The bulk of the endosperm is composed of cells with high starch contents and a little zein protein. The starch is composed of two types of molecules, amylose and amylopectin. Amylose consist of unbranched strings of glucose molecules whereas amylopectin molecules are branched strings of glucose units. This molecular structural difference affects the processed food uses of corn starch. Amylose molecules dissolve easier in hot water and do not form a paste whereas amylopectin tends to gel easier. Amylose molecules stain blue in iodine but the amylose molecules do not stain in iodine. Endosperm of most corn varieties will stain blue because of the presence of amylose, although it may only be 25% of the total starch in the endosperm.
The recessive mutant gene waxy (wx) inhibits the production of amylose molecules. This results in production of total amylopectin starch. It is easily identified by the iodine reaction. Being recessive, the gene must be present in both the female and male parent of the hybrid seed, requiring isolation from contamination during increases of parent seed, hybrid seed and grain production. Recessive gene amylose extender (ae) greatly increases the amylose portion of the endosperm starch. This also requires isolation in all steps from seed to grain production to maintain the highest amylose content. Both types of starch affect the final use of the corn grain from animal digestibility to industrial processing. The enzyme amylase will digest amylose and the straight chain component of amylopectin molecules but not the branches.
Three sweet corn genes (sugary, sugary-enhanced, shrunken-2) inhibit or delay the formation of starch in the endosperm. They are used alone and in combination for sweet corn varieties.
The main protein in endosperm cells is zein. It is synthesized in the endoplasmic reticulum of the endosperm cells as amino acids are connected into large molecules. Hardness of the kernel is linked to structure, amount and shape of these molecules. Many mutations have been identified that affect the amino acid content of the zein protein. Recessive gene opague-2(o2) for high-lysine amino acid inclusion in the zein is useful to provide an essential amino acid for animal feed. Unfortunately, it detracts from the hardness of grain. Another mutant called floury-2 (fl2) increases the methionine amino acid component of zein. Both of these genes are recessive and thus requires isolation for purity. Both of these genes do affect kernel hardness and thus can affect processing of grain.
Humans have made corn selections for genes that fit their uses and cultures for a long time. Selections have included endosperm genetics.
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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.