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. viciae, a pea symbiont, in the presence of a flavonoid receptor-coding gene nodD of this biov ar. No nod gene-inducing activity of pisatin could be detected up to 1 00 mu mol/L concentration. The suppressor activity as determined again st 0.1 mu mol/L naringenin was expressed only above 10 mu mol/L pisati n, i.e. at a concentration higher by a one order of magnitude than wit h the potent flavonoid suppressor rhamnetin and by two orders above th e isoflavonoid suppressor daidzein. Analogously, only a weak suppressi on was observed with NodD1 of R. meliloti, the alfalfa symbiont. The r esults do not support the notion about the role of plant defence subst ances in the control of rhizobial symbiosis.