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Nitrogen nutrition modifies the susceptibility of Arabidopsis thaliana to the necrotrophic fungus, Alternaria brassicicola
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International audience. The impact of the form of nitrogen (N) source (nitrate versus ammonium) on thesusceptibility toAlternaria brassicicola, a necrotrophic fungus, has been examined inArabidopsis thalianaat the rosette stage. Nitrate nutrition was found to increase fungallesions considerably. There was a similar induction of defence gene expression followinginfection under both N nutritions, except for thephytoalexin deficient 3gene, which wasoverexpressed with nitrate. Nitrate also led to a greater nitric oxide production occurringin plantaduring the saprophytic growth and lower nitrate reductase (NIA1) expression7 days after inoculation. This suggests thatnitrate reductase-dependent nitric oxideproduction had a dual role, whereby, despite its known role in the generic response topathogens, it affected plant metabolism, and this facilitated fungal infection. Inammonium-grown plants, infection withA. brassicicolainduced a stronger gene expressionof ammonium transporters and significantlyreduced the initially high ammonium contentin the leaves. There was a significant interaction between N source and inoculation (pres-ence versus absence of the fungus) on the total amino acid content, while N nutrition rec-onfigured the spectrum of major amino acids.Typically, a higher content of total aminoacid, mainly due to a stronger increase in asparagine and glutamine, is observed underammonium nutrition while, in nitrate-fed plants, glutamate was the only amino acid whichcontent increased significantly after fungalinoculation. N nutrition thus appears to controlfungal infection via a complex set of signalling and nutritional events, shedding light onhow nitrate availability can modulate disease susceptibility.
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