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

Abstract Sulfur dioxide (SO 2 ) was recently proposed as a novel bio-regulator in mammals. However, the possible advantageous effects of SO 2 in plant adaptation to heavy metal-contaminated environments are largely unknown. In the present study, using Na 2 SO 3 /NaHSO 3 derivatives as SO 2 donors, we investigated the possible roles and regulation mechanisms of SO 2 in alleviating Cd 2+ toxicity in foxtail millet seedlings. Exogenous SO 2 derivatives (0.5 mM) application significantly reduced the seedling growth inhibition caused by Cd 2+ stress. Cd 2+ -induced oxidative damage was also alleviated by SO 2 derivatives, which was supported by the decreased malondialdehyde (MDA) level in the leaves of seedlings pretreated with SO 2 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 2+ -elicited O 2 •－ and H 2 O 2 within the leaves of foxtail millet seedlings. Also, SO 2 derivative application promoted sulfur assimilation and increased the content of glutathione and phytochelatins, which may help to enhance Cd 2+ detoxification capacity in foxtail millet seedlings. Moreover, application of SO 2 derivatives caused down-regulation of the transcript expression levels of several genes involved in Cd 2+ uptake and translocation, such as NRAMP1 , NRAMP6 , IRT1 , IRT2 , HMA2 , and HMA4 , thus resulting in reduced Cd 2+ accumulation in the shoots and roots of Cd 2+ -stressed seedlings. Collectively, these results suggest that exogenous SO 2 derivative application can alleviate oxidative damage and restrict Cd 2+ buildup, thereby reducing Cd 2+ -induced growth inhibition in foxtail millet seedlings upon Cd 2+ exposure. This novel finding indicates that the usage of SO 2 derivatives may be an effective approach for enhancing Cd 2+ tolerance in foxtail millet and other crops.