Transcriptional Responses Of Whiteflies To Host Plant Transfer And The Role Of A Salivary Effector BtO56in This Process

The whitefly Bcmisia tabaci is a species complex that contains more than34cryptic species. Among them, the Middle East Asia Minor1(MEAM1) and Mediteranean (MED) species of the complex have spread rapidly around the world and become major invasive pests causing enormous economic losses in agriculture. In previous studies, our laboratory found that the host range of the MEAM1species may be much wider than that of the indigenous Asia Ⅱ3, and this difference in host range may assist invasion and displacement of Asia Ⅱ3by MEAM1in the field. In the natural environment, whiteflies are found to encounter different host plants during their spread, so it is important to understand the capacity of adaptation of different whitefly species to host transfer and the molecular mechanisms behind it.In the present study, we first compared the performance of the invasive MEAM1and the indigenous Asia Ⅱ3whitefly species following host plant transfer from a suitable host (cotton) to a poor host (tobacco) and then analyzed their transcriptional responses using Digital Gene Expression (DGE) profile technology. Moreever, by combining DGE profile with whitefly salivary transcriptome, we found a salivary effector protein, which may affect the whitefly host adaptability, and analyzed its function. The results are summarized as follows:(1) MEAM1has a stronger capacity of adaptation to host transfer. After transfer from cotton, a suitable host plant for both MEAM1and Asia Ⅱ3, to tobacco, a marginal host plant for both species, for24h, MEAM1performed much better than Asia Ⅱ3. Although the mortality and fecundity of both species were seriously affected, the survival and fecundity of MEAM1was significantly higher than those of Asia II3.(2) Transcriptional analysis showed that the patterns of gene regulation differed markedly between the two species of whiteflies. Significantly much more differentially expressed genes (DEGs) were observed in Asia Ⅱ3than in MEAM1. While most of the DEGs were up-regulated slightly in MEAM1, more DEGs in Asia Ⅱ3were sharply down-regulated. After transfer to tobacco for24h, the carbohydrate and energy metabolisms were repressed in Asia Ⅱ3but were not seriously affected in MEAM1. Moreover, the gene expression and protein metabolisms were activated in MEAM1. Compared to the constitutive high expression of detoxification genes in MEAM1, most of the detoxification genes were down-regulated in Asia Ⅱ3. Enzymatic activities of P450, GST and esterase further verified that the detoxification of MEAMl was much higher than that of Asia Ⅱ3. These results reveal obvious differences in responses of MEAM1and Asia Ⅱ3to host transfer.(3) By combining DGE profile with whitefly salivary transcriptome, we identified68 candidate salivary effector proteins that may affect whitefly host adaptability during host transfer. Using RNAi and transient overexpression in planta we found a candidate salivary protein BtQ56that can improve the whitefly performance on tobacco. Bioinformatics analysis indicates that BtQ56is a secretion protein with a signal peptide but not transmembrane domain. BtQ56gene was a novel gene without any homologous genes in GeneBank and specifically highly expressed in whitefly salivary glands. The expression level of BtQ56in MED and MEAM1whiteflies was much higher than that in Asia Ⅱ3. After host transfer, the BtQ56was up-regulated significantly in MED and ME AM1whiteflies but not in Asia Ⅱ3. Using Western blot, we detected BtQ56protein in cotton after whitefly feeding and proved that BtQ56is a saliva protein. The effect of BtQ56to plant defense marker genes were also analyzed. On tobacco plant with BtQ56overexpression, we found that while PR5gene in SA pathway was induced by BtQ56, while proteinase inhibitor gene in JA pathway was repressed after whitefly feeding. These results suggest that BtQ56can repress the transcription of proteinase inhibitor gene by inducing SA pathway and repressing JA pathway.Taken together, this study showed that the invasive whitefly has stronger capacity of adaptation than indigenous whitefly. When encountering host transfer, the carbohydrate and energy metabolisms and detoxification metabolisms were repressed seriously in Asia Ⅱ3species but not in MEAM1, indicating that MEAM1species had stronger ability of tolerance in these metabolisms. Moreover, the activation of gene expression and protein metabolisms in MEAM1may be an important factor for its stronger adaptation. In addition, the salivary effector BtQ56may plays a powerful role in whitefly response to host transfer.