Polymorphism of b-glucosidase isozymes Glu1 and Glu2 in maize (Zea mays L.)
--Shahid, M, Esen, A

Maize (Zea mays L.) b-glucosidase has two isoforms (Glu1 and Glu2). Glu1 is found in all parts of 15 day-old maize plants except in leaves, where the second isoform Glu2 is exclusively present. Glu1 is considered to be the most polymorphic enzyme, having, at least, 28 allozymes (Stuber and Goodman, Crop. Sci. 12:737-740, 1982). To investigate whether or not the Glu2 isozyme is also polymorphic like Glu1, seven maize inbreds, each homozygous for a different Glu1 allozyme, were analyzed for their Glu2 allozyme profile.

For the visualization of Glu1 allozymes, mesocotyls of 15-day-old light-grown seedlings were used, while the leaves (blades) of the same seedlings were used for the Glu2 allozyme studies. The plant parts were harvested on ice and immediately frozen and stored at -70 C until use. Mesocotyls and leaf blades were ground to a powder in a chilled mortar and extracted with 50 mM HEPES buffer, pH 7.0/14 mM 2-ME at a ratio of 1 g plant material to 3 ml buffer. The homogenate was kept on ice for 1h and then centrifuged at 14,000 rpm at 4 C for 30 min in a Beckman microcentrifuge. The supernatant was loaded onto a 6% native gel for electrophoresis and b-glucosidase zymogram development.

Figure 1 A shows the position of different Glu1 allozymes in a 6% native gel. It is obvious from the zymogram that Glu1-1 is the fastest allozyme, while Glu1-10 is the slowest and the other allozymes are in between them. The zymogram of Glu2 (Figure 1 B) indicates that this isozyme does not have any allozymic variation in these inbreds. The lack of allozymes of the Glu2 isozyme in maize suggests that mutations at the glu2 gene may be deleterious. This does not seem to be the case for the Glu1 isozyme which has about 28 different allozymes. The physiological and adaptive significance of extensive polymorphism at the glu1 gene and lack of polymorphism at the glu2 gene remains to elucidated.

Figure 1. A. Differential mobility of Glu1 allozymes in 6% native gel. The enzyme extracts were made from the mesocotyls of 15 day-old seedlings. B. Mobility of Glu2 allozymes in 6% native gel. The enzyme extracts were made from the leaves of 15 day-old seedlings. 1, OH7B (Glu1-1); 2, H25 (Glu1-2); 3, CO109 (Glu1-3); 4, F44 (Glu1-6); 5, K55 (Glu1-7); 6, CI44 (Glu1-9); and 7, W182 (Glu1-10). b-glucosidase activity was detected by incubating the gel in 1 mM solution of 4-methylumbelliferyl b-D-glucopyranoside.

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