Polygalacturonase-Mediated Solubilization and Depolymerization of
Pectic Polymers in
Tomato Fruit Cell Walls1
Regulation by pH and Ionic Conditions
Jong-Pil Chun and
Donald J. Huber*
Horticultural Sciences Department, Institute for Food and
Agricultural Sciences, P.O. Box 110690, University of Florida,
Gainesville, Florida 32611
The hydrolysis of cell wall pectins
by tomato (Lycopersicon esculentum) polygalacturonase
(PG) in vitro is more extensive than the degradation affecting these
polymers during ripening. We examined the hydrolysis of
polygalacturonic acid and cell walls by PG isozyme 2 (PG2) under
conditions widely adopted in the literature (pH 4.5 and containing
Na+) and under conditions approximating the apoplastic
environment of tomato fruit (pH 6.0 and K+ as the
predominate cation). The pH optima for PG2 in the presence of
K+ were 1.5 and 0.5 units higher for the hydrolysis of
polygalacturonic acid and cell walls, respectively, compared with
activity in the presence of Na+. Increasing K+
concentration stimulated pectin solubilization at pH 4.5 but had little
influence at pH 6.0. Pectin depolymerization by PG2 was extensive at pH
values from 4.0 to 5.0 and was further enhanced at high K+
levels. Oligomers were abundant products in in vitro reactions at pH
4.0 to 5.0, decreased sharply at pH 5.5, and were negligible at pH 6.0. EDTA stimulated PG-mediated pectin solubilization at pH 6.0 but did not
promote oligomer production. Ca2+ suppressed PG-mediated
pectin release at pH 4.5 yet had minimal influence on the proportional
recovery of oligomers. Extensive pectin breakdown in processed tomato
might be explained in part by cation- and low-pH-induced stimulation of
PG and other wall-associated enzymes.
1
This work was supported in part by U.S.
Department of Agriculture Competitive Grant no. 93-37304-9575. This is
journal series no. R06232 of the Florida Agricultural Experiment
Station.
*
Corresponding author; e-mail djh{at}gnv.ifas.ufl.edu; fax
1-352-392-6479.
Plant Physiol. (1998) 117: 1293-1299
Copyright Clearance Center: 0032-0889/98/117//07
© 1998 American Society of Plant Physiologists
