Arabidopsis transient expression analysis reveals that activation of GLABRA2 may require concurrent binding of GLABRA1 and GLABRA3 to the promoter of GLABRA2

Trichomes as models for studying plant cell differentiation

Trichomes, originating from epidermal cells, are present on nearly all terrestrial plants. They exist in diverse forms, are readily accessible, and serve as an excellent model system for analyzing the molecular mechanisms in plant cell differentiation, including cell fate choices, cell cycle control, and cell morphogenesis. In Arabidopsis, two regulatory models have been identified that function in parallel in trichome formation; the activator-inhibitor model and the activator-depletion model. Cotton fiber, a similar unicellular structure, is controlled by some functional homologues of Arabidopsis trichome-patterning genes. Multicellular trichomes, as in tobacco and tomato, may form through a distinct pathway from unicellular trichomes. Recent research has shown that cell cycle control participates in trichome formation. In this review, we summarize the molecular mechanisms involved in the formation of unicellular and multicellular trichomes, and discuss the integration of the cell cycle in its initiation and morphogenesis.

Nucleotide sequence variation of GLABRA1 contributing to phenotypic variation of leaf hairiness in Brassicaceae vegetables

GLABRA1 (GL1) belongs to the group of R2R3-MYB transcription factors and is known to be essential for trichome initiation in Arabidopsis. In our previous study, we identified a GL1 ortholog in Brassica rapa as a candidate for the gene controlling leaf hairiness by QTL analysis and suggested that a 5-bp deletion (B-allele) and a 2-bp deletion (D-allele) in the exon 3 of BrGL1 and a non-synonymous SNP (C-allele) in the second nucleotide of exon 3 possibly cause leaf hairlessness. In this study, we transformed a B. rapa line having the B-allele with the A-allele (wild type) or the C-allele of BrGL1 under the control of the CaMV 35S promoter. The transgenic plants with the A-allele showed dense coverage of seedling tissues including stems, young leaves and hypocotyls with trichomes, whereas the phenotypes of those with the C-allele were unchanged. In order to obtain more information about allelic variation of GL1 in different plant lineages and its correlation with leaf hairiness, two GL1 homologs, i.e., RsGL1a and RsGL1b, in Raphanus sativus were analyzed. Allelic variation of RsGL1a between a hairless line and a hairy line was completely associated with hairiness in their BC1F1 population. Comparison of the full-length of RsGL1a in the hairless and hairy lines showed great variation of nucleotides in the 3' end, which might be essential for its function and expression.

From landing lights to mimicry: the molecular regulation of flower colouration and mechanisms for pigmentation patterning

Flower colour is a key component for plant signaling to pollinators and a staggering variety of colour variations are found in nature. Patterning of flower colour, such as pigment spots or stripes, is common and is important in promoting pollination success. Developmentally programmed pigmentation patterns are of interest with respect to the evolution of specialised plant-pollinator associations and as models for dissecting regulatory signaling in plants. This article reviews the occurrence and function of flower colour patterns, as well as the molecular genetics of anthocyanin pigmentation regulation. The transcription factors controlling anthocyanin biosynthesis have been characterised for many species and an 'MBW' regulatory complex of R2R3MYB, bHLH and WD-Repeat proteins is of central importance. In particular, R2R3MYBs are key determinants of pigmentation intensity and patterning in plants. Progress is now being made on how environmental or developmental signal pathways may in turn control the production of the MBW components. Furthermore, additional regulatory proteins that interact with the MBW activation complex are being identified, including a range of proteins that repress complex formation or action, either directly or indirectly. This review discusses some of the recent data on the regulatory factors and presents models of how patterns may be determined.

The patterning of epidermal hairs in Arabidopsis--updated

Epidermal hairs of Arabidopsis thaliana emerge in regular spacing patterns providing excellent model systems for studies of biological pattern formation. A number of root-hair and leaf-trichome patterning mutants and tools for cell-specific and tissue-specific manipulation of patterning protein activities have been combined in cycles of experimentation and mathematical modelling. These approaches have provided insight into molecular mechanisms of epidermal patterning. During the last two years, endoreplication has, unexpectedly, been found to control cell-fate maintenance during trichome patterning. New genetic interactions between a downstream, positive transcriptional regulator and lateral inhibitors of trichome or non-root-hair fate specification have been uncovered. A lateral inhibitor and a new positive regulator have been identified as major loci affecting trichome patterning in natural Arabidopsis populations. Finally, factors that modify root-hair patterning from the underlying cell layer have been discovered.

Functional characterization of TRICHOMELESS2, a new single-repeat R3 MYB transcription factor in the regulation of trichome patterning in Arabidopsis

BACKGROUND

Single-repeat R3 MYB transcription factors (single-repeat MYBs) play important roles in controlling trichome patterning in Arabidopsis. It was proposed that single-repeat MYBs negatively regulate trichome formation by competing with GLABRA1 (GL1) for binding GLABRA3/ENHANCER OF GLABRA3 (GL3/EGL3), thus inhibiting the formation of activator complex TTG1(TRANSPARENT TESTA GLABRA1)-GL3/EGL3-GL1 that is required for the activation of GLABRA2 (GL2), whose product is a positive regulator of trichome formation. Previously we identified a novel single-repeat MYB transcription factor, TRICHOMELESS1 (TCL1), which negatively regulates trichome formation on the inflorescence stems and pedicels by directly suppressing the expression of GL1.

RESULTS

We analyzed here the role of TRICHOMELESS2 (TCL2), a previously-uncharacterized single-repeat MYB transcription factor in trichome patterning in Arabidopsis. We showed that TCL2 is closely related to TCL1, and like TCL1 and other single-repeat MYBs, TCL2 interacts with GL3. Overexpression of TCL2 conferred glabrous phenotype while knockdown of TCL2 via RNAi induced ectopic trichome formation on the inflorescence stems and pedicels, a phenotype that was previously observed in tcl1 mutants. These results suggested that TCL2 may have overlapping function with TCL1 in controlling trichome formation on inflorescences. On the other hand, although the transcription of TCL2, like TCL1, is not controlled by the activator complex formed by GL1 and GL3, and TCL2 and TCL1 proteins are more than 80% identical at the amino acid level, the expression of TCL2 under the control of TCL1 promoter only partially recovered the mutant phenotype of tcl1, implying that TCL2 and TCL1 are not fully functional equivalent.

CONCLUSIONS

TCL2 function redundantly with TCL1 in controlling trichome formation on inflorescences, but they are not fully functional equivalent. Transcription of TCL2 is not controlled by activator complex formed by GL1 and GL3, but MIR156 controlled SQUAMOSA PROMOTER BINDING PROTEIN LIKE (SPL) transcription factors. However, SPLs might require co-activators to regulate the expression of their target genes, including TCL1, TRY and possibly, TCL2.

Activity of transcription factor JACKDAW is essential for SHR/SCR-dependent activation of SCARECROW and MAGPIE and is modulated by reciprocal interactions with MAGPIE, SCARECROW and SHORT ROOT

Two GRAS family transcription factors, SHORT-ROOT (SHR) and SCARECROW (SCR), are required for ground tissue and quiescent center formation in Arabidopsis roots. The action of SHR and SCR is regulated by two INDETERMINATE DOMAIN (IDD) family proteins, JACKDAW (JKD) and MAGPIE (MGP). Although the reciprocal interaction of these transcription factors is considered to be involved in the modulation of SHR and SCR action by JKD and MGP, the underlying mechanism remains unclear. In this study, we use a transient assay with Arabidopsis culture cells to show that the physical interaction of these transcription factors modulate their transcriptional activity. Transient expression of LUC reporter genes with the proximal sequences upstream from the ATG codon of SCR and MGP in protoplasts were activated by JKD. Moreover, promoter activities were enhanced further by the addition of SHR and SCR to JKD, but not by the combination of SHR and SCR in the absence of JKD. Yeast one-hybrid analysis showed that JKD binds to the SCR and MGP promoter sequences, indicating the existence of another binding sequences of JKD different from the previously determined IDD binding sequence. Our findings suggest that JKD directly regulates SCR and MGP expression in cooperation with SHR, SCR and MGP.

Differential regulation of a MYB transcription factor is correlated with transgenerational epigenetic inheritance of trichome density in Mimulus guttatus

• Epigenetic inheritance, transgenerational transmission of traits not proximally determined by DNA sequence, has been linked to transmission of chromatin modifications and gene regulation, which are known to be sensitive to environmental factors. Mimulus guttatus increases trichome (plant hair) density in response to simulated herbivore damage. Increased density is expressed in progeny even if progeny do not experience damage. To better understand epigenetic inheritance of trichome production, we tested the hypothesis that candidate gene expression states are inherited in response to parental damage. • Using M. guttatus recombinant inbred lines, offspring of leaf-damaged and control plants were raised without damage. Relative expression of candidate trichome development genes was measured in offspring. Line and parental damage effects on trichome density were measured. Associations between gene expression, trichome density, and response to parental damage were determined. • We identified M. guttatus MYB MIXTA-like 8 as a possible negative regulator of trichome development. We found that parental leaf damage induces down-regulation of MYB MIXTA-like 8 in progeny, which is associated with epigenetically inherited increased trichome density. • Our results link epigenetic transmission of an ecologically important trait with differential gene expression states - providing insight into a mechanism underlying environmentally induced 'soft inheritance'.

Analysis of purified glabra3-shapeshifter trichomes reveals a role for NOECK in regulating early trichome morphogenic events

Transcriptome analysis using the Affymetrix ATH1 platform has been completed on purified trichomes from the gl3-sst mutant. These trichomes display immature features, such as glassy cell walls and blunted branches. The gl3-sst trichome transcriptome was greatly enriched for genes involved in lipid biosynthesis, including those mediating the synthesis of fatty acids and wax. In addition, gl3-sst trichomes displayed reduced expression of the R3 MYBs TRY and CPC, which normally function to limit trichome development. The expression of the MIXTA-like MYB gene NOK was elevated. Members of the MIXTA-like family promote conical cell outgrowth, and in some cases, trichome initiation in diverse plant species. In contrast, NOK limits trichome outgrowth in wild-type Arabidopsis plants. Similar to other MIXTA-like genes, NOK was required for the expansion of gl3-sst trichomes, as the gl3-sst nok double mutant trichomes were greatly reduced in size. Expression of NOK in nok mutants reduced branch formation, whereas in gl3-sst nok, NOK expression promoted trichome cell outgrowth, illustrating duel roles for NOK in both promoting and limiting trichome development. MIXTA-like genes from phylogenetically diverse plant species could substitute for NOK in both nok and gl3-sst nok backgrounds. These findings suggest that certain aspects of NOK and MIXTA-like gene function have been conserved.

Endoreplication controls cell fate maintenance

Cell-fate specification is typically thought to precede and determine cell-cycle regulation during differentiation. Here we show that endoreplication, also known as endoreduplication, a specialized cell-cycle variant often associated with cell differentiation but also frequently occurring in malignant cells, plays a role in maintaining cell fate. For our study we have used Arabidopsis trichomes as a model system and have manipulated endoreplication levels via mutants of cell-cycle regulators and overexpression of cell-cycle inhibitors under a trichome-specific promoter. Strikingly, a reduction of endoreplication resulted in reduced trichome numbers and caused trichomes to lose their identity. Live observations of young Arabidopsis leaves revealed that dedifferentiating trichomes re-entered mitosis and were re-integrated into the epidermal pavement-cell layer, acquiring the typical characteristics of the surrounding epidermal cells. Conversely, when we promoted endoreplication in glabrous patterning mutants, trichome fate could be restored, demonstrating that endoreplication is an important determinant of cell identity. Our data lead to a new model of cell-fate control and tissue integrity during development by revealing a cell-fate quality control system at the tissue level.

Differentiating cells often amplify their nuclear DNA content through a special cell-cycle variant, called endoreplication, in which cell division is skipped. Although this process is widespread from humans to plants, not much is currently known about the biological importance of endoreplication. Moreover, the control of cell-cycle activities has been thought to follow developmental decisions and the adoption of a specific cell fate. Here we have uncovered a previously unrecognized function of endoreplication in maintaining cell identity, presenting a striking example of how cell fate and cell-cycle progression are linked. Using leaf hairs on the reference plant Arabidopsis as a model, we show that compromising endoreplication leads to dedifferentiation of the newly forming leaf hair cell. Live observations of young Arabidopsis leaves revealed that dedifferentiating leaf hairs underwent repeated rounds of cell division and were re-integrated into the epidermal cell layer acquiring the typical characteristics of the surrounding epidermal cells. Conversely, promoting endoreplication in mutants that fail to develop hairs could at least partially restore their differentiation program. With this, our findings also pinpoint an important role of the social context of a cell, revealing a differentiation control system at the tissue level.

Trichome patterning in Arabidopsis thaliana from genetic to molecular models

The aerial organs of plants typically produce trichomes that may adopt various functions, including light, wind, frost, and herbivore protection. Trichomes are of epidermal origin regularly distributed on the surface. The mechanism by which trichome differentiation is triggered in individual cells in a field of protodermal cells is best studied in Arabidopsis thaliana. The genetic analysis has revealed a number of key genes controlling this patterning process, and further molecular analysis has enabled the in-depth cell-biological and biochemical analysis. The established models explain trichome patterning by the mutual interaction between positive and negative factors. Three activators, a bHLH (helix-loop-helix), a R2R3 MYB-related transcription factor, and a WD40 domain protein, form an active complex. The activity of this complex is counteracted by R3 MYB factors that compete with the R2R3 MYB for binding to the bHLH factor. The R3 MYBs can move between cells and thereby mediate cellular interactions. This general model cannot explain all genetic observations and recent data suggest the existence of several parallel patterning mechanisms. In this chapter we aim to summarize the current data and sketch possible alternative, not mutually exclusive theoretical models.

Distinct relationships between GLABRA2 and single-repeat R3 MYB transcription factors in the regulation of trichome and root hair patterning in Arabidopsis

*The patterning of epidermal cell types in Arabidopsis is an excellent model for studying the molecular basis of cell specification. Trichome and root hair formation is controlled by a transcriptional activator complex that induces the homeobox gene GLABRA2 (GL2) and some single-repeat R3 MYB genes (single MYB). However, it remains unclear how the actions of GL2 and single MYBs are coordinated to regulate epidermal patterning. *GL2 is thought to act downstream of single MYBs to regulate trichome and root hair development. In order to test this hypothesis genetically, double and higher order mutants between gl2 and single myb were generated. *In these mutants, the glabrous phenotypes observed in the gl2 single mutants were partially recovered, suggesting that single MYBs may not act solely through GL2 to regulate trichome development. On the other hand, double and higher order mutants between gl2 and single myb phenocopied the root hair phenotype of gl2 single mutants, suggesting that GL2 and single MYBs act in a common pathway to regulate root hair patterning. *These findings reveal distinct relationships between GL2 and single MYBs in the regulation of trichome vs root hair development, and provide new insights into the molecular mechanism of epidermal patterning.

One, two, three...models for trichome patterning in Arabidopsis?

Trichome patterning in Arabidopsis rosette leaves serves as a model system to study how individual cells are determined in a regular spacing pattern from initially equivalent cells. A conserved gene cassette regulates this spacing pattern. bHLH, MYB, and WD40 factors are positive regulators of trichome development that are inhibited by R3 single-repeat MYB proteins. One positive regulator, the WD40-protein, and the negative regulators are mobile and are transported in opposite directions: the WD40 factor moves to the negative regulators away from trichome initials. This movement behavior and the genetic and molecular interactions between the contributing genes and proteins make it very difficult to derive the mechanistic core behind pattern formation. Attempts to develop mathematical models toward this end are critically discussed.