Running title : Modulation of root elongation by N and P supply Full Title : Strigolactones are required for nitric oxide to induce root elongation in response to nitrogen-and phosphate-deficiency in rice

The response of the root system architecture to nutrient deficiencies is critical for sustainable agriculture. Nitric oxide (NO) is considered a key regulator of root growth, although the mechanisms remain unknown. Phenotypic, cellular, and genetic analyses were undertaken in rice to explore the role of NO in regulating root growth and strigolactone signaling under nitrogenand phosphate-deficient conditions (LN and LP). LNand LP-induced seminal root elongation paralleled NO production in root tips. NO played an important role in a shared pathway of LNand LP-induced root elongation via increased meristem activity. Interestingly, no responses of root elongation were observed in strigolactone d mutants compared to wild-type plants, although similar NO accumulation was induced by sodium nitroprusside (SNP) application. Application of Abamine (the strigolactone inhibitor) reduced seminal root length and pCYCB1;1::GUS expression induced by SNP application in wild-type; furthermore, comparison with wild-type showed lower strigolactone-signaling genes in nia2 mutants under control and LN treatments and similar under SNP application. Western blot analysis revealed that NO, similar to strigolactone, triggered proteasome-mediated degradation of D53 protein levels. Therefore, we presented a novel signaling pathway in which NO activated seminal root elongation