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Summary The spikelet number and heading date are two crucial and correlated traits for yield in wheat. Here, a quantitative trait locus (QTL) analysis was conducted in F8 recombinant inbred lines (RILs) derived from crossing two common wheats with different spikelet numbers. A total of 15 stable QTL influencing total spikelet number (TSN) and heading date (HD) were detected. Notably, FT‐D1, a well‐known flowering time gene in wheat, was located within the finely mapped interval of a major QTL on 7DS (QTsn/Hd.cau‐7D). A causal indel of one G in the third exon of FT‐D1 was significantly associated with total spikelet number and heading date. Consistently, CRISPR/Cas9 mutant lines with homozygous mutations in FT‐D1 displayed an increase in total spikelet number and heading date when compared with wild type. Moreover, one simple and robust marker developed according to the polymorphic site of FT‐D1 revealed that this one G indel had been preferentially selected to adapt to different environments. Collectively, these data provide further insights into the genetic basis of spikelet number and heading date, and the diagnostic marker of FT‐D1 will be useful for marker‐assisted pyramiding in wheat breeding.

Summary Vernalization and photoperiod pathways converging at FT1 control the transition to flowering in wheat. Here, we identified a gain‐of‐function mutation in FT‐D1 that results in earlier heading date (HD), and shorter plant height and spike length in the gamma ray‐induced eh1 wheat mutant. Knockout of the wild‐type and overexpression of the mutated FT‐D1 indicate that both alleles are functional to affect HD and plant height. Protein interaction assays demonstrated that the frameshift mutation in FT‐D1eh1 exon 3 led to gain‐of‐function interactions with 14‐3‐3A and FDL6, thereby enabling the formation of florigen activation complex (FAC) and consequently activating a flowering‐related transcriptomic programme. This mutation did not affect FT‐D1eh1 interactions with TaNaKR5 or TaFTIP7, both of which could modulate HD, potentially via mediating FT‐D1 translocation to the shoot apical meristem. Furthermore, the ‘Segment B’ external loop is essential for FT‐D1 interaction with FDL6, while residue Y85 is required for interactions with TaNaKR5 and TaFTIP7. Finally, the flowering regulatory hub gene, ELF5, was identified as the FT‐D1 regulatory target. This study illustrates FT‐D1 function in determining wheat HD with a suite of interaction partners and provides genetic resources for tuning HD in elite wheat lines.

Summary FLOWERING LOCUS T (FT), a multifunctional regulator in crops, modulates multiple key agronomic traits such as flowering time or heading date and plant height; however, its role in grain development regulation is unclear. Herein, through genome‐wide association studies (GWAS), we identified TaFT‐D1, which encodes a phosphatidylethanolamine‐binding protein (PEBP), as a candidate gene for grain weight in wheat. A one‐bp insertion/deletion (InDel) (G/‐) in the third exon of TaFT‐D1, resulting in different protein lengths, was significantly associated with grain weight. TaFT‐D1 knockout via the CRISPR‐Cas9 system reduced grain size and weight, and TaFT‐D1 increased grain size by promoting cell proliferation and starch synthesis. Transcriptome analysis revealed a significant decrease in the expression of cell cycle‐ and starch synthesis‐related genes, including TaNAC019‐3A, TaSWEET15‐like‐7B, TaCYCD4;1 and TaCYCD3;2, in the taft‐d1 knockout line. TaFT‐D1 interacted with the bZIP transcription factor TaFDL2, and the tafdl2 mutant presented relatively small grains, suggesting that TaFDL2 is a positive regulator of grain size. Moreover, TaFDL2 bound to the promoters of downstream cell cycle‐ and starch synthesis‐related genes, activating their expression, whereas TaFT‐D1 increased this activation via TaFDL2. Interaction assays demonstrated that TaFT‐D1, Ta14‐3‐3A and TaFDL2 formed a regulatory complex. Furthermore, the TaFT‐D1(G) allele was significantly correlated with greater thousand‐grain weight and earlier heading. This favourable allele has undergone strong positive selection during wheat breeding in China. Our findings provide novel insights into how TaFT‐D1 regulates grain weight and highlight its potential application for yield improvement in wheat.