Plant nitrogen limitation impairs apoplastic defense and affects the ability of pathogenic microorganisms to attack them

Global climatic and societal changes are profoundly transforming crop systems. Beyond their impact on yield and plant quality, they also alter plant–microorganism interactions, a key issue for understanding and improving crop protection against disease.
Nutritional stress linked to nitrogen limitation modifies the incidence and/or severity of most diseases, as well as the effectiveness of protection strategies such as biocontrol. For necrotrophic pathogens, such as the fungus Botrytis cinerea, responsible for grey mould, or the bacterium Erwinia amylovora, the causal agent of fire blight, the impact of nitrogen limitation on infection varies with host plant species. To develop sustainable and effective crop protection strategies, it is essential to understand the impact of abiotic stresses such as nitrogen limitation on both the establishment of host plant defenses and pathogen virulence, as well as on the different stages of the infection process. To address these questions, the apoplast — a key compartment in plant–microorganism interactions — was characterized in detail during infection of Arabidopsis thaliana with E. amylovora and B. cinerea. This liquid compartment, which surrounds plant cells and bathes the cell wall, plays a major role in early interactions between plant cells and both pathogenic and beneficial microorganisms. Despite of its role, the apoplast compartment has remained little studied.

Results
The teams showed that leaf apoplastic fluids have distinct effects on pathogens depending on whether plants are cultivated under low or high nitrogen conditions. These findings were supported by metabolomic analyses showing that the composition of these extracts is profoundly altered both by nitrogen limitation and by infection.
Under conditions unfavourable to infection, the plant apoplast accumulates metabolites that inhibit the expression of E. amylovora virulence genes. This reduction in virulence gene expression leads to enhanced jasmonic acid–dependent defense responses and increased resistance.
During infection of A. thaliana by B. cinerea, nitrogen limitation alters the profile of apoplastic oligosaccharides through the action of a pectin lyase, BcPNL1. The generation and characterization of a B. cinerea mutant lacking BcPNL1 revealed its key role in the infection process in both A. thaliana and tomato — a previously unsuspected function for this enzyme. These changes in the oligosaccharide profile influence the activation of plant defenses and restrict fungal growth in planta.

Perspectives
Further in-depth characterization of apoplastic fluids, particularly through proteomic analyses, should clarify the central role of the apoplast in the integrated response to abiotic and biotic stresses, especially during the early stages of infection. This work will also include investigation of the role of foliar endophytes (bacteria and fungi) in this process.


Research developed at the Institute Jean-Pierre Bourgin for Plant Sciences in collaboration.