After sowing, cereal grains imbibe water and begin a series of events leading to germination of the embryo and subsequent mobilisation of the stored reserves in the endosperm. A chemical stimulus, produced in the embryo, diffuses to the aleurone layer surrounding the starchy endosperm and initiates the synthesis of a range of enzymes that digest cell walls, storage proteins and starch present in the dead cells of the endosperm. Lipids, nucleic acids and mineral reserves are also degraded. The simple sugars and peptides released by the activities of these hydrolytic enzymes are absorbed by the scutellum and are used by the growing embryo before the leaves emerge and photosynthesis begins. We know that a stimulus is derived from the embryo because nothing happens to the endosperm if the embryo is removed before planting. We also know that the plant hormone, gibberellin (GA), is produced by the embryo and that synthetic gibberellic acid (GA3) applied directly to endosperm can substitute for the presence of the embryo.
However, this simple model of an embryo stimulus diffusing to the aleurone and leading to the production of hydrolytic enzymes and mobilisation of the endosperm storage reserves is, in fact, too simplistic. The initial sites of production of hydrolytic enzymes during cereal germination are the epithelial cells of the scutellum, adjacent to the endosperm. Because of the close proximity to the site of GA production in the scutellum, we do not know whether or not this is a GA-dependent process. Later in development the source of hydrolytic enzyme activity is the aleurone layer. This reflects the pattern of dissolution of the contents of endosperm cells which begins next to the scutellum and moves as a wave advancing fastest beneath the aleurone layer on the dorsal side and flanks towards the brush end of the grain.
The pattern of digestion of the contents of these cells is reminiscent of the events that led to the production of the endosperm 'crushed cell layer' during embryo growth in the developing grain.
The brewing and distilling industries have benefited from using this information during the production of good quality malt. In this instance, wheat or barley grains are steeped in large tanks containing water, to increase the moisture content, with intermittent draining and air rests. The grain is then germinated under controlled conditions of moisture and temperature allowing the production of hydrolytic enzymes to digest the contents of the endosperm. In some cases GA3 is added to increase uniformity of germination and hasten the malting process. In order to restrict embryo growth, germination is halted by the controlled application of heat (kilning). The temperature has to be sufficiently high to arrest the activity of the hydrolytic enzymes but not to destroy them in the finished malt. Kilning temperature also affects malt colour and flavour, important components of beer quality.