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Solubilisation of tomato fruit pectins by ascorbate: a possible non-enzymic mechanism of fruit softening

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Abstract

The aim of this work was to test the hypothesis that endogenous ascorbate, released into the apoplast by membrane permeabilisation early in fruit ripening, could promote the solubilisation and depolymerisation of polysaccharides, and thus contribute to fruit softening. In vitro, ascorbate (1 mM), especially in the presence of traces of either Cu2+ or H2O2, solubilised up to 40% of the total pectin from the alcohol-insoluble residue of mature-green tomato (Lycopersicon esculentum Mill.) fruit. Solubilisation was due to the action of ascorbate-generated hydroxyl radicals (·OH), which can cause non-enzymic scission of polysaccharides. The pectins solubilised by ascorbate in vitro were polydisperse (4–1,000 kDa), partially esterified and galactose-rich. Excised pieces of living tomato fruit released ascorbate into the medium (apoplast); the ability of different tissues to do this increased in the order pericarp < placenta < locule. In all three tissues, but especially in the locule, the ability to release ascorbate increased during ripening. The Cu content of each tissue also increased during ripening, whereas neither Fe nor Mn showed a similar trend. We suggest that progressively increasing levels of Cu and ascorbate in the fruit apoplast would lead to elevated ·OH production there and thus to non-enzymic scission of pectins during ripening. Such scission could contribute to the natural softening of the fruit. De-esterified citrus pectin was more susceptible to ascorbate-induced scission in vitro than methylesterified pectin, suggesting a possible new significance for pectin methylesterase activity in fruit ripening. In conclusion, non-enzymic mechanisms of fruit softening should be considered alongside the probable roles of hydrolases, xyloglucan endotransglucosylases and expansins.

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Abbreviations

AIR:

alcohol-insoluble residue

Ara:

l-arabinose

DMSO:

dimethylsulphoxide

endo-PG:

endo-polygalacturonase

Gal:

d-galactose

GalA:

d-galacturonic acid

Glc:

d-glucose

k ·OH :

rate constant for reaction with the hydroxyl radical

K av :

elution from Sepharose column relative to the void volume (K av=0.0) and totally included volume (K av=1.0)

MG:

mature-green

PME:

pectin methylesterase

Rha:

l-rhamnose

RR:

red-ripe

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Acknowledgement

J.C.D. thanks the UK Biotechnology and Biological Sciences Research Council for the award of a studentship.

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  1. The Edinburgh Cell Wall Group, ICMB, The University of Edinburgh, Daniel Rutherford Building, The King's Buildings, Edinburgh, EH9 3JH, UK

    Jo C. Dumville & Stephen C. Fry

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  1. Jo C. Dumville
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  2. Stephen C. Fry
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Correspondence to Stephen C. Fry.

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Dumville, J.C., Fry, S.C. Solubilisation of tomato fruit pectins by ascorbate: a possible non-enzymic mechanism of fruit softening. Planta 217, 951–961 (2003). https://doi.org/10.1007/s00425-003-1061-0

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