The role of pectinase enzyme in the development of soft rot caused by Pseudomonas fluorescens in the purple variety of onions (Allium cepa)

Abstract

Pectic enzyme production was induced in vitro by culturing the pathogen (P flourescens) in minimal salt medium
containing citrus pectin as the only carbon source and in vivo by inoculating the pathogen into healthy onion bulbs.
The spent broth and onion rot extract were obtained and enzyme detection was done using viscometric method and
cup plate assay. The crude enzyme was partially purified by precipitating with ammonium sulphate and dialyzed
against water at 4o C for 24 h before use. The activity of the purified enzyme was determined based on the macerating
ability on Onion and Potato discs and later by estimation of reducing sugars (galacturonic acid) level. The type of
cleavage reaction of the enzyme was determined using the thiobarbituric acid reaction and the presence of
antimicrobial substance in the rot tissue was also investigated. The pathogen produced a hydrolytic polygalacturonase in vitro and in vivo. The purified enzyme showed appreciable activity with all the assay methods used. Activity
was highest at pH 5 in the culture filtrate sample and at pH 4 in the rot extract. Maceration was observed in potato and
onion discs. The detection of increasing concentration of polyphenols in the rot tissue during rot development
demonstrated the plant’s response to the pathogen