Nodulation gene inducers

A. J. Dharmatilake and W. D. Baker, Cheinotaxis of Rhizohium meliloti towards nodulation gene inducing compounds from alfalfa roots. Applied and Environmental Microbiology 5S 1153 (1992). 

Peters, N.K. and Long, S.R., Rhizobium meliloti nodulation gene inducers and inhibitors. Plant Physiol, 88, 396, 1988. 

Table 1 Nodulation gene inducers secreted by leguminous plants61...

B. G. Rolfe, Flavones induce expression of nodulation genes in Rhizobium, Nature 323 632 (1986). 

N. R. Peters, J. W. Frost, and S. R. Long, A plant flavone, luteolin, induces expression of Rhizohium nteliloti nodulation genes. Science 233 911 (1986). 

Recently, a new polyketide biosynthetic pathway in bacteria that parallels the well studied plant PKSs has been discovered that can assemble small aromatic metabolites.8,9 These type III PKSs10 are members of the chalcone synthase (CHS) and stilbene synthase (STS) family of PKSs previously thought to be restricted to plants.11 The best studied type III PKS is CHS. Physiologically, CHS catalyzes the biosynthesis of 4,2, 4, 6 -tetrahydroxychalcone (chalcone). Moreover, in some organisms CHS works in concert with chalcone reductase (CHR) to produce 4,2 ,4 -trihydroxychalcone (deoxychalcone) (Fig. 12.1). Both natural products constitute plant secondary metabolites that are used as precursors for the biosynthesis of anthocyanin pigments, anti-microbial phytoalexins, and chemical inducers of Rhizobium nodulation genes.12... 

Maxwell, C.A. et al., Molecular characterization and expression of isoliquiritigenin 2 -0-methyltransferase, an enzyme specifically involved in the biosynthesis of an inducer of Rhizobium meliloti nodulation genes. Plant J., 4, 971, 1993. 

Fig. 3. Three distinct groups of signals produced during infection and nodule development inducing early and late nodulin genes.

Legumes also secrete antagonists that inhibit the flavonoid-mediated activation of nodulation genes (Table 2).63 64 Some act as inducers in another symbiotic system. Antagonists generally have structures similar to those of inducers and inhibition can be overcome by increasing the concentration of inducers, hence they are considered as competitive inhibitors.63... 

The formation of root nodules associated with nitrogen fixation in legumes is an area of active study. Hormonal involvement in the developmental modifications needed to establish the nodule, has been reviewed [186]. Studies on the molecular characterization of nodulation have shown that early nodulation genes are induced by auxin transport inhibitors, and inhibitor treated roots produce pseudonodules [187]. Rhizobium species appear to produce lAA by a pathway involving the conversion of tryptophan to indole-... 

Peters NK, Frost JW, Long SR (1986) A plant flavraie, luleolin, induces expression of Rhizobium meliloti nodulation genes. Science 233 977—980... 

---