Aleurone layer mobilization

Fig. 1.—A Cross-section of the Outer Part of a Seed of Trigonella foenum-graecum Before Mobilization of the Galactomannan, Showing the Three-layered Seed-coat (S) and a Small Part of the Cotyledon (C), with the Endosperm in Between. [The aleurone layer (A) is the outer cell-layer of the endosperm, and the rest of the endosperm is composed of large cells that have thin, primary walls and are completely filled with the dark-stained galactomannan (G). Stained with the periodic acid-Schiff reagent x300 (reproduced, by permission, from Ref. 199).]...

The major products of a- and jS-amylolysis are a- and jS-maltose respectively, both of which are hydrolysed by a-glucosidase, an enzyme present in the embryo and aleurone tissue of ungerminated barley [78]. The enzyme increases in activity at both sites over the first 5 days after imbibition, but it seems that the a-glucosidase in the embryo is associated with starch utilization in the axis and, unlike the aleurone layer enzyme, is not involved with mobilization of the endosperm... 

It would obviously be of great interest to know whether two distinct patterns of reserve mobilization really do occur within cereals. It is still possible that there is fundamentally only one (GA-dependent) and that the endosperms of maize and sorghum contain sufficient endogenous GA (produced there during development) to stimulate maximum hydrolase production without a requirement for a further supply from the embryo, or without any additional effect of exogenously added GA. The effect of added GA on maize endosperm (aleurone layer) probably varies in different hybrid or inbred lines because some already have a high endogenous GA level while others, e.g. dwarf maize, are naturally deficient in GA. The latter show a three- to five-fold increase in hydrolase production in response to added GA [64] (see Chap. 7). 

A system for the mobilization of stored reserves in the starchy endosperm for use by the developing seedling. This might be comprised of two components proteinase synthesized and secreted by the aleurone layer (and again controlled by GA), and proteinase pre-formed in the endosperm itself and activated therein. 

We saw in Chapter 6 how the reserves in the endosperm of cereals are mobilized largely as a result of the activity of enzymes secreted by the aleurone layer. We also referred to the stimulation of aleurone layer activity by gibberellin coming from the embryo. Herein lies the basis of the control exerted by the embryo over food mobilization, which we will now examine more closely. 

This supposition has been confirmed. In the mobilization of the reserve materials of barley the following events take place the embryo discharges gibberellic acid into the outermost layer of the endosperm, the aleuron. As a result genes for the synthesis of various hydrolytic enzymes are activated in the aleuron. The process culminates in the synthesis of... 

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