Trichome Development, Genetics of

Tri c h o m e D eve l o pm e nt , G en e t ic s o f 2045 trees are often unrooted due to the limitations of available information.

domain and is assumed to execute its inhibitory function by directly moving to neighboring cells.

See also: Gene Trees; Genetic Distance; Phylogeny; Species Trees; Taxonomy, Numerical

Endoreduplication

Trichome Development, Genetics of M HuÈlskamp and J Mathur Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.1678

Plant hairs, also called trichomes, are specialized epidermal cells. On aerial organs trichomes have a protective role against insects or sun. In Arabidopsis thaliana trichomes are easily accessible and have become a genetic model system for the analysis of pattern formation and cell differentiation.

Genetic Dissection of Trichome Development in Arabidopsis In Arabidopsis, trichomes are unicellular, branched cells that are regularly distributed on most aerial surfaces. Systematic screens for trichome mutants in Arabidopsis revealed 37 complementation groups. The analysis of these mutants enabled the dissection of trichome development into distinct, genetically controlled steps (Figure 1): (1) initiation, (2) endoreduplication, (3) differentiation, (4) branching, (5) expansion, and (6) maturation.

Trichome Initiation On leaves, trichomes are initiated at the base in a field of dividing epidermal cells. The incipient trichome cells are separated by three to four epidermal cells and show a characteristic spacing pattern. Trichome patterning does not seem to involve cell lineage. Rather it is thought to be based on a mechanism where initially equivalent epidermal cells compete with each other via cell±cell interactions (Figure 2A). According to the current models GLABRA1 (GL1), a MYB-related transcription factor that is expressed in developing trichomes, and TRANSPARENT TESTA GLABRA1 (TTG1), a WD40 protein, function as positive regulators of trichome development. Epidermal cells surrounding a young trichome are inhibited from becoming trichomes by the negative regulator TRIPTYCHON (TRY) probably by downregulating the two positive regulators GL1 and TTG. TRY encodes a MYB-related protein lacking the activation

Incipient trichome cells stop cell divisions but proceed, on an average through four cycles of DNA replication (called endoreduplication). The number of endoreduplication cycles in trichomes is controlled by two genetic pathways. One pathway depends on the plant hormone gibberellin (GA). Mutants deficient in GA biosynthesis lack trichomes and a mutant, spindly (spy), that results in a constitutive activation of the GA signal transduction pathway displays trichomes with an increased DNA content (Figure 2B). In addition three genes control trichome endoreduplication in a GA independent pathway. Strikingly, two of them, GL1 and TRY, also play a role during trichome patterning, with GL1 promoting and TRY inhibiting additional endoreduplication cycles. In addition, the GL3 gene is required as a positive regulator. gl3 mutants undergo only three cycles of endoreduplication and since this phenotype can not be rescued by any other overreplicating mutant GL3 is assumed to act upstream of all other known genes.

Differentiation Mutants Five genes, GLABRA2, ROOT HAIRLESS1 (RHL1), RHL2, RHL3, and ECTOPIC ROOT HAIR3 (ERH3), appear to function early during trichome differentiation in the regulation of genes acting later. The corresponding mutants show a wide range of trichome phenotypes: trichome size and branching is generally reduced, and mutant trichomes often lack papillae on their surface. These phenotypic aspects resemble the single mutant phenotypes of other trichome morphogenesis mutants and it is therefore believed that the differentiation genes are required to integrate the function of later-acting trichome morphogenesis genes. Consistent with this idea is the finding that the cloning of the GLABRA2 gene revealed that it encodes a protein with sequence similarity to homeodomain transcription factors.

Branching Mutants Fifteen genes have been identified that function as positive or negative regulators of branch number. They fall into two groups. One group establishes a connection between the DNA content and branch number (Figure 2B,C). Accordingly, mutants with a reduced DNA content, e.g. glabra3, have fewer branches while mutants with an increased DNA content, e.g. triptychon, show more branches. Since changes in

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Figure 1 Trichome development mutants. Left: Schematic illustration of developmental steps during trichome formation. Right: Examples of mutants affecting various developmental steps. Abbreviations: try (triptychon), gl2 (glabra2), gl3 (glabra3), sti (stichel), klk (klunker) (a member of the distorted class of mutants), cha (chabli).

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Figure 2 Genetic models of trichome development. (A) Trichome cell selection. The genetic model postulates that GLABRA1 (GL1) and TRANSPARENT TESTA GLABRA1 (TTG) form a positive regulatory loop and trichome development is initiated by trichome differentiation genes such as GLABRA2 (GL2). Cell±cell communication is mediated by TRIPTYCHON (TRY) which is activated by TTG and downregulates GL1. As a result cells compete with each other to become a trichome cell. In the upper situation both cells are in an equilibrium. Below, the right, shaded cell has gained higher concentrations of GL1 and TTG and suppresses trichome development in the left cell. (B) Endoreduplication. The number of endoreduplication cycles is controlled by positive and negative regulators. Arrows indicate positive regulation events, blunted bars indicate negative regulatory events. Abbreviations: GA (gibberellin), GL1 (GLABRA1), GLABRA3 (GL3), SPY (SPINDLY), PYC (POLYCHOM), KAK (KAKTUS), RFI (RASTAFARI), TRIPTYCHON (TRY). (C) Branching. The number of branches is controlled by several independent pathways. Abbreviations: STI (STICHEL), FRC1 (FURCA1), FRC2 (FURCA2), FRC3 (FURCA3), FRC4 (FURCA4), STA (STACHEL), ZWI (ZWICHEL), AN (ANGUSTIFOLIA), NOK (NOEK).

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the DNA content in the wild-type background by using inhibitors of DNA replication or in tetraploid plants also result in a correlation between the DNA content and branch number it is unlikely that the mutants have two separate roles in the two processes. This suggests that either cell growth or cell size controls branch number. In the second group of mutants the DNA content is like in the wild-type. Genetically, they seem to act largely in independent pathways (Figure 2C). Only FURCA2 and STACHEL seem to function redundantly and downstream of ZWICHEL, FURCA4, and NOEK. The DNA content-related pathway seems to be mediated by ANGUSTIFOLIA. Presently the underlying molecular mechanisms are unknown. Only the ZWICHEL gene has been cloned. The ZWICHEL gene encodes a member of the kinesin superfamily of motor proteins that contains a calmodulin-binding site. It is therefore assumed to be either involved in the transport of important intracellular components or in the reorganization of microtubules prior to branch initiation. A role of the microtubules in branch formation is also suggested by drug inhibitor experiments: destabilization of microtubules results in unbranched trichomes and the stabilization of microtubules can trigger branch formation in the unbranched stichel mutant.

Trichome Expansion Eight genes, grouped in the DISTORTED class, are required to maintain the directionality of trichome cell expansion. Development of trichomes in distorted mutants is nearly normal until branch initiation while later growth is irregular resulting in mature trichomes displaying a twisted and distorted phenotype. Although none of these genes is cloned yet, an analysis of the cytoskeleton in these mutants suggests that the DISTORTED genes play a role in the organization of the actin cytoskeleton. All distorted mutants show strong abnormalities in the organization of the actin cytoskeleton. The biological relevance of the actin cytoskeleton in the expansion growth has been independently demonstrated with drug inhibitors. Drugs interferingwith theactin organizationresult in aphenotype indistinguishable from the distorted mutants.

Trichome Maturation During trichome maturation the cell wall thickens and small papilla are formed. This step is affected in five mutants, under developed trichome (udt), trichome birefringence (tbr), chablis (cha), chardonnay (cdo), and retsina (rts). These mutants appear transparent and may even collapse at some point. Only the tbr

mutant has been studied in some detail and was shown to be affected in cellulose deposition.

Further Reading

HuÈlskamp M (2000) How plants split hairs. Current Biology 10: R308±R310. HuÈlskamp M, Folkers U and Schnittger A (1999) Trichome development in Arabidopsis thaliana. International Review of Cytology 186: 147±178. Marks MD (1997) Molecular genetic analysis of trichome development in Arabidopsis. Annual Review of Plant Physiology and Plant Molecular Biology 48: 137±163. Oppenheimer D (1998) Genetics of plant cell shape. Current Opinion in Plant Biology 1: 520±524. Szymanski DB, Lloyd AM and Marks MD (2000) Progress in the molecular genetic analysis of trichome initiation and morphogenesis in Arabidopsis. Trends in Plant Sciences 5: 53.

See also: Arabidopsis thaliana: The Premier Model Plant; Cell Lineage

Trinucleotide Repeats: Dynamic DNA and Human Disease V Brown and S T Warren Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.1337

For many years a handful of heritable disorders puzzled geneticists by showing a tendency of the disease phenotype to become more severe or have earlier ageof-onset as the disease is passed on through subsequent generations in a family (Wells and Warren, 1998). This has been termed genetic anticipation, which is similar to the Sherman paradox described in fragile X syndrome, where the likelihood of having an affected child increases through subsequent generations of a pedigree (Warren and Sherman, 2000). In 1991, through research on spinal bulbar muscular atrophy and the fragile X syndrome, scientists discovered of a new class of genetic mutation termed trinucleotide repeat expansions or dynamic mutations (La Spada et al., 1991; Warren and Sherman, 2000). Understanding this novel type of mutation revealed in molecular terms the underlying mechanism of both genetic anticipation and the Sherman paradox. To date, at least 24 neurological diseases and 17 nonneurologic genetic diseases show evidence of genetic anticipation (Wells and Warren, 1998) and at least 20 genetic disorders have been linked to mutations in trinucleotide repeat tracts (Table 1).