The shrunken gene on chromosome 9 encodes the enzyme sucrose synthase (A), which is found at a high level in the endosperm of the developing maize kernel but not in the embryo. A second sucrose synthase (B) enzyme is found in both tissues (Chourey and Nelson, Genetics 14:1041, 1976).
DNA fragments of the shrunken gene cross-hybridize to a slightly larger mRNA species in polyA+-RNA from sh mutants. This transcript was interpreted to be a mRNA of the sucrose synthase B gene (McCormick et al., Mol.Gen.Genet. 187:494, 1982). We have analyzed the mRNA levels in different tissues of maize plants including kernels, shoots, roots and leaves. The genotype of the plant material used for polyA+-RNA isolation was Sh, or homozygous for the sh bz-m4 allele, in which the shrunken gene is deleted. Under the assumption that the slightly larger mRNA encodes the enzyme sucrose synthase B, we find identical low amounts in all tissues examined from sh bz-m4 plants. In polyA+-RNA from Sh plants, we find hybridizing mRNA not only in endosperm but also in roots and shoots of germinating maize kernels at high levels. In roots and shoots we find 10 times less hybridizing polyA+-RNA than in endosperm 20 days after pollination.
Interestingly, the level of mRNA in roots and shoots increases ca. 20 times upon anaerobic stress for 19 hours. In leaves the mRNA for sucrose synthase A is 1/100 of the amount found in endosperm at 20 days. The level is comparable to the expression of the sucrose synthase B gene. We conclude that the gene is regulated during development of the maize plant at the transcriptional level and can respond to environmental signals. In all tissues examined we find a protein which can be precipitated by sucrose synthase antiserum and has similar migration properties in acrylamide gels as those of sucrose synthase.
B. Springer, W. Werr, and P. Starlinger
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