Leaf trichome initiation and patterning by the MYB-bHLH-WD40 activator complex and R3 MYB lateral inhibitors

Arabidopsis leaf trichomes (unicellular epidermal hairs) are spaced by a cell-autonomous activator complex and mobile inhibitors. In the leaf primordium epidermis an MYB-bHLH-WD40 (MBW) activator complex forms from the R2R3-MYB GLABRA1 (GL1; MYB23 acts partially redundantly), the redundant bHLH proteins GLABRA3 (GL3) and ENHANCER OF GLABRA3 (EGL3), and the WD40-repeat protein TRANSPARENT TESTA GLABRA1 (TTG1); GL3/EGL3 bridge GL1 and TTG1. The active complex binds and activates the promoters of the HD-ZIP IV transcription factor GLABRA2 (GL2), which executes trichome differentiation, and of the single-repeat R3 MYB inhibitors CAPRICE (CPC), ENHANCER OF TRY AND CPC1 (ETC1) and TRIPTYCHON (TRY). The R3 MYB proteins lack an activation domain; they move into neighbouring epidermal cells, bind GL3/EGL3 and compete with GL1, so that inactive complexes form there and those cells remain pavement cells (lateral inhibition). TRY also represses its own promoter, and TTG1 depletion from cells surrounding a trichome by GL3-dependent trapping contributes a second, trapping/depletion patterning mechanism. TRY suppresses trichome clustering, CPC mainly sets trichome density, and try cpc double mutants convert all cells contacting a trichome into trichomes. Jasmonate (JAZ) and gibberellin (DELLA) repressors bind GL1 and GL3/EGL3 and modulate the complex; these hormone inputs, the root epidermis WER-GL3/EGL3-TTG1 module that specifies non-hair cells, and the seed coat/anthocyanin MBW complexes built on TT8 and TT2/PAP MYBs share components with this module but are outside its boundary.

Concrete Arabidopsis module grounded in the gene reviews for GL1, GL3, EGL3, TTG1, GL2, TRY and CPC (UniProt accessions from each gene folder's uniprot.txt). Partially redundant members without gene reviews (MYB23, ETC1, ETC2, ETC3, TCL1, TCL2, CPL3) are named in the description but not modelled as annotons. TT8 is excluded: it is the seed-coat/anthocyanin bHLH and is not required for leaf trichome initiation (gl3 egl3 is already glabrous; tt8 adds only the seed and anthocyanin defects). No GO-CAM in gocams/index.tsv contains any member gene, so no gocam_associations are given. No PANTHER family ids are asserted; PTHR47998 (official name "TRANSCRIPTION FACTOR MYB51-LIKE ISOFORM X1", see interpro/panther/PTHR47998/) was examined for the R3 MYB inhibitors and rejected: it is a broad MYB family (3365 proteins, 44 subfamilies, named for an R2R3 MYB) rather than one specific to the single-repeat inhibitor clade. Its InterPro representative structure, PDB 7FDO ("Crystal structure of transcription factor CPC in complex with EGL3"), is direct structural evidence for the CPC-EGL3 sequestration complex underlying the GO:0140416 call. The R3 MYB inhibitor molecular function is modelled as GO:0140416 transcription regulator inhibitor activity (competitive sequestration of the bHLH partner); whether R3 MYB-containing complexes also sit on DNA as inactive complexes remains unresolved.