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

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 15Nglycine was widespread among plant species from subantarctic to tropical communities. In species from subantarctic herbfield, subtropical coral cay, subtropical rainforest and wet heathland, 15Nglycine 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 15Nglycine 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.