Sulfur dioxide derivatives alleviate cadmium toxicity by enhancing antioxidant defence and reducing Cd uptake and translocation in foxtail millet seedlings.

Sulfur dioxide (SO) was recently proposed as a novel bio-regulator in mammals. However, the possible advantageous effects of SO in plant adaptation to heavy metal-contaminated environments are largely unknown. In the present study, using NaSO/NaHSO derivatives as SO donors, we investigated the possible roles and regulation mechanisms of SO in alleviating Cd toxicity in foxtail millet seedlings. Exogenous SO derivatives (0.5 mM) application significantly reduced the seedling growth inhibition caused by Cd stress. Cd-induced oxidative damage was also alleviated by SO derivatives, which was supported by the decreased malondialdehyde (MDA) level in the leaves of seedlings pretreated with SO derivatives. These responses were related to the enhanced activities of representative antioxidant enzymes, including catalase and superoxide dismutase, as well as the up-regulation of ascorbate-glutathione cycle, which contributed to the scavenging of Cd-elicited O and HO within the leaves of foxtail millet seedlings. Also, SO derivative application promoted sulfur assimilation and increased the content of glutathione and phytochelatins, which may help to enhance Cd detoxification capacity in foxtail millet seedlings. Moreover, application of SO derivatives caused down-regulation of the transcript expression levels of several genes involved in Cd uptake and translocation, such as NRAMP1, NRAMP6, IRT1, IRT2, HMA2, and HMA4, thus resulting in reduced Cd accumulation in the shoots and roots of Cd-stressed seedlings. Collectively, these results suggest that exogenous SO derivative application can alleviate oxidative damage and restrict Cd buildup, thereby reducing Cd-induced growth inhibition in foxtail millet seedlings upon Cd exposure. This novel finding indicates that the usage of SO derivatives may be an effective approach for enhancing Cd tolerance in foxtail millet and other crops.