New insights into the resistance of four citrus genotypes toXanthomonas citrisubsp.citri

The resistance of different citrus genotypes to citrus canker, caused byXanthomonas citrisubsp.citri(Xcc), is quite different. In this study, four citrus genotypes with different resistance, including resistant kumquat 'Luofu' (Fortunella margarita), moderately resistant Ponkan '18-1-10' (Citrus reticulata), moderately susceptible sweet orange 'Newhall' (C. sinensis) and susceptible grapefruit 'Duncan' (C. paradisi) were used to elucidate the resistance mechanism. The antibacterial analysis in vitro showed low concentration of H(2)O(2)promoted growth ofXcc, whereas high concentration inhibited growth. In addition, NADPH oxidases, also known as respiratory burst oxidase homologues (rbohs), are required for H(2)O(2)burst during infection. Subsequently,Xccaccumulation, H(2)O(2)production and expression ofCsrbohDwas measured to elucidate their roles in citrus-Xccinteraction. Through the Spearman correlation and multiple linear regression analysis, we found all three factors were significantly correlated with variety resistance, which was significantly affected by H(2)O(2)content and expression ofCsrbohD. Taken together, these data indicate a feedback regulation ofCsrbohDand H(2)O(2)in citrus. When the H(2)O(2)content was lower than the threshold, it was not sufficient to suppressXccgrowth, and moreXccpopulations suppressed H(2)O(2)production, thus forming a vicious circle. By contrast, it can suppressXccgrowth, thenCsrbohDwas suppressed to reduce the oxidative damage.