Plants can improve their antiherbivore defence by taking insect egg deposition as cue of impending feeding damage. Previous studies showed that Pieris brassicae larvae feeding upon egg‐deposited Brassicaceae perform worse and gain less weight than larvae on egg‐free plants. We investigated how P. brassicae oviposition on Arabidopsis thaliana affects the plant's molecular and chemical responses to larvae. A transcriptome comparison of feeding‐damaged leaves without and with prior oviposition revealed about 200 differently expressed genes, including enhanced expression of PR5, which is involved in salicylic acid (SA)‐signalling. SA levels were induced by larval feeding to a slightly greater extent in egg‐deposited than egg‐free plants. The adverse effect of egg‐deposited wild‐type (WT) plants on larval weight was absent in an egg‐deposited PR5‐deficient mutant or other mutants impaired in SA‐mediated signalling, that is, sid2/ics1, ald1, and pad4. In contrast, the adverse effect of egg‐deposited WT plants on larvae was retained in egg‐deposited npr1 and wrky70 mutants impaired further downstream in SA‐signalling. Oviposition induced accumulation of flavonols in WT plants with and without feeding damage, but not in the PR5‐deficient mutant. We demonstrated that egg‐mediated improvement of A. thaliana's antiherbivore defence involves SA‐signalling in an NPR1‐independent manner and is associated with accumulation of flavonols.

In the field, biotic and abiotic stresses frequently co-occur. As a consequence, common molecular signalling pathways governing adaptive responses to individual stresses can interact, resulting in compromised phenotypes. How plant signalling pathways interact under combined stresses is poorly understood. To assess this, we studied the consequence of drought and soil flooding on resistance of Solanum dulcamara to Spodoptera exigua and their effects on hormonal and transcriptomic profiles. The results showed that S. exigua larvae performed less well on drought-stressed plants than on well-watered and flooded plants. Both drought and insect feeding increased abscisic acid and jasmonic acid (JA) levels, whereas flooding did not induce JA accumulation. RNA sequencing analyses corroborated this pattern: drought and herbivory induced many biological processes that were repressed by flooding. When applied in combination, drought and herbivory had an additive effect on specific processes involved in secondary metabolism and defence responses, including protease inhibitor activity. In conclusion, drought and flooding have distinct effects on herbivore-induced responses and resistance. Especially, the interaction between abscisic acid and JA signalling may be important to optimize plant responses to combined drought and insect herbivory, making drought-stressed plants more resistant to insects than well-watered and flooded plants.

Plants can respond to insect oviposition, but little is known about which responses directly target the insect eggs and how. Here, we reveal a mechanism by which the bittersweet nightshade Solanum dulcamara kills the eggs of a generalist noctuid herbivore. The plant responded at the site of oviposition by Spodoptera exigua with formation of neoplasms and chlorotic tissue, accumulation of reactive oxygen species and induction of defence genes and proteins. Transcriptome analysis revealed that these responses were reflected in the transcriptional reprogramming of the egg-laden leaf. The plant-mediated egg mortality on S. dulcamara was not present on a genotype lacking chlorotic leaf tissue at the oviposition sites on which the eggs are exposed to less hydrogen peroxide. As exposure to hydrogen peroxide increased egg mortality, while catalase supplementation prevented the plants from killing the eggs, our results suggest that reactive oxygen species formation directly acts as an ovicidal plant response of S. dulcamara.

Plant responses to insect egg depositions are known to shape subsequent defensive responses to larvae hatching from the eggs. Elm (Ulmus minor) leaves, on which elm leaf beetles laid their eggs, mount a more efficient defence against larvae hatching from the eggs. However, the molecular mechanisms of this egg‐mediated, improved defence are insufficiently understood and have so far only been studied in annual plants. We analysed the dynamics of transcriptomic changes in larval feeding‐damaged elm leaves with and without prior egg deposition using de novo assembled RNA‐seq data. Compared to egg‐free leaves, egg deposition‐treated leaves showed earlier and/or faster transcriptional regulations, as well as slightly enhanced differential transcriptional regulation after the onset of larval feeding. These early responding transcripts were overrepresented in gene ontology terms associated with post‐translational protein modification, signalling and stress (defence) responses. We found evidence of transcriptional memory in initially egg deposition‐induced transcripts whose differential expression was reset prior to larval hatching, but was more rapidly induced again by subsequent larval feeding. This potential memory effect of prior egg deposition, as well as the earlier/faster and enhanced feeding‐induced differential regulation of transcripts in egg deposition‐treated leaves, may contribute to the egg‐mediated reinforcing effect on the elm's defence against larvae. Hence, our study shows that a plant's experience of a stress‐indicating environmental cue (here: insect eggs) can push the dynamics of the plant's transcriptomic response to subsequent stress (here: larval feeding). Such experience‐mediated acceleration of a stress‐induced plant response may result in improved stress resistance.

Plants can respond to eggs laid by herbivorous insects on their leaves by preparing (priming) their defense against the hatching larvae. Egg-mediated priming of defense is known for several plant species, including Brassicaceae. However, it is unknown yet for how long the eggs need to remain on a plant until a primed defense state is reached, which is ecologically manifested by reduced performance of the hatching larvae. To address this question, we used Arabidopsis thaliana, which carried eggs of the butterfly Pieris brassicae for 1–6 days prior to exposure to larval feeding. Our results show that larvae gained less biomass the longer the eggs had previously been on the plant. The strongest priming effect was obtained when eggs had been on the plant for 5 or 6 days, i.e., for (almost) the entire development time of the Pieris embryo inside the egg until larval hatching. Transcript levels of priming-responsive genes, levels of jasmonic acid-isoleucine (JA-Ile), and of the egg-inducible phytoalexin camalexin increased with the egg exposure time. Larval performance studies on mutant plants revealed that camalexin is dispensable for anti-herbivore defense against P. brassicae larvae, whereas JA-Ile – in concert with egg-induced salicylic acid (SA) – seems to be important for signaling egg-mediated primed defense. Thus, A. thaliana adjusts the kinetics of its egg-primed response to the time point of larval hatching. Hence, the plant is optimally prepared just in time prior to larval hatching.