Isoflavonoid phytoalexin pisatin is not recognized by the flavonoid receptor NodD of Rhizobium leguminosarum bv. viciae

Isoflavonoid (+)-pisatin, the main phytoalexin of pea (Pisum sativum), was tested for its inducer and suppressor activity in respect of the nodABCIJ operon of Rhizobium leguminosarum bv. viceae, a pea symbiont, in the presence of a flavonoid receptor-coding gene nodD of this biovar. No nod gene-inducing activity of pisatin could be detected up to 100 micro mol/L concentration. The suppressor activity as determined against 0.1 micro mol/L naringenin was expressed only above 10 micro mol/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 of R. meliloti, the alfalfa symbiont.