Wheat grain yield is generally sink-limited during grain filling.  The grain-filling rate (GFR) plays a vital role but is poorly studied due to the difficulty of phenotype surveys.  This study explored the grain-filling traits in a recombinant inbred population and wheat collection using two highly saturated genetic maps for linkage analysis and genome-wide association study (GWAS).  Seventeen stable additive quantitative trait loci (QTLs) were identified on chromosomes 1B, 4B, and 5A.  The linkage interval between IWB19555 and IWB56078 showed pleiotropic effects on GFR₁, GFR_max, kernel length (KL), kernel width (KW), kernel thickness (KT), and thousand kernel weight (TKW), with the phenotypic variation explained (PVE) ranging from 13.38% (KW) to 33.69% (TKW).  198 significant marker-trait associations (MTAs) were distributed across most chromosomes except for 3D and 4D.  The major associated sites for GFR included IWB44469 (11.27%), IWB8156 (12.56%) and IWB24812 (14.46%).  Linkage analysis suggested that IWB35850, identified through GWAS, was located in approximately the same region as QGFRmax2B.3-11, where two high-confidence candidate genes were present.  Two important grain weight (GW)-related QTLs colocalized with grain-filling QTLs.  The findings contribute to understanding the genetic architecture of the GFR and provide a basic approach to predict candidate genes for grain yield trait QTLs.

The NAM-B1 gene is a member of the NAC (NAM, ATAF, and CUC) transcription factor family and plays an important role in regulating wheat grain protein content (GPC).  The ancestral NAM-B1 allele has been discovered in many tetraploid wild emmer (Triticum turgidum ssp. dicoccoides) accessions and few domesticated emmer accessions (T. turgidum ssp. dicoccum), however, it is rarely found in hexaploid bread wheat (Triticum aestivum L.).  There are no systematic reports on the distribution of NAM-B1 alleles in Chinese wheat cultivars.  In this study, the NAM-B1 alleles in 218 Chinese cultivars were investigated.  The cultivars were collected from five major wheat regions (12 provinces), covering most of the winter wheat growing regions in China.  The results showed that the NAM-B1 gene is present in 53 (24.3%) cultivars and absent in the remaining 165 (75.7%) cultivars.  Further analysis revealed that in contrast to the wild-type allele, the NAM-B1 gene in Chinese wheat cultivars contained a 1-bp insertion in the coding region.  This caused a frame-shift mutation and introduced a stop codon in the middle of the gene, rendering it non-functional.  Polymorphisms were detected in DNA sequences of 21 cultivars among these 53 cultivars.  However, cDNA sequence analysis suggested that these variations in the exon region were not able to restore NAM-B1 gene (1-bp insertion) function.  Thus, exploring the distribution of NAM-B1 gene variations (1-bp insertion and deletion) can provide some information for improving the quality of winter wheat in China and other countries.