Effects of the Alleles at OsAGPS2 and OsSUT1 on the Grain Filling in Extra‐Heavy Panicle Type of Rice

Abstract

Rice (Oryza sativa L.) 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, OsAGPS2 and 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. OsAGPS2 and OsSUT1 had 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 OsAGPS2 and OsSUT1, designated as AS2‐2 and SUT1‐2, respectively. Second, new molecular markers based on the polymorphisms for OsAGPS2 and OsSUT1 were developed. Finally, three EHPT cultivars with AS2‐2 SUT1‐2 genotype 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 AS2‐2 and SUT1‐2 alleles can contribute to the improvement of poor grain filling of EHPTs assisted with the developed markers.

Non-random distribution of the alleles for good grain filling at OsAGPS2 and OsSUT1 among a wide range of rice (Oryza sativa L.) cultivars

“…Kato et al (2010) reported that the same series of nucleotide polymorphisms at this locus was detected in particular cultivars that have numerous spikelets in a panicle (extra-heavy panicle types) and show good grain filling. Therefore, this series of polymorphisms was recognized as a haplotype or an allele for good grain filling, and was designated as AS2-2 (the counterpart allele is AS2-1).…”

“…This series of polymorphisms in SUT1 was also found in the same cultivars as in the case of AS2 and was regarded as another allele for good grain filling designated as SUT1-2 (the counterpart allele is SUT1-1). The degree of grain filling was significantly better in three cultivars with a double homozygous genotype, AS2-2 SUT1-2, compared with the other three cultivars with AS2-1 SUT1-1 (Kato et al 2010). Interestingly, all of the cultivars with AS2-2 SUT1-2 were indica, whereas those with AS2-1 SUT1-1 were indica or japonica, although only six cultivars were examined.…”

“…The rice (Oryza sativa L.) gene of OsAGPS2 (AS2) on chromosome 8 (Os08g0345800, 15754394..15760588) encodes the small subunit of ADPglucose pyrophosphorylase, which plays an important role in sucrose-starch conversion and starch biosynthesis in developing endosperm (Ohdan et al 2005), and should be associated with the degree of grain filling (Kato et al 2007(Kato et al , 2010. Kato et al (2010) reported that the same series of nucleotide polymorphisms at this locus was detected in particular cultivars that have numerous spikelets in a panicle (extra-heavy panicle types) and show good grain filling.…”

“…Therefore, this series of polymorphisms was recognized as a haplotype or an allele for good grain filling, and was designated as AS2-2 (the counterpart allele is AS2-1). In addition, Kato et al (2010) revealed that the same kind of polymorphism series was detected in a gene encoding a rice sucrose transporter, OsSUT1 (SUT1) on chromosome 3 (Os03g017099, 3796696..3798433), which regulates the loading and unloading of sucrose through cell membranes at sink termini of sieve tubes and endosperm cells (Aoki et al 2003). This series of polymorphisms in SUT1 was also found in the same cultivars as in the case of AS2 and was regarded as another allele for good grain filling designated as SUT1-2 (the counterpart allele is SUT1-1).…”

“…This study investigated the distribution of the two alleles for good grain filling, AS2-2 and SUT1-2, among a wide range of rice cultivars including indica and japonica, using molecular markers designed directly from the nucleotide polymorphisms in the respective loci (Kato et al 2010). The results for the allele distribution among cultivars will provide information to explore gene sources for the breeding of extra-heavy panicle types with good grain filling.…”