Facilitation of auxin biosynthesis and metabolism by salt stress in rice plants

To investigate the effects of salt stress on the auxin in rice plants, we analyzed indole-3-acetic acid (IAA) and its metabolites in hydroponically grown rice plant seedlings (Oryza sativa cv. Nipponbare) and in the culture medium using LC-ESI/MS/MS. We found that the IAA level in the culture medium was increased 39-fold due to salt stress treatment, while those in the shoots and roots were increased by less than 2-fold. The culture medium also contained high amounts of oxidized metabolites such as OxIAA, DiOxIAA, DiOxIAA-Glu, and DiOxIAA-Asp. The total amount of IAA and its metabolites in the shoots, roots, and culture medium was significantly increased with salt treatment. A salt stress-responsive increase in the IAA metabolites was observed in 9 tested cultivars of rice. The results indicate that salt stress facilitates the metabolic turnover of IAA in rice plants and may open new insight into the role of auxin.