Whitefly-hostplant-begomovirus Interactions And The Roles Of Glucosinolates In These Interactions

The whitefly (Bemisia tabaci) is a species complex comprised of at least 36 cryptic species, of which some species such as MEAM1 and MED have been spreading around the world in the past 30 years because of their strong reproductive ability, broad host range, and high resistance to insecticides. They have evolved a specific relationship with begomoviruses, which they transmit in a circulative, persistent manner. Outbreaks of whiteflies are often followed by pandemics of begomoviruses. The reprogramming of gene expression and the regulation of plant physiology may influence the population dynamics of whiteflies and in turn the spread of the virus itself. Jasmonate signaling pathway and the secondary metabolite production that it regulates, such as glucosinolates, plays a vital role in plant’s defense against pathogen infection and insect herbivory.In the present study, we firstly examined the effect of plant JA defenses on the gene expression of whiteflies and the performance of whiteflies on JA-elicited tobacco plants; then we used RNA-seq to investigate the gene expression profile of tomato to whitefly infestation, virus infection and virus-whitefly co-infestation; and lastly we used the model plant Arabidopsis thaliana to investigate the influence of whitefly feeding and begomovirus pathogenicity factor βC1 gene on the production of glucosinolates and the role of glucosinolates on the performance of whiteflies in this context. The results are summarized as follows:1. The influence of JA defense pathway on the gene expression and fitness of whitefliesWe conducted a comprehensive investigation on the responses of the MEAM1 whitefly to plant JA-regulated defenses, using RNA-seq technology. After feeding for 3 days on JA-elicited tobacco, whiteflies have 242 genes up-regulated and 450 genes down-regulated. Further analysis of these differentially expressed genes (DEGs) revealed that the primary metabolism such as carbon hydrate metabolism, amino acid metabolism, lipid metabolism etc. were all significantly repressed, and genes associated with energy metabolism were all down-regulated. Meanwhile, nearly all the DEGs involved with detoxification were up-regulated, suggesting that the primary metabolism including energy production in whiteflies was repressed while it still managed to increase the detoxification activity. On the other hand, genes involved with cell cycle, and neural diseases were also significantly regulated. All these results indicate that while whiteflies actively detoxify the plant defenses to reduce the damage of plant defensive chemicals on them, plant defensive chemicals may still have some toxic effect on whiteflies leading to higher mortality of whiteflies on JA-elicited tobacco.2. Responses of tomato to whitefly feeding, virus infection and virus-whitefly co-infestationWe performed digital gene expression profile to examine the molecular responses of tomato plants to whitefly feeding, virus infection and virus-whitefly co-infestation. The three treatments resulted in 324,686 and 996 DEGs, respectively. Among the three treatments, co-infestation has the most pronounced effect on tomato. There were overlapping of DEGs between treatments as well as treatment-specific DEGs, and a large proportion of the overlapping genes have the same transcript pattern. Virus infection and co-infestation significantly suppressed photosynthesis rates and Phenylpropanoid/ Flavonoid biosynthesis, which might be a direct sign of perturbation of pigment biosynthesis leading to chlorosis of leaves. And also many genes involved in cell wall regulation and carbon fixation, both of which might be involved in plant pathogen resistance, were significantly influenced. Resistance genes and pathogenesis-related genes were up-regulated in these treatments, indicating that tomato might use basal resistance as well as gene-to-gene resistance to recognize and defend against virus and whiteflies. In the analysis of genes involved with plant hormones, we found that genes in JA pathway were repressed in plants infected by the virus and plants co-infested by the virus and whitefly. Surprisingly, hormones involved in plant growth, such as indole-3-acetic acid, abscisic acid, cytokinin, gibberellin and brassinosteroid, were more profoundly influenced in these three treatments. Effects on these plant growth hormones might be involved in the stunting, reduction of leaf size, and upward cupping/curling phenotype in begomovirus infected tomato. Besides, there are a whole set of genes that were significantly regulated but have yet no functional annotation or clear role in virus-plant-insect interactions, such as the histone related genes.3. The role of glucosinolates in whitefly-hostplant-virus interactionWe tested the content of glucosinolates in Arabidopsis plants in response to whitefly feeding and glucosinolates content in 35S:HA-βC1/Co1-0 transgenic plants. We found that whitefly feeding induced the production of glucosinolates while βC1 gene repressed the production of glucosinolates. In Arabidopsis vegetative growth, the aliphatic glucosinolates in rosette leaves were the majority at the beginning and started to decline as the plants grew, while the content of indole glucosinolates remained little changed or even increased as the plants grew. Whitefly performance bioassay showed that lower level of glucosinolates did not improve the fecundity significantly. However, in an indole glucosinolate over-production mutant, whiteflies have significantly reduced fecundity and growth rate. The βC1 gene improved whitefly performance, probably via the repression of JA pathway and secondary metabolites such as glucosinolates and terpenes.