Abstract
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.
Highlights
Nitrogen (N) is a major essential nutrient for plant growth (Stitt, 1999)
These results suggest that the root growth and total N concentration of rice plants are regulated by NO3− (Supplementary Figure 1)
Compared to NH4+, Nitric oxide (NO)-associated green fluorescence signals in root tip (RT) and the lateral roots (LRs) regions were stronger under NO3− supply, which suggests that production of NO in roots is induced by NO3−
Summary
Nitrogen (N) is a major essential nutrient for plant growth (Stitt, 1999). Ammonium (NH4+) and nitrate (NO3−) are the major sources of N for plants (Kronzucker et al, 2000). Rice (Oryza sativa L.) is a major staple food globally. Rice roots are exposed to both NH4+ and NO3−, and are efficient at acquiring NO3− by nitrification in the rhizosphere (Kirk and Kronzucker, 2005; Duan et al, 2007). It has been predicted that 40% of the total N acquired by rice roots grown under flooded conditions is NO3− (Kronzucker et al, 2000; Kirk and Kronzucker, 2005).
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