水稻籽粒灌浆速率是复杂的数量性状和重要的农艺性状，影响着稻米产量和品质，但其遗传调控机理尚不清楚，可供育种利用的优异等位基因也非常有限。申请人前期开展了水稻籽粒灌浆速率的全基因组关联分析，并鉴定了一个稳定遗传QTL GFR3a的候选基因，发现其敲除突变体的籽粒灌浆速率显著下降，单倍型显著性自然变异存在于启动子区。基于此，本项目拟通过转基因互补实验以及单倍型基因转录与蛋白表达水平分析，进一步验证该候选基因；构建含有不同自然变异的启动子驱动的GFR3a转基因株系及过量表达株系，比较其籽粒灌浆速率从而确定启动子区的关键功能性变异，并对该变异位点招募的反式作用因子进行鉴定；对相关转基因株系的颖果进行同化物积累测定、组织学与细胞学观察以及转录组分析，解析GFR3a在籽粒灌浆速率调控中的具体功能。本项目将进一步揭示籽粒灌浆速率的遗传调控机理，并为快速灌浆水稻品种的精准育种提供理论依据和基因资源。

Grain-filling rate (GFR) is a complex quantitative trait and an important agronomic trait, greatly affecting grain yield and quality in rice. However, the genetic mechanism of GFR regulation remains unclear and elite alleles of rapid GFR for rice breeding are limited. Previously, we performed genome-wide association analysis of GFR in rice, and identified the candidate gene of a stable QTL GFR3a. The knockout mutants of the candidate gene display a significant decrease in GFR, and remarkable natural variations between haplotypes of the gene locate in the promoter. In this project, the candidate gene of GFR3a will be further confirmed by complementation test and comparison of its transcription and translation levels between different haplotypes. Subsequently, we will construct the GFR3a transgenic lines driven by native promoters with different natural variations as well as over-expression lines, and investigate their GFR phenotypes to determine the key functional variation in the promoter. We will also identify the trans-acting factor which is recruited by the motif related to this key functional variation. To reveal the detailed function of GFR3a in GFR regulation, we will perform assimilate content measurement, cytological and histological observation, and transcriptome analysis of the grain-filling caryopses of the related transgenic lines. Our study of GFR3a will further reveal the genetic mechanism of GFR regulation in rice, and provide valuable theoretical knowledge and gene resources to promote rice precision breeding for rapid GFR.