Insect egg depositions on leaves are reliable warning cues of subsequent larval herbivory. However, no knowledge was available when we started project B4 whether also a stress signal that is not directly related to herbivory, i.e. exposure of a plant to cold that may precede herbivory in spring, affects the plant response to later herbivory. The results of our project in phase I showed that egg deposition by Pieris brassicae upon Arabidopsis thaliana, but not exposure of the plant to cold, improved the plant´s anti-herbivore defence against larvae. Thus, egg deposition can “warn” the plant of impending larval herbivory. We investigated for how long the plant maintains its transcriptional response to egg deposition when eggs are removed from the leaves. In nature, predators remove eggs from a leaf, or heavy rainfall is washing them off. Only one day after experimental removal of eggs from the leaves, the plant´s transcriptome was reset to the untreated state, and the transcriptional response to feeding damage and the defence efficiency was the same as in plants that never received eggs. Hence, removal of eggs serves as a reliable signal for the plant that the danger of impending herbivory by hatching larvae is over. In contrast, cold was “remembered” one day after re-exposure to warm temperatures, indicated by an altered gene expression pattern upon larval feeding. Nevertheless, larval performance was not affected by this “cold memory”, possibly because the specialised P. brassicae larvae may be adapted to cold-dependent responses of their host plants. Current experiments investigate putative "ecological targets" of the cold-dependent response to wounding by herbivory.
In phase II we intend to deepen our knowledge on the plant´s response to P. brassicae eggs and the characteristics of the stimulus that warns a plant of future herbivory.
In search of a functional link between the egg-mediated alteration of the plant transcriptomic responses to feeding damage and the increased anti-herbivore defence, we currently investigate the regulation of candidate genes in A. thaliana and aim to test in phase II their relevance for the egg-mediated priming of anti-herbivore defence by investigating the responses of respective mutants to P. brassicae. In addition we aim to enlighten the epigenetic memory marks of candidate genes.
To elucidate the ecological factors that shape the egg-mediated plant´s response to feeding damage, we will on the one hand study the impact of the plant´s longevity on its primability and persistence of a primed state. For these studies, we will use the perennial herb Arabis alpina, a relative of A. thaliana, and compare its responses to P. brassicae egg deposition and feeding with those of A. thaliana. These studies will shed light on the plant species-specificity of responses to these priming and triggering stimuli and on the role of plant longevity for plant memory of insect egg deposition. On the other hand, we will investigate how predation of eggs affects the anti-herbivore defence of A. thaliana. Insect predators (e.g. coccinellids feeding upon the eggs) may leave remains of their prey. It is unknown whether these leftovers still prime the plant´s anti-herbivore defence, or whether the effects of predation of eggs resemble those mediated by experimental removal of eggs.
To gain deeper knowledge on the active egg-associated stimulus that warns a plant of future herbivory, we will test whether (i) benzyl cyanide present in the egg glue - a secretion released with the eggs - elicits the same responses of A. thaliana to subsequent herbivory as the egg deposition does. This objective is based on the recent finding that P. brassicae larvae feeding upon Brassica nigra plants treated with benzyl cyanide were shown to perform worse than those feeding on untreated plants. We will further investigate whether (ii) microorganisms associated with the eggs contribute to the warning effect that P. brassicae egg deposition has on the anti-herbivore defence of A. thaliana.