ALDONIC ACIDS - A NOVEL FAMILY OF NOD GENE INDUCERS OF MESORHIZOBIUM-LOTI, RHIZOBIUM-LUPINI, AND SINORHIZOBIUM-MELILOTI

Molecular signals, such as flavonoids (or nonflavonoid type), nod gene -inducers, and bacterial lipochitin oligosaccharides (LCOs) act as mod ulators of species specificity during early stages of infection in Rhi zobium spp.-legume interactions. The fact that signaling in Lupinus al bus remains to be determined prompted us to investigate the flavonoid signal responsible for nod gene induction in Rhizobium lupini. A scree ning method was used based on the measurement of beta-galactosidase ac tivity of R. lupini strains harboring nodC::lacZ fusions in the presen ce of (1) authentic lupin isoflavones, (ii) carbohydrate-like inducers , and (iii) high-pressure liquid chromatography (HPLC)-fractionated lu pin seed effusates and root exudates, as putative nod gene inducers. T he results indicate that both erythronic and tetronic acids (4-C sugar acids) led to low but significant increases in beta-galactosidase act ivities, compared with the controls. In addition, lupi-wighteone, a mo noprenylated isoflavone, exerts a synergistic effect with the carbohyd rate-like inducers, compared with other isoflavone treatments. The nat ural occurrence of aldonic acids in L. albus root exudates and seed ef fusates has been demonstrated by HPLC analysis. When tested with nodC: :lacZ fusions, tetronic acid resulted in nod gene induction in Sinorhi zobium meliloti. In addition, a combination of luteolin and tetronic a cid promotes further increases in S. meliloti nod gene expression, as shown by beta-galactosidase assays. Incorporation studies with [C-14]L CO precursors confirmed the inductive role of both erythronic and tetr onic acids in promoting LCO biosynthesis in R. lupini cultures, and of tetronic acid in Mesorhizobium loti and S. meliloti. Hydrolysis of th e LCOs with various enzymes substantiated their putative identities. T hese results are discussed in relation to the impact of these unusual signal molecules on our knowledge of flavonoid signaling in Rhizobium- legume symbiosis.