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Title: Variation in the salivary proteomes of differentially virulent greenbug (Schizaphis graminum Rondani) biotypes

Author
item Nicholson, Scott
item Puterka, Gary

Submitted to: Journal of Proteomics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/9/2013
Publication Date: 6/13/2014
Citation: Nicholson, S.J., Puterka, G.J. 2014. Variation in the salivary proteomes of differentially virulent greenbug (Schizaphis graminum Rondani) biotypes. Journal of Proteomics. 105:186-203.

Interpretive Summary: The greenbug (Schizaphis graminum Rondani) is a major pest of wheat, barley, and sorghum, causing significant crop losses throughout its range. The damage inflicted by the greenbug consists of widespread leaf bleaching, plant stunting, and death, leading to a yield reduction from infested fields. Like other aphids, the greenbug uses its saliva, which contains many different proteins, to manipulate its host plant. This saliva deactivates plant defenses against aphid feeding and injury. However, most other grain aphids do not directly damage their host. Greenbug saliva contains proteins, called phytotoxins, that directly damages the host plant. The symptoms of phytotoxicity are uniform when present, but certain greenbug strains, or biotypes, are able to damage crop lines that are resistant to other greenbug biotypes. A greenbug biotype's ability to cause phytotoxic damage on a crop line that other greenbug biotypes do not damage is called virulence, and those biotypes that can damage greater numbers of crop lines are more virulent than those that damage fewer crop lines. To determine what causes the differing virulence of greenbug biotypes, and also to discover potential greenbug salivary phytotoxins, we analyzed the saliva of four separate greenbug biotypes by three different techniques. One- and two-dimensional electrophoresis demonstrated protein content variance in each examined greenbug biotype. Mass spectrometric analysis demonstrated salivary protein variance between biotypes in six proteins, and also identified thirty-two individual proteins in the saliva of the studied biotypes. This study 1) determined that greenbug salivary proteins differ among biotypes, providing evidence that salivary content determines virulence, 2) analyzes the known function of the identified proteins to predict their activity in the host plant, 3) demonstrates that greenbug saliva is considerably different from the saliva of other aphids, and 4) develops a model distinguishing virulence from phytotoxicity. This work will be useful in understanding greenbug phytotoxicity and virulence for use in the development of greenbug resistance in crops.

Technical Abstract: Greenbug (Schizaphis graminum Rondani) biotypes are classified by their differential virulence to wheat, barley, and sorghum varieties possessing greenbug resistance genes. Virulent greenbug biotypes exert phytotoxic effects upon their hosts during feeding, directly inducing physiological and metabolic alterations and accompanying foliar damage. Comparative analyses of the salivary proteomes of four differentially virulent greenbug biotypes C, E, G, and H showed significant proteomic divergence between biotypes. Thirty-two proteins were identified by LC-MS/MS; the most prevalent of which were three glucose dehydrogenase paralogs (GDH), lipophorin, complementary sex determiner, three proteins of unknown function, carbonic anhydrase, fibroblast growth factor receptor, and abnormal oocyte (ABO). Seven nucleotide-binding proteins were identified, including ABO which is involved in mRNA splicing. Quantitative variation among greenbug biotypes was detected in six proteins; two GDH paralogs, carbonic anhydrase, ABO, and two proteins of unknown function. Our findings reveal that the greenbug salivary proteome differs according to biotype and diverges substantially from those reported for other aphids. The proteomic profiles of greenbug biotypes suggest that interactions between aphid salivary proteins and the plant host result in suppression of plant defenses and cellular transport, and may manipulate transcriptional regulation in the plant host, ultimately allowing the aphid to maintain phloem ingestion.