Signal regulation involved in sulfur dioxide-induced guard cell apoptosis in Hemerocallis fulva.

Chronic and acute exposure to SO₂ is associated with increased risks of various damages to plants. In the present study, epidermal strip experiment was employed to investigate SO₂-induced guard cells apoptosis and the signal regulation in Hemerocallis fulva. The results showed that with the increase of treatment concentrate of SO₂ derivates (a mixture of sodium sulfite and sodium bisulfite, 3:1, mmol L⁻¹/mmol L⁻¹, 1.0-5.0 mmol L⁻¹), the physiological activity of the guard cells declined and cell death occurred. While the concentration of SO₂ derivatives exceeded 2.0 mmol L⁻¹, the percentage of cell death increased significantly (P<0.05). Typical features of apoptosis including nuclear condensation, nuclear elongation, fragmentation etc. were found. Meanwhile, concomitant presence of nitric oxide (NO), reactive oxygen species (ROS) and Ca²⁺ level increment appeared. However, SO₂-induced cell death can be effectively blocked by either of the following substances with their respective optimal concentrations: antioxidant ascorbic acid (Asc; 0.05 mmol L⁻¹) or catalase (CAT; 200 U mL⁻¹), nitric oxide (NO) scavenger 2-(4-carboxyphenyl)-4, 4, 5, 5- tetramethylmidiazoline-1-oxyl-3-oxide (c-PTIO; 0.20 mmol L⁻¹), nitrate reductase inhibitor NaN₃ (0.20 mmol L⁻¹), Ca²⁺ chelating agent EGTA (0.05 mmol L⁻¹) or plasma membrane Ca²⁺ channel blocker LaCl₃ (0.05 mmol L⁻¹). In addition to a significant decrease in cell death rate, a reduction in the levels of reactive oxygen species (ROS), NO and Ca²⁺ was observed. Further study showed that compared to treatment with SO₂ alone, Asc treatment led to a decrease in NO and Ca²⁺ levels and NaN₃ treatment led to a decrease in ROS and Ca²⁺ levels, but the NO and ROS levels of the LaCl₃ treatment changed little. All results suggested that NO, ROS and Ca²⁺ were involved in the apoptosis induced by SO₂ in H. fulva. The process might be related to the burst of NO or ROS, which would activate the plasma Ca²⁺ channel and result in the increase of intercellular Ca²⁺.