Polyamine Anabolism in Germinating Glycine max (L.) Seeds : Dynamics of Cadaverine and Putrescine Formation in the Embryonic Axis.
Active polyamine biosynthesis occurs in the embryonic axis, but not in the cotyledons, during germination of Glycine max (L.) cv Williams seeds and subsequent growth of the young seedlings. The hypocotyl and radicle synthesize and accumulate considerable amounts of cadaverine (Cad) and putrescine (Put) during the early stages of growth. Most of the amino acid precursors for the diamines are supplied from breakdown of the cotyledonary protein.In the axis, Cad synthesis which is catalyzed by l-lysine decarboxylase precedes the onset of growth (dry weight accumulation) of the axis and its accumulation continues as the growth progresses. After 2 days of imbibition, Cad is synthesized and accumulated at 37.4 nanomoles per axis per hour for at least 6 days. The rate then gradually decreases as the supply of free l-lysine from the cotyledons diminishes. Approximately 40 to 50% of the lysine resulting from breakdown of the cotyledonary protein ends up in Cad in the hypocotyl and radicle. Cad represents about 3.5% of the axis nitrogen derived from the cotyledons, which is equivalent to about 50% of the lysine content ( approximately 7%) of the seed protein.The synthesis and accumulation of Put in the axis also precedes the onset of growth of the axis. Both l-arginine decarboxylase and l-ornithine decarboxylase are involved in catalyzing the Put formation. The increased content of spermidine (Spd), but not spermine (Spm), parallels growth of the axis. Spm is maintained at a nearly undetectable level. After 2-day imbibition, Put and Spd are synthesized and accumulated at 0.94 and 0.17 nanomoles per axis per hour, respectively. The rate of Put accumulation, like that of Cad, decreases about 8 days after imbibition. The hypocotyl and radicle contain millimolar concentrations of Cad and Put which are primarily associated with the elongated zones. In contrast, Spd level or the molar ratio of Spd:Put appears to decrease as cell differentiation or elongation progresses.
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Department of Agronomy, University of Kentucky, Lexington, Kentucky 40546-009.