Effects of continuous versus pulsed drought on mycorrhiza-crop-aphid interactions

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Global change scenarios predict scarcer water availability together with increasing temperatures, leading to longer drought periods. Imbalances in water supply mediate changes in resource allocation of plants and cause various physiological responses, such as closure of stomata and suppressed photosynthetic rates. These responses affect the biosynthesis of plant metabolites, which has consequences on herbivores and pathogens that depend on the plant quality. For example, the plant stress hypothesis predicts that aphids perform better on drought-stressed plants. Furthermore, most plants are associated with symbiotic fungi, forming an arbuscular mycorrhiza (AM), which can, amongst other effects, alleviate drought stress of plants. While many studies have investigated the effects of continuous drought stress on plant growth, physiology and chemistry, pulsed drought stress events are much more realistic. Moreover, the combined effects of drought and AM on aphids have been neglected. Therefore, in this proposal the effects of different drought regimes (pulsed drought stress versus continuous stress versus well-irrigated) on morphological, physiological or chemical responses of the crop plant wheat and their main aphid pest will be studied in presence or absence of AM. Effects of drought regime and AM status on plant growth traits and relative changes in hydration status will be monitored. Aphids will be reared on plants of the different treatments and various performance traits and the population growth measured to test the plant stress, pulsed stress and plant vigour hypotheses. In parallel, the chemical composition of phloem exudates will be analysed using metabolomics approaches. Multivariate statistics will allow us to make predictions about the role of specific phloem metabolomes for aphids. Subsequently, the role of the ratio of carbohydrates to amino acids to specific secondary metabolites will be tested using artificial diets. Metabolites will be selected that differ in concentration between the phloem of the plant treatments. Understanding the mycorrhiza-crop-aphid interactions under expected climate change scenarios may help to develop improved planting strategies and to establish cultivars with a high drought tolerance and high aphid resistance. Furthermore, the knowledge about the mechanisms underlying effects of pulsed drought stress on aphid population growth may allow for prediction of outbreaks.