Plant lectins induce the production of a phytoalexin in pisum sativum

The effects of several plant lectins on the production of a pea phytoalexin, pisatin, were examined. Con A, PHA, PNA and PSA each induced the production of pisatin in pea epicotyl tissues, demonstrating that plant lectins can act as elicitors. The production of pisatin in response to PHA, PNA or PSA was not affected by the simultaneous presence of the respective hapten sugars, whereas haptens specific for Con A, such as α-D-mannose and methyl-α-D-mannoside, abolished the induction of pisatin by Con A. These results indicate that the elicitor effect of Con A is attributable to its ability to bind to specific carbohydrates in pea cells. Induction of the production of pisatin by Con A was markedly inhibited by the suppressor derived from a pea pathogen, Mycosphaerella pinodes, and by several inhibitors related to signal-transduction pathways. It is suggested, therefore, that the Con A-induced production of pisatin in pea tissues might be associated with activation of a signal-transduction pathway. An additive effect on the accumulation of pisatin was observed when Con A was present with a polysaccharide elicitor from M. pinodes, suggesting that exogenous Con A does not compete with the recognition site(s) for the fungal elicitor in pea cells. The present data also indicate that Con A may be useful for characterization of the signal-transduction system that leads to the synthesis of phytoalexin in pea epicotyl tissues.