Blue-Green Fluorescence for Downy Mildew Detection and Phenotyping in Lettuce: Linking Optical, Physiological and Molecular Changes

Effective phenotyping for disease resistance is crucial for plant breeding, but traditional visual assessment is often inaccurate and inefficient. This is particularly challenging in lettuce (Lactucae sativa) breeding for resistance to lettuce downy mildew caused by the oomycete Bremia lactucae. Due to the obligate biotrophic lifestyle of B. lactucae, visible signs and symptoms like emerging sporangiophores or chlorosis develop late and are difficult to track by eye. Thus, there is a need for more precise phenotyping methods. We discovered that B. lactucae-infected lettuce exhibits increased blue-green fluorescence (BGF) under UV-A excitation from 6 days post-inoculation, preceding other visible symptoms. To elucidate its origin and enable interpretation of this fluorescence signal in the context of plant-pathogen interactions we explored transcriptomic and metabolomic changes that correlate with the occurrence of BGF. We found an induction of the phenylpropanoid pathway and the accumulation of caffeoylquinic acids (chlorogenic and dicaffeoylquinic acids). The overlapping fluorescence spectra of BGF tissue and caffeoylquinic acids, along with their shared localization in mesophyll vacuoles, strongly suggest that these compounds contribute to the observed fluorescence. To leverage BGF for sensor-based phenotyping, we developed an imaging system to capture and quantify BGF. The BGF leaf area correlates with downy mildew severity, demonstrating the potential for BGF imaging to enable early and objective assessment of downy mildew infection. This approach offers a powerful tool for quantitative phenotyping in lettuce breeding for durable downy mildew resistance.