Apoplast multi-omics profiling during fungal infection uncovers new players of basal and early induced immunity

The extracellular space in plant tissues, known as the apoplast, remains one of the least characterized cellular compartments. The apoplastic fluid (APF), akin to mammalian extracellular fluid, serves as the primary interface between pathogens and their host. It allows the exchange of signalling molecules, coordinates host cell responses, and enables the circulation of pathogen effectors that modulate the immune response. We describe here the first multi-omics analysis of the APF content just 6 h after the onset of A. thaliana infection with B. cinerea, a fast-killing necrotrophic fungus. By varying plant nitrogen nutrition, known to affect both plant defenses and pathogen virulence, we identify candidates that do not stand out under optimal conditions. Our analysis uncovers novel nitrogen-dependent mechanisms that regulate both basal and early induced apoplastic immunity, revealing the presence of previously unidentified, potentially protective apoplastic metabolites as well as the intercellular transport of nuclear proteins, thereby offering new insights into early apoplastic immune responses.