Negative impact of drought stress on a generalist leaf chewer and a phloem feeder is associated with, but not explained by an increase in herbivore-induced indole glucosinolates

Plants are constantly exposed to multiple biotic and abiotic stresses, such as drought and herbivory. However, plant responses to these stresses have usually been studied in isolation. Here, we take a multidisciplinary approach addressing ecological and chemical aspects of plant responses to generalist herbivores and several intensities of drought. We hypothesize that in brassicaceous plants, the effects of drought stress on herbivores can be explained by an increase in indole glucosinolates. Four-week-old Arabidopsis thaliana plants were drought stressed for one week or watered as normal. Three types of drought stress were compared: (1) no watering for 1 week and then rewatered to saturation (low drought); (2) no watering for 1 week and then rewatered to 60% of soil water content (high drought); (3) watering every other day to 60% of soil water content (continuous drought). All three types of drought stress negatively affected both the larval mass of the leaf chewer Mamestra brassicae and the population growth of the phloem feeder Myzus persicae. This was associated with increased levels of herbivore-induced indole glucosinolates compared to infested control plants. Interestingly, the levels of total indole glucosinolates did not change in uninfested plants, except for the indole 4-methoxy-glucobrassicin that was induced by continuous drought. Two-choice experiments also showed that caterpillars of M. brassicae, but not aphids, avoided drought-stressed plants only after feeding on them, but not by visual/olfactory cues. However, on a knockout mutant blocked in the production of indole glucosinolates (cyp79B2 cyp79B3), the effect of drought on herbivore performance was similar to that on wild-type plants. The results of this study show that drought stress induced higher levels of indole glucosinolates; however, these levels were not responsible for higher resistance to generalist herbivores in drought-stressed plants.