ntrC Contributes to Nitrogen Utilization, Stress Tolerance, and Virulence in Acidovorax citrulli .

Bacterial fruit blotch (BFB), caused by , severely damages watermelon, melon, and other cucurbit crops worldwide. Nitrogen, one of the most important limiting elements in the environment, is necessary for the growth and reproduction of bacteria. As a nitrogen-regulating gene, plays an important role in maintaining bacterial nitrogen utilization and biological nitrogen fixation. However, the role of has not been determined for . In this study, we constructed a deletion mutant and a corresponding complementary strain in the background of the wild-type strain, Aac5. Through phenotype assays and qRT-PCR analysis, we investigated the role of in in nitrogen utilization, stress tolerance, and virulence against watermelon seedlings. Our results showed that the Aac5 deletion mutant lost the ability to utilize nitrate. The mutant strain also exhibited significantly decreased virulence, in vitro growth, in vivo colonization ability, swimming motility, and twitching motility. In contrast, it displayed significantly enhanced biofilm formation and tolerance to stress induced by oxygen, high salt, and copper ions. The qRT-PCR results showed that the nitrate utilization gene ; the Type III secretion system-related genes , and ; and the pili-related gene were significantly downregulated in the deletion mutant. The nitrate utilization gene , and the flagellum-related genes , and were significantly upregulated in the deletion mutant. The expression levels of gene in the MMX-q and XVM2 media were significantly higher than in the KB medium. These results suggest that the gene plays a pivotal role in the nitrogen utilization, stress tolerance, and virulence of .