Aphid honeydew alters plant defence responses

Plants respond to insect herbivory by ramping up the production of defence-related jasmonic acid (JA). The pea aphid, Acyrthosiphon pisum, is able to avoid triggering JA as it feeds. The aim of our study was to examine whether or not aphids simply avoid triggering plant defence signalling or whether they are able to manipulate plant signalling in a way to alter effectual plant defences. To resolve this, we tested the ability of pea aphids to suppress the accumulation of damage-induced JA. We then tested exogenous application of aphid honeydew as a possible mechanism for JA suppression. While broad bean plants, Vicia faba, accumulated JA in response to artificial damage, the presence of concurrent aphid feeding suppressed JA accumulation. Furthermore, exogenous application of aphid honeydew induced salicylic acid (SA) and suppressed JA accumulation. We also report the presence of SA within honeydew itself. We conclude that both aphid feeding and exogenous honeydew deposition suppress JA accumulation in response to damage. We demonstrate that honeydew application results in an increased accumulation of SA within plant tissue and find that levels of SA present within honeydew do not fully account for this increase. This finding demonstrates, for the first time, that aphid honeydew suppresses induced plant defence.