Elevated Carbon Dioxide Alleviates Aluminum Toxicity by Decreasing Cell Wall Hemicellulose in Rice (Oryza sativa)

Carbon dioxide (CO₂) is involved in plant growth as well as plant responses to abiotic stresses; however, it remains unclear whether CO₂ is involved in the response of rice (Oryza sativa) to aluminum (Al) toxicity. In the current study, we discovered that elevated CO₂ (600 μL·L^-1) significantly alleviated Al-induced inhibition of root elongation that occurred in ambient CO₂ (400 μL·L^-1). This protective effect was accompanied by a reduced Al accumulation in root apex. Al significantly induced citrate efflux and the expression of OsALS1, but elevated CO₂ had no further effect. By contrast, elevated CO₂ significantly decreased Al-induced accumulation of hemicellulose, as well as its Al retention. As a result, the amount of Al fixed in the cell wall was reduced, indicating an alleviation of Al-induced damage to cell wall function. Furthermore, elevated CO₂ decreased the Al-induced root nitric oxide (NO) accumulation, and the addition of the NO scavenger c-PTIO (2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) abolished this alleviation effect, indicating that NO maybe involved in the CO₂-alleviated Al toxicity. Taken together, these results demonstrate that the alleviation of Al toxicity in rice by elevated CO₂ is mediated by decreasing hemicellulose content and the Al fixation in the cell wall, possibly via the NO pathway.