Reactive oxygen species (ROS) are key signalling intermediates in plant
metabolism, defence, and stress adaptation. In plants, both the
chloroplast and mitochondria are centres of metabolic control and ROS
production, which coordinate stress responses in other cell
compartments. The herbicide and experimental tool, methyl viologen (MV)
induces ROS generation in the chloroplast under illumination, but is
also toxic in non-photosynthetic organisms. We used MV to probe plant
ROS signalling in compartments other than the chloroplast. Taking a
genetic approach in the model plant Arabidopsis (Arabidopsis thaliana),
we used natural variation, QTL mapping, and mutant studies with MV in
the light, but also under dark conditions, when the chloroplast electron
transport is inactive. These studies revealed a light-independent
MV-induced ROS-signalling pathway, suggesting mitochondrial involvement.
Mitochondrial Mn SUPEROXIDE DISMUTASE was required for ROS-tolerance
and the effect of MV was enhanced by exogenous sugar, providing further
evidence for the role of mitochondria. Mutant and hormone feeding assays
revealed roles for stress hormones in organellar ROS-responses. The
radical-induced cell death1 mutant, which is tolerant to MV-induced ROS
and exhibits altered mitochondrial signalling, was used to probe
interactions between organelles. Our studies suggest that mitochondria
are involved in the response to ROS induced by MV in plants.