Identification and Profiling of Known and Novel Fiber MicroRNAs during the Secondary Wall Thickening Stage in Cotton (Gossypium hirsutum) via High-Throughput Sequencing

Upland cotton (Gossypium hirsutum L.) is an allotetraploid species originated from interspecific hybridization between AA-genome diploid (G.arboretum) and DD-genome diploid (G.raimondii) (Wendel et al.,1992).Cotton fibers are singlecelled trichomes that emerge from the ovule epidermal cells.Indexed by the number of days post-anthesis (dpa),fiber morphogenesis includes four distinct but overlapping steps:initiation (0-3 dpa),elongation (3-20 dpa),secondary cell wall thickening (15-45 dpa) and maturation (40-60 dpa) (Yang et al.,2008,Du et al.,2013).The efficiency and duration of each morphogenesis stage is important to the quality attributes of the mature fiber.Cell elongation is critical for fiber length,whereas secondary cell wall thickening is important for fiber fineness and strength (Meinert and Delmer,1977).