Influences of silicon on activities of key defense enzymes in rice leaves infected by Magnaporthe grisea in relation to rice blast resistance

A series of hydroponics experiments were performed in a controlled rice-Magnaporthe grisea pathosystem to study the effects of silicon (Si) on disease development, plant growth, activities of peroxidase (POD), polyphenoloxidase (PPO) and phenylalanine ammonia-lyase (PAL). Silicon (Si) application significantly decreased rice blast incidence and severity index, with control efficiency of 60.59%. Shoot dry weight of rice plants amended with 1.7 mmol/L Si was significantly higher than that of the non-Si-amended plants by 10 d after inoculation with M. grisea. However, No significant difference was observed in root dry weight between the Si+ (with Si added) and Si- (without Si added) treatments. Application of silicon was beneficial for rice growth and alleviated damage resulting from infection by M. grisea. POD activity increased after inoculation and reached a maximum by 5 d. POD activity in leaves of Si+ plants was significantly lower than that of Si- plants after inoculation. The result showed that the change of POD activity was not directly related to rice blast resistance, but might be directly related to damage degree of riceleaves infected by M. grisea. PPO activity in Si+ and Si- rice plants both slowly increased after inoculation and had a peak by 5 d. PPO activity in leaves of Si+ plants was significantly higher than in Si- plants from 4 d and onward. PAL activity in Si+ and Si- rice plants rapidly increased after inoculation and had a peak by 24 h, thereafter began to decline. However, PAL activity in Si+ plants was 1.44-fold higher than in Si- plants. It is hypothesized that silicon can mediate activities of key defense enzymes to induce some natural defense responses in rice and play an active and physiological role in enhancing resistance to rice blast.