OsMADS5 interacts with OsSPL14/17 to inhibit rice root elongation by restricting cell proliferation of root meristem under ammonium supply

Nitrogen (N) is a vital major nutrient for rice (Oryza sativa). Rice responds to different applications of N by altering its root morphology, including root elongation. Although ammonium (NH4+) is the primary source of N for rice, NH4+ is toxic to rice roots and inhibits root elongation. However, the precise molecular mechanism that NH4+ -inhibited root elongation of rice is not well understood. Here, we identified a rice T-DNA insert mutant of OsMADS5 with a longer seminal root (SR) under sufficient N conditions. Reverse-transcription quantitative PCR analysis revealed that the expression level of OsMADS5 was increased under NH4+ compared with NO3- supply. Under NH4+ conditions, knocking out OsMADS5 (cas9) produced a longer SR, phenocopying osmads5, while there was no significant difference in SR length between wild-type and cas9 under NO3- supply. Moreover, OsMADS5-overexpression plants displayed the opposite SR phenotype. Further study demonstrated that enhancement of OsMADS5 by NH4+ supply inhibited rice SR elongation, likely by reducing root meristem activity of root tip, with the involvement of OsCYCB1;1. We also found that OsMADS5 interacted with OsSPL14 and OsSPL17 (OsSPL14/17) to repress their transcriptional activation by attenuating DNA binding ability. Moreover, loss of OsSPL14/17 function in osmads5 eliminated its stimulative effect on SR elongation under NH4+ conditions, implying OsSPL14/17 may function downstream of OsMADS5 to mediate rice SR elongation under NH4+ supply. Overall, our results indicate the existence of a novel modulatory pathway in which enhancement of OsMADS5 by NH4+ supply represses the transcriptional activities of OsSPL14/17 to restrict SR elongation of rice.