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.

Two-dimensional patterning by a trapping/depletion mechanism: the role of TTG1 and GL3 in Arabidopsis trichome formation.

Trichome patterning in Arabidopsis serves as a model system to study how single cells are selected within a field of initially equivalent cells. Current models explain this pattern by an activator-inhibitor feedback loop. Here, we report that also a newly discovered mechanism is involved by which patterning is governed by the removal of the trichome-promoting factor TRANSPARENT TESTA GLABRA1 (TTG1) from non-trichome cells. We demonstrate by clonal analysis and misexpression studies that Arabidopsis TTG1 can act non-cell-autonomously and by microinjection experiments that TTG1 protein moves between cells. While TTG1 is expressed ubiquitously, TTG1-YFP protein accumulates in trichomes and is depleted in the surrounding cells. TTG1-YFP depletion depends on GLABRA3 (GL3), suggesting that the depletion is governed by a trapping mechanism. To study the potential of the observed trapping/depletion mechanism, we formulated a mathematical model enabling us to evaluate the relevance of each parameter and to identify parameters explaining the paradoxical genetic finding that strong ttg1 alleles are glabrous, while weak alleles exhibit trichome clusters.

A competitive complex formation mechanism underlies trichome patterning on Arabidopsis leaves.

Trichome patterning in Arabidopsis serves as a model system for de novo pattern formation in plants. It is thought to typify the theoretical activator-inhibitor mechanism, although this hypothesis has never been challenged by a combined experimental and theoretical approach. By integrating the key genetic and molecular data of the trichome patterning system, we developed a new theoretical model that allows the direct testing of the effect of experimental interventions and in the prediction of patterning phenotypes. We show experimentally that the trichome inhibitor TRIPTYCHON is transcriptionally activated by the known positive regulators GLABRA1 and GLABRA3. Further, we demonstrate by particle bombardment of protein fusions with GFP that TRIPTYCHON and CAPRICE but not GLABRA1 and GLABRA3 can move between cells. Finally, theoretical considerations suggest promoter swapping and basal overexpression experiments by means of which we are able to discriminate three biologically meaningful variants of the trichome patterning model. Our study demonstrates that the mutual interplay between theory and experiment can reveal a new level of understanding of how biochemical mechanisms can drive biological patterning processes.

Functional diversity of R3 single-repeat genes in trichome development.

Trichome and root hair patterning are governed by a conserved cassette of bHLH and MYB factors, the WD40 protein TTG1, and six single-repeat MYB R3 factors that are thought to counteract them. In this work we focus on the single-repeat R3 factor ETC3 and show that its major role is in the regulation of trichome density in a redundant manner. Diversification of the ETC3 gene has occurred at the promoter level, as etc3 mutants can be rescued by expressing ETC3 under the control of the TRY or CPC promoter. ETC3 movement was detected between epidermal cells as well as between the epidermis and underlying tissues. Finally, we found marked differences in the ability of the single-repeat R3 factors to interfere with the dimerisation of GL1 and GL3 in a yeast three-hybrid system, with CPC being the most potent inhibitor followed by ETC1, TRY, ETC3 and ETC2. Mathematical analysis predicts that this behaviour has a major impact on protein mobility, suggesting a tight reverse correlation between inhibitory function and the diffusion/transport range of the inhibitors. This prediction is supported by a comparison of CPC and ETC3 mobility in egl3 gl3 double mutants and 35S:GL3 lines.

Functional diversification of MYB23 and GL1 genes in trichome morphogenesis and initiation.

The functional diversification of duplicated genes is one of the driving forces in evolution. To understand the molecular mechanisms of gene diversification, we studied the functional relationship of the two Arabidopsis paralogous MYB-related genes GL1 and MYB23. We show that MYB23 controls trichome branching and trichome initiation at leaf edges. The latter is controlled redundantly together with GL1. We show that the two proteins are functionally equivalent during trichome initiation but not during trichome branching. RT-PCR and reporter construct analysis revealed spatial, temporal and genetic differences in transcriptional regulation of the GL1 and MYB23 genes. Presented data indicate that the diversification of GL1 and MYB23 gene functions occurred at the level of cis-regulatory sequences with respect to trichome initiation, and that, in parallel, the diversification with respect to regulation of trichome branching also involved changes in respective proteins.

ENHANCER of TRY and CPC 2 (ETC2) reveals redundancy in the region-specific control of trichome development of Arabidopsis.

An evolutionarily conserved set of proteins consisting of MYB and bHLH transcription factors and a WD40 domain protein is known to act in concert to control various developmental processes including trichome and root hair development. Their function is difficult to assess because most of them belong to multigene families and appear to act in a redundant fashion. In this study we identified an enhancer of the two root hair and trichome patterning mutants triptychon (try) and caprice (cpc), enhancer of try and cpc2 (etc2). The ETC2 gene shows high sequence similarity to the single-repeat MYB genes CPC and TRY. Overexpression results in the suppression of trichomes and overproduction of root hairs similarly as observed for TRY and CPC suggesting that ETC2 has similar biochemical properties. The etc2 single mutant shows an increase in trichome number on leaves and petioles. Double and triple mutant analysis indicates that the ETC2 gene acts redundant with TRY and CPC in trichome patterning.