Abstract: 【Objective】 Cloning and function analysis of MYB transcription factor in response to drought stress were conducted to understand cotton tolerance to drought stress and its further application in cotton breeding.【Method】With a drought induced GbMYB5 as a query, BLASTX search in the non-redundant protein sequences (nr) database in NCBI was performed to search possible drought induced MYB transcription factors in cotton. Real time PCR confirmed that a MYB transcription factor GhRAX3 responded to drought stress. The agro-infiltration based transient expression of a fused GhRAX3-GFP4 in tobacco cell was used to detect subcellular localization of GhRAX3. A virus induced gene silencing system mediated by cotton leaf crumple virus was used to depress the expression of GhRAX3 in cotton, and the water loss rate, relative water content, total antioxidant activity, malondialdehyde content and electrolyte leakage in leaves of GhRAX3-silenced cotton were subsequently investigated as well as their phenotypes in response to drought stress. 【Result】BLASTX with GbMYB5 as a query revealed that GhRAX3, which encodes an R2R3-MYB transcription factor, had a query cover of 38% and a identity of 79%. GhRAX3 was quickly responded to the induction of 18% (v/v) PEG 6000 by up-regulating 5-fold at 0.5 h post PEG 6000 treatment and up to 33-fold at 24 h post PEG 6000 treatment. The subcellular localization of the fused GhRAX3-GFP4 in tobacco cell indicated that GhRAX3 was located in the cell nucleus. When subjected to virus induced gene silencing, the expression level of GhRAX3 was depressed to 41% of the normal level. The water loss rate in the detached leaves of GhRAX3 silencing (viz. VIGS) cotton plants was significantly higher that those in the wilt type (WT) and the empty vector agro-infiltrated (EM) cotton plantlets during the 0-7 h duration after detaching. Under the 18% (v/v) PEG 6000 treatment for 24 h or water-withhold for 15 d, theVIGS cotton plantlets showed a more severe wilt symptom when compared with the WT and the EM cotton plantlets. At 7 d post water-withhold treatment, the relative water contents, total antioxidant activity, malondialdehyde content and electrolyte leakage in leaves were further investigated in the VIGS, WT and EM plants. The relative water content and total antioxidant activity in the VIGS plants was 82% and 1.29 U·mg^-1 protein, respectively. By contrast, the relative water content in the WT and EM plants was 89% and 91%, respectively. And the total antioxidant activity in the WT and EM plants was 3.44 U·mg^-1 protein and 3.19 U·mg^-1 protein, respectively. The relative water content and total antioxidant activity in the VIGS plants were significantly lower than those in the WT and EM plants. The electrolyte leakage and malondialdehyde content in the VIGS plants was 78.54% and 74.20 nmol·mg^-1 FW, respectively. While the electrolyte leakage in the WT and EM plants was 44.98% and 47.45%, respectively. And the malondialdehyde content in the WT and EM plants was 44.90 nmol·mg^-1 FW and 47.29 nmol·mg^-1 FW, respectively. The electrolyte leakage and malondialdehyde content in the VIGS plants were significantly higher than those in the WT and EM plants. 【Conclusion】GhRAX3 was responded to drought stress. The depression of GhRAX3 compromised the tolerance of cotton plants to drought stress as a result of significant decrease in relative water content and total antioxidant activity and a significant increase in the electrolyte leakage and malondialdehyde content under drought stress.

Key words: cotton, MYB transcription factor, induction expression, virus induced gene silencing, drought stress