Genetic isozyme variations in Zea mays

Abstract

In an effort to narrow the gap in knowledge between the primary action of the gene and" the phenotype (in maize), use has been made of the technique of starch gel electrophoresis in combination with various enzyme staining procedures. The first to apply these methods to plant biochemical genetic studies was Schwartz (1960), who described genetic variations in esterases in Zea mays. In the present thesis slightly modified techniques were used to describe variations in the enzymes leucine aminopeptidase (LAP), catalase, esterase, peroxidase and amylase, in maize. Four different Molecular forms (isozymes) of LAP were found in maize endosperm (zones A, B, C, and D). The LAP A and LAP D enzymes show electrophoretic variations, and each was found to be controlled by a pair of alleles acting without dominance. Linkage between the two loci has been demonstrated. Tissue studies showed that the A and B zones were distributed throughout the organism, while the C zone was common only to the embryo and endosperm. The 0 zone was found only in the endosperm. The presence of tissue-specific variants of LAP and also of esterase, peroxidase and catalase, as demonstrated in 'these studies, lend supporting evidence for the existence of multiple molecular forms of various enzymes within" the same organism and within the same tissue; a point of great significance in developmental biology. The polymorphism found in catalase is of special interest due to the formation of hybrid enzymes in the heterozygote, which have electrophoretic mobilities intermediate to those of the parental types. The number and the activity concentration of these hybrid enzymes suggested that maize catalases exist as tetramers and that the hybrid patterns could be the result of random combination of two different catalase subunits. That catalase exists functionally as a tetramer is supported by experiments in which mixtures of two variants were frozen in NaCl and were dissociated into the "presumed" monomers. On thawing, reassociation into functional tetramers occurred. Such findings suggest that the primary product of catalase genes may be the monomer and that the enzymatically active unit is an aggregate (tetramer) of such monomers. The amylases are also of special interest developmentally, since their activity appears to be confined to the early period of germination and at a time when starch degradation is expected. Preliminary immunochemical studies are suggestive of differences in antigenic specificity of the variants mentioned. The possible significance of these findings in reference to gene action, development, and evolution of isozymes is discussed.