Sulfur dioxide improves drought tolerance through activating Ca2+ signaling pathways in wheat seedlings

Sulfur dioxide (SO2) and drought are two important co-occurring abiotic stresses affecting the growth and productivity of plants. Here, we will investigate the role of Ca2+ in regulating antioxidant defense during drought or SO2/drought stress, and the effect of SO2 pretreatment on the physiological response of wheat seedlings to drought stress. The results showed that exogenous Ca2+ increased the activities of SOD, CAT and POD, and reduced the contents of H2O2 and MDA in drought-treated wheat seedlings, suggesting Ca2+ could improve drought tolerance by promoting antioxidant defense in plants. Moreover, exogenous Ca2+ up-regulated the expression of two stress-responsive transcription factor (TF) genes, ERF1 and MYB30, to cope with drought stress. Exposure of wheat seedlings to 10 mg m(-3) SO2 significantly enhanced the activities of SOD, CAT and POD. The contents of H2O2 and MDA remained at control levels, showing that SO2 at this concentration led to an activation of the antioxidant defense system and did not cause oxidative damage to the seedlings. Furthermore, 10 mg m(-3) SO2 pretreatment increased the expression of CCaMK and CPK10, enhanced the activities of SOD and POD, and reduced the accumulation of H2O2 and MDA in drought-treated wheat seedlings, showing a role of SO2 in protection of plants against drought stress. However, with removal of Ca2+ by spraying EGTA on the SO2-pretreated wheat seedlings, the expression of transcription factor genes and activities of antioxidant enzymes were decreased, and the contents of H2O2 and MDA enhanced to the level of drought treatment alone, suggesting a role of Ca2+ in the SO2-induced alleviation of drought stress. Together, these results indicated that exogenous Ca2+ increased defense-related gene expression and enzyme activity in response to drought stress, and that pre-exposure to appropriate levels of SO2 could improve drought tolerance through activation of Ca2+ signaling pathways in plants. This study would provide new strategy for enhancing plant resistance to environmental stress.