A favorable natural variation in <i>CCD7</i> from orchardgrass confers enhanced tiller number

Tiller number is a crucial determinant that significantly influences the productivity and reproductive capacity of forage. The regeneration potential, biomass production, and seed yield of perennial forage species are highly reliant on the development of tillering. Strigolactones (SLs) are recently discovered carotenoid-derived phytohormones that play a crucial role in the regulation of tillering in annual crops. However, the modulation of tiller growth in perennial forage by SLs remains insufficiently investigated. In this study, we identified two alleles of the SLs biosynthesis gene, DgCCD7^A and DgCCD7^D, which encode CAROTENOID CLEAVAGE DIOXYGENASE 7 (CCD7), from two distinct subspecies of orchardgrass (Dactylis glomerata) exhibiting contrasting tillering phenotype and SLs content. The functionality of the DgCCD7^A allele derived from high-tillering phenotypic orchardgrass was found to be diminished compared to that of DgCCD7^D from the low-tillering type in rescuing the increased branching phenotype of CCD7-defective mutants in Arabidopsis and rice (Oryza sativa). Notably, the introduction of DgCCD7^A in rice resulted in an increase in tiller number without significantly compromising grain yield. Moreover, we demonstrated that the L309P variation in DgCCD7^A is a rare natural variant exclusively found in orchardgrass. Our findings revealed that DgCCD7^A, a rare favorable natural variation of CCD7 in orchardgrass, holds significant potential for breeding application in improving the plant architecture of perennial forage and crops.