Sulfur dioxide derivatives alleviate cadmium toxicity by enhancing antioxidant defence and reducing Cd2+ uptake and translocation in foxtail millet seedlings.

Sulfur dioxide (SO2) was recently proposed as a novel bio-regulator in mammals. However, the possible advantageous effects of SO2 in plant adaptation to heavy metal-contaminated environments are largely unknown. In the present study, using Na2SO3/NaHSO3 derivatives as SO2 donors, we investigated the possible roles and regulation mechanisms of SO2 in alleviating Cd2+ toxicity in foxtail millet seedlings. Exogenous SO2 derivatives (0.5 mM) application significantly reduced the seedling growth inhibition caused by Cd2+ stress. Cd2+-induced oxidative damage was also alleviated by SO2 derivatives, which was supported by the decreased malondialdehyde (MDA) level in the leaves of seedlings pretreated with SO2 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 Cd2+-elicited O2•－ and H2O2 within the leaves of foxtail millet seedlings. Also, SO2 derivative application promoted sulfur assimilation and increased the content of glutathione and phytochelatins, which may help to enhance Cd2+ detoxification capacity in foxtail millet seedlings. Moreover, application of SO2 derivatives caused down-regulation of the transcript expression levels of several genes involved in Cd2+ uptake and translocation, such as NRAMP1, NRAMP6, IRT1, IRT2, HMA2, and HMA4, thus resulting in reduced Cd2+ accumulation in the shoots and roots of Cd2+-stressed seedlings. Collectively, these results suggest that exogenous SO2 derivative application can alleviate oxidative damage and restrict Cd2+ buildup, thereby reducing Cd2+-induced growth inhibition in foxtail millet seedlings upon Cd2+ exposure. This novel finding indicates that the usage of SO2 derivatives may be an effective approach for enhancing Cd2+ tolerance in foxtail millet and other crops.