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)—a pathogen-elicited cell death linked to resistance. HR-eliciting Pseudomonas syringae pv. phaseolicola was infiltrated into leaves of tobacco fed with either [Image: see text] or [Image: see text]. The speed of cell death was faster in [Image: see text]-fed compared with [Image: see text]-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 [Image: see text]-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 [Image: see text] feeding. PR1a is a biomarker for the defence signal salicylic acid (SA), and expression was reduced in [Image: see text]-fed compared with [Image: see text] fed plants at 24h after inoculation. This pattern correlated with actual SA measurements. Conversely, total amino acid, cytosolic and apoplastic glucose/fructose and sucrose were elevated in [Image: see text]- treated plants. Gas chromatography/mass spectroscopy was used to characterize metabolic events following different N treatments. Following [Image: see text] nutrition, polyamine biosynthesis was predominant, whilst after [Image: see text] nutrition, flux appeared to be shifted towards the production of 4-aminobutyric acid. The mechanisms whereby [Image: see text] feeding enhances SA, NO, and polyamine-mediated HR-linked defence whilst these are compromised with [Image: see text] , which also increases the availability of nutrients to pathogens, are discussed.