Starch is the most important source of calories on the planet and a vital storage compound in plants. Despite its importance , we do not fully understand how starch is synthesized, how starch synthesis is initiated and what controls starch structure. Many genes in the starch biosynthesis pathway have been isolated and multiple forms of starch synthase and branching enzyme have been identified. For example, five starch synthase genes and three branching enzyme genes have been cloned from maize. To fully illustrate the mechanism of starch biosynthesis, we need to understand the functions of individual enzyme as well as the concerted actions of multiple forms of enzymes in starch synthesis. Since maize is the number one supply of starch for food and non-food industries and also a good source for genetic and biochemical studies, here we will use maize as a model plant to discuss the mechanism of starch biosynthesis, particularly the initiation of starch synthesis, the functions and interaction of multiple isozymes of starch synthase and branching enzyme.

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