Nitric Oxide Affects Rice Root Growth by Regulating Auxin Transport Under Nitrate Supply

Nitrogen (N) is a major essential nutrient for plant growth, and rice is an important food crop globally. Although ammonium (NH₄⁺) is the main N source for rice, nitrate (NO₃^-) is also absorbed and utilized. Rice responds to NO₃^- supply by changing root morphology. However, the mechanisms of rice root growth and formation under NO₃^- supply are unclear. Nitric oxide (NO) and auxin are important regulators of root growth and development under NO₃^- supply. How the interactions between NO and auxin in regulating root growth in response to NO₃^- are unknown. In this study, the levels of indole-3-acetic acid (IAA) and NO in roots, and the responses of lateral roots (LRs) and seminal roots (SRs) to NH₄⁺ and NO₃^-, were investigated using wild-type (WT) rice, as well as osnia2 and ospin1b mutants. NO₃^- supply promoted LR formation and SR elongation. The effects of NO donor and NO inhibitor/scavenger supply on NO levels and the root morphology of WT and nia2 mutants under NH₄⁺ or NO₃^- suggest that NO₃^--induced NO is generated by the nitrate reductase (NR) pathway rather than the NO synthase (NOS)-like pathway. IAA levels, [³H] IAA transport, and PIN gene expression in roots were enhanced under NO₃^- relative to NH₄⁺ supply. These results suggest that NO₃^- regulates auxin transport in roots. Application of SNP under NH₄⁺ supply, or of cPTIO under NO₃^- supply, resulted in auxin levels in roots similar to those under NO₃^- and NH₄⁺ supply, respectively. Compared to WT, the roots of the ospin1b mutant had lower auxin levels, fewer LRs, and shorter SRs. Thus, NO affects root growth by regulating auxin transport in response to NO₃^-. Overall, our findings suggest that NO₃^- influences LR formation and SR elongation by regulating auxin transport via a mechanism involving NO.