were detected in the R. padi homogenate (Table 5) . A new method was created containing only these peptides, and three biological replicate samples of digested homogenates from aphids reared on infected or healthy plants were analyzed. The chromatographic retention time for each peptide was highly reproducible among all six samples ( Table 5 ). The transitions are generally free from interference when they were detected (Fig. S2 , Table S2 ) in the digested homogenates that were derived from aphids fed on infected plant tissue. Only one peptide from thaumatin-like protein 5, 33-kD secretory protein and pyruvate dehydrogenase E1 and E2 was detected (Table 5) , whereas two peptides from cellulase were detected (Table 5 , Figure S2 ). Until the oat genome is sequenced, we do not know exactly what protein we are monitoring, i.e., a single plant protein isoform or a mixture of proteins sharing the same peptide. Total peak areas for the selected peptide ions and their CID fragments were used to calculate a fold-change, and the log [2] of the fold-change is reported so that a positive value indicates an enrichment of the peptide in the extract from aphids fed on CYDV-RPV infected plants, and a negative value indicates a lower level of the peptide in the extract from aphids fed on CYDV-RPV infected plants. Pyruvate dehydrogenase E1 and E2 were found to associate with only transmissible virions (Table 4 ). Peptides derived from both of these proteins were found to accumulate to higher levels in aphids fed on CYDV-RPV infected plant tissue (Table 5 ). In fact, the peptide from pyruvate dehydrogenase E2, GLGMIAEEVK, was only detected in aphids fed on infected plant tissue (Table 5) and not in aphids fed on healthy plants. Cellulose synthase, thaumatin-like protein, and the 33-kD secretory protein co-purified with both transmissible and non-transmissible virions (Table 4 ). Cellulose synthase was detected at similar levels in aphids reared on healthy and infected plants, with fold-changes consistent across two different peptides from this protein (20.3 and 0). In contrast, peptides specific to thaumatin-like protein 5 and 33-kD secretory protein were found to accumulate to higher levels in aphids that had fed on infected plants (Table 5 ). These data suggest that the amount of these proteins was increased in aphids fed on CYDV-RPV infected tissue, with fold-changes of 2.0 and 1.6, respectively. Peptides from remorin, cysteine-rich repeat secretory protein 55-like, structural maintenance of chromosomes protein 3, predicted ribosomal protein, and b-D-glucosidase were not detected. SRM is excellent at reproducibly monitoring peptide signals; however, upon finding interesting differences such as the ones described above, the next step is validation and absolute quantification via the use of stable isotope labeled peptides.