Functional Analysis of Cotton DELLA-Like Genes that are Differentially Regulated during Fiber Development

DELLA proteins act as repressors in the gibberellin (GA) signaling pathway. GA could relieve this repression by triggering DELLA protein degradation via the ubiquitin-proteasome pathway. In this study, we cloned four genes from allotetraploid cotton that encode DELLA-like proteins whose functions have not been characterized previously. These four genes were designated as GhGAI3a, GhGAI3b, GhGAI4a, and GhGAI4b. The deduced proteins encoded by these genes contain all domains present in typical DELLA proteins. Sequence analysis showed that GhGAI3a belongs to the D sub-genome, whereas GhGAI3b belongs to the A sub-genome. The results from the analysis of their expression patterns in allotetraploid cotton showed that their expression was regulated during the fiber initiation and elongation stages. GhGAI3a was expressed at a high level at the fiber initiation stage; however, its transcript level was significantly reduced in fiber cells during the early fiber elongation stage. To test the function of GhGAI3a, we generated transgenic Arabidopsis lines that carried a construct in which expression of GhGAI3a was under the control of the cauliflower mosaic virus 35S promoter. The transgenic Arabidopsis lines exhibited smaller leaves and shorter stems and stamens. In addition, the GhGAI3a overexpression lines showed a defect in GA-induced hypocotyl elongation. These results suggest that GhGAI3a acts as a repressor of the GA signaling pathway.