Small RNA Regulation of Homoeologous Cotton Fiber Gene MYB2: Translational Research from Arabidopsis Trichome to Cotton Seed Hair Development

Cotton fibers are derived from protodermal cells in cotton seedcoat and similar to Arabidopsis leaf trichomes.  Cotton fibers are single cells and compose of nearly pure cellulose. The developmental regulation of cotton fiber and Arabidpsis leaf trichome shares common mechanisms at transcriptional and posttranscriptional levels. MYB2 is a cotton fiber-related gene and is functionally similar to the Arabidopsis glabrous1(GL1) that promotes the development of trichome in Arabidopsis thaliana and fiber in cotton. The most widely cultivated allotetraploid cotton Gossypium hirsutum (AADD) is derived from two extant species, G. arboreum (AA) and G. raimondii (DD). The former produces fiber, while the latter does not. The mechanism for this is unknown. Here we show functional divergence between homoeologous genes GhMYB2A(derived from AA genome) and GhMYB2D(derived from DD genome) in leaf trichome development. GhMYB2A and GhMYB2D-coding sequences are conserved except for 8 single nucleotide polymorphisms (SNPs), including several near the binding sites of two adjacent miRNAs. The difference of these two homoeologues in functional complementation in the gl1 glabrous phenotype is related to the cleavage efficiency by miRNAs. Moreover, cleavage of target transcripts generates ta-siRNAs, which are enriched during cotton fiber initiation and are predicted to play a role in fiber development. These data provide the genetic evidence for coevolution between miRNA-mediated silencing and target specificity of homoeologous genes that are important to selection and domestication of morphological traits including trichome and fiber.