An ecogenomic analysis of herbivore-induced plant volatiles in Brassica juncea.

Upon herbivore feeding, plants emit complex bouquets of induced volatiles that may repel insect herbivores as well as attract parasitoids or predators. Due to differences in the temporal dynamics of individual components, the composition of the herbivore-induced plant volatile (HIPV) blend changes with time. Consequently, the response of insects associated with plants is not constant either. Using Brassica juncea as the model plant and generalist Spodoptera spp. larvae as the inducing herbivore, we investigated herbivore and parasitoid preference as well as the molecular mechanisms behind the temporal dynamics in HIPV emissions at 24, 48 and 72 h after damage. In choice tests, Spodoptera litura moth preferred undamaged plants, whereas its parasitoid Cotesia marginiventris favoured plants induced for 48 h. In contrast, the specialist Plutella xylostella and its parasitoid C. vestalis preferred plants induced for 72 h. These preferences matched the dynamic changes in HIPV blends over time. Gene expression analysis suggested that the induced response after Spodoptera feeding is mainly controlled by the jasmonic acid pathway in both damaged and systemic leaves. Several genes involved in sulphide and green leaf volatile synthesis were clearly up-regulated. This study thus shows that HIPV blends vary considerably over a short period of time, and these changes are actively regulated at the gene expression level. Moreover, temporal changes in HIPVs elicit differential preferences of herbivores and their natural enemies. We argue that the temporal dynamics of HIPVs may play a key role in shaping the response of insects associated with plants.

A novel indirect defence in Brassicaceae: structure and function of extrafloral nectaries in Brassica juncea.

While nectaries are commonly found in flowers, some plants also form extrafloral nectaries on stems or leaves. For the first time in the family Brassicaceae, here we report extrafloral nectaries in Brassica juncea. The extrafloral nectar (EFN) was secreted from previously amorphic sites on stems, flowering stalks and leaf axils from the onset of flowering until silique formation. Transverse sections at the point of nectar secretion revealed a pocket-like structure whose opening was surrounded by modified stomatal guard cells. The EFN droplets were viscous and up to 50% of the total weight was sugars, 97% of which was sucrose in the five varieties of B. juncea examined. Threonine, glutamine, arginine and glutamate were the most abundant amino acids. EFN droplets also contained glucosinolates, mainly gluconapin and sinigrin. Nectar secretion was increased when the plants were damaged by chewing above- and belowground herbivores and sap-sucking aphids. Parasitoids of each herbivore species were tested for their preference, of which three parasitoids preferred EFN and sucrose solutions over water. Moreover, the survival and fecundity of parasitoids were positively affected by feeding on EFN. We conclude that EFN production in B. juncea may contribute to the indirect defence of this plant species.