Defending flowers against multiple attack: from phytohormones to plant ecology

My thesis aimed at investigating the defensive and reproductive strategies of plants when flowers are under multiple attack by two florivorous insect species (aphid, caterpillar) and a phytopathogenic bacterium. The study of Brassica nigra, an annual outcrossing Brassicaceae, indicated that inflorescences under attack were more resistant to caterpillars than to aphids. This project identified jasmonates as the main phytohormones mediating plant responses to attack, and jasmonates were particularly upregulated in inflorescences exposed to single or dual attack by caterpillars. Glucosinolates were not induced in inflorescences and these defensive compounds likely did not mediate induced direct resistance against florivores. Attack induced changes in the volatile emission of plants in the flowering stage, especially when caterpillars were among the attackers. However, plants maintained their interaction with parasitoid wasps that mediated indirect defences against florivores. Moreover, changes in primary metabolism may have contributed to plant compensation for damage inflicted by the attackers and overall, to plant tolerance to attacks. Finally, plants maintained interactions with pollinators despite the phytochemical changes induced upon attack.