Transcriptome and soluble sugar content analyses reveal the defense response of cotton leaves to Verticillium dahliae

Verticillium dahliae is a soil-borne phytopathogenic fungus causing destructive Verticillium wilt disease that greatly threats cotton production worldwide. The mechanism of cotton resistance to Verticillium wilt is very complex and requires further research. In this study, RNA-sequencing was used to investigate the defense responses of cotton leaves using varieties resistant (Zhongzhimian 2 or Z2) or susceptible (Xinluzao 7 or X7) to V. dahliae. The leaf samples were collected at 48 and 72hpi (hours post infection) from the two varieties infected by V. dahliae (strain Vd991) or treated by water. Compared to X7, Z2 had less genes responding to V. dahliae infection at 72hpi and had no DEGs found at 48hpi. Several reported disease resistance pathways were found to be up-regulated in Z2, with some of those pathways down-regulated in X7. Unexpectedly, several photosynthesis-related pathways, including photosynthesis-antenna proteins, photosynthesis, and porphyrin and chlorophyll metabolism, were significantly up-regulated in leaves of X7 infected by V. dahliae, leading to different profile of the soluble sugars, particularly the glucose content, which was significantly decreased at 72hpi and 48hpi in X7 and Z2, respectively. These results suggest that the leaves of resistant variety have a slower and different response to V. dahliae compared to that of susceptible variety, and that translocation of sugars produced by photosynthesis in cotton leaves might vary in the two varieties. Additionally, several HUB genes regulating disease response were identified, which might serve as potential candidates for breeding cotton disease resistance.