Nitrate Modulates Lateral Root Formation by Regulating the Auxin Response and Transport in Rice

Nitrate (NO3-) plays a pivotal role in stimulating lateral root (LR) formation and growth in plants. However, the role of NO3- in modulating rice LR formation and the signalling pathways involved in this process remain unclear. Phenotypic and genetic analyses of rice were used to explore the role of strigolactones (SLs) and auxin in NO3--modulated LR formation in rice. Compared with ammonium (NH4+), NO3- stimulated LR initiation due to higher short-term root IAA levels. However, this stimulation vanished after 7 d, and the LR density was reduced, in parallel with the auxin levels. Application of the exogenous auxin alpha-naphthylacetic acid to NH4+-treated rice plants promoted LR initiation to levels similar to those under NO3- at 7 d; conversely, the application of the SL analogue GR24 to NH4+-treated rice inhibited LR initiation to levels similar to those under NO3- supply by reducing the root auxin levels at 10 d. D10 and D14 mutations caused loss of sensitivity of the LR formation response to NO3-. The application of NO3- and GR24 downregulated the transcription of PIN-FORMED 2 (PIN2), an auxin efflux carrier in roots. LR number and density in pin2 mutant lines were insensitive to NO3- treatment. These results indicate that NO3- modulates LR formation by affecting the auxin response and transport in rice, with the involvement of SLs.