NIT24 and NIT29-Dependent Auxin Biosynthesis Promotes Virulence and Auxin-SA Antagonism in Rice During Xanthomonas oryzae pv. oryzae Infection.

Auxin production by plant-associated bacteria is increasingly recognized as a determinant of host colonization, yet its integration into virulence networks remains incompletely understood. Nitrilase-dependent auxin (IAA) biosynthesis contributes to virulence in Xanthomonas oryzae pv. oryzicola (Xoc), but whether this mechanism operates in X. oryzae pv. oryzae (Xoo) has remained unclear. Here, we identify two conserved nitrilases, NIT24 and NIT29, in the Xoo strain PXO99A and examine their contributions to IAA production and virulence. Deletion of either gene reduces IAA accumulation, and under the tested conditions ΔNIT29 tended to show a stronger reduction, together with attenuated virulence characterized by shorter lesions and decreased in planta bacterial growth. Hormone profiling indicates a shift from auxin toward salicylic acid (SA)-associated responses in nitrilase-deficient infections, accompanied by reduced expression of auxin-responsive genes and enhanced activation of SA-related defense markers. In addition, nitrilase mutants display defects in biofilm formation, cellulase activity, and chemotactic motility, and exhibit altered expression of type III secretion system regulators, transcription activator-like effectors, and diffusible signal factor quorum-sensing components. Together, these findings support that nitrilase-mediated auxin biosynthesis contributes to multiple aspects of Xoo virulence and bacterial fitness and is associated with modulation of host hormone balance, suggesting a conserved role for auxin in X. oryzae pathosystems and its involvement in the interplay between bacterial physiology and host immune responses.