Effects of glucosinolates and flavonoids on colonization of the roots of Brassica napus by Azorhizobium caulinodans ORS571

Plants of Brassica napus were assessed quantitatively for their susceptibil ity to lateral root crack colonization by Azorhizobium caulinodans ORS571(p XLGD4) (a rhizobial strain carrying the lacZ reporter gene) and for the con centration of glucosinolates in their roots by high-pressure liquid chromat ography (HPLC). High- and low glucosinolate-seed (HGS and LGS) varieties ex hibited a relatively low and high percentage of colonized lateral roots, re spectively. HPLC showed that roots of HGS plants contained a higher concent ration of glucosinolates than roots of LGS plants. One LGS variety showing fewer colonized lateral roots than other LGS varieties contained a higher c oncentration of glucosinolates than other LGS plants. Inoculated HGS plants treated with the flavonoid naringenin showed significantly more colonizati on than untreated HGS plants. This increase was not mediated by a naringeni n-induced lowering of the glucosinolate content of HGS plant roots, nor did naringenin induce bacterial resistance to glucosinolates or increase the g rowth of bacteria. The erucic acid content of seed did not appear to influe nce colonization by azorhizobia. Frequently, leaf assays are used to study glucosinolates and plant defense; this study provides data on glucosinolate s and bacterial colonization in roots and describes a bacterial reporter ge ne assay tailored easily to the study of ecologically important phytochemic als that influence bacterial colonization. These data also form a basis for future assessments of the benefits to oilseed rape plants of interaction w ith plant growth-promoting bacteria, especially diazotrophic bacteria poten tially able to extend the benefits of nitrogen fixation to nonlegumes.