Possible Roles Of Several Salivary Components From Bemisia Tabaci B-biotype In Its Invasion Mechanism

Bemisia tabaci (Gennadius) biotype B was introduced to China at the end of20th century andposed huge threat to the ecosystem and agricultural industry. Strong competitiveness and high hostadaptation were the main causes for its outbreak. B.tabaci biotype B is piercing-sucking insect, whichwill secrete gell saliva and watery saliva when it sucks phloem sap from the plants. This liquidsexchange between whitefly and plant is one of the most important ways how invasive whitefly affectsthe local biota. Invasion hypothesis are the key theories of invasion biology. Inherent superiorityhypothesis describes the potential advantages of successful invader, while novel weapon hypothesisemphasis the allelopathic effect of invader on the local community. Thus the key scientific questions inthis study are: Is saliva a novel weapon for B.tabaci biotype B? Can this be the Inherent superiority forB.tabaci biotype B when compare to the indigenous species Trialeurodes vaporariorum?In this study, we first tested the effects of rearing time, density, sucrose concentration of diet andpH value of diet on the mortality of the B.tabaci biotype B and T. vaporariorum. The results showedrearing400whiteflies on10%sucrose concentration and pH6.5diet for2days is the best combinationfor saliva collection. This artificial feeding system and biochemical methods were used to identify andanalyze different enzymes in whitefly saliva. For salivary hydrolase, both B.tabaci biotype B and T.vaporariorum have higher salivary alkaline phosphatase activities on their preferred hosts compared tothose from unsuitable hosts. B. tabaci B-biotype maintained higher salivary alkaline phosphataseactivity than T. vaporariorum on tomato and cotton. The salivary alkaline phosphatase activities fromboth whiteflies increased when the sucrose concentration in diet decrease, which suggest alkalinephosphatase plays a role in sucrose metabolism. For salivary pectinase, Pectin methylesterase andpolygalacturonase were not identified in the saliva of two whitefly species in this study.For salivary oxides, polyphenol oxidase and peroxidase were found in the gel and watery saliva ofboth B. tabaci B-biotype and T.vaporariorum. For tomato colonies, the polyphenol oxidase andperoxidase activities in the watery saliva of B. tabaci B-biotype were2.27and1.34fold higher thanthose of T. vaporariorum. The polyphenol oxidase activities against specific phenolic compoundscommonly found in plants were compared. while the order of activity remained the same for bothwhitefly species, gallic acid>quercetin> pyrocatechol> caffeic acid>chlorogenic acid, the level ofactivity was markedly different with B. tabaci B-biotype consistently demonstrating much higher levelsof activity We also measured polyphenol oxidase activity in both species after they had feed on plantsthat were undamaged or had been previously damaged with either a plant pathogen (Phytophthorainfestans) infection, mechanical damage, B. tabaci B-biotype infestation or exogenous salicylic acid.For B. tabaci B-biotype, polyphenol oxidase activities in watery saliva increased229%,184%,152%,and139%in response to the four treatments while those of T. vaporariorum only increased133%,119%,113%,103%, respectively. B. tabaci B-biotype infestation can significantly increase the total phenolicscontent of tomato leaves. Meanwhile, feeding on tomato pre-infested with B. tabaci B-biotype had no significant effect on the survival rate of B. tabaci B-biotype but can decrease the survival rate of T.vaporariorum significantly.Laccase belong to the polyphenol oxidase family and is known to be present in the saliva andsalivary gland of several insect species. The full-length cDNA sequence of Laccase-1gene(BtLac-1)was cloned from B.tabaci biotype B, which was sequenced to be3261bp and encoded911amino acid.BtLac-1has the typical features of Cu-oxidase domain, which belongs to blue copper-containingpolyphenol oxidases family. Homology analysis of amino acid sequence showed that BtLac-1shares58%identity with Nephotettix cincticeps and Acyrthosiphon pisum. The sequence analyses suggest thatB.tabaci biotype B produce soluble laccase in an active form. This indicates that the salivary laccasecauses rapid oxidation of phenolic substances just after being released into the saliva. In general, theseresults suggest that B. tabaci B-biotype is better adapted with the greater alkaline phosphatase andpolyphenol oxidase activities to metabolize sucrose or detoxify high concentrations of phenolics than T.vaporariorum, and have a significant advantage to help itself hold high fitness on plants, resulting rapidand successful invasion.