Functional Specialization of the TRANSPARENT TESTA GLABRA1 Network Allows Differential Hormonal Control of Laminal and Marginal Trichome Initiation in Arabidopsis Rosette Leaves1[W]

Trichome initiation in Arabidopsis (Arabidopsis thaliana) is controlled by the TRANSPARENT TESTA GLABRA1 (TTG1) network that consists of R2R3- and R1-type MYB-related transcription factors, basic helix-loop-helix (bHLH) proteins, and the WD40 protein TTG1. An experimental method was designed to investigate the molecular mechanisms by which jasmonates, cytokinins, and gibberellins modulate Arabidopsis leaf trichome formation. All three phytohormones provoked a seemingly common effect on cell patterning by promoting trichome initiation but caused strikingly distinct effects on cell and trichome maturation. The phytohormonal control was mediated by transcriptional regulation of the established TTG1 complex and depended on the R2R3-MYB factor GLABRA1. However, unsuspected degrees of functional specialization of the bHLH factors and a resultant differential molecular regulation of trichome initiation on leaf lamina and leaf margins were revealed. Trichome formation on leaf lamina relied entirely on GLABRA3 and ENHANCER OF GLABRA3. Conversely, TRANSPARENT TESTA8 (TT8) was particularly important for marginal trichome development. This hitherto unknown role for TT8 in trichome formation further underscored the functional redundancy between the three TTG1-dependent bHLH proteins.