Strigolactone biosynthesis requires the symbiotic GRAS-type transcription factors NSP1 and NSP2

Legume GRAS-type transcription factors NSP1 and NSP2 are essential for Rhizobium Nod factor-induced nodulation. Both proteins are considered to be Nod factor response factors regulating gene expression upon symbiotic signalling. However, legume NSP1 and NSP2 can be functionally replaced by non-legume orthologs; including rice (Oryza sativa) OsNSP1 and OsNSP2. This shows that both proteins are functionally conserved in higher plants, suggesting an ancient function that was conserved during evolution. Here we show that NSP1 and NSP2 are indispensable for strigolactone biosynthesis in the legume Medicago truncatula as well as rice. Mutant nsp1-nsp2 plants hardly produce strigolactones. The lack of strigolactone biosynthesis coincides with strongly reduced DWARF27 expression in both species. Rice and Medicago represent distinct phylogenetic lineages that split ~150 million years ago. Therefore we conclude that regulation of strigolactone biosynthesis by NSP1 and NSP2 is an ancestral function conserved in higher plants. Since strigolactone biosynthesis is highly regulated by environmental conditions like phosphate starvation, NSP1 and NSP2 will be important tools in future studies on the molecular mechanisms by which environmental sensing is translated into regulation of strigolactone biosynthesis. As NSP1 and NSP2 are single copy genes in legumes, it implies that a single protein complex fulfills a dual regulatory function of different downstream targets; symbiotic and non-symbiotic, respectively.