LAUSR

MAIN CONCLUSION Exogenous calcium enhances rice tolerance to acid rain stress by regulating isozymes composition and transcriptional expression of ascorbate peroxidase and glutathione reductase. Calcium (Ca) participates in signal transduction in plants under abiotic stress, and addition of Ca2+ is beneficial to alleviate damage of plants caused by acid rain. To clarify the effect of exogenous Ca2+ on tolerance of plants to acid rain stress, we investigated regulation of Ca2+ (5 mM) on activities, isozymes composition and transcriptional expression of ascorbate peroxidase (APX) and glutathione reductase (GR), redox state, and H2O2 concentration and growth in rice leaves and roots under simulated acid rain (SAR) stress. SAR (pH 3.5/2.5) decreased the total activities of APX and GR in rice by decreasing the concentration of APX isoforms (APXII in leaves and APXIII in roots) as well as activation degree of GR isozymes and transcription level of GR1, indicating that SAR (pH 3.5/2.5) destroyed the redox state in rice cells and induced H2O2 excessive accumulation, and inhibited growth of rice. Exogenous Ca2+ alleviated SAR-induced inhibition on activities of APX and GR by regulating the concentration, activation, and transcription of their isozymes, and then maintained the redox level of cells and protected cells from oxidative damage, being beneficial to the growth of rice. Therefore, the promotion of exogenous Ca2+ on activities of APX and GR can be important to enhance rice tolerance to acid rain by maintaining redox state and avoiding oxidative damage.