The plant defensin gene AtPDF2.1 mediates ammonium metabolism by regulating glutamine synthetase activity in Arabidopsis thaliana

Background: In plants, ammonium metabolism is particularly important for converting absorbed nitrogen into amino acids. However, the molecular mechanism underlying this conversion remains largely unknown. Results: Using wild type Arabidopsis thaliana (Col-0) and AtPDF2.1 mutants (pdf2.1-1 and pdf2.1-2), we found that the small cysteine-rich peptide AtPDF2.1, a plant defensin, is involved in regulating ammonium metabolism in the shoot. Ammonium significantly induced the expression of AtPDF2.1 in the shoot and root, particularly in root xylem vascular bundles, as demonstrated by histochemical analysis. Subcellular localization analysis revealed that AtPDF2.1 was localized to the cell wall. Ammonium concentration was higher in the shoot of mutants than in the shoot of Col-0, but no differences were found for total nitrogen content, root ammonium concentration, and the expression of the ammonium transporter gene AtAMT2.1. The activity of glutamine synthetase was significantly decreased in mutants, and the glutamine synthetase family genes GLN1.3 and GLN1.5 were significantly downregulated in mutants compared to Col-0. The activity of nitrate reductase showed no difference between mutants and Col-0. Conclusions: Overall, these data suggest that AtPDF2.1 affects ammonium metabolism by regulating the expression of GLN1.3 and GLN1.5 through a yet unidentified mechanism.