Induction as well as suppression: How aphid saliva may exert opposite effects on plant defense.

Aphids ingest from the sieve tubes and by doing so they are confronted with sieve-tube occlusion mechanisms, which are part of the plant defense system. Because aphids are able to feed over longer periods, they must be able to prevent occlusion of the sieve plates induced by stylet penetration. Occlusion probably depends upon Ca(2+)-influx into the sieve element (SE) lumen. Aphid behavior, biochemical tests and in vitro experiments demonstrated that aphid's watery saliva, injected during initial phase of a stylet penetration into the SE lumen, contains proteins that are able to bind calcium and prevent calcium-induced SE occlusion. In this addendum, we speculate on the consequences of saliva secretion for plant resistance. (a) The release of elicitors (e.g., oligogalacturonides) due to cell wall digestion by gel saliva enzymes may increase the resistance of cortex, phloem parenchyma cells and companion cells (CC) around the puncture site. (b) Ca(2+)-binding by aphid watery saliva may suppress the local defense responses in the SEs. (c) Signaling cascades triggered in CCs may lead to systemic resistance.