Galactomannan, soluble sugar and starch mobilization following germination of Trigonella foenum-graecum seeds

Galactomannan reserves in the endosperm of intact fenugreek (Trigonella foenum-graecum) seeds were mobilized in part by endo-β-D-mannanase (mannan endo-1,4-beta mannosidase, EβM, EC 3.2.1.78) activity. The hydrolytic products, mannose and galactose, applied to isolated endosperms at physiologically relevant concentrations enhanced EβM activity; but at higher concentrations, EβM activity and its secretion were inhibited. Increases in glucose, fructose, and sucrose in the embryo reflected the mobilization of the galactosyl-sucrose sugars therein and, presumably, the uptake of the hydrolytic products of galactomannan breakdown from the endosperm. Concurrent with the accumulation of both axial and cotyledonary starch, with time and with the incubation of isolated seedlings in sugar solutions, there were increases in the steady state amounts of transcripts of both the large and small subunits of adenosine diphosphate glucose pyrophosphorylase (glucose-1-phosphate adenylyl-transferase, EC 2.7.7.27), the key regulatory enzyme of starch synthesis. The corresponding sugar, adenosine diphosphoglucose, accumulated only within the cotyledons after starch began to decrease. Activity of the starch degradative enzyme β-amylase (EC 3.2.1.2) increased in the cotyledon as starch started to decline, although low amounts of α-amylase (EC 3.2.1.1) were constitutively present.