Phase-specific enhancement of carotenoids and abscisic acid promotes secondary cell wall synthesis by activating key transcription factors and ethylene biosynthesis in cotton fiber

• Up-regulation of GhPSY2D and GhDXS6D enhances caroteniod and ABA accumulation in developing cotton fibers. • ABA initiates secondary cell wall formation in cotton fibers by activating key transcription factors (GhTCP4A, GhFSN1 and GhMYB7D) Cotton ( Gossypium ) is an important economic crop providing most of the natural fiber for the global textile industry. The secondary cell wall (SCW) comprises the major dry weight of cotton fiber, and is a key determinant of cotton yield and quality. In this study, a fiber-specific promoter, proFbl2A , was employed to control the expression of a fusion gene of phytoene synthase and 1-deoxy-D-xylulose-5-phosphate synthase ( GhPSY2D and GhDXS6D , respectively) in cotton fibers of the SCW synthesis stage, resulting in higher carotenoid and abscisic acid (ABA) levels in the transgenic cotton fibers. The SCW synthesis initiated earlier in the ABA-up-regulated cotton fibers than the wild-type control, along with the expression of SCW stage-specific genes and key SCW regulators. Consistently, several positive bZIP transcription factors of ABA signaling (GhbZIP27b, GhbZIP37b, and GhbZIP66b), were found to bind to and activate the promoters of key SCW regulators ( GhTCP4A, GhFSN1 , and GhMYB7D ). Furthermore, these bZIPs could also interact with and promote the expression of two ethylene synthase genes ( GhACS10 and GhACO3 ). Our data demonstrated that enhancement of carotenoid and ABA could advance SCW initiation by activating key transcription factors, and promote SCW thickening via ethylene biosynthesis in cotton fibers.