Specificity in Chemical Communication between Plants in Respose to Herbivory and Patogen Infection

Abstract

Research has demonstrated that plants can perceive and respond to complex blends of above- or belowground volatile organic compounds (VOCs) emitted by neighbouring conspecific or heterospecific plants, a phenomenon known as "plant communication". These responses often involve the priming or induction of defences in "receiver" plants exposed to VOCs from herbivore-damaged "emitter" neighbours, ultimately leading to increased resistance to herbivory. Plant communication has been documented in over 30 plant species, including several agricultural crops and forestry species, and is now well-accepted in plant defence research. Additionally, in the past decade, several authors have proposed agricultural management methods that exploit these signalling mechanisms in search of alternative and more sustainable crop protection against pests and diseases (e.g. in integrated pest management). That said, applied research focusing on the use of plant VOCs to reduce pest damage is still at its infancy. Of up most importance is to address the drivers behind variability and context-dependency of plant-plant VOC-mediated signalling, ultimately shedding insight into when and how these interactions are more likely to occur and thus the predictability and effectiveness of VOC-based management actions. In this PhD dissertation, I investigated biotic and abiotic factors that can affect VOC-mediated plant communication in response to herbivory and pathogen infection using potato (Solanum tuberosum) and wild cotton (Gossypium hirsutum) as model species, and beet armyworm (Spodoptera exigua), cotton worm (Alaba arguillacea), Fusarium solani and Phytophthora infestans as attacker agents. Specifically, I tested whether: (i) communication between different varieties of potato plants in response to S. exigua damage is contingent on plant genetic relatedness (ii) communication between wild cotton plants in response to A. arguillacea damage is contingent on herbivore density and/or herbivore damage-pattern, (iii) communication between potato plants in response to S. exigua damage is affected by soil nutrient increase; and (iv) communication between potato plants is attacker-specific, infecting plants with either F. solani or P. infestans pathogens. To this end, I conducted a series of greenhouse experiments manipulating these biotic and abiotic factors and documented their effects on VOC emissions and their effects on neighbouring plants. First, I found that insect herbivory affected the total amount and/or composition of VOC blends emitted in both potato and wild cotton, and that these induced VOCs boosted resistance against herbivory in neighbouring receiver plants. Second, I found that plant genotypic relatedness in potato and herbivory amount and pattern (within plant dispersion) in cotton did not affect communication outcomes, despite varietal and herbivory treatment differences in total VOCs released and/or VOC composition, respectively. Third, soil fertilization had similarly no detectable effect on plant-plant signalling effects in potato, and in this case furthermore did not influence VOC induction. Fourth, in contrast to insect damage, pathogen infection did not induce changes in VOC emissions or receiver resistance in potato plants. Overall, these findings show contrasting effects on VOCs and signalling between insect herbivores and pathogens, and in the case of herbivory suggest that signalling is buffered against variation in biotic and abiotic drivers, in some cases despite altered patterns of induced emissions. These findings provide initial indications on context-dependency and specificity in plant VOC emissions and plant-plant signalling (or lack of thereof), knowledge that can be used to avoid interference with or promote these signalling interactions to reduce pest damage under different management scenarios