ETHYLENE BIOSYNTHESIS AND CELL-WALL DIGESTION IN CITRUS PEEL (OLIGOSACCHARIDE, ELICITOR, POLYGALACTURONASE)

Increased ethylene production is often associated with pathogens that cause cell wall dissolution. Cell wall lysing enzymes are also associated with ethylene production, softening during ripening of certain fruits, or cell wall lysis during abscission of plant parts. Control over such events through the manipulation of ethylene would be of benefit to the horticultural industry.;Sugar fragments are known to induce phytoalexin production in different plant tissues. Pectic fragments, released from citrus pectin by acid hydrolysis or pectolyase digestion, induced ethylene production when injected into citrus peel. Oligosaccharides of around nine to ten sugar units were found to be the most potent elicitors of ethylene synthesis, although a range of fragment sizes induced more ethylene than the summed ethylene production of individual fragments. This suggests synergism between the individual fragments in terms of ethylene production. Such elicitor fragments would be useful for manipulation of ethylene to promote such events as degreening of citrus, abscission of citrus fruits, and possibly resistance against pathogen invasion.;Ethylene was produced by orange peel in response to injections of solutions containing pectolyase (obtained from Aspergillus japonicus) as well as some other commercial cell wall lysing enzyme mixtures. These cell wall lysing enzyme mixtures showed a complex protein profile when analyzed by SDS-PAGE. Polygalacturonase (PG), isolated from citrus peel that had been infected by the citrus sour rot fungus, Goetrichum candidum, and a similar enzyme isolated from the pectolyase mixture, caused ethylene to be produced when injected into orange peel tissue. Neither pectolyase nor fungal PG induced ethylene production when applied to orange callus tissue. Transmission electron microscopy revealed middle lamellae dissolution and SDS gels showed differences in protein profiles in orange peel treated with pectolyase when compared to controls.