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.