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Amino acids are delivered into developing wheat grains to support the accumulation of storage proteins in the starchy endosperm, and transporters play important roles in regulating this process. RNA-seq, RT-qPCR, and promoter-GUS assays showed that three amino acid transporters are differentially expressed in the endosperm transfer cells (TaAAP2), starchy endosperm cells (TaAAP13), and aleurone cells and embryo of the developing grain (TaAAP21), respectively. Yeast complementation revealed that all three transporters can transport a broad spectrum of amino acids. RNAi-mediated suppression of TaAAP13 expression in the starchy endosperm did not reduce the total nitrogen content of the whole grain, but significantly altered the composition and distribution of metabolites in the starchy endosperm, with increasing concentrations of some amino acids (notably glutamine and glycine) from the outer to inner starchy endosperm cells compared with wild type. Overexpression of TaAAP13 under the endosperm-specific HMW-GS (high molecular weight glutenin subunit) promoter significantly increased grain size, grain nitrogen concentration, and thousand grain weight, indicating that the sink strength for nitrogen transport was increased by manipulation of amino acid transporters. However, the total grain number was reduced, suggesting that source nitrogen remobilized from leaves is a limiting factor for productivity. Therefore, simultaneously increasing loading of amino acids into the phloem and delivery to the spike would be required to increase protein content while maintaining grain yield.

The amylose content, size distribution, and processing properties of isolated starch granules were compared between eight near‐isogenic lines (NILs) of a wheat cultivar with different null‐waxy (wx) alleles. The increases in the amylose content (0.6–34.6%), volume percentage of A‐type starch granules (54.8–67.6%), low (118–179 cP) and final viscosities (160–329 cP), and setback value (43–146 cP) were positively related to the number of waxy alleles. All the viscosity values, except those of the peak time and the pasting temperature, of the purified A‐type starch granules were much higher than those of the B‐type granules, and so was the swelling power. The amylose content and pasting properties of the purified A‐ and B‐type starch granules showed similar patterns to those of flour in response to different null‐waxy alleles. In addition, wx‐B1 had greater effects on the amylose content, starch granule diameter, and pasting traits than the other alleles did. The diameter of biscuits baked with the flour of NILs varied between 46.29 mm (the wild‐type) and 44.09 mm (the waxy‐type), but their hardness decreased from 2895 to 1707 g as the number of the null‐wx alleles increased. The varied pasting traits of NILs were indirectly related to the modified size distributions of starch granules, but were directly ascribed to the regulated content of amylose due to deletion of the wx genes.