The rice transcription factor Nhd1 regulates root growth and nitrogen uptake by activating nitrogen transporters

Plants adjust root architecture and nitrogen (N) transporter activity to meet the variable N demand, but their integrated regulatory mechanism remains unclear. We have previously reported that a floral factor in rice (Oryza sativa), N-mediated heading date-1 (Nhd1), regulates flowering time. Here, we show that Nhd1 can directly activate the transcription of the high-affinity ammonium (NH4+) transporter 1;3 (OsAMT1;3) and the dual affinity nitrate (NO3-) transporter 2.4 (OsNRT2.4). Knockout of Nhd1 inhibited root growth in the presence of NO3- or a low concentration of NH4+. Compared to the wild-type (WT), nhdl and osamt1;3 mutants showed a similar decrease in root growth and N uptake under low NH4+ supply, while nhdl and osnrt2.4 mutants showed comparable root inhibition and altered NO3- translocation in shoots. The defects of nhdl mutants in NH4+ uptake and root growth response to various N supplies were restored by overexpression of OsAMT1;3 or OsNRT2.4. However, when grown in a paddy field with low N availability, nhdl mutants accumulated more N and achieved a higher N uptake efficiency (NUpE) due to the delayed flowering time and prolonged growth period. Our findings reveal a molecular mechanism underlying the growth duration-dependent NUpE.