The Spermine Synthase OsSPMS1 Regulates Seed Germination, Grain Size, and Yield.

Polyamines, including putrescine (Put), spermidine (Spd), and spermine (Spm), play essential roles in a wide variety of prokaryotic and eukaryotic organisms. Rice (Oryza sativa) contains four putative spermidine/spermine synthase (SPMS)-encoding genes (OsSPMS1, OsSPMS2, OsSPMS3, and OsACAULIS5), but none have been functionally characterized. In this study, we used a reverse genetic strategy to investigate the biological function of OsSPMS1. We generated several homozygous RNA interference (RNAi) and overexpression (OE) lines of OsSPMS1. Phenotypic analysis indicated that OsSPMS1 negatively regulates seed germination, grain size, and grain yield per plant. The ratio of Spm to Spd was significantly lower in the RNAi lines and considerably higher in the OE lines than in the wild-type (WT), suggesting that OsSPMS1 may function as a spermine synthase. S-adenosyl-L-methionine is a common precursor of polyamines and ethylene biosynthesis. The 1-aminocyclopropane-1-carboxylic acid (ACC) and ethylene contents in seeds significantly increased in RNAi lines and decreased in OE lines, respectively, compared with wild-type. Additionally, the reduced germination rates and growth defects of OE lines could be rescued with ACC treatment. These data suggest that OsSPMS1 affects ethylene synthesis and may regulate seed germination and plant growth by affecting the ACC and ethylene pathways. Most importantly, an OsSPMS1 knockout mutant showed an increase in grain yield per plant in a high-yield variety, Suken118, suggesting that OsSPMS1 is an important target for yield enhancement in rice.