Differential elicitation of defense responses by pectic fragments in bean seedlings

The cell walls of two near-isogenic lines of bean (Phaseolus vulgaris L.) seedlings, susceptible or resistant to the bean anthracnose pathogen Colletotrichum lindemuthianum, were digested with the pure endopolygalacturonase (endoPG; EC 3.2.1.15) isolated from the fungus. The solubilized pectic fragments were separated according to their charge and size. Analysis of their uronic acid contents showed that their elution patterns were quite dissimilar, depending on whether they originated from the resistant or the susceptible host plant. Their sugar compositions revealed that neutral sugars were more abundant in the fragments released from the resistant plant than from the susceptible one, while the reverse was true for acidic residues. The fragments solubilized from the resistant plant induced an increase of pathogenesis-related (PR) proteins when challenged on resistant or susceptible bean seedlings, both at the transcript and enzyme-activity levels. On the other hand, pectic fragments released from susceptible bean cell walls exhibited either no significant activity or only a weak elicitor effect on the defence of susceptible or resistant bean seedlings. The differential elicitor effect observed between pectic fragments was inversely correlated to their acidity. Thus, endoPG-released pectic fragments from bean cell walls exhibited the same ability as the endoPG itself (C. Lafitte et al., 1993, Mol Plant-Microb Interact 6: 628-634) to elicit defence responses in a cultivar-specific manner.