Heat shock-induced ascorbic acid accumulation in leaves increases cadmium tolerance of rice (Oryza sativa L.) seedlings

Ascorbic acid (AsA) is the most abundant antioxidant in plants and plays a role in responding to oxidative stress. It has been shown that AsA plays a role in protecting against abiotic stresses. Rice seedlings stressed with 5 μM CdCl₂ showed typical Cd toxicity (chlorosis and increase in malondialdehyde content). Rice seedlings pretreated with heat shock at 45°C (HS) or H₂O₂ under non-HS conditions resulted in the increase in ascorbic acid (AsA) content and the AsA/dehydroascorbate ratio in rice leaves. Exogenous application of AsA or L-galactonone-1, 4-lactone (GalL), a biosynthetic precursor of AsA, under non-HS conditions, which resulted in an increase in AsA content in leaves, enhanced subsequent Cd tolerance of rice seedlings. Pretreatment with imidazole, an inhibitor of NADPH oxidase, under HS conditions significantly decreased H₂O₂ and AsA contents in leaves and reduced subsequent Cd tolerance of rice seedlings. We also observed that pretreatment with lycorine, which is known to inhibit the conversion of GalL to AsA, significantly inhibited HS-induced AsA accumulation in leaves and reduced HS-induced protection against subsequent Cd stress of rice stress. It appears that HS- or H₂O₂-induced protection against subsequent Cd stress of rice seedlings is mediated through AsA. The time-course analyses of HS in rice seedlings demonstrated that the accumulation of H₂O₂ preceded the increase in AsA. Based on the data obtained in this study, it could be concluded that the early accumulation of H₂O₂ during HS signals the increase in AsA content, which in turn protects rice seedlings from oxidative damage caused by Cd.