Light induces lettuce seed germination through promoting nitric oxide production and phospholipase D-derived phosphatidic acid formation

Nitric oxide (NO) and phospholipase D (PLD) are essential regulators of seed germination in many plant species, but their roles and relationships in seeds that require light for germination are poorly understood. Here, we investigated how NO and PLD regulate lettuce (Lactuca sativa) seed germination under conditions that either promote (i.e., 23 °C in light) or restrict (i.e., 28 °C in light or 23 °C in darkness) germination. Our results showed that light induced NO production, increased PLD activity and phosphatidic acid (PA) levels in lettuce seeds, which resulted in seed germination. These processes were impaired at 23 °C in darkness or at 28 °C in light. The NO scavenger(s) inhibited light-induced PLD activation, PA formation and seed germination. Inhibiting PLD prevented light-induced PA formation and seed germination, whereas inhibiting phospholipase C (PLC) did not. Exogenous NO stimulated PLD activity, PA formation and seed germination at 23 °C in darkness or at 28 °C in light and its effects were offset by the NO scavenger and the PLD inhibitor, rather than by the PLC inhibitor. The results suggested that during lettuce seed germination, light-induced NO production stimulated germination through inducing PA formation via PLD, whereas darkness or high temperature inhibited light-induced germination by impairing NO generation, PLD activation and PA formation.