THE PLANT CELL, Vol 6, Issue 11 1623-1634, Copyright © 1994 by American Society of Plant Biologists
Reduction of Tomato Polygalacturonase [beta] Subunit Expression Affects Pectin Solubilization and Degradation during Fruit Ripening
C. F. Watson, L. Zheng and D. DellaPenna
Department of Plant Sciences, Forbes Hall, University of Arizona, Tucson, Arizona 85721
The developmental changes that accompany tomato fruit ripening include
increased solubilization and depolymerization of pectins due to the action
of polygalacturonase (PG). Two PG isoenzymes can be extracted from ripe
fruit: PG2, which is a single catalytic PG polypeptide, and PG1, which is
composed of PG2 tightly associated with a second noncatalytic protein, the
[beta] subunit. Previous studies have correlated ripening-associated
increases in pectin solubilization and depolymerization with the presence
of extractable PG1 activity, prior to the appearance of PG2, suggesting a
functional role for the [beta] subunit and PG1 in pectin metabolism. To
assess the function of the [beta] subunit, we produced and characterized
transgenic tomatoes constitutively expressing a [beta] subunit antisense
gene. Fruit from antisense lines had greatly reduced levels of [beta]
subunit mRNA and protein and accumulated <1% of their total extractable
PG activity in ripe fruit as PG1, as compared with 25% for wild type.
Inhibition of [beta] subunit expression resulted in significantly elevated
levels of EDTA-soluble polyuronides at all stages of fruit ripening and a
significantly higher degree of depolymerization at later ripening stages.
Decreased [beta] subunit protein and extractable PG1 enzyme activity and
increased pectin solubility and depolymerization all cosegregated with the
[beta] subunit antisense transgene in T2 progeny. These results indicate
(1) that PG2 is responsible for pectin solubilization and depolymerization
in vivo and (2) that the [beta] subunit protein is not required for PG2
activity in vivo but (3) does play a significant role in regulating pectin
metabolism in wild-type fruit by limiting the extent of pectin
solubilization and depolymerization that can occur during ripening. Whether
this occurs by direct interaction of the [beta] subunit with PG2 or
indirectly by interaction of the [beta] subunit with the pectic substrate
remains to be determined.
