To observe the contrasting effects of TaFlo2-A1a and TaFlo2-A1b on TGW at the gene expression level in flag leaves and developing grains, two polymorphic accessions were used. The expression level of TaFLO2 was positively correlated with TGW, which is consistent with previous results in rice . The haplotype TaFlo2-A1a, which exhibits low expression levels, represented the group of accessions that have low average TGW, and the haplotype TaFlo2-A1b, which exhibits high expression levels, represented the group of accessions that have high average TGW in both Chinese and Pakistani wheat populations. Quantitative RT-PCR assays showed that for both types of haplotypes, the expression level was maximum in 12-day-old flag leaves followed by expression in developing grains sampled at 5 DAF. However, the expression of both types of haplotypes decreased rapidly in the fertilized caryopses collected at 10, 15, 20 and 25 DAF. The expression level of TaFlo2-A1b was higher than that of TaFlo2-A1a at all tested stages but significantly differed only in flag leaves and developing grains at 5 DAF (Fig. 4a). Furthermore, the expression level was positively correlated in Chinese Spring and three cultivars (Pubing3228, Shannong23, and Zhengmai9405) in developing grains sampled at 5 DAF. The expression level was lowest in Chinese Spring (TGW, 21.3 g) and highest in the cultivar Zhengmai9405 (TGW, 64.1 g) (Fig. 4b). All these cultivars contained the positive haplotype TaFlo2-A1b. Together, these results suggested that the relative expression level of TaFlo2-A1 was highest in flag leaves but started to decrease in developing grains. However, the expression in developing grains at 5 DAF was positively correlated with TGW in cultivars carrying the positive haplotype.Fig. 4 a Expression of TaFlo2-A1 in flag leaves and developing grains. b Expression level of TaFlo2-A1b in cultivars with different TGW values at 5 DAF