Members of the MYBMIXTA-like transcription factors may orchestrate the initiation of fiber development in cotton seeds

MYBMIXTA-like (MML) transcription factors form the subgroup 9 of R2R3MYBs (Stracke et al., 2001) whose first characterized member was MIXTA from Antirrhinum majus. Various MML genes have been shown to be important regulators of epidermal cell differentiation in different plant species, including specifying cell shape in petals, vegetative trichome initiation and branching and seed fiber initiation (Martin et al., 2002; Machado et al., 2009; Walford et al., 2011). Indeed, the conical cells of petals look very much like young fibers shortly after they protrude from the epidermal surface of the cotton seed and begin to elongate, so it is likely there is some commonality in cellular regulation between the different tissue types. In tetraploid cotton, Gossypium hirsutum L. (Gh), the silencing or over-expression of two MML genes (GhMYB25 and GhMYB25Like) expressed predominantly in the ovule epidermis during fiber initiation affect the initiation or timing of expansion of fiber initials (Machado et al., 2009; Walford et al., 2011). Based on silencing phenotypes and gene expression data, GhMYB25Like may be one of the most upstream genes in a regulatory cascade currently known to involve GhMYB25, GhMYB109 (an R2R3 MYB from subgroup 15, Pu et al., 2008) and other types of transcription factors, such as the homeodomain leucine zipper (HDzip) factor GhHD-1, that may act in a protein complex with GhMYB25 (Zhang et al., 2010; Walford et al., 2012). The recent release of the genome sequence from the diploid G. raimondii (Gr), an extant species most closely related to the D-genome progenitor of tetraploid cotton, may help draw a more complete picture about the evolution of the MML gene subgroup in cotton and their apparent expansion and recruitment to specialized functions in epidermal seed fiber development.