Plant Molecular Biology 42: 317–328, 2000.
© 2000 Kluwer Academic Publishers. Printed in the Netherlands.
Polygalacturonase gene expression in kiwifruit: relationship to fruit
softening and ethylene production
Zhong-Yan Wang, Elspeth A. MacRae, Michele A. Wright, Karen M. Bolitho, Gavin S. Ross
and Ross G. Atkinson
HortResearch, Mt Albert Research Centre, Private Bag 92169, Auckland, New Zealand (
author for correspon-
Received 27 May 1999; accepted in revised form 20 October 1999
Key words: Actinidia chinensis, fruit, kiwifruit, polygalacturonase, ripening
In kiwifruit, much of the softening process occurs prior to the respiratory climacteric and production of ethylene.
This fruit therefore represents an excellent model system for dissecting the process of softening in the absence
of endogenous ethylene production. We have characterized the expression of three polygalacturonase(PG) cDNA
clones (CkPGA, B and C) isolated from fruit of Actinidia chinensis. Expression of CkPGA and B was detected by
northern analysis only in fruit producing endogenous ethylene, and by RT-PCR in other tissues including ﬂower
buds, petals at anthesis, and senescent petals. CkPGA promoter fragments of 1296, 860 and 467 bp fused to the
β-glucuronidase (uidA) reporter gene directed fruit-speciﬁc gene expression during the climacteric in transgenic
tomato. CkPGC gene expression was observed in softening fruit, and reached maximum levels (50-fold higher
than for CkPGA and B) as fruit passed through the climacteric. However, expression of this gene was also readily
detected during fruit development and in fruit harvested prior to the onset of softening. Using RT-PCR, expression
of CkPGC was also detected at low levels in root tips and in senescent petals. These results suggest that PG
expression is requirednotonlyduringperiodsof cell walldegeneration,butalso duringperiodsof cell wallturnover
Fruit ripening is a complex, genetically programmed
processinvolving increases in respiration and ethylene
production,changes in color and ﬂavor, and softening.
Softening is associated with structural changes in the
cell wall including reduction in the size of hemicellu-
lose, loss of galactose sidechains, and solubilization
and depolymerization of pectin (reviewed by Fischer
and Bennett, 1991). Polygalacturonase (PG) is one of
the enzymes involved in this process.
The role of PG in fruit ripening has been stud-
ied most extensively in tomato (reviewed by Hadﬁeld
The nucleotide sequence data reported will appear in the EMBL,
GenBank and DDBJ Nucleotide Sequence Databases under the
accession numbers AF152752 (CkPGA-1), AF152753 (CkPGA-
2), AF152758 (CkPGA-3), AF152754 (CkPGB-1), AF152755
(CkPGB-2), AF152756 (CkPGC-1) and AF152757 (CkPGC-2).
and Bennett, 1998). PG gene expression is initiated at
breaker stage when endogenous ethylene is ﬁrst pro-
duced and increases to >1% of mRNA in ripe fruit
(DellaPenna et al., 1987). PG activity levels have been
shown to roughly correlate with the degree of soft-
ening among different tomato cultivars and mutants
(Speirs et al., 1989). However, using a transgenic
approach, PG activity alone has been shown not to
be necessary or sufﬁcient for fruit softening (Smith
et al., 1988; Giovannoni et al., 1989). A recent re-
examination has shown that PG mRNA accumulation
is ethylene-regulated, and that the low threshold con-
centration of ethylene required for induction appears
to be exceeded even in transgenic antisense ACC
synthase tomato fruit (Sitrit and Bennett, 1998).
PG gene expression has also been studied in the
fruit of a number of other species including peach