Overexpression of GmAAP6a enhances tolerance to low nitrogen and improves seed nitrogen status by optimizing amino acid partitioning in soybean

Amino acid transport via phloem is one of the major source-to-sink nitrogen translocation pathways in most plant species. Amino acid permeases (AAPs) play essential roles in amino acid transport between plant cells and subsequent phloem or seed loading. In this study, a soybean AAP gene, annotated as GmAAP6a, was cloned and demonstrated to be significantly induced by nitrogen starvation. Histochemical staining of GmAAP6a:GmAAP6a-GUS transgenic soybean revealed that GmAAP6a is predominantly expressed in phloem and xylem parenchyma cells. Growth and transport studies using toxic amino acid analogs or single amino acids as a sole nitrogen source suggest that GmAAP6a can selectively absorb and transport neutral and acidic amino acids. Overexpression of GmAAP6a in Arabidopsis and soybean resulted in elevated tolerance to nitrogen limitation. Furthermore, the source-to-sink transfer of amino acids in the transgenic soybean was markedly improved under low nitrogen conditions. At the vegetative stage, GmAAP6a-overexpressing soybean showed significantly increased nitrogen export from source cotyledons and simultaneously enhanced nitrogen import into sink primary leaves. At the reproductive stage, nitrogen import into seeds was greatly enhanced under both sufficient and limited nitrogen conditions. Collectively, our results imply that overexpression of GmAAP6a enhances nitrogen stress tolerance and source-to-sink transport and improves seed quality in soybean. Co-expression of GmAAP6a with genes specialized in source nitrogen recycling and seed loading may represent an interesting application potential in breeding.