Metal-specific and NADPH oxidase dependent changes in lipoxygenase and NADPH oxidase gene expression in Arabidopsis thaliana exposed to cadmium or excess copper

Reactive oxygen species produced by NADPH oxidase and oxylipins derived from lipoxygenase activity can signal various stress conditions and have been implicated when plants are exposed to heavy metals. Transcriptional profiling of the 10 NADPH oxidase and 6 lipoxygenase genes was performed after exposure of Arabidopsis thaliana wildtype and NADPH oxidase mutants to 5 mu M CdSO(4) or 2 mu M CuSO(4) for 24 h. Under these short exposures to environmentally realistic concentrations of Cd or Cu, plants modulate signalling networks that regulate the onset of adaptive responses. Metal-specific NADPH oxidase genes were upregulated by Cd but downregulated by Cu, and metal-specific lipoxygenase gene expression was observed only after Cu exposure. Genes that are responsive to both metals were upregulated and may be responsive to general oxidative stress. For all metal-responsive genes except RBOHD, distinct responses were observed between leaves and roots, which may be due to different stress intensities and signalling mechanisms. Mutation of NADPH oxidase genes had opposing effects on gene expression after Cd or Cu exposure. Upregulation of LOX1 and LOX6 in the roots after exposure to Cd depended on NADPH oxidase gene expression, whereas LOX3 and LOX6 expression was induced more strongly in NADPH oxidase mutants after Cu exposure. Furthermore, NADPH oxidases regulated their own expression level and that of other members of the gene family when exposed to Cd or Cu. The results suggest interplay between reactive oxygen species and oxylipin signalling under Cd or Cu stress, and are useful as a basis for genetic studies to unravel metal-specific signalling mechanisms.