Abstract
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.
Highlights
Wheat contributes about 10% of the dietary intake of protein in the UK, and grain protein content (GPC) is a key contributor to breadmaking quality
The amino acid transporters TaAAP2, TaAAP13, and TaAAP21 are differentially expressed in wheat grain cells and plant organs
TaAAP2 and TaAAP13 were highly expressed during the middle stages of grain filling (14 and 21 DPA), while the expression of TaAAP21 increased during the late stage of grain filling (28 DPA)
Summary
Wheat contributes about 10% of the dietary intake of protein in the UK, and grain protein content (GPC) is a key contributor to breadmaking quality. The AtAAP1 transporter is highly expressed in the embryo epithelium (transfer cells) of Arabidopsis and involved in uptake and transport of amino acids from the endosperm into the embryo, with ataap mutants having lower seed protein content and accumulation of high level of amino acids in seeds [13]. Altered xylem-phloem transfer of amino acids in the ataap mutant increased seed yield and oil content in Arabidopsis by changing the source-sink translocation of amino acids [28]. Our previous work identified three amino acid transporters (TaAAP2, TaAAP13, and TaAAP21) which are highly expressed in different grain cells [10] In this study, their expression patterns and roles in amino acid transport and nitrogen accumulation in wheat grains were characterized by heterologous expression and transgenesis
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