Amino acid recovery does not prevent susceptibility to Pseudomonas syringae in nitrate reductase double-deficient Arabidopsis thaliana plants

Nitrate reductase (NR) double-deficient Arabidopsis thaliana (nia1 nia2) was previously shown to be susceptible to Pseudomonas syringae pv. maculicola (Psm). This abnormal phenotype was proposed to be caused by low endogenous levels of nitric oxide (NO). Amino acid contents are also reduced in the leaves of nia1 nia2 plants, suggesting an alternative cause for their pathogen susceptibility. In order to determine whether deficient nitrogen assimilation could account for impaired response to Psm, NR-deficient plants were cultivated with glutamine or arginine to increase the leaf amino acid contents to levels similar to or higher than wild-type plants. Despite this, the amino acid-recovered nia1 nia2 plants did not develop the hypersensitive response when infiltrated with an avirulent Psm, and severe evolution of disease symptoms and intense bacterial growth occurred in the leaves. The reduced NO emission by nia1 nia2 leaves, determined by the free form of the 4,5-diaminofluorescein probe, did not change significantly in amino acid-recovered mutants, nor did NO levels increase substantially upon Psm inoculation as in wild-type plants. These results suggest that the susceptibility to Psm of nia1 nia2 does not result from deficient amino acid content and may instead be a consequence of the reduced ability to synthesize NO.