THE FLAVONOID NARINGENIN STIMULATES THE INTERCELLULAR COLONIZATION OFWHEAT ROOTS BY AZORHIZOBIUM-CAULINODANS

We have studied intercellular colonization of wheat roots by Azorhizob ium caulinodans and other diazotrophic bacteria, using strains marked with the lacZ reporter gene to facilitate their detection and identifi cation. A. caulinodans was observed by light and electron microscopy t o enter the roots of wheat at high frequency at the points of emergenc e of lateral roots (lateral root cracks). After lateral root crack col onization, bacteria moved into intercellular spaces within the cortica l cell layer of roots. The flavonoid naringenin at 10 and 100 mmol m(- 3) significantly stimulated root colonization. The roles of the struct ural nodABC genes and the regulatory nodD gene were also studied; late ral root crack colonization of wheat was shown to be Nod factor- and N odD-independent. Similar frequencies of lateral root crack colonizatio n were observed following inoculation of wheat with Azospirillum brasi lense. Colonization by A. brasilense was stimulated by naringenin and also by other flavonoid molecules.