Interactions between plant defence signalling pathways: Evidence from bioassays with insect herbivores and plant pathogens

Sequential damage by attackers is hypothesized to result in reciprocal antagonism (crosstalk) between jasmonic acid ( JA ) and salicylic acid ( SA ) defence signalling pathways in plants. However, evidence for this crosstalk is not universal and several studies have found positive interactions (i.e. synergism) or no interaction whatsoever between JA and SA pathways. Here we conducted a meta‐analysis of studies on plant‐mediated effects of initial attackers on performance of subsequent attackers to test the hypothesis of crosstalk between plant hormonal signalling pathways. We found a significant negative mean effect size of JA ‐inducing initial attackers on both JA ‐ and SA ‐inducing subsequent attackers, but a non‐significant effect of SA ‐inducing initial attackers on both JA ‐ and SA ‐inducing subsequent attackers. Effects on subsequent herbivores were contingent on the biology of the initial attacker, with negative effects of JA ‐inducing initial herbivores but no effect of SA ‐inducing initial herbivores, whereas pathogens on average did not influence subsequent herbivores. Furthermore, negative effects of JA ‐inducing initial herbivores on JA ‐ and SA ‐inducing subsequent herbivores held when both attacks occurred on the same plant part, but when attacks were on different plant parts the effect of JA ‐inducing herbivores on SA ‐inducing herbivores was non‐significant. Synthesis . These results indicate that reciprocal antagonism between jasmonic acid and salicylic acid pathways is not universal, and suggest assymetries and specificity in the strength of plant‐mediated interactions.