| Cotton is an important economic crop in the world and its fibre can be used for producing various commodities. Many studies show that plant hormones, such as auxin, gibberellic acid, ethylene, and etc., play a vital role during the fiber growth and development. It is proved that the post-anthesis treatment of gibberellic acid alone increases the fiber production, but the molecular mechanism how gibberellic acid effect the initiation and development of fibre cell is still unknown. Bioactive gibberellins (GAs) are plant hormones that control the plant growth and affect many plant development processes. The DELLA protein, as a repressor in the GA signal pathway, can repress most growth and development processes regulated by GA. GA regulates the plant growth and development processes by inducing the degradation of DELLA proteins via the ubiquitin/proteasome pathway. Therefore DELLAs, playing an important role in GA signal transduction, are supposed to be necessary for fibers initiation and development. In this study, four genes encoding the DELLA-like proteins were isolated from cotton and their biological function on the fiber growth and development was identified indirectly. The study provides some evidences for us to characterize the genes that regulate fiber development, and offers some candidate genes that may play a key role in the development of fiber cell. The results are described as following:1. Four DELLA-like genes isolated from cotton were designated GhGAI3a, GhGAI3b, GhGAI4a and GhGAI4b. The sequences alignment between cDNA and genomic DNA sequences of GhGAIs shows that there were no introns in these four genes. The amino acid sequences alignment between GhGAIs and DELLA proteins from the other plant species indicated that all GhGAIs had the conserved domains which DELLA subfamily proteins had. The analysis of phylogeny trees displayd the closest genetic relationship between GhGAIs and the DELLA protein (MdRGL3b) from apple.2. The amino acid sequence alignment of GhGAIs with their orthologues in several different wild diploid cottons with A or D genome showed that two genes of each a/b pair are homologs to each other, GhGAI3a was from A sub-genome, GhGAI3b was from D sub-genome, and the origins of GhGAI4a and GhGAI4b were not determined.3. The promoters of GhGAI3 and GhGAI4 were isolated, and the analysis of the cis-acting element of these two promoters showed that there were both light responsive element and gibberellin-responsive element in their sequences. The result proved that GhGAI3 and GhGAI4 were possibly regulated by light and gibberellin signal.4. The transcriptome profiling of GhGAI3 and GhGAI4 in cotton revealed that GhGAI3 was significantly expressed in vegetative organ and in the ovules at fiber initial stage; GhGAI4 was significantly expressed in the ovlues at fiber elongation stage.5. The phenotypes of wild-type Arabidopsis with ectopic expressed GhGAI3a showed that GhGAI3a suppressed the development of leaf, shoot, flower, silique and hypocotyl in transgenic Arabidopsis, and reduced the hypocotyl response of transgenic Arabidopsis to GA signal in the light. All of these results indicate that the functions of GhGAI3a were similar to that of DELLA proteins in Arabidopsis, and that GhGAI3a might work as a repressor in GA signal pathway.6. The reduced expression level of GhGAI3a in transgenic Arabidopsis by GA treatment indicates that there might be mechanism that GhGAI3a was degraded at mRNA level.7. The GhGAI3a protein was expressed in E.coil, and 2mg purified GhGAI3a protein was obtained to prepare antibodies, which made it possible to research the mechanism that how GhGAI3a responses to GA signal at protein level. |
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