Journal of Experimental Botany, Vol 50, 767-775, Copyright © 1999 by Oxford University Press
L Chin, Z Ali and H Lazan
The fruit extracts of the slow-ripening cv. B 10 carambola
(Averrhoa carambola L.) contained a number of cell
wall hydrolases. The predominant ones appeared to be
ARTICLES
Cell wall modifications, degrading enzymes and softening of carambola fruit during ripening
Plant Molecular Physiology Group, Department of Botany, Faculty of Life Science, University Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia; Plant Molecular Physiology Group, Department of Biochemistry, Faculty of Life Science, University Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia; Corresponding author; Fax: +60 3 8293244; E-mail: hlazan@pkrisc.cc.ukm.my
-(1,4)-glucanase (as carboxymethylcellulase),
pectinesterase, -galactosidase, and
polygalacturonase (PG). Other significant hydrolases the activity of which
also increased with ripening were the glycosidases, 
-arabinosidase, -galactosidase, and 
-mannosidase, and
also the glycanases, 
-(1,4)-galactanase and xylanase. Throughout
ripening, as pectins and hemicelluloses were being differentially modified,
the levels of buffered-phenol cell wall materials, total polyuronides as
well as arabinose, galactose, xylose, and glucose decreased. At early
ripening phase (days 0-12) there was no apparent pectin solubilization, and
the loosely-bound water- and chelator-soluble pectins were the first pectic
polysaccharides to be affected. That of the former exhibited an upshift in
their molecular size profiles. At late ripening phase (days 12-24) when
tissue firmness had declined substantially, dramatic changes involving
pectins and hemicelluloses were evident. Pectins were solubilized, and this
increased solubility was accompanied by depolymerization of all pectin
classes and a decrease in the level of the Na2CO3-soluble polyuronides.
Coincident with this marked modifications of the tightly-bound, predominant
polyuronide fractions and hemicellulose was the increase in activities of
polygalacturonase and -(1,4)-glucanase, suggesting that these
enzymes may contribute to wall modifications late during ripening. Some of
the other wall hydrolases, namely, -arabinosidase,
-galactosidase and certain isoforms of
-galactosidase/galactanase, might be relevant to the early
ripening changes when pectin solubilization was limited.Keywords:
Averrhoa carambola L, cell wall
carbohydrates, ripening, softening, wall degrading enzymes.
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