Aleurone Cell Development Of Rice And Wheat

The periphery of the endosperm of rice and wheat forms an epidermis-like layer called the aleurone. During the development of caryopsis, the aleurone transports carbohydrates into the endosperm and embryo. During germination, the aleurone performs an important digestive function, secreting hydrolases to break down the starch and proteins stored in the starchy endosperm cells. Several features of cereal aleurone cells make them an attractive system for studying, while there are still lots of questions which need to be answered such as their differentiation, development and functions.The microstructure and ultrastructure of rice and wheat caryopsis in the different phase of both development and germination were studied by resin embedding combined with semi-thin section technology and ultra-thin technology. The changes of morphology and cell-wall-fluorescence were also studied by scan electron microscope (SEM) and fluorescence microscope, respectively. The main results were as follows:1. Aleurone cells were differentiated from the periphery cells of the endosperm. Their development was divided into five stages:①Endosperm cellularization: Cell wall of the free nucleus of endosperm occurred from outside towards inside.②Lipid bodies formation: The cell was highly vacuolated. The spherosome inside the cell accumulated fatty acid forming.③Aleurone grains formation: Phytin and proteins accumulated in the small vacuoles forming the aleurone grains.④Enrichment: A mass of aleurone grains and lipid bodies appeared and were enriched, the number of vacuoles reduced.⑤Maturation: The process of enrichment was over. These cells are cuboidal in shape, thick-walled, nucleus-existed and little starchy, showing typical characteristics of the aleurone.2. In wheat, several layers of transfer cells was formed near the vascular bundle, while the other region developed one or two layers of aleurone cells. In rice, three to four layers of aleurone cells was formed near the vascular bundle while the other region developed one or two layers of aleurone cells. Inside the aleurone, subaleurone cell was formed.3. The aleurone cells of indica rice developed faster than those of japonica rice. The number of aleurone layers from different grain size was different in the same series: the large grain variety usually had two aleurone layers, but the small grain variety only one layer. The aleurone cells of varied position in the same grain developed differently. Compared with the aleurone in the abdomen of the grain, the aleurone in the back developed faster and had more layers. The aleurone cells in the back were irregular in shape while the aleurone cells in the abdomen were more regular. The subaleurone lays were more in the back inside the aleurone than in the abdomen.4. Aleurone cells had a large amount of proteins, lipids and minerals such as P, K, Ca, Mg etc, but little starch. These cells rich in cell organelles were live cells with nucleus. They were thick-walled and regular in shape. Starchy endosperm cells were thin-walled and irregular in shape, containing lots of amyloplasts but little minerals and cell organelles. These cells underwent programmed cell death during the development of the seed.5. The waste of grain-filling which could not be absorbed by starchy endosperm cells such as lipids, minerals and some proteins accumulated in the periphery of the endosperm to form the aleurone layer. Superfluous waste resulted in the formation of subaleurone layer inside the aleurone.6. During the germination of the seed, aleurone grains worked out 4～6d after germination, but some substance can't be used by seedling. Rice endosperm is absorbed from one side that near by scutella to another.7. The functions of aleurone layer: transporting and accumulating nutrition, supplying biologically active molecules for germination, maintaining a relatively steady environment to keep the embryo alive, might involved in seed dormancy.