GbPDF1 Is Involved in Cotton Fiber Initiation via the Core cis-Element HDZIP2ATATHB2

Cotton (Gossypium spp.) fiber cells are seed trichomes derived from the epidermal layer of the cotton seed coat. The molecular components responsible for regulating fiber cell differentiation have not been fully elucidated. A cotton PROTODERMAL FACTOR1 gene (GbPDF1) was found to be expressed preferentially during fiber initiation and early elongation, with highest accumulation in fiber cells 5 d post anthesis. PDF1 silencing caused retardation of fiber initiation and produced shorter fibers and lower lint percentage compared with the wild type, indicating that the gene is required for cotton fiber development. Further analysis showed that a higher accumulation of hydrogen peroxide occurred in the RNA interference transgenic cotton lines. Meanwhile, the expression of several genes related to ethylene and pectin synthesis or sugar transport during cotton fiber growth was found to be significantly reduced in the PDF1-suppressed cotton. Three proteins interacting with GbPDF1 in yeast and in planta might involve cellular signaling or metabolism. GbPDF1 promoter::GUS constructs in transgenic cotton were predominantly expressed in the epidermis of ovules and developing fibers. Progressive deletions of the GbPDF1 promoter showed that a 236-bp promoter fragment was sufficient for basal GbPDF1 transcription in cotton. Mutation of putative regulatory sequences showed that HDZIP2ATATHB2, an element within the fragment, was essential for PGbPDF1-1 expression. The binding activity between this cis-element and nuclear extracts from fiber-bearing cotton ovules at 5 d post anthesis was specific. We conclude that GbPDF1 plays a critical role together with interaction partners in hydrogen peroxide homeostasis and steady biosynthesis of ethylene and pectin during fiber development via the core cis-element HDZIP2ATATHB2.