The peroxidase system in maize is characterized by a large number of isozymes whose expression depends both on tissue and developmental stage of the plant. Thirteen zones of peroxidase activity are recognized electrophoretically, and nine genetic loci which control the inheritance of individual peroxidase isozymes have been described (Brewbaker and Hamill-Johnson, 1972, MCGN 46:29-33; Brewbaker and Hasegawa, 1974, MCGN 48:35-37). The number of genes responsible for the coding of peroxidase isozymes, all basically heme containing glycoproteins, suggests that although these multiple enzyme forms are closely related evolutionarily they may have distinct physiological functions.
Px7 is a major form of peroxidase in many maize tissues, including coleoptile, leaf, husk, silk, pericarp, and roots. It is absent in embryo, endosperm, pollen, anther and tassel initial. In leaf tissue Px7 accounts for about one-third of the total peroxidase activity. In silk, Px7 is the predominant form of the enzyme, and accounts for over half the total peroxidase in this tissue.
The migration of Px7 in an electric field is quite slow. In normal electrophoretic separations, Px7 does not move out of the origin. Lowering acrylamide gel concentration to 6% at pH 8.4 and extending electrophoresis time to 40 hours results in migration of Px7 toward the anode and good resolution of allelic variants. Increasing pH to 9.1 accelerates anodal migration but resolution of variants suffers.
Two major alleles of Px7 have been observed (Table 1). The slow variant,
Px7-S, is common. A fast variant, Px7-F, has been observed in three lines
of sweet corn and Carioca 338. Backcrosses suggest these alleles are inherited
in a co-dominant manner:
|71-1285 (S/F) x 71-1258 (F/F)||14||17|
Several inbred lines have been observed which are null at the Px7 locus. These fall into two classes: those which are null in all tissues, and those where the enzyme is absent in one tissue, silk, and present in others such as leaves. Obviously the mechanisms by which nulls arise in these classes are quite different.
Px7 has a tendency to streak on acrylamide gels,
which suggests that it might be bound to some heterogeneous component.
The molecular weight of Px7, as determined by Sephadex chromatography,
is about 75,000. The molecular weights of the other maize peroxidases range
from 33,000-50,000. Although Px7 is the largest of the maize peroxidase
isozymes, it is retained within the pore structure of the molecular sieve
gels Sephadex G-100 and Sephadex G-200:
|Gel Pore Type||Exclusion MW||Elution Volume/Void Volume|
This indicates that it is not attached to large cellular structures such as cell walls or membranes. The F1 heterozygotes in crosses between Px7-F and Px7-S lines have only two bands which co-migrate with each of the parental alleles. The absence of a third intermediate heteromultimer band suggests that Px7 despite its large molecular weight behaves as a monomer.
Edwin H. Liu, Chifumi Nagai, and James L. Brewbaker
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