RHIZOBIUM INOCULATION INDUCES CONDITION-DEPENDENT CHANGES IN THE FLAVONOID COMPOSITION OF ROOT EXUDATES FROM TRIFOLIUM-SUBTERRANEUM

Rapid induction of chalcone synthase (predominantly CHS5) gene express ion occurs within 6 h following the inoculation of Rhizobium leguminos arum by. trifolii strain ANU843 on Trifolium subterraneum or wounding of plants (C. G. R. Lawson, M. A. Djordjevic, J. J. Weinman and B. G. Rolfe. 1994. Molecular Plant-Microbe Interactions 7, 498-507). Experim ents were conducted under the same conditions to examine the time of o nset of synthesis and excretion of flavonoids that might result from t his early CHS expression. Flavonoids in root tissues and root exudates were examined by HPLC analysis and the ability of fractionated and un fractionated material to induce nodulation gene expression in Rhizobiu m measured. There were no detectable changes in nod-gene-inducing acti vity of individual HPLC fractions of root exudates of 1 day dark-grown roots after Rhizobium inoculation. In contrast, after 3 days exposure to Rhizobium, analysis of specific HPLC fractions showed the presence of an additional nod-gene-inducing compound which the data indicate w as 4',7-dihydroxyflavone. A different and additional nod gene inducer was found in inoculated 5 day samples of root exudate of light-grown p lants indicating that light exposure changes the HPLC profiles as well as the nod-gene-inducing compound(s). Exudates collected from wounded plants were considerably different from those from Rhizobium-inoculat ed and uninoculated plants and contained no delectable nod gene induce rs. The late detection (at day 3) of Rhizobium-induced flavonoid excre tion may occur too late to be directly correlated with the observed ex pression of CHS 6 h after inoculation. In addition, the data suggest t hat although the CHS5 promotor responds to both wounding and Rhizobium inoculation, the biochemical consequences of CHS5 induction resulting from these treatments are different.