Enhanced Drought Tolerance Of Foxtail Millet Seedlings By Sulfur Dioxide Fumigation

Sulfur dioxide?SO₂?is a common air pollutant.In recent years,it has been reported that SO₂ can induce vasodilation,decrease blood pressure,and inhibit myocardial injury in animals.SO₂ enters plants mainly through the stomata.Previous studies have shown that SO₂ regulated plant responses to adversity stress.Drought stress inhibits plant growth and development,disturbing photosynthesis,respiration and nutrition metabolism processes,which can lead to crop yield reduction and even plant death.To date,the effects of SO₂ on crop adaptation to drought stress and the related mechanisms are still unclear.Foxtail millet?Setaria italica L.?is an important food crop in arid and semi-arid areas of northern China.It suffers from long-term of drought stress during its growth process.The effects of SO₂ exposure on the stomatal movement and physiological characteristics of foxtail millet seedlings were studied in the present research.Furthermore,we investigated the physiological and molecular changes that are induced by SO₂ fumigation to improve the drought tolerance of foxtail millet seedlings.The main results and conclusions are as follows:?1?The toxic effects of SO₂ on foxtail millet seedlings were concentrationdependent.The results showed that 10 mg·m^-3 SO₂ fumigation had no significant effect on leaf morphology,relative water content,stomatal aperture,proline accumulation,and antioxidant enzyme activity in foxtail millet seedlings.24-72 h of 30 mg·m^-3 SO₂ exposure resulted in obvious damage symptoms in the leaves,as well as decreased relative water content and reduced stomatal apertures.?2?Foxtail millet plants adapt to SO₂ stress by controlling stomatal opening,regulating proline metabolism and antioxidant enzyme activity when exposed to 30 mg·m^-3 SO₂.The results show that 30 mg·m^-3 SO₂ fumigation led to significant increase in proline content in the leaves of seedlings.Meanwhile,long term of SO₂ stress induced up-regulation of Si PDH transcript,while the expression of proline synthesis-related genes Si P5CS and Si P5 CR was significantly inhibited.Moreover,under exposure to 30 mg·m^-3 SO₂,markedly higher production rate of superoxide anion?O2.-?was observed in the leaves of seedlings,as compared with the control,which might enhance the activities of superoxide dismutase?SOD?and catalase?CAT?,thus maintaining the content of hydrogen peroxide?H₂O₂?at a normal level.?3?30 mg·m^-3 SO₂ pre-exposure could effectively enhance drought tolerance of foxtail millet seedlings.SO₂ fumigation could trigger stomatal closure in the leaves of foxtail millet seedlings,which may help to reduce transpirational water loss under drought stress conditions.Furthermore,enhanced drought tolerance induced by SO₂ may also be attributed to increased P5CS activity and decreased Pro DH activity and thus a higher accumulation of proline in drought-stressed seedlings.Additionally,SO₂ exerted its protective effects by activating some antioxidant enzymes,including CAT and POD,which could eliminate the overproduction of reactive oxygen species,and thus alleviate the oxidative damage induced by drought stress in foxtail millet seedlings.The aim of our study is to reveal the roles and relevant mechanisms of SO₂ in foxtail millets adaptation to drought stress.Our results may provide theoretical basis for improving drought tolerance of millet plants.