title: Aphid salivary proteases are capable of degrading sieve-tube proteins.
abstract: Sieve tubes serve as transport conduits for photo-assimilates and other resources in angiosperms and are profitable targets for piercing-sucking insects such as aphids. Sieve-tube sap also contains significant amounts of proteins with diverse functions, for example in signalling, metabolism, and defence. The identification of salivary proteases in Acyrthosiphon pisum led to the hypothesis that aphids might be able to digest these proteins and by doing so suppress plant defence and access additional nitrogen sources. Here, the scarce knowledge of proteases in aphid saliva is briefly reviewed. In order to provide a better platform for discussion, we conducted a few tests on in vitro protease activity and degradation of sieve-tube sap proteins of Cucurbita maxima by watery saliva. Inhibition of protein degradation by EDTA indicates the presence of different types of proteases (e.g. metalloproteses) in saliva of A. pisum. Proteases in the watery saliva from Macrosiphum euphorbiae and A. pisum were able to degrade the most abundant phloem protein, which is phloem protein 1. Our results provide support for the breakdown of sieve-element proteins by aphid saliva in order to suppress/neutralize the defence responses of the plant and to make proteins of sieve-tube sap accessible as a nitrogen source, as is discussed in detail. Finally, we discuss whether glycosylation of sieve-element proteins and the presence of protease inhibitors may confer partial protection against the proteolytic activity of aphid saliva.
authors.affiliation: Institute of General Botany and Plant Physiology, Friedrich-Schiller-University, Dornburger Str. 159, D-07743 Jena, Germany.
authors.forename: Alexandra C U
authors.initials: AC
authors.lastname: Furch
authors.affiliation: Department of General Botany, Justus-Liebig-University, Heinrich-Buff-Ring 17, D-35392 Giessen, Germany.
authors.forename: Aart J E
authors.initials: AJ
authors.lastname: van Bel
authors.affiliation: Institute of Phytopathology and Applied Zoology, Centre for BioSystems, Land Use and Nutrition, Justus-Liebig-University, Heinrich-Buff-Ring 26-32, D-35392 Giessen, Germany torsten.will@agrar.uni-giessen.de.
authors.forename: Torsten
authors.initials: T
authors.lastname: Will
mesh_terms: D000818:Animals; D001042:Aphids / Q000201:enzymology*; D010447:Peptide Hydrolases / Q000378:metabolism*; D052585:Phloem / Q000378:metabolism*; D010940:Plant Proteins / Q000378:metabolism*; D059748:Proteolysis; D012463:Saliva / Q000201:enzymology*
chemical_list: D010940:Plant Proteins; D010447:Peptide Hydrolases
keywords: Aphid; aphid saliva; aphid–plant interaction; protease; protease inhibitor; sieve-tube occlusion; sieve-tube protein.