Elevated CO2 enhances the host resistance against the western flower thrips, Frankliniella occidentalis, through increased callose deposition.
Elevated CO2 can alter plant resistance against insect herbivores. In this study, we investigated the effect of elevated CO2 on the callose synthesis involved in the resistance of Phaseolus vulgaris against Frankliniella occidentalis, which is one of the most important invasive insect pests worldwide. Elevated CO2 elevated the silver damage, callose deposition, and the expression level of CalS3 and CalS10 genes involved in callose synthase (CalS) in thrips-infested bean leaves, while reducing PR2 gene expression related to the hydrolysis of callose. In addition, both infestation by thrips and mechanical damage increased the callose deposition in leaves and induced CalS and β-1,3-glucanases (BG) expression at both transcriptional and translational levels. Under ambient CO2, callose content in the mechanically damaged plants (MDPs) and thrips-infested plants (TIPs) was positively correlated with CalS activity and the expression level of CalS3 and CalS10; BG activity was positively correlated with PR2 gene expression. Under elevated CO2, callose content in MDPs and TIPs was negatively correlated with BG activity which also negatively correlated with the expression level of CalS10 and PR2. F. occidentalis feeding can induce callose synthesis and deposition in P. vulgaris leaves, especially under elevated CO2. Specifically, genes associated with CalS defense are up-regulated, while the expression level of PR2 gene is downregulated. These results suggest that elevated CO2 can modulate callose synthesis leading to a higher level of resistance in host plants against thrips infestation.