Polygalacturonase inhibitors of Bartlett pear fruits: differential effects on Botrytis cinerea polygalacturonase isozymes, and influence on products of fungal hydrolysis of pear cell walls and on ethylene induction in cell culture

Abstract

The influence of polygalacturonase inhibitor protein (PGIP) from mature green Bartlett pears on the action of a mixture of cell wall hydrolases from Botrytis cinerea on isolated pear cell walls, and on a soluble extract of these walls, was studied. The aim of this work was to test the hypothesis of Cervone et al. that PGIP may modify cell wall degradation to result in the production of an increased proportion of pectic oligomers in the particular size class most effective for elicitation of defence responses. Products of the partially inhibited and uninhibited enzyme system from B. cinerea culture supernatant were compared as elicitors of ethylene production by pear fruit cells in suspension culture (which would indicate the presence of oligogalacturonides in the size class known to elicit plant defence responses), and on the basis of direct analytical data relating to oligomer length. Pear PGIP caused partial inhibition of the crude mixture of Botrytis enzymes, and increased the ratio of dimeric to monomeric uronide products. However, no accumulation of larger oligomeric breakdown intermediates was detected, and no impact on ethylene elicitor activity of the digestion products was observed. Differential inhibition of the B. cinerea polygalacturonase isozymes by pear PGIP was demonstrated following their separation by isoelectric focusing. The fact that some isozymes remained relatively uninhibited may have accounted for the absence of any accumulation of oligomeric breakdown intermediates.