A B-Box (BBX) Transcription Factor from Cucumber, CsCOL9 Positively Regulates Resistance of Host Plant to Bemisia tabaci.

B-box (BBX) transcription factors play crucial roles in plant growth, development, and defense responses to biotic and abiotic stresses. In this study, we cloned a BBX transcription factor gene, CsCOL9I, from cucumber and analyzed its role in the plant's defense against the feeding of Bemisia tabaci. CsCOL9 is expressed throughout all developmental stages in cucumber, with the highest expression in the leaves. CsCOL9 is induced by B. tabaci feeding, salicylic acid (SA), methyl jasmonate (MeJA), and hydrogen peroxide (H2O2). Cucumber plants with CsCOL9 silence (TRV2-CsCOL9) and overexpression (1301-CsCOL9) were obtained and analyzed. After CsCOL9 silencing, survival rates and host selectivity for B. tabaci increased; however, the expression levels of genes encoding enzymes (CsSOD, CsRBOH, CsPOD), activities of superoxide dismutase (SOD) and peroxidase (POD), and content of H2O2 in plants were all reduced. CsCOL9 overexpression led to decreased survival rates and host selectivity for B. tabaci. Conversely, the expression levels of genes (CsSOD, CsRBOH and CsPOD), activities of SOD and POD, and content of H2O2 increased after CsCOL9 overexpression in plants. Collectively, our results demonstrate CsCOL9 positively regulates cucumber resistance to B. tabaci by activating reactive oxygen species bursts. This study lays a theoretical foundation for the application of CsCOL9 in cucumber resistance breeding and green pest control of B. tabaci.