Water Deficiency and Induced Defense Against a Generalist Insect Herbivore in Desert and Mediterranean Populations of Eruca sativa

In natural and agricultural ecosystems, plants are often simultaneously or sequentially exposed to combinations of stressors. Here we tested whether limited water availability (LWA) affects plant response to insect herbivory using two populations of Eruca sativa from desert and Mediterranean habitats that differ in their induced defenses. Considering that such differences evolved as responses to biotic and possibly abiotic stress factors, the two populations offered an opportunity to study ecological aspects in plant response to combined stresses. Analysis of chemical defense mechanisms showed that LWA significantly induced total glucosinolate concentrations in the Mediterranean plants, but their concentrations were reduced in the desert plants. However, LWA, with and without subsequent jasmonate elicitation, significantly induced the expression of proteinase inhibitor in the desert plants. Results of a no-choice feeding experiment showed that LWA significantly increased desert plant resistance to Spodoptera littoralis larvae, whereas it did not affect the relatively strong basal resistance of the Mediterranean plants. LWA and subsequent jasmonate elicitation increased resistance against the generalist insect in Mediterranean plants, possibly due to both increased proteinase inhibitor expression and glucosinolate accumulation. The effect of LWA on the expression of genes involved in phytohormone signaling, abscisic acid (ABA-1) and jasmonic acid (AOC1), and the jasmonate responsive PDF1.2, suggested the involvement of abscisic acid in the regulation of defense mechanisms in the two populations. Our results indicate that specific genotypic responses should be considered when estimating general patterns in plant response to herbivory under water deficiency conditions.