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PLANT PHYSIOLOGY , Vol 115, Issue 2 717-725, Copyright © 1997 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Effect of Antisense Suppression of Endopolygalacturonase Activity on Polyuronide Molecular Weight in Ripening Tomato Fruit and in Fruit Homogenates
D. A. Brummell and J. M. Labavitch
Pomology Department, University of California, Davis, California 95616
Fruit of tomato (Lycopersicon esculentum Mill.) in which
endopolygalacturonase (PG) activity had been suppressed to <1% of
wild-type levels were slightly firmer than nontransgenic controls later in
ripening. Enzymically inactive cell walls were prepared from these ripening
fruit using Tris-buffered phenol. When extracted with chelator followed by
Na2CO3, the amounts of pectin solubilized from cell walls of nontransgenic
control or from transgenic antisense PG fruit were similar. Size-exclusion
chromatography analysis showed that, relative to controls, in antisense PG
fruit polyuronide depolymerization was delayed in the chelator-soluble
fraction throughout ripening and reduced in the Na2CO3-soluble fraction at
the overripe stage. Reduced pectin depolymerization rather than altered
extractability thus may have contributed to enhanced fruit firmness.
Substantially larger effects of suppressed PG activity were detected in
tomato fruit homogenates processed to paste. In control paste the majority
of the polyuronide was readily soluble in water and was very highly
depolymerized. In antisense PG paste the proportion of polyuronide
solubilized by water was reduced, and polyuronides retained a high degree
of polymerization. The suppression of fruit PG activity thus has a small
effect on polyuronide depolymerization in the fruit but a much larger
effect in paste derived from these fruit. This indicates that in the cell
wall PG-mediated degradation of polyuronide is normally restricted but that
in tissue homogenates or in isolated cell walls this restriction is removed
and extensive pectin disassembly results unless PG is inactivated.
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