The Ecological Mechanism Of Secondary Symbiont Hamiltonellain The Transmission Of Tomato Yellow Leaf Curl Virus By Bemisia Tabaci Biotype Q

The phloem-feeding whitefly, Bemisia tabaci,is a major agricultural pest of many agriculturalcrops and the damage in terms of the spread of plant virus has become a major obstacle to the development of agricultural production. The B. tabaci harbors several bacterial symbionts. Among thesecondary (facultative) symbionts, Hamiltonella has high prevalence and high infection frequencies, suggesting that it maybe important for the biology and ecology of its hosts. We used the previous establishedHamiltonella-infected and Hamiltonella-uninfected strains with the same nuclear background to investigate the influence of Hamiltonella to nutrition provisioning to host, defend plant defenses and natural enemies and the mitualism of B. tabaci biotype Q and Tomato yellow leaf curl virus (TYLCV). The results are significant for clarifying the mechanism of insect-virus-symbiont interactions and developing new ways to effective control virus transmission by vectors. The specific results are as follows:1. Hamiltonellaincreases the growth rate of its host Bemisia tabacibiotype Qduring periods of nutritional stressWe assessed the effects of Hamiltonella infection on the growth of B. tabacibiotype Q reared on low-, standard-, or high-nitrogen diets.When B. tabacibiotype Q was reared on a standard-nitrogen diet, no cost or benefit was associated with Hamiltonella infection. But, ifwe reared whiteflies on low-nitrogen diets, Hamiltonella-infected whiteflies often grew better than uninfected whiteflies.Furthermore, nitrogen levels in field-collected whiteflies indicated that the nutritional conditions in the field werecomparable to the low-nitrogen diet in our laboratory experiment.2. Hamiltonella suppresses induced plant defences by Bemisia tabacibiotype Qin tomatoThe downregulation of plant defences was associated with increased B. tabacibiotype Q fecundity andsurvival. Feeding by Hamiltonella-infected B. tabacibiotype Q suppressed JA and JA-related anti-herbivore-induced defences in tomato relative to uninfected controls. Interestingly, suppression of defences was not observed in SA-deficient NahG plants, indicatingthat suppression of JA-regulated defences depends on the SA signalling pathway.That saliva-only treatments ofdamaged tomato leaves from Hamiltonella-infected whiteflies suppressed induced plant defencescompared to saliva from uninfected controls, suggesting that elicitors in saliva were responsible.Characterization of the putative salivary factor(s) revealed they are likely small (< 3-kDa)and nonproteinaceous.3. Hamiltonella favor of Bemisia tabacibiotype Qevade parasitic waspsHamiltonella did not reduce B. tabacibiotype Q susceptibility to parasitoid Encarsia formosa, nor did it affect the fitness of wasps that successfullycompleted development. Despite this, experienced femalesof parasitoid E. formosa preferentially oviposited intouninfected B. tabacibiotype Q hosts when given a choice between geneticallyidentical individuals with or without Hamiltonella.4. Effects of TYLCV infection and feeding by Bemisia tabacibiotype Q on the tomato quality and defense statusHoneydew excreted by viruliferous B. tabacibiotype Q had a sugar:amino acid ratio half that of honeydew from uninfectedwhiteflies. Their honeydew did not differin the percentage of essential amino acids, nor in any of the 16 individual aminoacids; free amino acid concentrations were significantlower in the epidermis and mesophyll tissues ofinfected versus uninfected plants. There were, however, no differences insimple carbohydrates (sucrose, glucose, and fructose); as a result, the sugar:amino acid ratio in theepidermis and mesophyll tissues of infected plants was2.5-fold higher than that of uninfected plants; in contrast to the epidermis and mesophyll tissue,free amino acid concentrations were significant higher in thephloem sap of infected plants. Because infected plants also had higher concentrationsof simple carbohydrates, there were nobetween-treatment differences in the sugar:amino acidratio of their phloem sap. TYLCV infection reduces plant defensiveresponse to B. tabaciinfestation.