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.