Total catalase (H₂0₂:H₂0₂ oxidoreductase, EC 1.11.1.6) activity in maize scutellum increases dramatically during the first three or four days of germination and decreases gradually thereafter (Scandalios, J. Hered. 65:28-32, 1974). A similar pattern of scutellar catalase development has been observed for excised intact embryos (embryonic axis attached to scutellum) cultivated on solid nutrient medium in the dark. The influence of exogenously applied plant hormone--i.e., indoleacetic acid (IAA), gibberellic acid (GA3), benzyladenine (BA) and abscisic acid (ABA)--on the developmental pattern of catalase were studied in an attempt to establish any relationship between the action of these hormones and those regulatory mechanisms already known to affect the expression of catalase in maize scutellum (Scandalios, Isozymes, Vol. 111, 213-238, ed. C. L. Markert, 1975). Plant hormones were applied by incubating intact embryos of maize inbred line W64A, on solid nutrient medium containing different hormones at 10 µM. The developmental profile of catalase activity in intact embryos following the application of ABA, BA, GA₃, or IAA is shown in Figure 1. Although treatment with GA₃ or BA does not cause a significant change in the developmental pattern of catalase activity, IAA promotes both the initial rise in and the retention of catalase activity. Application of ABA to intact embryos delays both the rise and subsequent fall in catalase activity in scutella during early germination.

In addition, ABA alters the developmental pattern of catalase isozymes in scutella (Fig. 2). The expression of the Cat1 gene product (isozyme V) is prolonged after ABA treatment. On the other hand, the expression of the Cat2 gene product (isozyme Z) is delayed by ABA treatment. These data suggest that ABA may be involved in regulating the differential expression of catalase genes in maize scutellum.

It has been previously shown that catalase is turning over in the maize scutellum during germination (Quail and Scandalios, Proc. Nat. Acad. Sci. USA, 68:1402-1406, 1971). Data from density-labeling experiments indicate that catalase is synthesized de novo in the scutella of ABA-treated intact embryos. Comparing the density shift and the band width at the half-height peak of catalase distribution in CsCl gradients of ABA-treated scutella with those of the untreated control, it is suggested that ABA may be involved in regulating the rate of synthesis and the rate of degradation of catalase in maize scutella during early germination. Therefore, the turnover rates of a specific catalase gene product in response to the treatment of ABA are being investigated by density-labeling techniques and using the porphyrinogenic drug 2-allyl-2-isopropylacetamide (AIA) to inhibit catalase synthesis in an attempt to further evaluate mechanisms of differential gene expression.

Figure 1.

Figure 2.

Ray Whay Yen and John G. Scandalios

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