Transcriptome analysis reveals the effect of grafting on gossypol biosynthesis and gland formation in cotton

Gossypol is a unique secondary metabolite and sesquiterpene in cotton, which is mainly synthesized in the root system of cotton and exhibits many biological activities. Previous research found that grafting affected the density of pigment glands and the gossypol content in cotton. This study performed a transcriptome analysis on cotton rootstocks and scions of four grafting methods. The gene expression of mutual grafting and self-grafting was compared to explore the potential genes involved in gossypol biosynthesis. A total of six differentially expressed enzymes were found in the main pathway of gossypol synthesis-sesquiterpene and triterpene biosynthesis (map00909): lupeol synthase (LUP1, EC:5.4.99.41), beta-amyrin synthase (LUP2, EC:5.4.99.39), squalene monooxygenase (SQLE, EC:1.14.14.17), squalene synthase (FDFT1, EC:2.5.1.21), (-)-germacrene D synthase (GERD, EC:4.2.3.75), ( +)-delta-cadinene synthase (CADS, EC:4.2.3.13). By comparing the results of the gossypol content and the density of the pigment gland, we speculated that these six enzymes might affect the biosynthesis of gossypol. It was verified by qRT-PCR analysis that grafting could influence gene expression of scion and stock. After suppressing the expression of the LUP1, FDFT1, and CAD genes by VIGS technology, the gossypol content in plants was significantly down-regulated. These results indicate the potential molecular mechanism of gossypol synthesis during the grafting process and provide a theoretical foundation for further research on gossypol biosynthesis.