MAPK-dependent JA and SA signalling in Nicotiana attenuata affects plant growth and fitness during competition with conspecifics

As expected, irWIPK and LOX3-silenced plants out-performed their competing WT plants. Surprisingly, irSIPK plants, which have the largest reductions in JA signaling, did not. Phytohormone profiling of leaves revealed that irSIPK plants accumulated higher levels of SA compared to WT. To test the hypothesis that these high levels of SA, and their presumed associated fitness costs of pathogen associated defenses in irSIPK plants had nullified the JA-deficiency-mediated growth benefits in these plants, we genetically reduced SA levels in irSIPK plants. Reducing SA levels partially recovered the biomass and fitness deficits of irSIPK plants. We also evaluated whether the increased fitness of plants with reduced SA or JA levels resulted from increased nitrogen or CO2 assimilation rates, and found no evidence that greater intake of these fitness-limiting resources were responsible.Signaling mediated by WIPK, but not SIPK, is associated with large fitness costs in competing N. attenuata plants, demonstrating the contrasting roles that these two MAPKs play in regulating the plants’ growth-defense balance. We discuss the role of SIPK as an important regulator of plant fitness, possibly by modulating SA-JA crosstalk as mediated through ethylene signaling.Plants have evolved effective defense strategies to ward off natural enemies, including pathogens and herbivores. Allocation of fitness-limiting resources to anti-pathogen and anti-herbivore resistance frequently imposes costs on plants, which are readily seen as reductions in plant growth and fitness. These fitness costs of defense production play a fundamental role in most plant defense theories (reviewed in [1]). Instead of producing costly defense metabolites permanently, plants often activate defense pathways only in response to signals that implicate the presence of attackers. Such plastic defense pathways, so called induced defenses, are generally believed to have evolved as a resource-saving strategy (reviewed in [2]).