Abstract
Plant roots have evolved adaptive strategies mediated by transcriptional networks to cope with fluctuating nitrogen (N) forms and availability. However, the mechanisms linking root-foraging responses to N use efficiency (NUE) in crops remain poorly understood. Here, we show that rice exhibits enhanced root elongation under nitrate compared with ammonium, particularly under low N supply, suggesting a specific regulatory role for nitrate in root morphogenesis. We identify the transcription factor OsMADS61 as a key regulator of nitrate-dependent root morphological and physiological responses, as well as NUE, especially under N-limited conditions. OsMADS61 acts as a transcriptional activator of nitrate metabolism by directly binding to OsNRT2.1 and OsNR2 promoters. Nitric oxide produced via the nitrate reductase pathway, under the control of nitrate-responsive OsMADS61, precisely triggers cell proliferation in the root meristem. Moreover, single-nucleotide polymorphisms in the OsMADS61 promoter may be associated with differential root-foraging responses to nitrate availability. Therefore, enhancing N-adaptive root responses to optimize N uptake and assimilation represents a promising strategy for breeding crops with high NUE.