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From central–peripheral to adaxial–abaxial
548
Research Update
TRENDS in Plant Science Vol.6 No.12 December 2001
From central–peripheral to adaxial–abaxial Masao Tasaka Higher plants are constructed of three organs – the stem, the root and the leaf. The stem and the root have two axes, apical–basal and central–peripheral, which cross orthogonally. Leaves develop from the shoot apical meristem as lateral organs that have three different axes, apical–basal, adaxial–abaxial and right–left. Recent data point to the possibility that the adaxial–abaxial axis in the leaf is formed from the central–peripheral axis in the stem.
The plant body is composed of several organs including the stem, root and leaf. The stem and root have the shoot apical meristem (SAM) and the root apical meristem (RAM), respectively, at their tip, and both form an ‘apical–basal’ axis (Fig. 1a). Cells in these organs are ranged along the apical–basal axis – the primary axis of elongation – according to their ages. There is a ‘central–peripheral’ axis orthogonal to the apical–basal axis in stems and roots. In dicots, the tissues from centre to periphery are the stele (including vascular bundles), ground tissue and epidermis. The ground tissue and epidermis are radially symmetrical. Because the stem and root are joined through the hypocotyl, these two axes are continuous. In Arabidopsis thaliana, the stem and the root partially share a common molecular mechanism, the SHORT ROOT (SHR) and SCARECROW (SCR) genes, which control the differentiation of stem and root tissues to generate the radial pattern of ground tissues such as the endodermis and the cortex1–3. During dicot embryogenesis, a single zygote develops into a mature embryo with distinct parts consisting of SAM, cotyledon, hypocotyl, root and RAM along the apical–basal axis. During this process, the embryo seems to establish the apical–basal axis initially with distinct polarity (Fig. 1b). Then, each of the embryonic organs and meristems differentiate at appropriate positions along the axis. The radial organization must also be established along the central–peripheral axis. In Arabidopsis embryos, SHR and SCR function to form http://plants.trends.com
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Fig. 1. Axes in dicot plant. (a) Apical–basal (a–b) axes in shoot, root and leaf, central–peripheral (c–p) axis in stem, and adaxial–abaxial (ad–ab) and right–left (r–l) axes in leaf. (b) Apical–basal and central–peripheral axes in heartstage embryo and adaxial–abaxial axis in cotyledon primordia. Top view of the heart stage embryo (left). (c) Vertical section of the shoot apical meristem (SAM). SAM is zoned into three parts: the central zone (CZ), the peripheral zone (PZ) and the rib zone (RZ). Leaf primordia are numbered as P0, P1 and P2 depending on their ages.
ground tissues in the hypocotyl as well as in the stem and the root1,3. Essentially, plant embryogenesis can be viewed as initial differentiation of the shoot and root fates. Two axes, apical–basal and central–peripheral, established during embryogenesis continuously function after germination for the rest of the plant’s life. The apical portion of the dicot embryo has the spatial bilateral symmetry to make two cotyledon primordia and a central shoot apical meristem. The apical region is proposed to have two independent patterns, radial and bilateral. These two are combined to develop two lateral cotyledon primordia at peripheral regions and the SAM in the middle4,5. The third major plant organ is the leaf, which is developed from the SAM as a lateral organ (Fig. 1c). The leaf has an apical–basal axis, an ‘adaxial–abaxial’ (dorsal–ventral) axis and a ‘right–left’ axis. There are three zones in the SAM – central, peripheral and rib – which are divided based on their position, and the
frequency and orientation of cell division. Leaves develop as primordia from the peripheral zone of the SAM with a specific phyllotaxis. The leaf primordia first elongates as a club in the apical orientation, and then leaf blades spread to the left and right perpendicular to the adaxial–abaxial axis. An important and exciting question is why are these new axes formed only in lateral organs? There are many genes with biased expression patterns in young dicot leaves. These genes are grouped into two types. One type, including PINHEAD (PHN) (also known as ZWILLE, ZLL) and SERRATE (SE ) in Arabidopsis, is expressed in the adaxial side of young leaves6,7. In the early embryo, PNH/ZLL is expressed throughout the embryo. As embryogenesis progresses, expression is restricted to the central provascular cells and to the adaxial site of cotyledon primordia. After germination, PNH/ZLL is strongly expressed in developing vascular strands and low-level expression is detected in the SAM and in
1360-1385/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S1360-1385(01)02155-0
Research Update
the adaxial side of leaf primordia. The pnh mutant is characterized by filamentous organs in place of leaves6. A second class of differentially expressed genes includes FILAMENTOUS FLOWER (FIL), YABBY3 (YAB3), CRABS CLAW (CRC) and INNOR NO OUTER (INO), which are expressed in the abaxial side of lateral organs8–11. In the fil mutant, some floral buds and floral organs are transformed into a filamentous structure whereas the fil yab3 double mutant has an enhanced phenotype with filamentous organs completely adaxilized9. The FIL, YAB3, CRC and INO proteins are highly homologous to each other, and grouped together as the YABBY family. These proteins are thought to function redundantly in the abaxialization of lateral organs. Continuity between the central–peripheral axis in the stem and the adaxial–abaxial axis in the leaf
Two recent publications have presented an interesting and important idea as to how the adaxial–abaxial axis of the leaf is formed12,13. Kathryn Barton and her colleagues studied PHABULOSA (PHB) and PHAVOLUTA (PHV) genes12. Both phb and phv mutants are dominant and cells on the abaxial leaf side change their characters to adaxial. PHB and PHV genes encode ATHB14 and ATHB9 proteins, respectively, which are members of the homeodomain-leucine zipper (HD-Zip) protein family, and belong to a subfamily that contains a sterol–lipid-binding domain. PHB is normally expressed not only on the adaxial side of leaf primordia but also in a central region of mid and late embryo development. These observations indicate that these genes promote adaxial identity. Further evidence came from an analysis of the KANADI (KAN) gene by Scott Poethig and his colleagues13. The recessive kan mutant was isolated as an enhancer of crc (Ref. 14). Leaves of the kan single mutant exhibit adaxialized characters on the abaxial side, indicating that the KAN promotes abaxial identity. The KAN gene encodes a nuclear-localized protein of the GARP family of putative transcription factors. Interestingly, KAN is expressed in the abaxial side of the leaf and in the peripheral region of embryos at the late globular and early heart stages. The most http://plants.trends.com
TRENDS in Plant Science Vol.6 No.12 December 2001
exciting point in this article is that when KAN was expressed ectopically in the embryo by the constitutive CaMV 35S promoter, the embryos had neither SAM nor vasculature and showed peripheral identity even in the central region. ‘…when KAN was expressed ectopically in the embryo by the constitutive CaMV 35S promoter, the embryos had neither SAM nor vasculature and showed peripheral identity even in the central region.’
These results strongly suggest that the adaxial–abaxial axis of lateral organs such as leaves is formed by the same mechanisms used to generate the central–peripheral axis of the embryo and the stem. The central–peripheral axis is the basis of the radial distribution of tissues in the hypocotyl, the stem and the root. This axis is formed in the early embryo and continuously functions in the SAM and the RAM. When leaf primordia develop at the peripheral zone of the SAM, a part of the radial pattern in the basal part of the SAM is cut out and the central–peripheral axis is remade as the abaxial–adaxial axis (Fig. 2). This idea is also supported by several other types of evidence. The distribution of xylem and phloem in the leaf vascular strand is identical to their orientation in the stem. Furthermore, auxiliary meristems always develop on the adaxial side of the leaf. This position can be considered analogous to the position of the SAM in the stem, which is in the middle of the top part of the stem. The SAM develops initially in the apical center of the embryo and remains at the center for the rest of the plant’s life. However, in the phb mutant, lateral shoot meristems are produced even in the abaxial side of the leaf because of the adaxialization of the leaf 15. In transgenic seedlings overexpressing KAN, the internal tissue of transgenic cotyledons resembles abaxial spongy mesophyll even on the adaxial side13. Although the evidence amassed to date is intriguing, this hypothesis needs to be tested carefully using additional molecular markers from the stele and the ground tissues. The genes expressed in either adaxial or abaxial regions, such as SE, PHB, PHV, KAN and members of the YABBY gene family, appear to play important roles in either adaxialization or abaxialization of
549
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TRENDS in Plant Science
Fig. 2. Continuity between the central–peripheral (c–p) axis in the stem and the adaxial–abaxial (ad–ab) axis in the leaf. (a) Horizontal section of basal part of the shoot apical meristem (SAM). (b) Vertical section of SAM. (c) Horizontal section of stem with a lateral shoot (L) and leaf. Abbreviations: X, xylem; Ph, phloem.
lateral organs depending on their expression patterns. Proteins encoded by these genes probably function to respond to positional information along the adaxial–abaxial axis. Identifying the downstream events mediated by these proteins is the next important question because most of these proteins are putative transcription factors. The downstream genes should specify cells depending on their positions. Other key questions include: how is the spatial expression of these genes controlled and how is the central–peripheral axis in the
550
Research Update
TRENDS in Plant Science Vol.6 No.12 December 2001
embryo, the stem and the root established and maintained? Acknowledgements
My sincere thanks to Philip Benfey, Hidehiro Fukaki and Mituhiro Aida for critical reading and comments on my manuscript. References 1 Wysocka-Diller, J.W. et al. (2000) Molecular analysis of SCARECROW function reveals a radial patterning mechanism common to root and shoot. Development 127, 595–603 2 Di Laurenzio, I. (1996) The SCARECROW gene regulates an asymmetric cell division that is essential for generating the radial organization of Arabidopsis roots. Cell 86, 423–433 3 Helariutta, Y. (2000) The SHORT-ROOT gene controls radial patterning of Arabidopsis root through radial signaling. Cell 101, 555–567 4 Long, J.A. and Barton, M.K. (1998) The development of apical embryonic pattern in Arabidopsis. Development 125, 3027–3035
5 Aida, M. et al. (1999) Shoot apical meristem and cotyledon formation during Arabidopsis embryogenesis: interaction among the CUP-SHAPED-COTYLEDON and SHOOT MERISTEMLESS genes. Development 126, 1563–1570 6 Lynn, K. et al. (1999) The PINHEAD/ZWILLE gene acts pleiotropically in Arabidopsis development and has overlapping functions with ARGONAUTE1 gene. Development 126, 469–481 7 Prigge, M.J. and Wagner, D.R. (2001) The Arabidopsis SERRATE gene encodes a zinc-finger protein required for normal shoot development. Plant Cell 13, 1263–1279 8 Sawa, S. et al. (1999) FILAMENTOUS FLOWER, a meristem and organ identity gene of Arabidopsis, encodes a protein with a zinc finger and HMGrelated domains. Genes Dev. 13, 1079–1088 9 Siegfried, K.R. (1999) Members of the YABBY gene family specify abaxial cell fate in Arabidopsis. Development 126, 4117–4128 10 Bowman, J.L. and Smyth, D.R. (1999) CRABS CLAW, a gene that regulates carpel and nectary development in Arabidopsis, encodes a novel protein with zinc finger and helix–loop–helix domains. Development 126, 2387–2396
11 Baker, S.C. and Robinson-Beers, K. (1997) Interactions among genes regulating ovule development in Arabidopsis thaliana. Genetics 145, 1109–1124 12 McConnell, J.R. et al. (2001) Role of PHABULOSA and PHAVOLUTA in determining radial patterning in shoots. Nature 411, 709–713 13 Kerstetter, R.A. et al. (2001) KANADI regulates organ polarity in Arabidopsis. Nature 411, 706–709 14 Eshed, Y. et al. (1999) Distinct mechanisms promote polarity establishment in carpels of Arabidopsis. Cell 99, 199–209 15 McConnell, J.R. and Barton, M.K. (1998) Leaf development and meristem formation in Arabidopsis. Development 125, 2935–2942
Masao Tasaka Graduate School of Biological Science, Nara Institute of Science and Technology, Ikoma, Nara 630-0101, Japan. e-mail: [email protected]
Root hairs, trichomes and the evolution of duplicate genes Elizabeth A. Kellogg The MYB-class proteins WEREWOLF and GLABRA1 are functionally interchangeable, even though one is normally expressed solely in roots and the other only in shoots. This shows that their different functions are the result of the modification of cis-regulatory sequences over evolutionary time. The two genes thus provide an example of morphological diversification created by gene duplication and changes in regulation.
How does a plant know to make root hairs on a root epidermis and trichomes on a leaf epidermis? Why don’t they ever get it mixed up? This question is all the more compelling because so much of the relevant machinery is the same at the two ends of the plant. An Arabidopsis protein, TRANSPARENT TESTA GLABRA (TTG), regulates trichome production in the leaf epidermis and root hair production in the root epidermis, but in opposite directions1. Mutations in TTG cause loss of trichomes on the leaf, but proliferation of root hairs. GLABRA2 (GL2), which operates downstream of TTG, also has opposite effects in the shoot and in the root, producing malformed trichomes on leaves http://plants.trends.com
and ectopic root hairs in the root1,2. The question is, how can TTG and GL2 have such different effects in different parts of the plant? WEREWOLF and GLABRA1 are interchangeable proteins
Functional comparisons of the MYB-class proteins WEREWOLF (WER) and GLABRA1 (GL1) have deepened the mystery. GL1 is expressed only in epidermal cells in the shoot, and is necessary for the production of trichomes on leaves; it acts at the same point genetically as TTG and itself regulates GL2 (Refs 3,4). Conversely, WER is expressed only in the root and hypocotyls in a subset of epidermal cells where it suppresses root hairs (in the root) and stomatal cells (in the hypocotyl)5. Like GL1, WER acts at the same point as TTG and itself regulates GL2. GL1 and WER might thus be alternate components of a pathway connecting TTG to GL2 and thence to determination of epidermal cell identity. A model for WER action was proposed by Myeong Min Lee and John Schiefelbein5 (Fig. 1) in which TTG activates an unknown bHLH protein, which then interacts with WER to activate GL2, thus blocking
root-hair formation in non-root hair cells. In cells where WER is not expressed, another protein with a truncated MYB domain, CAPRICE (CPC), represses GL2 and permits root-hair formation. An obvious and testable corollary is that GL1 could replace WER in the root. In a carefully controlled set of experiments, Lee and Schiefelbein6 have now shown that the transcriptional units of GL1 and WER are interchangeable, and that specificity is conferred by their upstream and downstream regulatory sequences. A pair of constructs was created in which the transcriptional unit of each gene was connected to the regulatory sequences (both 5′ and 3′) of the other gene. When the WER (regulatory)–GL1 (gene) construct was introduced into a wer mutant, the plants produced wild-type numbers of root hairs and stomata in roots and hypocotyls, respectively, and trichomes remained unaffected. Conversely, the GL1 (regulatory)–WER (gene) construct was able to rescue a gl1 mutation, and produced wild-type numbers of trichomes in the leaves. The constructs each regulated a GL2::GUS reporter gene in a pattern appropriate for each tissue.
1360-1385/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S1360-1385(01)02157-4
Research Update
TRENDS in Plant Science Vol.6 No.12 December 2001
From central–peripheral to adaxial–abaxial Masao Tasaka Higher plants are constructed of three organs – the stem, the root and the leaf. The stem and the root have two axes, apical–basal and central–peripheral, which cross orthogonally. Leaves develop from the shoot apical meristem as lateral organs that have three different axes, apical–basal, adaxial–abaxial and right–left. Recent data point to the possibility that the adaxial–abaxial axis in the leaf is formed from the central–peripheral axis in the stem.
The plant body is composed of several organs including the stem, root and leaf. The stem and root have the shoot apical meristem (SAM) and the root apical meristem (RAM), respectively, at their tip, and both form an ‘apical–basal’ axis (Fig. 1a). Cells in these organs are ranged along the apical–basal axis – the primary axis of elongation – according to their ages. There is a ‘central–peripheral’ axis orthogonal to the apical–basal axis in stems and roots. In dicots, the tissues from centre to periphery are the stele (including vascular bundles), ground tissue and epidermis. The ground tissue and epidermis are radially symmetrical. Because the stem and root are joined through the hypocotyl, these two axes are continuous. In Arabidopsis thaliana, the stem and the root partially share a common molecular mechanism, the SHORT ROOT (SHR) and SCARECROW (SCR) genes, which control the differentiation of stem and root tissues to generate the radial pattern of ground tissues such as the endodermis and the cortex1–3. During dicot embryogenesis, a single zygote develops into a mature embryo with distinct parts consisting of SAM, cotyledon, hypocotyl, root and RAM along the apical–basal axis. During this process, the embryo seems to establish the apical–basal axis initially with distinct polarity (Fig. 1b). Then, each of the embryonic organs and meristems differentiate at appropriate positions along the axis. The radial organization must also be established along the central–peripheral axis. In Arabidopsis embryos, SHR and SCR function to form http://plants.trends.com
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Fig. 1. Axes in dicot plant. (a) Apical–basal (a–b) axes in shoot, root and leaf, central–peripheral (c–p) axis in stem, and adaxial–abaxial (ad–ab) and right–left (r–l) axes in leaf. (b) Apical–basal and central–peripheral axes in heartstage embryo and adaxial–abaxial axis in cotyledon primordia. Top view of the heart stage embryo (left). (c) Vertical section of the shoot apical meristem (SAM). SAM is zoned into three parts: the central zone (CZ), the peripheral zone (PZ) and the rib zone (RZ). Leaf primordia are numbered as P0, P1 and P2 depending on their ages.
ground tissues in the hypocotyl as well as in the stem and the root1,3. Essentially, plant embryogenesis can be viewed as initial differentiation of the shoot and root fates. Two axes, apical–basal and central–peripheral, established during embryogenesis continuously function after germination for the rest of the plant’s life. The apical portion of the dicot embryo has the spatial bilateral symmetry to make two cotyledon primordia and a central shoot apical meristem. The apical region is proposed to have two independent patterns, radial and bilateral. These two are combined to develop two lateral cotyledon primordia at peripheral regions and the SAM in the middle4,5. The third major plant organ is the leaf, which is developed from the SAM as a lateral organ (Fig. 1c). The leaf has an apical–basal axis, an ‘adaxial–abaxial’ (dorsal–ventral) axis and a ‘right–left’ axis. There are three zones in the SAM – central, peripheral and rib – which are divided based on their position, and the
frequency and orientation of cell division. Leaves develop as primordia from the peripheral zone of the SAM with a specific phyllotaxis. The leaf primordia first elongates as a club in the apical orientation, and then leaf blades spread to the left and right perpendicular to the adaxial–abaxial axis. An important and exciting question is why are these new axes formed only in lateral organs? There are many genes with biased expression patterns in young dicot leaves. These genes are grouped into two types. One type, including PINHEAD (PHN) (also known as ZWILLE, ZLL) and SERRATE (SE ) in Arabidopsis, is expressed in the adaxial side of young leaves6,7. In the early embryo, PNH/ZLL is expressed throughout the embryo. As embryogenesis progresses, expression is restricted to the central provascular cells and to the adaxial site of cotyledon primordia. After germination, PNH/ZLL is strongly expressed in developing vascular strands and low-level expression is detected in the SAM and in
1360-1385/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S1360-1385(01)02155-0
Research Update
the adaxial side of leaf primordia. The pnh mutant is characterized by filamentous organs in place of leaves6. A second class of differentially expressed genes includes FILAMENTOUS FLOWER (FIL), YABBY3 (YAB3), CRABS CLAW (CRC) and INNOR NO OUTER (INO), which are expressed in the abaxial side of lateral organs8–11. In the fil mutant, some floral buds and floral organs are transformed into a filamentous structure whereas the fil yab3 double mutant has an enhanced phenotype with filamentous organs completely adaxilized9. The FIL, YAB3, CRC and INO proteins are highly homologous to each other, and grouped together as the YABBY family. These proteins are thought to function redundantly in the abaxialization of lateral organs. Continuity between the central–peripheral axis in the stem and the adaxial–abaxial axis in the leaf
Two recent publications have presented an interesting and important idea as to how the adaxial–abaxial axis of the leaf is formed12,13. Kathryn Barton and her colleagues studied PHABULOSA (PHB) and PHAVOLUTA (PHV) genes12. Both phb and phv mutants are dominant and cells on the abaxial leaf side change their characters to adaxial. PHB and PHV genes encode ATHB14 and ATHB9 proteins, respectively, which are members of the homeodomain-leucine zipper (HD-Zip) protein family, and belong to a subfamily that contains a sterol–lipid-binding domain. PHB is normally expressed not only on the adaxial side of leaf primordia but also in a central region of mid and late embryo development. These observations indicate that these genes promote adaxial identity. Further evidence came from an analysis of the KANADI (KAN) gene by Scott Poethig and his colleagues13. The recessive kan mutant was isolated as an enhancer of crc (Ref. 14). Leaves of the kan single mutant exhibit adaxialized characters on the abaxial side, indicating that the KAN promotes abaxial identity. The KAN gene encodes a nuclear-localized protein of the GARP family of putative transcription factors. Interestingly, KAN is expressed in the abaxial side of the leaf and in the peripheral region of embryos at the late globular and early heart stages. The most http://plants.trends.com
TRENDS in Plant Science Vol.6 No.12 December 2001
exciting point in this article is that when KAN was expressed ectopically in the embryo by the constitutive CaMV 35S promoter, the embryos had neither SAM nor vasculature and showed peripheral identity even in the central region. ‘…when KAN was expressed ectopically in the embryo by the constitutive CaMV 35S promoter, the embryos had neither SAM nor vasculature and showed peripheral identity even in the central region.’
These results strongly suggest that the adaxial–abaxial axis of lateral organs such as leaves is formed by the same mechanisms used to generate the central–peripheral axis of the embryo and the stem. The central–peripheral axis is the basis of the radial distribution of tissues in the hypocotyl, the stem and the root. This axis is formed in the early embryo and continuously functions in the SAM and the RAM. When leaf primordia develop at the peripheral zone of the SAM, a part of the radial pattern in the basal part of the SAM is cut out and the central–peripheral axis is remade as the abaxial–adaxial axis (Fig. 2). This idea is also supported by several other types of evidence. The distribution of xylem and phloem in the leaf vascular strand is identical to their orientation in the stem. Furthermore, auxiliary meristems always develop on the adaxial side of the leaf. This position can be considered analogous to the position of the SAM in the stem, which is in the middle of the top part of the stem. The SAM develops initially in the apical center of the embryo and remains at the center for the rest of the plant’s life. However, in the phb mutant, lateral shoot meristems are produced even in the abaxial side of the leaf because of the adaxialization of the leaf 15. In transgenic seedlings overexpressing KAN, the internal tissue of transgenic cotyledons resembles abaxial spongy mesophyll even on the adaxial side13. Although the evidence amassed to date is intriguing, this hypothesis needs to be tested carefully using additional molecular markers from the stele and the ground tissues. The genes expressed in either adaxial or abaxial regions, such as SE, PHB, PHV, KAN and members of the YABBY gene family, appear to play important roles in either adaxialization or abaxialization of
549
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Fig. 2. Continuity between the central–peripheral (c–p) axis in the stem and the adaxial–abaxial (ad–ab) axis in the leaf. (a) Horizontal section of basal part of the shoot apical meristem (SAM). (b) Vertical section of SAM. (c) Horizontal section of stem with a lateral shoot (L) and leaf. Abbreviations: X, xylem; Ph, phloem.
lateral organs depending on their expression patterns. Proteins encoded by these genes probably function to respond to positional information along the adaxial–abaxial axis. Identifying the downstream events mediated by these proteins is the next important question because most of these proteins are putative transcription factors. The downstream genes should specify cells depending on their positions. Other key questions include: how is the spatial expression of these genes controlled and how is the central–peripheral axis in the
550
Research Update
TRENDS in Plant Science Vol.6 No.12 December 2001
embryo, the stem and the root established and maintained? Acknowledgements
My sincere thanks to Philip Benfey, Hidehiro Fukaki and Mituhiro Aida for critical reading and comments on my manuscript. References 1 Wysocka-Diller, J.W. et al. (2000) Molecular analysis of SCARECROW function reveals a radial patterning mechanism common to root and shoot. Development 127, 595–603 2 Di Laurenzio, I. (1996) The SCARECROW gene regulates an asymmetric cell division that is essential for generating the radial organization of Arabidopsis roots. Cell 86, 423–433 3 Helariutta, Y. (2000) The SHORT-ROOT gene controls radial patterning of Arabidopsis root through radial signaling. Cell 101, 555–567 4 Long, J.A. and Barton, M.K. (1998) The development of apical embryonic pattern in Arabidopsis. Development 125, 3027–3035
5 Aida, M. et al. (1999) Shoot apical meristem and cotyledon formation during Arabidopsis embryogenesis: interaction among the CUP-SHAPED-COTYLEDON and SHOOT MERISTEMLESS genes. Development 126, 1563–1570 6 Lynn, K. et al. (1999) The PINHEAD/ZWILLE gene acts pleiotropically in Arabidopsis development and has overlapping functions with ARGONAUTE1 gene. Development 126, 469–481 7 Prigge, M.J. and Wagner, D.R. (2001) The Arabidopsis SERRATE gene encodes a zinc-finger protein required for normal shoot development. Plant Cell 13, 1263–1279 8 Sawa, S. et al. (1999) FILAMENTOUS FLOWER, a meristem and organ identity gene of Arabidopsis, encodes a protein with a zinc finger and HMGrelated domains. Genes Dev. 13, 1079–1088 9 Siegfried, K.R. (1999) Members of the YABBY gene family specify abaxial cell fate in Arabidopsis. Development 126, 4117–4128 10 Bowman, J.L. and Smyth, D.R. (1999) CRABS CLAW, a gene that regulates carpel and nectary development in Arabidopsis, encodes a novel protein with zinc finger and helix–loop–helix domains. Development 126, 2387–2396
11 Baker, S.C. and Robinson-Beers, K. (1997) Interactions among genes regulating ovule development in Arabidopsis thaliana. Genetics 145, 1109–1124 12 McConnell, J.R. et al. (2001) Role of PHABULOSA and PHAVOLUTA in determining radial patterning in shoots. Nature 411, 709–713 13 Kerstetter, R.A. et al. (2001) KANADI regulates organ polarity in Arabidopsis. Nature 411, 706–709 14 Eshed, Y. et al. (1999) Distinct mechanisms promote polarity establishment in carpels of Arabidopsis. Cell 99, 199–209 15 McConnell, J.R. and Barton, M.K. (1998) Leaf development and meristem formation in Arabidopsis. Development 125, 2935–2942
Masao Tasaka Graduate School of Biological Science, Nara Institute of Science and Technology, Ikoma, Nara 630-0101, Japan. e-mail: [email protected]
Root hairs, trichomes and the evolution of duplicate genes Elizabeth A. Kellogg The MYB-class proteins WEREWOLF and GLABRA1 are functionally interchangeable, even though one is normally expressed solely in roots and the other only in shoots. This shows that their different functions are the result of the modification of cis-regulatory sequences over evolutionary time. The two genes thus provide an example of morphological diversification created by gene duplication and changes in regulation.
How does a plant know to make root hairs on a root epidermis and trichomes on a leaf epidermis? Why don’t they ever get it mixed up? This question is all the more compelling because so much of the relevant machinery is the same at the two ends of the plant. An Arabidopsis protein, TRANSPARENT TESTA GLABRA (TTG), regulates trichome production in the leaf epidermis and root hair production in the root epidermis, but in opposite directions1. Mutations in TTG cause loss of trichomes on the leaf, but proliferation of root hairs. GLABRA2 (GL2), which operates downstream of TTG, also has opposite effects in the shoot and in the root, producing malformed trichomes on leaves http://plants.trends.com
and ectopic root hairs in the root1,2. The question is, how can TTG and GL2 have such different effects in different parts of the plant? WEREWOLF and GLABRA1 are interchangeable proteins
Functional comparisons of the MYB-class proteins WEREWOLF (WER) and GLABRA1 (GL1) have deepened the mystery. GL1 is expressed only in epidermal cells in the shoot, and is necessary for the production of trichomes on leaves; it acts at the same point genetically as TTG and itself regulates GL2 (Refs 3,4). Conversely, WER is expressed only in the root and hypocotyls in a subset of epidermal cells where it suppresses root hairs (in the root) and stomatal cells (in the hypocotyl)5. Like GL1, WER acts at the same point as TTG and itself regulates GL2. GL1 and WER might thus be alternate components of a pathway connecting TTG to GL2 and thence to determination of epidermal cell identity. A model for WER action was proposed by Myeong Min Lee and John Schiefelbein5 (Fig. 1) in which TTG activates an unknown bHLH protein, which then interacts with WER to activate GL2, thus blocking
root-hair formation in non-root hair cells. In cells where WER is not expressed, another protein with a truncated MYB domain, CAPRICE (CPC), represses GL2 and permits root-hair formation. An obvious and testable corollary is that GL1 could replace WER in the root. In a carefully controlled set of experiments, Lee and Schiefelbein6 have now shown that the transcriptional units of GL1 and WER are interchangeable, and that specificity is conferred by their upstream and downstream regulatory sequences. A pair of constructs was created in which the transcriptional unit of each gene was connected to the regulatory sequences (both 5′ and 3′) of the other gene. When the WER (regulatory)–GL1 (gene) construct was introduced into a wer mutant, the plants produced wild-type numbers of root hairs and stomata in roots and hypocotyls, respectively, and trichomes remained unaffected. Conversely, the GL1 (regulatory)–WER (gene) construct was able to rescue a gl1 mutation, and produced wild-type numbers of trichomes in the leaves. The constructs each regulated a GL2::GUS reporter gene in a pattern appropriate for each tissue.
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