant Physiol 2015, 169:2187-2199. 28. Bell EM, Lin WC, Husbands AY, Yu L, Jaganatha V, Jablonska B, Mangeon A, Neff MM, Girke T, Springer PS: Arabidopsis lateral organ boundaries negatively regulates brassinosteroid accumulation to limit growth in organ boundaries. Proc Natl Acad Sci U S A 2012, 109:21146-21151.
40. Atamian HS, Creux NM, Brown EA, Garner AG, Blackman BK, Harmer SL: Circadian regulation of sunflower heliotropism, floral orientation, and pollinator visits. Science 2016, 353:587-590. Sunflower heads track the sun during the day and re-orient during the night to face east at dawn. Disruption of sun-tracking by rotating pots at night decreased biomass and pollinator attraction, showing that this is a physiologically and ecologically important response. Re-orientation at night depended on the circadian clock and continued for several days in growth rooms with fixed light source. Re-orientation was driven by gibberellin-dependent, differential elongation of opposite sides of the stem. The alternating growth of both sides of the stem resulted from the interplay between a spatial cue provided by light and a temporal pattern of growth set by the circadian clock. 41. Hall H, Ellis B: Transcriptional programming during cell wall maturation in the expanding Arabidopsis stem. BMC Plant Biol 2013, 13:14. 42. Hall HC, Cheung J, Ellis BE: Immunoprofiling reveals unique cell-specific patterns of wall epitopes in the expanding Arabidopsis stem. Plant J 2013, 74:134-147.
29. Gendron JM, Liu J-S, Fan M, Bai M-Y, Wenkel S, Springer PS, Barton MK, Wang Z-Y: Brassinosteroids regulate organ boundary formation in the shoot apical meristem of Arabidopsis. Proc Natl Acad Sci U S A 2012, 109:21152-21157.
43. Minic Z, Jamet E, San-Clemente H, Pelletier S, Renou J-P, Rihouey C, Okinyo DP, Proux C, Lerouge P, Jouanin L: Transcriptomic analysis of Arabidopsis developing stems: a close-up on cell wall genes. BMC Plant Biol 2009, 9:6.
30. Dockter C, Gruszka D, Braumann I, Druka A, Druka I, Franckowiak J, Gough SP, Janeczko A, Kurowska M, Lundqvist J et al.: Induced variations in brassinosteroid genes define barley height and sturdiness, and expand the green revolution genetic toolkit. Plant Physiol 2014, 166:1912-1927.
44. Marı´n-de la Rosa N, Pfeiffer A, Hill K, Locascio A, Bhalerao RP, Miskolczi P, Grønlund AL, Wanchoo-Kohli A, Thomas SG, Bennett MJ: Genome wide binding site analysis reveals transcriptional coactivation of cytokinin-responsive genes by DELLA proteins. PLoS Genet 2015, 11:e1005337.
31. Zheng X, Zhao Y, Shan D, Shi K, Wang L, Li Q, Wang N, Zhou J, Yao J, Xue Y et al.: MdWRKY9 overexpression confers intensive dwarfing in the M26 rootstock of apple by directly inhibiting brassinosteroid synthetase MdDWF4 expression. New Phytol 2018, 217:1086-1098.
45. Phyo P, Wang T, Kiemle SN, O’Neill H, Pingali SV, Hong M, Cosgrove DJ: Gradients in wall mechanics and polysaccharides along growing inflorescence stems. Plant Physiol 2017 http://dx.doi.org/10.1104/pp.17.01270. The authors measured growth rates and mechanical properties (elastic and plastic compliance) of the Arabidopsis inflorescence stem and correlated these with changes in cell wall properties along the apicalbasal axis. Cellulose content was constant and small-angle neutron scattering showed little change in the alignment of cellulose microfibrils, suggesting a role of fibril alignment in the orientation, but not in the rate of cell wall extension. In contrast, faster growth correlated with increased side branching, methylesterification and mobility of pectins, measured in intact cell walls using solid-state NMR spectroscopy. Overall, the data indicated that faster stem elongation was associated with weaker pectincellulose interactions, which strengthened as the stem matured and growth diminished.
32. Scarpella E: The logic of plant vascular patterning, polarity, continuity and plasticity in the formation of the veins and of their networks. Curr Opin Genet Dev 2017, 45:34-43. 33. Dengler NG: The shoot apical meristem and development of vascular architecture. Botany 2006, 84:1660-1671. 34. Vernoux T, Brunoud G, Farcot E, Morin V, Van den Daele H, Legrand J, Oliva M, Das P, Larrieu A, Wells D et al.: The auxin signalling network translates dynamic input into robust patterning at the shoot apex. Mol Syst Biol 2011, 7:508. 35. Smith HMS, Hake S: The interaction of two homeobox genes, BREVIPEDICELLUS and PENNYWISE, regulates internode patterning in the Arabidopsis inflorescence. Plant Cell 2003, 15:1717-1727. 36. Etchells JP, Moore L, Jiang WZ, Prescott H, Capper R, Saunders NJ, Bhatt AM, Dickinson HG: A role for BELLRINGER in cell wall development is supported by loss-of-function phenotypes. BMC Plant Biol 2012, 12:212. 37. Hall H, Ellis B: Developmentally equivalent tissue sampling based on growth kinematic profiling of Arabidopsis inflorescence stems. New Phytol 2012, 194:287-296. 38. a´dnı´kova´ P, Smet D, Zhu Q, Van der Straeten D, Benkova´ E: Strategies of seedlings to overcome their sessile nature: auxin in mobility control. Front Plant Sci 2015, 6:218. 39. Okamoto K, Ueda H, Shimada T, Tamura K, Kato T, Tasaka M, Morita MT, Hara-Nishimura I: Regulation of organ straightening and plant posture by an actin–myosin XI cytoskeleton. Nat Plants 2015, 1:15031. This paper shows that the plant stem has a myosin-dependent mechanism to correct its own posture. Bending of the shoot in response to gravity was followed by a straightening response when wild-type plants were rotated in a clinostat to eliminate a constant gravity vector. This straightening response was eliminated in the double mutant myosin xif myosin xik, whose stems showed no obvious anatomical or mechanical differences. Expression of myosins XIf and XIk overlapped in elongating internodes within in a thin band of immature fibre cells that surround the inner stem tissues (pith and xylem). Actin-dependent cytoplasmic streaming was inhibited in the mutant fibre cells, and disruption of the actin cytoskeleton also affected self-straightening. Based on these results, the authors propose that bending of fibre cells is sensed by the acto-myosin cytoskeleton to trigger a straightening response. Current Opinion in Plant Biology 2018, 45:11–17
46. Peaucelle A, Louvet R, Johansen JN, Salsac F, Morin H, Fournet F, Belcram K, Gillet F, Ho¨fte H, Laufs P et al.: The transcription factor BELLRINGER modulates phyllotaxis by regulating the expression of a pectin methylesterase in Arabidopsis. Development 2011, 138:4733-4741. 47. Galletti R, Verger S, Hamant O, Ingram GC: Developing a ‘thick skin’: a paradoxical role for mechanical tension in maintaining epidermal integrity? Development 2016, 143:3249-3258. 48. Beauzamy L, Louveaux M, Hamant O, Boudaoud A: Mechanically, the shoot apical meristem of Arabidopsis behaves like a shell inflated by a pressure of about 1 MPa. Front Plant Sci 2015, 6:1038. 49. Baskin TI, Jensen OE: On the role of stress anisotropy in the growth of stems. J Exp Bot 2013, 64:4697-4707. 50. Maeda S, Gunji S, Hanai K, Hirano T, Kazama Y, Ohbayashi I, Abe T, Sawa S, Tsukaya H, Ferjani A: The conflict between cell proliferation and expansion primarily affects stem organogenesis in Arabidopsis. Plant Cell Physiol 2014, 55:1994-2007. 51. Tran D, Galletti R, Neumann ED, Dubois A, Sharif-Naeini R, Geitmann A, Frachisse J-M, Hamant O, Ingram GC: A mechanosensitive Ca 2+ channel activity is dependent on the developmental regulator DEK1. Nat Commun 2017, 8:1009. DEK1 is a conserved plant gene encoding a membrane protein with a calpain Ca++ dependent protease domain, which can be released by proteolysis and has been associated with mechanosensing channels in animals. Patch-clamped protoplast membranes were used to investigate a role for DEK1 in mechanosensing, revealing the presence of a pressureactivated Ca++ channel, whose activity required the transmembrane domain of DEK1. The authors proposed that mechanical stimuli activate the DEK1 Ca++ channel, leading to increased Ca++ on the cytoplasmic www.sciencedirect.com
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side of the plasma membrane and cleavage of the calpain domain, which activates downstream responses of plant cells to mechanical stress. 52. Amanda D, Doblin MS, MacMillan CP, Galletti R, Golz JF, Bacic A, Ingram GC, Johnson KL: Arabidopsis DEFECTIVE KERNEL1 regulates cell wall composition and axial growth in the inflorescence stem. Plant Direct 2017, 1. 53. Savaldi-Goldstein S, Peto C, Chory J: The epidermis both drives and restricts plant shoot growth. Nature 2007, 446:199-202.
signaling (axr3-1) in specific tissues had different effects on hypocotyl elongation in light and shade, with a more pronounced effect in shade when axr3-1 was expressed in the epidermis. Auxin induced distinct responses in inner tissues and in the epidermis, which showed increased local synthesis of auxin and BR. In line with the effect on BR synthesis, auxin activated expression of the BR-responsive transcription factor BZR1 and exogenous BR partially rescued hypocotyl growth in plants expressing axr3-1 in the epidermis. The authors propose that auxin functions in the epidermis at least in part by activating BR signaling to control growth in other tissues.
54. Nobusawa T, Okushima Y, Nagata N, Kojima M, Sakakibara H, Umeda M: Synthesis of very-long-chain fatty acids in the epidermis controls plant organ growth by restricting cell proliferation. PLoS Biol 2013, 11:e1001531.
56. Tameshige T, Ikematsu S, Torii KU, Uchida N: Stem development through vascular tissues: EPFL–ERECTA family signaling that bounces in and out of phloem. J Exp Bot 2016, 68:45-53.
55. Procko C, Burko Y, Jaillais Y, Ljung K, Long JA, Chory J: The epidermis coordinates auxin-induced stem growth in response to shade. Genes Dev 2016, 30:1529-1541. Auxin induces hypocotyl elongation in the shade, but its exact site of action was not clear. Localised expression of a dominant inhibitor of auxin
57. Uchida N, Lee JS, Horst RJ, Lai H-H, Kajita R, Kakimoto T, Tasaka M, Torii KU: Regulation of inflorescence architecture by intertissue layer ligand–receptor communication between endodermis and phloem. Proc Natl Acad Sci U S A 2012, 109:6337-6342.
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