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.