Several biotic and abiotic elicitors act synergistically in the induction of phytoalexin accumulation in soybean

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Abstract
Plants often respond to microbial infection by producing antimicrobial compounds called phytoalexins. Plants also produce phytoalexins in response to in vitro treatment with molecules called elicitors. Specific elicitors, including a hexa-β-glucosyl glucitol derived from fungal cell walls, the pectin-degrading enzyme endopolygalacturonic acid lyase, and oligogalacturonides obtained by either partial acid hydrolysis or enzymatic degradation of plant cell walls or citrus polygalacturonic acid, induce soybean (Glycine max. L.) cytoledons to accumulate phytoalexins. The experiments reported here demonstrate that the elicitor-active hexa-β-glucosyl glucitol acts synergistically with several biotic and abiotic elicitors in the induction of phytoalexins in soybean cotyledons. At concentrations below 50 ng/ml, the hexa-β-glucosyl glucitol does not induce significant phytoalexin accumulation. When assayed in combination with either endopolygalacturonic acid lyase or with a decagalacturonide released from citrus polygalacturonic acid by this lyase, however, the observed elicitor activity of the hexa-β-glucosyl glucitol is as much as 35-fold higher than the sum of the responses of these elicitors assayed separately. A similar synergism was also demonstrated for the combination of the hexa-β-glucosyl glucitol with dilute solutions of sodium acetate, sodium formate, or sodium propionate buffers. These buffers are thought to damage or kill plant cells, which may cause the release of oligogalacturonides from the plant cell wall. The results suggest that oligogalacturonides act as signals of tissue damage and, as such, can enhance the response of plant tissues to other elicitor-active molecules during the initiation of phytoalexin accumulation.