Asymptomatic colonization by Botrytis cinerea reveals polarized plant-fungus interfaces before widespread disease

Botrytis cinerea is a necrotrophic pathogen whose destructive growth is typically studied under conditions that rapidly generate visible disease. Yet symptomless associations are well documented, the fungal and host processes operating before widespread tissue collapse remain poorly resolved. We developped an intact Arabidopsis thaliana seedling pathosystem that extends the interval between infection establishment and widespread visible disease. Fungal colonization was quantified using qPCR and a constitutively bioluminescent reporter strain, together with longitudinal plant phenotyping, hormone profiling and multiscale fluorescence microscopy. Substantial fungal colonization occurred while seedlings remained largely green and continued to grow, whereas robust whole seedling accumulation of salicylic and jasmonic acids occurred later. During this interval, B. cinerea displayed diverse tissue-associated infection morphologies. At individual contact sites, fungal walls showed heterogeneous exposure of chitin-associated epitopes, while host cells exhibited focal plasma-membrane polarization toward fungal contact sites, together with cortical actin reorganization. These results show that the interval preceding widespread visible disease is an active and spatially heterogeneous interaction state, in which substantial fungal development and highly organized local host responses are already established. Intact seedlings therefore provide a powerful system for resolving plant-pathogen interactions that are largely obscured in rapid disease assays.