Effects of the Alleles at OsAGPS2and OsSUT1on the Grain Filling in Extra‐Heavy Panicle Type of Rice

Rice (Oryza sativaL.) cultivars of extra‐heavy panicle type (EHPT), which have numerous spikelets in a panicle, often fail to realize their high‐yield potential due to poor grain filling in general, while some EHPTs showed relatively better grain filling. This study first surveyed nucleotide polymorphisms in rice genes possibly related to grain filling, that is, genes for ADPglucose pyrophosphorylase (AGPase) subunits, OsAGPS2and OsAGPL2, and a sucrose transporter gene, OsSUT1, to clarify the causes underlying such genetic variation. Many types of polymorphisms were detected in all three genes, but none of them affected amino acid sequences. OsAGPS2and OsSUT1had an identical series of polymorphisms in two EHPT cultivars, Milyang 23 and Nanjing 11, both of which have a high degree of grain filling, while another series for these genes was detected in other EHPTs showing a low grain filling. The former polymorphisms could constitute alleles for good grain filling in OsAGPS2and OsSUT1, designated as AS2‐2and SUT1‐2, respectively. Second, new molecular markers based on the polymorphisms for OsAGPS2and OsSUT1were developed. Finally, three EHPT cultivars with AS2‐2 SUT1‐2genotype showed a higher degree of grain filling and higher activity of AGPase than three other EHPTs with another genotype at these loci. These results suggest that these AS2‐2and SUT1‐2alleles can contribute to the improvement of poor grain filling of EHPTs assisted with the developed markers.