The role of the redox-active tripeptide
glutathione in plant defence against pathogens has been studied extensively; however, the impact of changes in cellular
glutathione redox potential on signalling processes during defence reactions has remained elusive. This study explored the impact of elevated
glutathione content on the cytosolic redox potential and on early defence signalling at the level of
mitogen-activated protein kinases (MAPKs), as well as on subsequent defence reactions, including changes in
salicylic acid (SA) content, pathogenesis-related gene expression,
callose depositions, and the hypersensitive response. Wild-type (WT) Nicotiana tabacum L. and transgenic high-
glutathione lines (HGL) were transformed with the cytosol-targeted sensor GRX1-roGFP2 to monitor the cytosolic redox state. Surprisingly, HGLs displayed an oxidative shift in their cytosolic redox potential and an activation of the tobacco MAPKs
wound-induced
protein kinase (WIPK) and
SA-induced protein kinase (SIPK). This activation occurred in the absence of any change in free SA content, but was accompanied by constitutively increased expression of several defence genes. Similarly, rapid activation of MAPKs could be induced in WT tobacco by exposure to either reduced or
oxidized glutathione. When HGL plants were challenged with adapted or non-adapted Pseudomonas syringae pathovars, the cytosolic redox shift was further amplified and the defence response was markedly increased, showing a priming effect for SA and
callose; however, the initial and transient hyperactivation of MAPK signalling was attenuated in HGLs. The results suggest that, in tobacco, MAPK and SA signalling may operate independently, both possibly being modulated by the
glutathione redox potential. Possible mechanisms for redox-mediated MAPK activation are discussed.