Plant-mediated above- & belowground interactions through induced systemic defense in "Cardamine" species (Brassicaceae)

To ward off herbivore attack, that damages plant tissues in both above- and belowground (AG-BG) compartment, plants have evolved a diverse array of defense traits, including mechanical and chemical defenses. Induction of chemical defenses in response to herbivore attack at local tissues is a known phenomenon; however, we recently began to recognize the concept of systemic induced defense that crosses the root-shoot divide. The extent to which the induced systemic defenses are regulated is affected by the both biotic, as well as the abiotic component of the environment. The research presented within this dissertation is an attempt for better understanding plant-mediated AG-BG interactions through systemic inducibility of chemical defenses with insect herbivores, and across changing environments. I addressed the major objectives of this thesis within four chapters. First, I investigated the effect of root induction on subsequent expression of defensive secondary metabolites (glucosinolates - GSLs) in the leaves and induced systemic resistance against AG herbivores of different diet breadth, in Cardamine hirsuta (Brassicaceae). In addition, I tested whether induction of systemic defenses from root to shoots and consequent resistance against herbivores has genetic basis (Chapter I & II). I found that BG induction increased AG resistance against the generalist but not the specialist herbivore and found substantial plant family-level variation for inducibility of GSLs in the leaves and resistance against the generalist herbivore. I showed that the systemic induction of several GSLs tempered the negative effects of herbivory on total seed set production. Specifically, plant families possessing the ability for increased production of certain GSL compounds in the induced state could hinder the negative fitness effect of AG herbivory. Second, I investigated the effect of climate in shaping the expression of growth and defense phenotype across elevation gradient at the intraspecific level in C. pratensis. Next, I examined the ecological relevance of induced systemic resistance by testing the effect of root induction on consequent expression of GSLs and subsequent systemic resistance against natural herbivory within natural populations in the field. Furthermore, I looked for the existence of genotypic variation in systemic inducibility of GSLs and resistance, by conducting a reciprocal transplant experiment (Chapter III & IV). I found that climatic conditions regulate expression of growth traits in C. pratensis, while production of defensive traits were rather genetically fixed. I demonstrated that ecotypes of plants originating from different altitudes differed in their phytochemical make-up and observed significant suppression of AG herbivory in response to root induction. These findings were confirmed in the following common garden experiment, and in addition, I found genotypic variation in systemic inducibility of GSLs from root-to-shoot for high elevation ecotypes, and in contrast, I observed genetic convergence in response of different families to induction for low elevation ecotypes suggesting different selection pressures are acting on plants at different elevations. Third, to understand the role of shared evolutionary history and/or shared ecological niches on driving the variation in constitutive diversity of GSLs as well as their inducibility, at interspecific level, I combined targeted metabolomics analyses of GSL compounds, with insect herbivore bioassays, across 14 different Cardamine species. More so, I investigated the consequence of constitutive and systemic induced defenses on herbivores of different diet breadth and feeding guilds, across different species (Chapter V & VI). My findings demonstrated that GSL-based plant defense strategies, at constitutive level, converge into similar forms within each elevation, highlighting that during the radiation of a group, habitat filtering and plant–herbivore interaction shaped the nature of phytochemical variation of Cardamine species in the Alps. Moreover, the pattern of inducibility of GSLs from BG to AG in Cardamine species follows that of the root herbivory, which was shown to be declining along elevation gradients. Finally, by extending my investigation to the third trophic level (Chapter VII), I demonstrated significant variation in production of indirect defensive VOCs in response to BG herbivory and the consequent BG predator recruitment to the roots across several Cardamine species. I further tested the extent to which BG predator recruitment was modified by presence of AG herbivory and sought for specify of root defense strategy among species. While, I was unable to detect a specific pattern of BG predator recruitment across species of different ecological niches, my findings clearly demonstrated the variation in root induced indirect defense influenced by AG herbivory. The findings of this dissertation enhance our understanding on how plant-mediated AG-BG interactions with insect herbivores are regulated by means of induced systemic expression of secondary metabolites under variable environmental conditions. The novelty of combining both climatic and biotic factor influencing induced systemic defense shed further light on how the deployment of plant defenses locally adapt to biotic and abiotic conditions across different ecosystems and should inspire further and deeper investigations on elucidating the mechanisms governing the ecology and evolution of plant-insect interactions.