Seeds play a vital role in the perpetuation of plant species, both in natural environments and agriculture. However, they often face challenges from biotic stresses, such as seedborne pathogenic fungi. The transgenerational transmission of these seedborne fungi, along with their dissemination during seed commercialization, can contribute to the emergence of global epidemic diseases, resulting in substantial economic losses. One such pathogenic fungus is Alternaria brassicicola, responsible for causing black spot disease in Brassicaceae. This necrotrophic seedborne fungus negatively impacts plant development, consequently affecting crop production and seed quality. To gain a deeper understanding of the interaction between seeds and necrotrophic fungi, the Arabidopsis thaliana/Alternaria brassicicola model pathosystem has been extensively studied. Recent transcriptomic and genetic studies revealed a distinctive immune response of seeds against this pathogen, divergent from the immune response described in the plant vegetative phase. In germinating seeds and seedlings, the necrotrophic fungus seems to induce susceptible response [1]. It was observed at the RNA level that to the detriment of jasmonic acid pathway, Alternaria induced the salicylic acid and glucosinolate (GSL) pathways. Genetic evidence identified that seedling necrosis, potentially beneficial to the growth of the necrotrophic agent, is hypersensitive response mediated by GSL pathway. Transmission of necrotrophic fungal from seed to seedling unveils novel regulations of the immune response, that would illustrate a trade-off between seedling establishment and fungal infection. This knowledge is promising to sustainably ensure the sanitary quality of crops.

Little is known about the effect of nitrogen nutrition on seedling susceptibility to seed-borne pathogens. We have previously shown that seedlings grown under high nitrate (5 mM) conditions are less susceptible than those grown under low nitrate (0.1 mM) and ammonium (5 mM) in the Arabidopsis-Alternaria brassicicola pathosystem. However, it is not known how seedling metabolism is modulated by nitrogen nutrition, nor what is its response to pathogen infection. Here, we addressed this question using the same pathosystem and nutritive conditions, examining germination kinetics, seedling development, but also shoot ion contents, metabolome and selected gene expression. Nitrogen nutrition clearly altered the seedling metabolome. A similar metabolomic profile was observed in inoculated seedlings grown at high nitrate levels and in not inoculated-seedlings. High nitrate levels also led to specific gene expression patterns (e.g. polyamine metabolism), while other genes responded to inoculation regardless of nitrogen supply conditions. Furthermore, the metabolites best correlated with high disease symptoms were coumarate, tyrosine, hemicellulose sugars and polyamines, and those associated with low symptoms were organic acids (tricarboxylic acid pathway, glycerate, shikimate), sugars derivatives and β-alanine. Overall, our results suggest that the beneficial effect of high nitrate nutrition on seedling susceptibility is likely due to nutritive and signaling mechanisms affecting developmental plant processes detrimental to the pathogen. In particular, it may be due to a constitutively high tryptophan metabolism, as well as down regulation of oxidative stress caused by polyamine catabolism.