Aluminium can activate grapevine defence through actin remodelling

A grapevine cell line, where actin filaments are labelled by GFP, was used to show that aluminium causes actin remodeling through activation of a NADPH oxidase in the plasma membrane, followed by activation of phytoalexin synthesis genes. Elimination of actin filaments by Latrunculin B disrupts the gene activation, inhibition of MAPK signalling by the inhibitor PD98059 as well. Interestingly, aluminum also induces transcripts for ISOCHORISMATE SYNTHASE, as well as of PR1, which are known to be responsive to salicylic acid. However, aluminium-triggered defence is not as SA responses accompanied by cell death. Also in grapevine leaf discs from two genotypes contrasting in stilbene inducibility, aluminum can induce accumulation of a central grapevine phytoalexins, the stilbene aglycone trans-resveratrol, preceded by a rapid induction of transcripts for RESVERATROL SYNTHASE and the regulating transcription factor MYB14. The amplitude of this induction reflects the general stilbene inducibility of these genotypes, indicating that the aluminum effect is not caused by unspecific toxicity, but by activation of specific signalling. The findings show that, actin filaments activate a specific branch of defence signalling acting in concert with calcium-dependent PAMP triggered immunity. This pathway links apoplastic oxidative burst through MAPK signalling with the activation of defence-related transcripts.