Excessive storage of carbohydrates in plants used by humans for food has been selected over time by moving sugars into starch. Kernel endosperm is the main site for starch storage in corn. At least 11 major genes have been characterized in corn by the enzymes that they code (Maydica 50(2005):497-506).
The enzyme Adenosine dipbospbate glucose pyropbosphorylse (AGP) affects the rate of starch synthesis and is coded by at least two genes. This enzyme is also found in other plants, such as potatoes with the same function determining the rate the starch synthesis. The enzyme is located mostly in the liquid surrounding the cell organelles (amyloplasts) where the starch is ultimately stored. The recessive mutant of one of these two genes (shrunken2) reduces the rate of starch synthesis, resulting Supersweet corn. The dominant form of this gene (Sh2) thus was found to regulate the rate of synthesis of starch in the corn endosperm.
Other genes affecting starch synthesis in corn kernels have been found after mutants were identified. Recessive mutant form of the Sugary1 (Su1) gene reduces the branching of the starch molecules resulting in more sugars in the endosperm. This led to the conclusion that the dominant form of the gene is associated with starch synthesis.
Corn starch is composed of highly branched strings of carbohydrate molecules (amylopectin) and non-branched molecules (amylose). The recessive mutant form of the Waxy gene (wx) results in only the amylopectin form of starch in the corn endosperm. Amylose starch stains blue when exposed to iodine. A recessive version of the iodine affinity gene (ia) increases the amylose portion of starch in the endosperm. The amylose-extender gene recessive (ae) further increases the amylose portion from 25% found in most corn varieties to 50-80% of the starch in high amylose corn varieties.
Genetic diversity of corn is expressed in nearly all aspects of its production.
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About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.