Enhanced drought tolerance of foxtail millet seedlings by sulfur dioxide fumigation.

Recently, sulfur dioxide (SO2) has been considered to be a beneficial bio-regulator in animals. However, the positive roles of SO2 in plant adaptation to drought stress are still unclear. In this study, we investigated the physiological and molecular changes that are induced by SO2 fumigation to improve the drought tolerance of foxtail millet seedlings. The relative water content in the leaves of drought-stressed seedlings was significantly improved by pre-exposure to 30 mg/m3 SO2. These responses might be related to decreased stomatal apertures and a reduced leaf transpiration rate, which were induced by SO2 under drought conditions. In addition, the SO2 pretreatment markedly enhanced proline accumulation in the leaves of drought-stressed seedlings, which was supported by increased Δ1-pyrroline-5-carboxylate synthetase (P5CS) activity, decreased proline dehydrogenase (ProDH) activity, and the corresponding transcripts. Moreover, the SO2 application upregulated the enzyme activity of catalase (CAT) and peroxidase (POD) in the leaves of drought-stressed plants, as well as their transcripts, which contributed to the scavenging of hydrogen peroxide (H2O2) and alleviated drought-induced oxidative damage, as indicated by the decreased malondialdehyde (MDA) level in SO2-pretreated plants. Together, these results indicate that the application of SO2 might enhance drought tolerance by reducing stomatal apertures, increasing proline accumulation, and promoting antioxidant defence in foxtail millet seedlings. This study presents new insight into the beneficial roles of SO2 in plant responses to drought stress.