NITROGEN-METABOLISM IN LIGNIFYING PINUS-TAEDA CELL-CULTURES

The primary metabolic fate of phenylalanine, following its deamination in plants, is conscription of its carbon skeleton for lignin, suberin , flavonoid, and related metabolite formation. Since this accounts for similar to 30-40% of all organic carbon, an effective means of recycl ing the liberated ammonium ion must be operative. In order to establis h how this occurs, the uptake and metabolism of various N-15-labeled p recursors (N-15-Phe, (NH4Cl)-N-15, N-15-Gln, and N-15-Glu) in lignifyi ng Pinus taeda cell cultures was investigated, using a combination of high performance liquid chromatography, N-15 NMR, and gas chromatograp hy-mass spectrometry analyses. It was found that the ammonium ion rele ased during active phenylpropanoid metabolism was not made available f or general amino acid/protein synthesis. Rather it was rapidly recycle d back to regenerate phenylalanine, thereby providing an effective mea ns of maintaining active phenylpropanoid metabolism with no additional nitrogen requirement. These results strongly suggest that, in lignify ing cells, ammonium ion reassimilation is tightly compartmentalized.