Effects of Strigolactones on Grain Yield and Seed Development in Rice

Strigolactones (SLs) are well known as a class of endogenous phytohormones that regulate tiller bud outgrowth. Reduction of inorganic phosphate (Pi) induces the accumulation of SLs, which inhibit tiller bud outgrowth in wild-type (WT) rice plants, but not in SL mutants. This suggests that SLs are important for plant adaptation to Pi-deficient conditions. Thus, we investigated the effects of SLs on grain yield and seed size in WT and dwarf (d) mutant rice plants treated with various Pi concentrations. In both WT and d mutants, plant growth, chlorophyll levels, panicle number, number of hulls, and total grain number decreased as Pi decreased, indicating that SL is not required to mediate these Pi responses. The d mutants produced more panicles than the WT control, but there was no increase in grain yield, and the seed-setting rate decreased. Removal of outgrowing tillers did not affect grain yield in d mutants. GR24 (a synthetic SL) treatment rescued grain yield in d mutants. The d3 and d53 mutants had the lowest grain yields among d mutants. Furthermore, the endosperm of d mutants was 25% smaller than that of WT plants; there were no significant differences in embryo length between WT and d mutant plants, but the endosperm cell area of the d mutants was approximately 30% smaller than that of WT plants. We propose that SLs control grain yield and rice endosperm development.