Silicon alleviates aluminum-induced inhibition of photosynthetic and growth attributes in rice by modulating competitive pathways between ethylene and polyamines and activating antioxidant defense

Silicon (Si) has been reported to mitigate aluminium (Al3+) toxicity in rice; however, the mechanism underlying this beneficial effect has not been fully elucidated. In this study, Si addition increased the total level of both free and conjugated putrescine (Put) content in rice leaves by 89.3 % through up-regulation of the key synthesis genes in both of arginine decarboxylase (ADC) and ornithine decarboxylase (ODC) pathways under Al stress. The production of total Put increased by 10.3 % under Si treatment but decreased by 11.7 % under Al treatment compared to the control. Similarly, Si increased total spermidine (Spd) and spermine (Spm) levels by 154.9 % and 83.5 %, respectively, through up-regulation of S-adenosyl-Met-decarboxylase genes (SAMDC1 and SAMDC2), spermidine synthase genes (SPDS1 and SPDS2), and spermine synthase gene (SPMS) under Al stress. Compared with Al treatment alone, Si significantly increased the levels of free, conjugated and total polyamines (PAs) in leaves under Al stress by 106.1 %, 86.6 % and 99.3 %, respectively. The increase of PAs induced by Si maintained redox balance and improved photosynthetic capacity, ultimately increasing rice growth by 28.6 % under Al stress. Conversely, Si reduced Al-induced increase in 1-aminocyclopropane-1-carboxylate (ACC) content and ethylene production by 23.9 % and 43.8 %, respectively, through down-regulation of ACC synthase genes (ACS1 and ACS2) and ACC oxidase ACO genes (ACO1 and ACO4). In addition, Si mitigated the Al-induced oxidative damage by reducing the accumulation of reactive oxygen species (ROS) through activation of enzymatic (superoxide dismutase and catalase) and non-enzymatic (ascorbate-glutathione cycle) antioxidant defence systems. We therefore propose that Si attenuates Al-induced damage on rice photosynthetic apparatus by modulating competitive interactions between ethylene and PA biosynthesis and activating ROS scavenging capacity.