A putative multi-sensor hybrid histidine kinase, BarA Ac , inhibits the expression of the type III secretion system regulator HrpG in Acidovorax citrulli

dctA, dctB, and dctD contribute to the utilization of C4-dicarboxylates, carbon, nitrogen, as well as virulence in Acidovoraxcitrulli

Bacterial fruit blotch (BFB), a destructive bacterial disease triggered by Acidovorax citrulli, affects cucurbit crops like watermelon and melon. The absorption and use of carbon sources are foundational for bacteria to successfully colonize host plants. C4-dicarboxylates are critical carbon and energy substances, and their transport is completed by the C4-dicarboxylate transport system (Dct) which plays an important role in typical bacterial metabolism. However, the role of dct genes have not been determined for A. citrulli. To clarify the biological roles of the Dct system-related genes in A. citrulli, we developed dctA1, dctA2, dctB, and dctD deletion mutants, as well as dctA1A2 double deletion mutant, with their corresponding complementary strains in the A. citrulli wild-type strain Aac5 in this study. The functions of Dct-related genes in A. citrulli were analyzed through phenotype assays, including pathogenicity, C4-dicarboxylates utilization, carbon and nitrogen utilization, biofilm formation, swimming motility, and qRT-PCR analysis. Compared to the wild-type strain, the pathogenicity, utilization of C4-dicarboxylates, growth ability in vivo and in vitro, and seed adhesion ability of the mutant strains were significantly limited, while the biofilm formation ability was significantly improved. Additionally, the utilization of select carbon sources (glucose, maltose, and sucrose) and nitrogen sources ((NH4)2SO4, NH4Cl, CH4N2O, and KNO3) was significantly enhanced. qRT-PCR results demonstrated that the deletion of Dct-related genes resulted in significant downregulation of the expression of T3SS-related genes (hrpG and hrpE), the pili-related genes (pilA and pilN), and some flagellum-related genes (fliC, flhC, and flhD). These findings suggested that Dct-related genes were involved in C4-dicarboxylate utilization, carbon and nitrogen use, and the pathogenicity of A. citrulli.

Evidence for a Functional HipBA Toxin-Antitoxin System in Acidovorax citrulli

Bacterial fruit blotch (BFB) is a highly destructive seed-borne and seed-transmitted disease caused by the Gram-negative bacterium Acidovorax citrulli that has caused substantial economic losses for the cucurbit industry in China. Despite its potential for economic damage, little is known about the bacterium's molecular mechanisms of pathogenicity. Toxin-antitoxin (TA) systems are critical for the bacterial stress response. These systems are composed of two genes, toxin and antitoxin, that encode a stable toxin protein and a labile antitoxin protein, respectively. In this study, the genes for the putative HipBA TA system were identified in A. citrulli genomes through bioinformatic analysis. A series of molecular biology experiments have demonstrated that the HipBA TA system exists in A. citrulli Aac5. Furthermore, the transcription of hipA and hipB in A. citrulli Aac5 were induced by pH stress, chloramphenicol stress, and during plant infection. Overall, our results have revealed an active type II TA system, HipBA, in A. citrulli Aac5, and provided insights into its biological functions. These findings contribute to a better understanding of TA systems in plant pathogens.

Acidovorax citrulli type III effector AopU interferes with plant immune responses and interacts with a watermelon E3 ubiquitin ligase

Acidovorax citrulli is a seed-borne bacterium that causes bacterial fruit blotch of watermelon and other cucurbit plants worldwide. It uses a type III secretion system to inject type III effectors (T3Es) into plant cells, which affect the host immune responses and facilitate pathogen colonization. However, the current understanding of the specific molecular mechanisms and targets of these effectors in A. citrulli is limited. In this study, we characterized a novel T3E called AopU in A. citrulli group II strain Aac5, which shares homology with XopU in Xanthomonas oryzae. The Agrobacterium-mediated gene transient expression system was used to study the effect of AopU on host immunity. The results showed that AopU localized on the cell membrane and nucleus of Nicotiana benthamiana, inhibited reactive oxygen species burst induced by flg22 and the expression of marker genes associated with pathogen-associated molecular pattern-triggered immunity, but activated salicylic acid and jasmonic acid signal pathways. Further investigations revealed that AopU interacts with E3 ubiquitin ligase ClE3R in watermelon, both in vitro and in vivo. Interestingly, the deletion of aopU did not affect the virulence of A. citrulli, suggesting that AopU may have functional redundancy with other effectors in terms of its role in virulence. Collectively, these findings provide new insights into the mechanism of plant immune responses regulated by A. citrulli T3Es.

pilA Gene Contributes to Virulence, Motility, Biofilm Formation, and Interspecific Competition of Bacteria in Acidovorax citrulli

Acidovorax citrulli, the causative agent of bacterial fruit blotch, can be divided into two main groups based on factors such as pathogenicity and host species preference. PilA is an important structural and functional component of type IV pili (T4P). Previous studies have found significant differences in pilA DNA sequences between group I and group II strains of A. citrulli. In this study, we characterized pilA in the group I strain pslb65 and the group II strain Aac5. pilA mutants, complementation strains, and cross-complementation strains were generated, and their biological phenotypes were analyzed to identify functional differences between pilA in the two groups. pilA deletion mutants (pslb65-ΔpilA and Aac5-ΔpilA) showed significantly reduced pathogenicity compared with the wild-type (WT) strains; pslb65-ΔpilA also completely lost twitching motility, whereas Aac5-ΔpilA only partially lost motility. In King's B medium, there were no significant differences in biofilm formation between pslb65-ΔpilA and WT pslb65, but Aac5-ΔpilA showed significantly reduced biofilm formation compared to WT Aac5. In M9 minimal medium, both mutants showed significantly lower biofilm formation compared to the corresponding WT strains, although biofilm formation was recovered in the complementation strains. The biofilm formation capacity was somewhat recovered in the cross-complementation strains but remained significantly lower than in the WT strains. The interspecies competitive abilities of pslb65-ΔpilA and Aac5-ΔpilA were significantly lower than in the WT strains; Aac5-ΔpilA was more strongly competitive than pslb65-ΔpilA, and the complementation strains recovered competitiveness to WT levels. Furthermore, the cross-complementation strains showed stronger competitive abilities than the corresponding WT strains. The relative expression levels of genes related to T4P and the type VI secretion system were then assessed in the pilA mutants via quantitative PCR. The results showed significant differences in the relative expression levels of multiple genes in pslb65-ΔpilA and Aac5-ΔpilA compared to the corresponding WT stains. This indicated the presence of specific differences in pilA function between the two A. citrulli groups, but the regulatory mechanisms involved require further study.

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

Bacterial fruit blotch (BFB), caused by Acidovorax citrulli, 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, ntrC plays an important role in maintaining bacterial nitrogen utilization and biological nitrogen fixation. However, the role of ntrC has not been determined for A. citrulli. In this study, we constructed a ntrC deletion mutant and a corresponding complementary strain in the background of the A. citrulli wild-type strain, Aac5. Through phenotype assays and qRT-PCR analysis, we investigated the role of ntrC in A. citrulli in nitrogen utilization, stress tolerance, and virulence against watermelon seedlings. Our results showed that the A. citrulli Aac5 ntrC deletion mutant lost the ability to utilize nitrate. The ntrC 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 nasS; the Type III secretion system-related genes hrpE, hrpX, and hrcJ; and the pili-related gene pilA were significantly downregulated in the ntrC deletion mutant. The nitrate utilization gene nasT, and the flagellum-related genes flhD, flhC, fliA, and fliC were significantly upregulated in the ntrC deletion mutant. The expression levels of ntrC gene in the MMX-q and XVM2 media were significantly higher than in the KB medium. These results suggest that the ntrC gene plays a pivotal role in the nitrogen utilization, stress tolerance, and virulence of A. citrulli.