PLANT PHYSIOLOGY , Vol 105, Issue 4 1189-1195, Copyright © 1994 by American Society of Plant Biologists
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MOLECULAR BIOLOGY AND GENE REGULATION |
Differential Expression of the Two Subunits of Tomato Polygalacturonase Isoenzyme 1 in Wild-Type and rin Tomato Fruit
L. Zheng, C. F. Watson and D. DellaPenna
Department of Plant Sciences, University of Tucson, Arizona 85721
The [beta] subunit of tomato (Lycopersicon esculentum Mill.) fruit
polygalacturonase 1 is a cell wall glycoprotein that binds to and
apparently regulates the catalytic PG2 polypeptide in vivo. [beta] Subunit
and polygalacturonase 2 (PG2) expression have been investigated in both
wild-type and ripening inhibitor (rin) mutant fruit. During fruit
development and ripening, [beta] subunit expression was unrelated to
expression of the catalytic PG2 protein. In wild-type fruit, [beta] subunit
mRNA and protein were first detected early in development and increased to
maximal levels before PG2 mRNA and protein were detected. At the onset of
ripening [beta] subunit mRNA decreased dramatically, but [beta] subunit
protein levels remained stable. In rin fruit, which fail to ripen, [beta]
subunit expression was similar to that in wild type, although PG2 mRNA and
protein were not detected. These data suggest that [beta] subunit
expression is ethylene independent and regulated primarily by developmental
cues. This conclusion is supported by results from ethylene-treated
immature (20 days after pollination) wild-type and rin fruit in which no
significant differences were observed in [beta] subunit expression patterns
in response to ethylene treatment. Surprisingly, RNA blot analysis
indicated that catalytic PG2 mRNA was induced in immature rin fruit after 3
d of exogenous ethylene treatment. In addition, [beta] subunit mRNA and
protein were also detected at lower levels in root, leaf, and flower
tissues of both genotypes, suggesting a broader functional role for the
protein.
