Nitric oxide reduces the aluminum-binding capacity in rice root tips by regulating the cell wall composition and enhancing antioxidant enzymes.

Plant cell wall, the first interface or barrier for toxic ions entering into protoplast, suffers from risk. Nitric oxide (NO) plays an important role in plant growth and responses to abiotic stresses, however, it is not clear whether NO is connected with the response of cell wall to aluminum (Al) tolerance in rice (Oryza sativa L.). In this study, we found that the application of 50 µM Al induces nitrate reductase (NR) activity and endogenous NO production, but not nitric oxide synthase (NOS) activity in two rice genotypes. Pretreatment with 100 µM NO donor (sodium nitroprusside, SNP) reduced Al-induced inhibition of root elongation by 32.3% and 91.7%, and Al accumulation in root-tip by 38.4% and 44.3% in Nipponbare and Zhefu802, respectively. The addition of SNP significantly decreased Al-induced accumulation of pectin, hemicellulose 1 and hemicellulose 2 by 43.1%, 13.1% and 19.2% in Zhefu802 and by 16.9%, 13.4% and 14.0% in Nipponbare, compared with roots treated with Al alone, as well as pectin methylesterase (PME) activity. Therefore, the content of Al absorbed in cell walls was decreased, indicating that the Al-induced structure damage to cell walls was alleviated. Furthermore, the activities of peroxidase (POD), superoxide dismutase (SOD) and catalase (CAT) treated by Al were all increased by SNP pretreatment, and the lipid peroxidation and damage to plasma membrane of root tips detected with Schiff's reagent and Evans blue reduced. In contrast, the effect was abolished when NO scavenger (cPTIO), and NR inhibitor (NaN3), were added. These results indicated that by regulating the Al-binding capacity to cell walls and lipid peroxidation, the structure of cell walls can be stabilized and that Al toxicity in rice can be alleviated with increased NO.