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.