The Nutrition-related Mechanisms Underlying TYLCCNV-MEAM1Whitefly-tobacco Plant Interactions

In the virus-vector-plant tripartite interaction systems, viruses can affect the population growth of insect vectors in a direct way by infecting the vectors as well as in an indirect way by infecting their host plant. These interactions may play important roles in determining the population dynamics of both insect vectors and plant pathogens. In the past20years, outbreaks of invasive whiteflies in many regions of the world were often accompanied with epidemics of plant diseases caused by begomoviruses. The implications of begomovirus-whitefly interactions in the invasions by whiteflies and epidemics of begomoviruses have attracted much interest. In our laboratory, an earlier study with Tomato yellow leaf curl China virus (TYLCCNV), Middle East-Aisa Minor1(MEAM1) whitefly and tobacco plants showed that the whitefly population increased at a substantially higher rate on virus-infected plants than on uninfected plants, but the viruliferous whiteflies performed worse than non-viruliferous whiteflies. The objectives of this study were to reveal the potential nutrition-related mechanisms underlying TYLCCNV-MEAM1whitefly-tobacco plants interactions.In this study, we conducted experiments to investigate whether the nutritional quality of phloem sap in tobacco plants changed following TYLCCNV-infection and/or whiteflies-infestation, to compare the nurtritional composition of honeydew excreted by whiteflies feeding on TYLCCNV-infected and uninfected tobacco plants, and to compare the nurtritional composition of honeydew excreted by viruliferous and non-viruliferous whiteflies feeding on cotton, a non-host plant for TYLCCNV. The objectives were to analyze the effects of tripartite interactions on nutritional composition of phloem sap in plants and on nutritional assimilation by whiteflies. In view of the important roles trehalase and trehalose play in insect’s nutritional physiology, we compared the trehalase activities and trehalose content between whiteflies feeding on virus-infected and uninfected plants, and between viruliferous and non-viruliferous whiteflies. we also cloned two genes (BtTres) encoding trehalase and measured the expression of BtTres at different stages. The results are summarized as follows: (1) The effect of TYLCCNV-infection and/or whiteflies-infestation on nutritional quality of phloem sap in tobacco plants.The quality of phloem sap in plants did not change significantly following virus-infection and/or whitefly feeding, indicating the nutritional quality of phloem sap is not involved in the TYLCCNV-MEAM1whitefly indirect interaction.(2) The effect of TYLCCNV-infected tobacco plants on nutritional assimilation by MEAM lwhitefliesAlthough the quality of phloem sap in plants did not change significantly following virus-infection and/or whitefly feeding, the whiteflies feeding on TYLCCNV-infected plants excreted comparatively a lower proportion of amino acids, especially essential amino acids, or proportionally a higher level of sugars than those feeding on uninfected plants, suggesting that whiteflies feeding on virus-infected plants are able to achieve a more balanced nutritional assimilation. In addition, whiteflies feeding on older plants assimilated a lower proportion of amino acids, especially essential amino acids, or a higher propotion of sugars than those feeding on younger plants, suggesting that plant age, irrespective of infection of TYLCCNV, affected nutritional assimilation by whiteflies.(3) The direct effect of TYLCCNVon nutritional assimilation by MEAM lwhitefliesAnalysis of nutritional composition of honeydew revealed that viruliferous whiteflies assimilated a more imbalanced nutrition with higher sugars:amino acids ratio than non-viruliferous whiteflies, even though the amino acid profiles and percentages of essential amino acids were not significantly affected. The imbalanced nutritional assimilation may be detrimental to whiteflies. In addition, our data indicate that the improved nutritional assimilation by whiteflies feeding on virus-infected tobacco was caused by indirect effect of the virus, and the positive indirect effects overwhelmed the negative direct effects when they both occur.(4) the effect of TYLCCNV on trehalase and trehalose in MEAM1whitefliesThe trehalase activity was higher, but the trehalose content was lower in whiteflies feeding on virus-infected plants than those feeding on uninfected plants. The trehalase activity did not differ significantly between viruliferous and non-viruliferous whiteflies, but the trehalose content in viruliferous whiteflies was significantly higher than non-viruliferous whiteflies. As the trehalase activity was positively correlated with plant suitability, and the trehalose content was positively correlated with proportion of assimilated sugars, we assumed that trehalose, but not trehalase, is involved in regulation of nutritional assimilation by whiteflies. The trehalase activity, however, may be a indicator of plant suitability. In addtion, we obtained the full-length cDNA sequnce of two BtTres with RACE technique. The deduced protein sequence indicated that BtTre-1and BtTre-2are soluble and membrane-bound trehalase, respectively. Both BtTres reached the maximal level at pupae stage, probably because the trehalases are involved in provision of chitin and energy for eclosion at this atage. Besides, BtTre-2may play a more important role than BtTre-1, as the expession of BtTre-2is significantly higher than BtTre-1at all stages, but their physiological functions remain unknown.Overall, within TYLCCNV-MEAM1whitefly-tobacco plants system, the virus-infection and/or whitefly feeding did not change significantly the quality of phloem sap in plants, but the virus could directly and indirectly afftect the nutritional assimilation by whiteflies. The balance of nutritional assimilation was positively correlated with the performance of whiteflies, suggesting the balance of nutritional assimilation is one of the mechanisms underlying tripartite interactions. In addition, trehalase activity and trehalose content were also affected by virus, but they played different roles in tripartite interactions. However, the physiological mechanisms underlying the direct and indirect effects of TYLCCNV on nutritional assimilation, trehalase activity and trehalose content in MEAM1whiteflies remains unknown.