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Starch serves as the main storage carbohydrate to support various cellular activities throughout various stages of plant development. In germinating seedlings, starch synthesis in the columella cells of the root cap plays an important role in sensing gravity. Gravity sensing begins with the biosynthesis of a highly dense form of starch and its accumulation in the amyloplasts of the columella cells. These starch-filled amyloplasts are called statoliths; accordingly, the specialized columella cells are called statocytes. The statocytes located in the center of the root cap act as a primary site of gravity sensing (Sack, 1991; Blancaflor et al., 1998). In addition, starch biosynthesis in anthers plays an important role in pollen and grain/seed development (Qu et al., 2021).
Starch is synthesized in plastids, chloroplasts in photosynthetic tissues, and amyloplasts in non-photosynthetic tissues). Typically, glucose-1-phosphate (G1P) or glucose-6-phosphate (G6P) can serve as the starting point of starch biosynthesis in plastids/amyloplasts. Starch granules are made of amylose (linear a-1,4-polyglucans) and amylopectin (a-1,6-branched polyglucans). Amylose contains up to several thousand ?-glucosyl units linked almost exclusively in ?(1->4) linkage with very few branches of ?(1->6) linkage. Amylose accounts for 30% of starch in rice grain. Amylopectin is a branched molecule and contains up to several million glucosyl residues. Amylopectin accounts for 70% of starch in rice grain. The specific details of each reaction presented in this pathway are provided in reaction summaries with citations.
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