Nitric oxide promotes MPK6-mediated caspase-3-like activation in cadmium-induced Arabidopsis thaliana programmed cell death

Nitric oxide (NO), a vital cell-signalling molecule, has been reported to regulate toxic metal responses in plants. This work investigated the effects of NO and the relationship between NO and mitogen-activated protein kinase (MAPK) in Arabidopsis (Arabidopsis thaliana) programmed cell death (PCD) induced by cadmium (Cd2+) exposure. With fluorescence resonance energy transfer (FRET) analysis, caspase-3-like protease activation was detected after Cd2+ treatment. This was further confirmed with a caspase-3 substrate assay. Cd2+-induced caspase-3-like activity was inhibited in the presence of the NO-specific scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), suggesting that NO mediated caspase-3-like protease activation under Cd2+ stress conditions. Pretreatment with cPTIO effectively inhibited Cd2+-induced MAPK activation, indicating that NO also affected the MAPK pathway. Interestingly, Cd2+-induced caspase-3-like activity was significantly suppressed in the mpk6 mutant, suggesting that MPK6 was required for caspase-3-like protease activation. To our knowledge, this is the first demonstration that NO promotes Cd2+-induced Arabidopsis PCD by promoting MPK6-mediated caspase-3-like activation.