Proteomics and functional analyses of the Arabidopsis - Pseudomonas syringae pv. tomato interactions reveal that Arabidopsis nitrilases are required for plant defense and R gene-mediated resistant responses to microbial pathogens. A high-throughput in planta
proteome screen has identified Arabidopsis
nitrilase 2 (AtNIT2), which was de novo-induced by Pseudomonas syringae pv. tomato (Pst)
infection. The AtNIT2, AtNIT3, and AtNIT4 genes, but not AtNIT1, were distinctly induced in Arabidopsis leaves by Pst
infection. Notably, avirulent Pst DC3000 (avrRpt2)
infection led to significant induction of AtNIT2 and AtNIT4 in leaves. Pst DC3000 and Pst DC3000 (avrRpt2) significantly grew well in leaves of
nitrilase transgenic (nit2i-2) and mutant (nit1-1 and nit3-1) lines compared to the wild-type leaves. In contrast, NIT2 overexpression in nit2 mutants led to significantly high growth of the two Pst strains in leaves. The
nitrilase transgenic and mutant lines exhibited enhanced susceptibility to Hyaloperonospora arabidopsidis
infection. The nit2 mutation enhanced Pst DC3000 (avrRpt2) growth in
salicylic acid (SA)-deficient NahG transgenic and sid2 and npr1 mutant lines.
Infection with Pst DC3000 or Pst DC3000 (avrRpt2) induced lower levels of
indole-3-acetic acid (IAA) in nit2i and nit2i NahG plants than in wild-type plants, but did not alter the IAA level in NahG transgenic plants. This suggests that Arabidopsis
nitrilase 2 is involved in IAA signaling of defense and R gene-mediated resistance responses to Pst
infection. Quantification of SA in these transgenic and mutant plants demonstrates that Arabidopsis
nitrilase 2 is not required for SA-mediated defense response to the virulent Pst DC3000 but regulates SA-mediated resistance to the avirulent Pst DC3000 (avrRpt2). These results collectively suggest that Arabidopsis
nitrilase genes are involved in plant defense and R gene-mediated resistant responses to microbial pathogens.