To deal with (a)biotic stresses such as herbivores, pathogens or environmental changes, plants have evolved several defense strategies. Currently, most studies are focused on plant defense responses to a single attack or infection and knowledge of the effects of multiple stresses is scarce. Plants defenses are constitutive or inducible. Constitutive defenses are composed of physical barriers such as trichomes or cuticule and of toxic secondary metabolites, such as the glucosinolates (GS). Inducible defenses are triggered specifically after infection or feeding to induce an efficient response. In response to single attacks in Arabidopsis thaliana, jasmonic acid (JA) and ethylene (ET) signaling pathways are triggered after necrotroph and herbivorous challenge whereas the salicylic acid (SA) pathway is triggered after biotroph invasion. To create a flexible defense response, notably in case of combined stresses, these different pathways can have a synergistic or an antagonistic effect on each other, a phenomenon named crosstalk. Moreover, plants are known to emit biogenic volatile organic compounds (BVOCs) into the atmosphere for their own protection against herbivores by attracting parasitoids.
The aim of my thesis was to characterize plant defense responses under multiple stresses. I performed whole-genome analyses of Brassica nigra plants challenged by Pieris brassicae egg extract, the bacterial pathogen Xanthomonas campestris pv. campestris (Xcc), Brevicoryne brassicae aphids or ozone followed by P. brassicae larval feeding. Surprisingly, results indicated that the primary stress has a weak effect on transcriptional responses to a secondary stress. In addition, expression of genes involved in the biosynthesis of GS and BVOCs were not affected by any treatment.
In the second part, I focused on the effects of P. brassicae egg application or Xcc infection on performance of the specialist P. brassicae and the generalist Spodoptera littoralis. Strikingly, insects performed differently depending on the stress applied, the site of feeding and their feeding specialization. Moreover, GS, SA and JA were induced specifically at the site of herbivory or infection. Furthermore, we showed that a SA antagonistic effect on the JA pathway, and inversely, occurred depending of the pretreatment and the insect applied.
In conclusion, this work demonstrated that B. nigra defense response is localized to the treatment area and can differ from adjacent tissue. Importantly, the response to combined stresses is highly specific to the history of attacks.