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Abstract Background Trichome patterning in Arabidopsis thaliana is governed by three types of activators, R2R3MYB, bHLH and WD40 proteins, and six R3MYB inhibitors. Among the inhibitors TRIPTYCHON (TRY) seems to fulfill a special function. Its corresponding mutants produce trichome clusters whereas all other inhibitors are involved in trichome density regulation. Results To better understand the role of TRY in trichome patterning we analyzed its transcriptional regulation. A promoter analysis identified the relevant regulatory region for trichome patterning. This essential region contains a fragment required for a double negative feedback loop such that it mediates the repression of TRY/CPC auto-repression. By transforming single cells of pTRY:GUS lines with p35S:GL1, p35S:GL3 and p35S:TTG1 in the presence or absence of p35S:TRY or p35S:CPC we demonstrate that TRY and CPC can suppress the TRY expression without the transcriptional down regulation of the activators. We further show by promoter/CDS swapping experiments for the R3MYB inhibitors TRY and CPC that the TRY protein has specific properties relevant in the context of both, cluster formation and trichome density. Conclusions Our identification of a TRY promoter fragment mediating a double negative feedback loop reveals new insight in the regulatory network of the trichome patterning machinery. In addition we show that the auto-repression by TRY can occur without a transcriptional down regulation of the activators, suggesting that the differential complex formation model has a biological significance. Finally we show that the unique role of TRY among the inhibitors is a property of the TRY protein.

The GLABRA3 gene is a major regulator of trichome patterning in Arabidopsis thaliana. The regulatory regions important for the trichome-specific expression of GL3 have not been characterized yet. In this study, we used a combination of marker and rescue constructs to determine the relevant promoter regions. We demonstrate that a 1 kb 50 region combined with the second intron is sufficient to rescue the trichome mutant phenotype of gl3 egl3 mutants. Swap experiments of the second intron suggest that it is not sufficient to generally enhance the expression level of GL3. This implies that the second intron contains regulatory regions for the temporal and spatial regulation of GL3. The corresponding GUS-marker constructs revealed trichome-specific expression in young trichomes. [ABSTRACT FROM AUTHOR]

Trapping of a protein in a specialized cell is shown to cause a depletion of this protein in surrounding cells, thus regulating the spacing pattern produced in plant hair cells. [ABSTRACT FROM AUTHOR]

Protein-protein interactions are fundamental to virtually every aspect of cellular functions. Blue, green and yellow bimolecular fluorescence complementation (BiFC) systems based on GFP and its variants allow the investigation of protein-protein interactions in vivo. We have developed the first red BiFC system based on an improved monomeric red fluorescent protein (mRFP1-Q66T), expanding the range of possible applications for BiFC. [ABSTRACT FROM AUTHOR]

Trichome and root hair patterning is governed by a gene regulatory network involving TTG1 and several homologous MYB and bHLH proteins. The bHLH proteins GL3 and EGL3 are core components that serve as a regulatory platform for the activation of downstream genes. In this study we show that a homologue of GL3 and EGL3, AtMYC1, can regulate the intracellular localisation of GL1 and TRY. AtMYC1 protein is predominantly localised in the cytoplasm and can relocate GL1 from the nucleus into the cytoplasm. Conversely, AtMYC1 can be recruited into the nucleus by TRY and CPC, concomitant with a strong accumulation of TRY and CPC in the nucleus. When AtMYC1 is targeted to the nucleus or cytoplasm by nuclear localisation or export signals (NLS or NES), respectively, the intracellular localisation of GL1 and TRY also changes accordingly. The biological significance of this intracellular localisation is suggested by the finding that the efficiency of rescue of trichome number is significantly altered in NES and NLS fusions as compared with wild-type AtMYC1. Genetic analysis of mutants and overexpression lines supports the hypothesis that AtMYC1 represses the activity of TRY and CPC.

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., (Copyright 2010 Elsevier Inc. All rights reserved.)