Isoflavonoid phytoalexin pisatin is not recognized by the flavonoid receptor NodD ofRhizobium leguminosarum bv.viciœ

Isoflavonoid (+)-pisatin, the main phytoalexin of pea (Pisum sativum), was tested for its inducer and suppressor activity in respect of thenodABCIJ operon ofRhizobium leguminosarum bv.viciœ, a pea symbiont, in the presence of a flavonoid receptor-coding genenodD of this biovar. Nonod gene-inducing activity of pisatin could be detected up to 100 μmol/L concentration. The suppressor activity as determined against 0.1 μmol/L naringenin was expressed only above 10 μmol/L pisatin,i.e. at a concentration higher by a one order of magnitude, than with the potent flavonoid suppressor rhamnetin and by two orders above the isoflavonoid, suppressor daidzein. Analogously, only a weak suppression was observed with NodD1 ofR. meliloti, the alfalfa symbiont. The results do not support the notion about the role of plant defence substances in the control of rhizobial symbiosis.