The form of nitrogen nutrition affects resistance against Pseudomonas syringae pv. phaseolicola in tobacco

Different forms of nitrogen (N) fertilizer affect disease development; however, this study investigated the effects of N forms on the hypersensitivity response (HR)üa pathogen-elicited cell death linked to resistance. HR-eliciting Pseudomonas syringae pv. phaseolicola was infiltrated into leaves of tobacco fed with either NO3 - or NH4 + . The speed of cell death was faster in NO^-₃ -fed compared with NH⁺₄ -fed plants, which correlated, respectively, with increased and decreased resistance. Nitric oxide (NO) can be generated by nitrate reductase (NR) to influence the formation of the HR. NO generation was reduced in NH ⁺₄ -fed plants where N assimilation bypassed the NR step. This was similar to that elicited by the disease-forming P. syringae pv. tabaci strain, further suggesting that resistance was compromised with NH ⁺₄ feeding. PR1a is a biomarker for the defence signal salicylic acid (SA), and expression was reduced in NH⁺₄ -fed compared with NO^-₃ fed plants at 24 h after inoculation. This pattern correlated with actual SA measurements. Conversely, total amino acid, cytosolic and apoplastic glucose/fructose and sucrose were elevated in NH - treated plants. Gas chromatography/mass spectroscopy was used to characterize metabolic events following different N treatments. Following NO^-₃ nutrition, polyamine biosynthesis was predominant, whilst after NH⁺₄ nutrition, flux appeared to be shifted towards the production of 4-aminobutyric acid. The mechanisms whereby NO ^-₃ feeding enhances SA, NO, and polyamine-mediated HR-linked defence whilst these are compromised with NH⁺₄ , which also increases the availability of nutrients to pathogens, are discussed.