Leaf Developmental Stage Influences Disease Resistance in Tomato

The processes of morphogenesis and differentiation are crucial for leaf development, with the duration of the morphogenetic window influencing final leaf shape. Leaves at different developmental stages exhibit distinct morphological and physiological characteristics that may influence their ability to resist pathogens, and disease resistance has been linked to developmental stage in many plant species. To understand how leaf development impacts disease resistance, we examined the immunity of leaves at distinct developmental stages, exploring the role of hormonal pathways and the impact of leaf structure and microbial interactions on disease resistance. Our findings reveal that leaves of different developmental stages exhibit distinct disease responses to various pathogens, determined primarily by the ratio between salicylic acid and jasmonic acid. Higher relative jasmonic acid content in later developing leaves was found to result in increased disease resistance to necrotrophs, while higher relative salicylic acid content in earlier developing leaves rendered them more resistant to biotrophs. This phenomenon occurred across plant ages, in several species, and also impacted the plants' response to biocontrol agents, depending on the pathway being primed. We found that structural variations among leaves can also affect disease response, due to differential recognition by the invading pathogen, and possibly also due to alterations in the leaf microbiome. Our results uncover some of the factors influencing developmental immunity in tomato, and highlight the importance of considering plant development when managing disease resistance.