Acidovorax citrulli (Ac) is the causative agent of bacte- rial fruit blotch disease in watermelon. Since resistant cultivars have not yet been developed, the virulence factors/mechanisms of Ac need to be characterized. This study reports the functions of a putative pyridoxal phosphate-dependent aminotransferase (PpdaAc) that transfers amino groups to its substrates and uses pyridoxal phosphate as a coenzyme. It was observed that a ppdaAc knockout mutant had a significantly reduced virulence in watermelon when introduced via germinated-seed inoculation as well as leaf infiltration. Comparative proteomic analysis predicted the cellular mechanisms related to PpdaAc. Apart from causing virulence, the PpdaAc may have significant roles in energy production, cell membrane, motility, chemo- taxis, post-translational modifications, and iron-related mechanisms. Therefore, it is postulated that PpdaAc may possess pleiotropic effects. These results provide new insights into the functions of a previously unidenti- fied PpdaAc in Ac.