The degradation of onion cell walls by extracellular enzymes of Botrytis allii and Sclerotium cepivorum [proceedings].

Botrytis allii and Sclerotium cepivorum, which respectively cause neck rot and white rot of onions, produce a range of cell-wall-degrading enzymes in onion tissue (Hancock et al., 1964; Beck, 1976). The two fungi have been grown on liquid media using purified onion cell walls, prepared by an adaptation of the method of Jarvis et al. (1977), as the sole carbon source for B. allii and the major carbon source for S. cepivorum. A 0-95X-satd. (NH&S04 precipitate of each culture medium was prepared at daily intervals for 10 days after inoculation and after gel filtration was examined for polysaccharide-degrading activity by determining reducing-sugar release from a range of substrates. For both fungi, the first enzyme to appear in quantity and the one that had the highest activity over a period of 10 days was polygalacturonase [poly-(I +4)-a-Dgalactosiduronate glycanohydrolase]. 1,4,/l,Xylanase, pectinase [poly(methyl Dgalactosiduronate)glycanohydrolase] and galactanase [( 1 +4)-jl-~-galactan galactanohydrolase] appeared at similar rates, although the relative amounts of the individual enzymes in the two fungi varied. Cellulase activity only became significantly high after the sixth day, and L-arabanase activity was relatively low throughout the experiment. The desalted enzyme preparations were incubated in buffer with cell walls and the release of carbohydrate into solution measured after incubation for 4 h. The release of individual sugars was followed by g.1.c. analysis after hydrolysis of both solutions and cell-wall residues. Cell-wall carbohydrate was solubilized by the enzyme preparations in increasing amounts depending on the age of the culture. The major differences between the solubilization of the wall by the two fungi was found by the enzyme preparations of day 4 and day 5, and these differences appear to be related both to the rate of release of uronic acid and the production of polygalacturonase by the two fungi. The rate of release of the other individual cell-wall sugars appears to correlate with the rate of production of the enzymes that hydrolyse their polymeric products. These studies emphasize the importance of polygalacturonase activity in the solubilization of onion cell walls by both fungi.