Plants are the primary source of food for humans and feed for livestock. A major part of the human diet all over the world consists of cereals and legumes. 70% of human food comprises cereals and legumes and the remaining 30% comes from animals. Plants accumulate storage substances such as starch, lipids and proteins in certain phases of development. Storage proteins accumulate in both vegetative and reproductive tissues and serve as a reservoir to be used in later stages of plant development. Seed storage proteins are a group that comprises proteins generated mainly during seed production and stored in the seed that serve as nitrogen sources for the developing embryo during germination. The average protein content of cereal grains is 10-15 % of their dry weight that of leguminose seeds 20-25, while it is only 3-5 % in normal leaves. Besides seeds, storage proteins can also be found in root and shoot tubers. No clear definition what a storage protein is exists. The term was coined for all those proteins whose share in the total protein amount of the cell is greater than 5%. Typically, seed storage proteins have the following properties:
â€¢ The proteins have no enzymatic activities.
â€¢ They serve as nitrogen sources for the germinating seed.
â€¢ They occur normally in an aggregated state within a membrane surrounded vesicle (protein bodies, aleuron grains).
â€¢ They are often built from a number of different polypeptide chains.
Seed proteins were empirically classified by T.B. Obsorne based on their solubility as follows:
â€¢ Albumins: Water extractable (1.6S-2S);
â€¢ Globulins: Extractable in dilute salt solutions (7S-13S);
â€¢ Prolamins: Extractable in aqueous alcohol;
â€¢ Glutelins: Most difficult to solubilize; Extractable by weakly acidic or alkaline or dilute SDS solution.
Seed storage proteins are important for human nutrition so an interest in the production of mutants with increased protein content or an increased amount of essential amino acids (lysine, methionine tryptophan etc.) exists. Leguminoses contain mostly two types of storage proteins, legumin and vicelin. The legumins - as well as the vicelins - are very similar in the different leguminose species. Gramineae contain a third type: prolamin and, depending on the origin, it is distinguished between the zeines (from Zea mays), the hordeines (from Hordeum vulgare) etc. In contrast to legumins and vicelins that are mainly located in the cotyledons of seeds, prolamines are found in the endosperm. The first detailed studies on plant storage proteins were carried through by Obsorne. Legumins occur in the seeds of many dicot families and that a compound similar to legumin is also produced in monocots. These molecules are usually polymer. They are typically constructed from two subunits, the acidic and the basic polypeptides. The quaternary structure is composed of six acidic and six basic polypeptides that are linked by disulfide bonds. The accumulation of seed storage protein is beneficial for the survival of plants and also an important source of dietary plant proteins
Seed proteins are generally incomplete in nutrition due to their deficiency in several essential amino acids, for example, lysine and tryptophan in cereals and methionine and cysteine in legumes. Attempts to breed crops with increased levels of lysine and methionine have been less than satisfactory. Modern biotechnology offers alternative approaches for rectifying this nutrition deficiency. Several transgenic strategies aimed at modifying the amino acid composition of plant proteins and enhancing the content of specific essential amino acid for nutrition improvement have been developed which include synthetic proteins, modification of protein sequences, over-expression of heterologous or homologous proteins, and metabolic engineering of the free essential amino acid pool and protein sink.
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