Brassinosteroids and Auxin Down-Regulate DELLA Genes in Fiber Initiation and Elongation of Cotton

Plant hormones play important roles in cotton fiber growth and development.However,the interaction of phytohormones is largely unknown in fiber cells up to now.DELLA proteins are critical component in GA （gibberellic acid） signal transduction,which are also regulated by other phytohormones,such as auxin and ethylene.To understand the regulation of DELLA genes in cotton fiber growth and development,we cloned four DELLA genes from upland cotton fibers （Gossypium hirsutum L.）,named GhGAI1,GhGAI2,GhGAI3,and GhGAI4.Alignment of the four predicted proteins with other reported DELLA proteins in various species displayed that they shared conserved domains and high homology.Expression profiles of the four GhGAIs in various tissues and organs as well as cotton fibers in different stages displayed that GhGAI1 has higher transcriptional levels than other GhGAIs in all detected samples.Furthermore,the expression level of GhGAI1 was significantly reduced in 0 dpa （day post anthesis） ovules by addition of IAA and epi-BL,and exogenous epi-BL decreased GhGAI1 level in 7 dpa fiber.Similarly,the levels of the other three GhGAIs in 0 dpa ovules and 7 dpa fibers were also regulated by applied phytohormones.In addition,the levels of GhGAI1 were higher in Xuzhou142 fl mutant （fuzzless-lintless） than in FL （Gossypium hirsutum vs.Xuzhou 142） from-1 to 3 dpa ovules,suggesting that GhGAI1 engaged in cotton fiber cell initiation.These results indicated that DELLA genes are involved in the process of fiber cell initiation and elongation regulated by different phytohormones.