Effects Of GhGA20ox1 Overexpression On Secondary Wall Biosynthesis Of Cotton Fiber

Cotton is an important industrial material and play an important role in our contry economy. Fiber qulity and yield have been the major targets of cotton for a long time. However, it is diffcult to improve these two traits simultaneously by traditional breeding, because they belong to quantitative traits and are negatively correlated. Gene engineering is an efficient strategy which can overcome reproductive obstacles between specieses and transfer good genes into cotton. Previously, many metabolic and regulatory genes related to fiber development have been cloned. But it is still to be elucidated how these genes effect fiber development.Gibberellin (GA) is an importan plant hormone involved in various processes of plant development. It was showed that GA play important role in regulating fiber development. Exogenous applied GA promote fiber elongation and increase fiber number per ovule. Overexpression of GhGA20oxl, a cotton GA 20-oxidase gene, efficiently increase endogenous GA cotent, and promote fiber iniation and early elongation.In the present researh, we described the effects of econdary wall biosynthesis and explored related molecular mechanism through physiological, cytological and transcriptional analyses.The main results were as followings:1. Fiber qulity- and yield-related traits in GhGA20oxl overexpressing cottonTo clarify the effects of GhGA20ox1 overexpression on seed and fiber, we determined seed traits and fiber quality. Compared to control, transgenic fiber show lower lint percentage, decreased micronaire value and increased fiber length and tenacity. By microscopic observation on the transverse section of mature fibers, we found that transgenic fiber have thinner cell wall and shorter perimeter. Those results show GhGA20ox1 overexpression can promote fiber elongation and inhibit cell wall thickening.2. Overexpressing GhGA20ox1 may delay secondary wall biosynthesisTo definite the effect of GhGA20ox1 overexpression on fiber development process, we compared the dynamics of cellulose biosynthesis and relative expressional level of marker genes(CesA1, CesA2 and Rac13) from 10DPA to 22DPA. We found rapid increase of fiber cellulose content in control and transgenic cotton occurred at 16-18DPA and 18-20DPA, respectively. It suggested that rapid cellulose biosynthesis period in transgenic cotton was delayed 1～2d. Meanwhile, the rapid increase of relative expressional levels of the three marker genes was also delayed. Taken togather, overexpressing GhGA20ox1 may delay secondary wall biosythesis in transgenic fibers.3. Digital gene expression analysis of GhGA20ox1 overexpressing fiberTo elucidate how GhGA20ox1 overexpression effect fiber development at transcriptional level, gene expressional level in 16DPA fiber from control and transgenic cotton was compared by digital gene expression analysis. We identified 96 differential expression genes including 25 up-regulated and 76 down-regulated genes. Those genes were involved in 33 pathways including sugar metabolism, secondary metabolism, oxidative phosphorylation, and so on. In flavonoid biosynthesis pathway, CCR was up-regulated and C4H, CHS, F3'H and F3'5'H were down-regulated. It implied that GhGA20ox1 overexpression prevent flavonoid biosynthsis in late period of fiber elongation...