Inducible tomato defences persist in detached leaves, despite differential plant variety and gene-dependent expression

Herbivore and pathogen attacks often lead to the induction of plant defences. Given the intricate nature of such responses, plant integrity is assumed to be a pre-requisite for the successful mounting of defences, a hypothesis supported by work done in plant-pathogen interactions. However, the relevance of plant integrity in plant-herbivore interactions, particularly in direct plant defences, is unclear and lacks empirical molecular validation. To test this, we measured the expression of plant defence-related genes (PPO-D and PI-IIc, involved in the jasmonate signal pathway and PR-1a, involved in the salicylic acid signal pathway) in intact plants and detached leaves of two tomato varieties infested or not with the herbivorous spider mite Tetranychus urticae . We also tested whether defences persisted in detached leaves after one, four, seven or fourteen days of infestation. We found that tomato defences were induced in both intact plants and detached leaves and that the expression of all three genes in detached leaves increased over time, being the increment higher in the two late-expression genes, PI-IIc and PR-1a. Such results suggests that both intact plants and detached leaves can be used in studies addressing herbivore interactions with plant defences. However, induction levels in intact plants and detached leaves varied with plant variety and the gene assessed, ranging from higher expression in detached leaves than in intact plants to the reverse. Hence, studies aiming at identifying the role of specific genes should account for the differential, variety-dependent, role of plant integrity in their expression.