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 <i>Xanthomonas oryzae</i> pv. <i>oryzicola</i> (<i>Xoc</i>), but whether this mechanism operates in X. oryzae pv. oryzae (Xoo) has remained unclear. Here, we identify two conserved nitrilases, <i>NIT24</i> and <i>NIT29</i>, in the <i>Xoo</i> 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 <i>Xoo</i> virulence and bacterial fitness and is associated with modulation of host hormone balance, suggesting a conserved role for auxin in <i>X. oryzae</i> pathosystems and its involvement in the interplay between bacterial physiology and host immune responses.