Glycine metabolism by plant roots and its occurrence in Australian plant communities

Abstract

Soluble organic nitrogen, including protein and amino acids, was found to be a ubiquitous form of soil N in diverse Australian environments. Fine roots of species representative of these environments were found to be active in the metabolism of glycine. The ability to incorporate [¹⁵N]glycine was widespread among plant species from subantarctic to tropical communities. In species from subantarctic herbfield, subtropical coral cay, subtropical rainforest and wet heathland, [¹⁵N]glycine incorporation ranged from 26 to 45 % of ¹⁵NH₄⁺ incorporation and was 2- to 3-fold greater than ¹⁵NO₃^- incorporation. Most semiarid mulga and tropical savanna woodland species incorporated [¹⁵N]glycine and ¹⁵NO₃^- in similar amounts, 18–26 % of ¹⁵NH₄⁺ incorporation. We conclude that the potential to utilise amino acids as N sources is of widespread occurrence in plant communities and is not restricted to those from low temperature regimes or where N mineralisation is limited. Seedlings of Hakea (Proteaceae) were shown to metabolise glycine, with a rapid transfer of ¹⁵N from glycine to serine and other amino compounds. The ability to take up and metabolise glycine was unaffected by the presence of equimolar concentrations of NO₃^- and NH₄⁺. Isonicotinic acid hydrazide (INH) did not inhibit the transfer of ¹⁵N- label from glycine to serine indicating that serine hydroxymethyltransferase was not active in glycine catabolism. In contrast aminooxyacetate (AOA) strongly inhibited transfer of ¹⁵N from glycine to serine and labelling of other amino compounds, suggesting that glycine is metabolised in roots and cluster roots of Hakea via an aminotransferase.