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 <i>Acidovorax citrulli</i> 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 <i>A. citrulli</i> genomes through bioinformatic analysis. A series of molecular biology experiments have demonstrated that the HipBA TA system exists in <i>A. citrulli</i> Aac5. Furthermore, the transcription of <i>hipA</i> and <i>hipB</i> in <i>A. citrulli</i> 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 <i>A. citrulli</i> Aac5, and provided insights into its biological functions. These findings contribute to a better understanding of TA systems in plant pathogens.