To test the hypothesis that sodium sulfite treatment changes the host-virus interactome, LC-MS/MS was used to analyze tryptic digests of non-transmissible and transmissible virion preparations purified from infected oat plants. Although the genome of oat is not available, we were able to use predicted proteins from available genomes of related cereals for homology-based protein identification of the plant proteins found in the virus preparations. Twenty proteins were identified in the transmissible, but not the non-transmissible virus preparations (Table 4 ). Sixteen proteins ( Table 4 ) are candidate proteins that may interact with transmissible virus directly or in complex. These include three chloroplast proteins (326533372, 20302473, and 2565305), seven mitochondrial proteins (115472339, 115474041, 115471693, 326500100, 115477529, 115448577, and 357139868), two isoforms of remorin (357164942 and 115456099), one predicted cytoplasmic protein (115449199) and three proteins with predicted functions in the nucleus (357144283, 357121487, and 226531758). Six host proteins were found in both transmissible and non-transmissible virus preparations. These include a 33-kD secretory protein, thaumatin-like protein 5 (TLP-5), ATP synthase CF1 beta subunit, cellulose synthase, triosephosphate isomerase (TIM), and a cysteine-rich repeat secretory protein (Table 4 ). Two proteins, prophobilinogen synthase and adenosylhomocysteinase, were identified exclusively from preparations of sodium-sulfite treated, non-transmissible virions. Proteins homologous to most of these proteins are involved in plant defense and have been previously identified in plant phloem sap proteomes [26, 27, 28, 29] . Among the potential virus genome-derived protein products, peptides from only the two virus structural proteins (CP and RTP) were identified (not shown).