Functionally differentiated GL2-interacting-repressor 1 homoeologs regulate epidermal hair development of Gossypium hirsutum

Cotton (Gossypium. spp) stem trichomes and seed fibers are unicellular epidermal hairs regulated by relevant molecular networks. Genetic analysis of the glabrous stem landrace (Palmeri37) of Gossypium hirsutum L. has pinpointed GL2-interacting-repressor 1 (GhGIR1D), featuring a RING-like zinc finger domain, as the candidate gene underlying the stem glabrous trait. Overexpressing and silencing experiments confirm GhGIR1D as a negative regulator specifically for stem trichome initiation, without influencing leaf trichome or seed fiber. High endogenous expression of GhGIR1D is associated with a SNP-573 T/G variation within the promoter region, and GhGIR1D Hap I confers the absence of stem trichomes. The homoeologous protein, GhGIR1A, inhibits trichome and fuzz fiber initiation by disrupting the GhHD1D-GhHOX3A module (a complex of 2 Homeodomain-Leucine Zipper IV transcription factors, HD-ZIP IV), and repressing downstream transcription of GhRDL1. Trichome density was enhanced in GhHD1A overexpression transgenic lines but reduced in ghhd1a mutants, demonstrating the positive regulatory role of GhHD1A on stem trichome initiation. GhHD1 displays distinct effects on stem trichome and fuzz fiber initiation due to its tissue-specific dosage. In the ghhd1a mutants, repression of GhGIR1D restores the wild-type pubescent phenotype, suggesting the presence of a potential negative feedback loop between GhGIR1D and GhHD1A, or that GhGIR1D and GhHD1A may function within the same regulatory pathway with opposite functions in regulation of trichome development. These findings enhance our comprehension of the GhGIR1-GhHD1-GhHOX3 interaction module in epidermal hair initiation and development.