Two distinct, genetically defined catalases of Zea mays are differentially expressed in the scutellum during development of the seedling (Scandalios, Physiological Genetics, Chapter 2, 1979). These are CAT-1 and CAT-2 coded for by the Cat1 and Cat2 loci, respectively. In a typical line (e.g., W64A), CAT-1 is expressed in the dry seed and early days post-imbibition. CAT-2 production becomes evident on days 2 to 3 post-imbibition, when a 5-banded zymogram pattern is observed (Figure 1a). As development of the seedling proceeds, CAT-1 activity declines while CAT-2 activity increases. This results in a "shift" in the zymogram pattern from CAT-1 expression to CAT-2 expression (Figure 1a).
A naturally occurring mutant line (Tx303) has recently been characterized which exhibits a more rapid shift from CAT-1 to CAT-2 expression than is commonly observed in most lines. By day 4 post-imbibition, only Cat2 is expressed on the zymograms (Figure 1b). It has been found that this mutant line does not differ significantly from W64A for various parameters related to CAT-2 expression. For instance, the developmental activity profile for catalase, primarily controlled by the production of CAT-2 gene product, does not differ between the two lines. CAT-2 protein developmental profiles (determined by rocket immunoelectrophoresis) do not differ between the two lines. Also, the developmental profiles of two glyoxysomal associated enzymes (malate synthase and isocitric lyase) do not differ between the two lines. The glyoxysomes were investigated because maize catalase is known to be associated with these organelles (Scandalios, J. Hered. 65:28, 1974). These data suggest that the regulation of the levels of CAT-2 protein in the scutellum of the two lines is similar. Therefore, the more rapid shift observed in the mutant line is probably due to a regulatory effect on the levels of CAT-1 protein likely by a more rapid degradation of CAT-1 protein.
Preliminary genetic analysis indicates that the trait is likely controlled by a single dominant gene. This and linkage studies are currently underway.
Joel M. Chandlee and John G. Scandalios
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