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<p> As the root microbiome’s role in plant defenses against herbivory becomes clearer, scientific focus has lingered on a single side of plant defenses: resistance. Its counterpart, tolerance, is comparatively overlooked despite its power as an evolutionarily sustainable mitigator of herbivore damage. This thesis seeks to supplement our limited understanding of the extent to which tolerance to herbivory may be influenced by rhizosphere microbial communities. First, in an agricultural field setting, I (1) quantified domesticated tomato cultivar and wild ancestor tolerance to herbivory form the specialist tobacco hornworm (<em>Manduca sexta</em>) and (2) characterized the bacterial and fungal rhizosphere communities associating with high and low tolerance plants. In a subsequent greenhouse experiment, I grew these same tomato lines in either sterilized or unsterilized soil and re-challenged plants with tobacco hornworms to tease apart the contributions from host plant and rhizosphere microbiome in expressing tolerance to herbivory. In the field, wild tomato lines excelled at tolerating hornworm herbivory, while their domesticated counterparts suffered 26% yield losses under herbivory. Rhizosphere community characteristics were most reliably shaped by timepoint of rhizosphere sampling, and more subtly by tomato line and herbivory treatments. Fungal and bacterial community traits that associated with high tolerance lines include (1) high diversity, (2) resistance to community shifts under herbivory, and (3) the abundance of ASVs belonging to <em>Strenotrophomonas</em>, <em>Sphingobacterium</em>, and <em>Sphingomonas</em>. When re-challenging these lines with hornworm herbivory in the greenhouse, expressed tolerance to tobacco hornworm damage was inverted from field trends. Though wild lines suffered yield losses when grown in +microbiome treatments, we found no consistent interactions between herbivory and microbiome treatments that might indicate that +microbiome treatments either helped or hampered plant expression of tolerance to herbivory under greenhouse conditions. These experiments shed light on what role, if any, the rhizosphere microbiome plays in plant tolerance to herbivory. Ultimately, understanding the qualities of tolerance-conferring microbiomes can (1) open avenues through which plant defenses may be amended in pest management, either through microbial inoculants or plant breeding efforts aimed at enhancing crop recruitment of beneficial microbiomes; and (2) ameliorate our understanding of the tripartite interactions between host plants, their rhizospheres, and their specialist herbivores. </p>

Cyclodipeptides (CDPs) are the smallest, most stable cyclic peptides that are synthesized as secondary metabolites by bacteria. The aim of this study was to investigate the effect of the Pseudomonas argentinensis (SA190) and four (CDPs), named as cis-cyclo-(Pro-Phe) (Cyclo2), cis-cyclo-(Pro-Leu) (Cyclo3), cis-cyclo-(Pro-Tyr) (Cyclo4) and cis-cyclo-(Pro-Val) (Cyclo5), with three concentrations (1µM, 100nM, and 10nM), on the growth of Arabidopsis thaliana under normal plant growth conditions [1/2MS media], salt conditions [125 mM NaCl] and drought conditions [25% PEG]. Moreover, we determined the most effective CDPs with optimal concentration. It was found that cis-cyclo-(Pro-Tyr) (Cyclo4) at a concentration of 100nM had an effect on the plant growth and can mimic the effect of SA190 under normal [1/2MS media] conditions. Also, cis-cyclo-(Pro-Tyr) (Cyclo4) at a concentration of 1µM can mimic the effect of SA190 under salt conditions [125mM NaCl]. Finally, cis-cyclo-(Pro-Val) (Cyclo5) at a concentration of 1µM can mimic the effect of SA190 under drought conditions [25% PEG].

The strength and direction of interactions between organisms vary spatially across the landscape. Traditionally, the focus has been on how trait variation affects the interactions between species. However, differences in abiotic and biotic environmental factors may also alter the distribution, phenology and behavior of the interacting species. To be able to understand why an interaction varies across the landscape, the effects of trait variation has to be separated from the effects of the environmental context. In this thesis, I try to separate the effects of context and trait differences on plant resistance against herbivory, through experimental and observational studies conducted with two cytotypes of the perennial herb Cardamine pratensis and its main herbivore, Anthocharis cardamines. The results show that differences in plant resistance against oviposition under controlled conditions were mainly mediated by flower size; larger flowers were more attractive to the female butterfly. However, among-populations differences in oviposition under natural conditions were not related to the resistance observed under controlled conditions, or to ploidy type, flowering phenology or plant size. Within populations under natural conditions the oviposition patterns by A. cardamines was affected by the plant traits plant size and flowering phenology. The result of this thesis shows that among-population differences in intensity of plant-herbivore interactions were caused by differences in environmental context rather than by herbivore preferences for any phenotypic plant traits, while host plant selection within population was based on plant traits. This suggests that biotic and biotic context can have important effects on the intensity of plant-herbivore interactions. Although genetic traits influenced the outcome of the interaction within populations, it was the environmental context of the populations that determined largely if the interaction took place or not. / <p>At the time of the doctoral defence the following papers were unpublished and had  a status as follows: Paper 2: Manuscript in review in Plos One; Paper 3: Manuscript in review in <em></em>Ecological Entomology; Paper 4: Manuscript</p>

Arbuscular mycorrhizal (AM) symbiosis modulates plant‐herbivore interactions. Still, how it shapes the overall plant defence strategy and the mechanisms involved remain unclear. We investigated how AM symbiosis simultaneously modulates plant resistance and tolerance to a shoot herbivore, and explored the underlying mechanisms. Bioassays with Medicago truncatula plants were used to study the effect of the AM fungus Rhizophagus irregularis on plant resistance and tolerance to Spodoptera exigua herbivory. By performing molecular and chemical analyses, we assessed the impact of AM symbiosis on herbivore‐triggered phosphate (Pi)‐ and jasmonate (JA)‐related responses. Upon herbivory, AM symbiosis led to an increased leaf Pi content by boosting the mycorrhizal Pi‐uptake pathway. This enhanced both plant tolerance and herbivore performance. AM symbiosis counteracted the herbivore‐triggered JA burst, reducing plant resistance. To disentangle the role of the mycorrhizal Pi‐uptake pathway in the plant's response to herbivory, we used the mutant line ha1‐2, impaired in the H+‐ATPase gene HA1, which is essential for Piuptake via the mycorrhizal pathway. We found that mycorrhiza‐triggered enhancement of herbivore performance was compromised in ha1‐2 plants. AM symbiosis thus affects the defence pattern of M. truncatula by altering resistance and tolerance simultaneously. We propose that the mycorrhizal Pi‐uptake pathway is involved in the modulation of the plant defence strategy.