Section 6
Chapter 5,100

Degradation of peach pectic substances by rhizopus stolonifer 2. in vivo mechanism

Nguyen-The, C.; Souty, M.; Breuils, L.

Sciences des Aliments 5(4): 639-652


ISSN/ISBN: 0240-8813
Accession: 005099677

Previous work (NGUYEN-THE et al., 1985) had shown that infection of peaches by Rhizopus stolonifer causes a rapid degradation of fruit protopectin into soluble pectin. Breakdown of the host pectin by the fungus is likely to play an important role in R. stolonifer - peach interaction, since resistance of unripe peach is related to a resistance of their cell-wall to the enzymes of the pathogen (NGUYEN-THE et al., 1985). In order to study the mechanism of protopectin degradation, two main pectic fractions (protopectin and soluble pectin) were extracted from sound and infected fruit, by the method described in SOUTY et al. (1981). Each fraction was analyzed by gel-filtration chromatography (on Sepharose-CL-2B) and ion-exchange chromatography (on DEAE-Sepharose-CL-6B). Gel-filtration column was equilibrated with 0.2 M sodium acetate buffer pH 4 and eluted with the same buffer. Ion-exchange chromatography column was equilibrated with 0.05 M sodium acetate buffer pH 4.8 and elution was carried out by a step gradient of 0.05 M, 0.15 M, 0.25 M, 0.35 M, 0.05 M and 1 M sodium acetate buffer pH 4.8. Results show that R. stolonifer attacks rhamnogalacturonan parts of pectic substances, releasing small molecules eluted at the total volume of the gel-filtration column (figures 1a and 2a) while molecular weight of neutral sugars polymers remain important. Linkage between rhamnogalacturonan and neutral sugar side chains of pectic substances is not cut by the fungus, since infection does not lead to an increase of free neutral sugars (eluted by the first buffer in ion-exchange chromatography, figure 2b). It is concluded that polygalacturonases secreted by the pathogen in infected fruits (NGUYEN-THE, 1985) are the main role in pectic substances degradation. Enzymes would attach homogalacturonan parts of pectin, and branched parts would remain undamaged.

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