Abstract
In nature plants are often exposed to a multitude of biotic and abiotic stress factors (e.g. drought, herbivory and infections) that occur sequentially or simultaneously. To withstand these stresses plants have evolved defense mechanisms that allow them to cope with diverse stresses. However, still little is known about how these
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adaptive mechanisms cooperate to maximize plant survival under multistress conditions. Our research reveals that plants are well-capable of adjusting their defense strategy when encountering multiple stresses. To investigate how plants cope with multi-stress conditions, we analyzed the dynamics of whole-transcriptome profiles of Arabidopsis thaliana exposed to three individual and six sequential double stresses inflicted by combinations of: (i) infection by the necrotrophic fungus Botrytis cinerea, (ii) herbivory by caterpillars of Pieris rapae, and (iii) drought stress. Each of these stresses induced specific expression profiles over time, in which one-third of all differentially expressed genes was shared by at least two single stresses. Still, a first stress encounter leaves a transcriptional stress mark that can adversely affect plant defenses to a second stress event, indicating that plant multi-stress interactions are affected by simultaneous or sequential stresses. Aside from a flexible defense response, we also find natural variation in globally collected A. thaliana plants for their resistance to B. cinerea as individual stress or when preceded by drought stress or herbivory by P. rapae. In search for the genetics underlying the variation in plant susceptibility, we found genes that are potentially involved in plant adaptive responses to the applied sequential stresses. Genes with both known and putative roles in defense against each of the applied stresses were identified. In a subsequent study, we compared the tolerance of plants to diverse (a)biotic stresses and sequential stress combinations in order to find genes involved in plant multistress resistance. Plants that are resistant to multiple stresses are well-adapted to grow in diverse environmental settings and can therefore ensure efficient and sustainable food production in the future. With a genetic study we revealed genes involved in one or more (a)biotic stresses. Validation of one of the identified genes revealed its involvement in tolerance to multiple stresses, indicating that with our approach it is possible to find genes involved in plant multi-stress tolerance. To minimalize crop damage, prevention of stress encounters is another approach of reducing crop losses instead of looking for plant adaptive responses that allow plants to cope with multiple stresses. For instance, by preventing egg-deposition by butterflies, subsequent herbivory by their offspring is avoided. In search of plant traits that are unfavorable for P. rapae, we studied plant selection for egg-deposition on naturally occurring A. thaliana plants. We observed variation in egg-deposition that was partially influenced by daylight, the experimental cage setup, plant size, spontaneous chlorosis and necrosis and the presence of secondary metabolites. A genetic study revealed genes of which one gene was shown to significantly associate with plant defense against caterpillars, jasmonic acid induced defenses. Butterflies might be capable of detecting differences in jasmonic acid defenses and select host plants that will benefit their offspring the best. Future functional analysis of the JA biosynthesis gene and the other identified candidate genes will be required to reveal their role in host selection for oviposition by P. rapae. In sum, this research has shown that plants have an incredible ability to adapt to and integrate diverse stress signals into an effective defense response. Knowledge on these natural adaptive capabilities and the natural genetic variation hereof forms a valuable basis for the sustainable development of stress resilient future crops.
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