Decreased endogenous nitric oxide contributes to sulfur dioxide derivative-alleviated cadmium toxicity in foxtail millet roots

Cadmium (Cd) is an environmental pollutant that is highly toxic to animals and plants. Although sulfur dioxide (SO2) has recently been considered as a novel bio-regulator, whether and how SO2 regulates Cd-induced phytotoxicity in plant roots is largely unknown. In this study, SO2 derivatives (Na2SO3/NaHSO3, M/M = 3:1) were used to investigate the beneficial roles and detailed mechanisms of SO2 in the alleviation of Cd-induced root growth suppression in foxtail millet seedlings. Our results showed that exogenously applied SO2 derivatives (0.5 mM) reversed Cd inhibition of root elongation. Cd-induced endogenous nitric oxide (NO) production downregulated the enzyme activities of superoxide dismutase (SOD) and peroxidase (POD), as well as corresponding transcript levels, and increased the expression of genes involved in Cd uptake (SiNRAMP1, SiNRAMP6, SiIRT1, and SiIRT2) in foxtail millet roots. Upon Cd exposure, SO2 derivative application reduced NO generation by inhibiting nitrate reductase (NR) and nitric oxide synthase (NOS) activities, thus resulting in increased activities of SOD and POD and suppressed Cd uptake-related gene expression, which in turn reduced reactive oxygen species (ROS) accumulation and decreased Cd content in the roots. This study suggested a beneficial role of SO2 derivatives in protecting foxtail millet roots against Cd toxicity by decreasing endogenous NO; this might be applicable to enhancing crop tolerance to Cd pollutants.