Effects of Xanthomonas campestris pv. campestris on the photosynthesis of cabbage in the early stage of infection

Cabbage is one of the most important cruciferous vegetables. Black rot, caused by Xanthomonas campestris pv. campestris (Xcc) seriously affects cabbage production; therefore, it is important to study the resistance mechanism to black rot in cabbage. In this study, cabbage photosynthetic physiology changes in the early stages of Xcc infection were analyzed using physiological index determination and transcriptome analysis. The results showed that the Xcc infection can lead the chlorophyll degradation and decreased the net photosynthetic rate (Pn), and the decrease in Pn was mainly due to non-stomatal factors. In addition, the maximum photosynthetic efficiency (Fv/Fm), actual photosynthetic efficiency of photosystem II (PSII) gradually decreased, and the OJIP curve tended to be flat, indicating that Xcc infection reduced the activity of the PSII reaction center and affected the chlorophyll fluorescence intensity. Moreover, the primary photochemical reaction of cabbage leaves was damaged, and electron transfer on the PSII receptor side was inhibited after Xcc infection. NADP-GAPDH and FBPase activities in the Calvin cycle decreased after Xcc invasion. Using transcriptome analysis, 173, 77, 63, and 53 differentially expressed genes (DEGs) were enriched in "carbon metabolism", "photosynthesis", "carbon fixation in photosynthetic organisms", and "metabolism of porphyrins and chlorophyll" metabolic processes, respectively. These results suggest that Xcc infection affects cabbage photosynthesis via multiple pathways. Our results provide a theoretical basis for further research on black rot-resistant breeding and cabbage resistance mechanisms.