Fine Mapping And Functional Analysis Of SG101 (SMALL GRAIN 101) In Rice (Oryza Sativa L.)

Rice,one of the most important food crops,is the staple food for more than half of the world's population.With the growth of the population,the environment deteriorate and the land pollution is increasingly serious,how to increase the rice production to ensure food safety has became an important topic in our country.The grain size is mainly comprised of grain length,grain width and grain thickness,which is generally governed by quantitative trait loci?QTLs?.A small grain mutant,named small grain 101?sg101?,gained by EMS mutagenesis for Zhong Hua 11?ZH11?.The preliminary location of the gene SG101 was completed.Genetic analysis revealed that sg101 phenotype was controlled by a single recessive nuclear gene.Compared with wild type,the grain length and grain width of sg101 mutant show significant decreased,resulting in the decreased of grain weight.More over,mutant sg101 also displayed descreaing in panicle length,secondary branch number,plant height and seed setting rate.The paraffin section and scanning electron microscope of the ZH11 and sg101 spikelet showed that cell number in mutant sg101 was significant decreased,but the cell size was almost not changed,suggesting that SG101 may influence to the grain size by regulate the cell division.Quantitative RT-PCR detected of ZH11 and sg101 cell cycle related genes showed that the expression of type-A cell cycle genes:CYCA2:2,CYCA2:3 and type-D cell cycle genes:CYCD4:1,CYCD7:1 were decreased in sg101 mutant.Meanwhile,we analyzed the DNA profiles of ZH11 and sg101 leaf nuclei using a flow cytometer,the results showed that the percent of diploid nuclei in sg101 mutant incressed in G1 stage,suggesting that the cell cycle was delayed at the G₁ phase.Brassinolide?BR?assay showed that the leaf angle and coleoptile in sg101 was slowness response to BR.Quantitative PCR detecting the expression of rice radicle in BR biosynthetic and signal pathway genes showed that D2,OSCPOD,DWARF-4Q,BU1,BZRF,D61,PAVL1 and OSMDP1 in BR biosynthetic and signal pathway were down-regulated in sg101 mutant.To determine the molecular basis for the sg101 phenotypes,a map-based cloning approach was carried out to isolate the corresponding gene.The BC₂F₂ mapping population was generated by crossing SG101 with Taichung Native 1,and 1600 individual with a mutation phenotype was selected for gene mapping.The SG101locus was primary located on chromosome 1,and finally narrowed down to an approximate 265 kb region.The above results imply that the mutation in sg101 may has an impact in both BR signaling and synthetic pathyway,which leads to abnormal cell division.As a result,the mutant plant showed a phenotype of decreased cell number and reduced grain size.This research lays foundation for the further cloning and study of SG101.