Host-specific regulation of nodulation genes in <i>Rhizobium</i>
 is mediated by a plant-signal, interacting with the <i>nodD</i>
 gene product

115

The virulence (vir) genes of Agrobacterium tumefaciens are induced by low-molecular-weight phenolic compounds and monosaccharides through a two-component regulatory system consisting of the VirA and VirG proteins. However, it is not clear how the phenolic compounds are sensed by the VirA/VirG system. We tested the vir-inducing abilities of 15 different phenolic compounds using four wildtype strains ofA. tumefaciens-KU12, C58, A6, and Bo542. We analyzed the relationship between structures of the phenolic compounds and levels of vir gene expression in these strains. In strain KU12, vir genes were not induced by phenolic compounds containing 4'-hydroxy, 3'-methoxy, and 5'-methoxy groups, such as acetosyringone, which strongly induced vir genes of the other three strains. On the other hand, vir genes of strain KU12 were induced by phenolic compounds containing only a 4'-hydroxy group, such as 4-hydroxyacetophenone, which did not induce vir genes of the other three strains. The vir genes of strains KU12, A6, and Bo542 were all induced by phenolic compounds containing 4'-hydroxy and 3'-methoxy groups, such as acetovanillone. By transferring different Ti plasmids into isogenic chromosomal backgrounds, we showed that the phenolic-sensing determinant is associated with Ti plasmid. Subcloning Table 2 show that the induction response to different phenolic compounds varies among the different strains.Under our standard induction conditions, vir genes in both the octopine-type strain A6 and the succinamopine-type strain Bo542 were strongly induced by all phenolic compounds in group A. The nopaline-type strain C58 showed a moderate level of vir gene induction by acetosyringone and sinapinic acid. However, no response to syringaldehyde, syringic acid, and 2,6-dimethoxyphenol was observed. Surprisingly, we found that all phenolic compounds in group A, including acetosyringone, the most commonly used vir gene inducer in the laboratory, were unable to induce the vir genes of strain KU12. All strains except C58 responded to acetovanillone and ferulic acid of group B, and strains KU12 and Bo542 were both induced by guaiacol. However, only strain Bo542 was induced by vanillin of group B. In our hands, strain A6 was not induced by vanillin, a phenolic compound classified as a strong vir gene inducer by Melchers et al. (29) using A. tumefaciens strain LBA2516, which contains an octopine-type Ti plasmid, pTiB6, that is closely related to pTiA6 (12). The reason for this discrepancy is not clear. Of special interest is the observation that the vir genes of strain KU12 were highly induced by a group of compounds in group C that lack both methoxy groups. These compounds included 4-hydroxyacetophenone, p-coumaric acid, and phenol, which had previously been reported to be non-vir gene inducers (29). In support of this latter report, the three otherA. tumefaciens strains were indeed not induced by the phenolic compounds in group C.We tested vir gene induction by other phenolic compounds that lacked the hydroxyl group at the R4 posi...

Section: Discussionsupporting

confidence: 80%

Genetic evidence for direct sensing of phenolic compounds by the VirA protein of Agrobacterium tumefaciens.

Lee

Jin²,

Sim³

et al. 1995

115

“…Recently, it has been reported that different nodD genes show different inducer specificities (34). It might be expected that acquisition of a B. japonicum nodD would broaden the The results shown here indicate that genes involved in the earliest steps in nodule formation are not fully expressed in the culture conditions traditionally used in laboratory studies of the B. japonicum/soybean symbiosis and in the preparation of commercial inocula for use in agriculture.…”

Section: "'Iscussionmentioning

confidence: 73%

“…nodD is a regulatory gene that is constitutively expressed and is required for induction of other nod genes (32)(33)(34). Recently, it has been reported that different nodD genes show different inducer specificities (34).…”

Section: "'Iscussionmentioning

confidence: 99%

Induction of Bradyrhizobium japonicum common nod genes by isoflavones isolated from Glycine max

Kosslak¹,

Bookland²,

Barkei³

et al. 1987

376

225

The early events in legume nodulation by Rhizobium spp. involve a conserved gene cluster known as the common nod region. A broad-host-range plasmid (pEA2-21) containing a Bradyrhizobium japonicum nodDABC-acZ translational fusion was constructed and used to monitor nod gene expression in response to soybean root extract. Two inducing compounds were isolated and identified. Analysis using ultraviolet absorption spectra, proton nuclear magnetic resonance, and mass spectrometry showed that the two inducers were 4',7-dihydroxyisoflavone (daidzein) and 4',5,7-trihydroxyisoflavone (genistein). Induction was also seen with some, but not all, of the flavonoid compounds that induce nod genes in fast-growing Rhizobium strains that nodulate clover, alfalfa, or peas. When pEA2-21 was introduced into Rhizobium trifohii, it was inducible by flavones but not by daidzein and genistein. In Rhizobium fredii, pEA2-21 was induced by isoflavones and flavones. Thus, the specificity of induction appears to be influenced by the host-strain genome.Members of the bacterial genus Rhizobium form symbiotic associations with leguminous plants that result in the formation of nitrogen-fixing root nodules. In three agronomically important Rhizobium/legume associations, R. trifolii/clover, R. meliloti/alfalfa, and R. leguminosarum/pea, important bacterial nodulation genes (1-4) and plant compounds that induce them (5-7) have been identified. In these associations flavones (5-7) or flavanones (7) have been found to induce the nodABC genes, as well as other nod genes involved in host specificity (1). Isoflavones have been found to inhibit the induction of nodABC in R. leguminosarum (7).The Bradyrhizobium japonicum/soybean symbiosis is of considerable agricultural importance. In contrast to Rhizobium spp., Bradyrhizobium species are slow-growing (8), nitrogen-fixation and nodulation genes are located on the chromosome (9) and not on plasmids (10)(11)(12), and less is known about the genetic requirements for nodulation (13)(14)(15). In particular, the compounds produced by the soybean host that interact with the common nod genes have not been characterized.In this study, a nodABC-lacZ translational fusion was used to monitor nod gene expression in B. japonicum in response to soybean root extract. Two major components from soybeans (Glycine max cv. Williams) were isolated that induced the expression of the nodABC-lacZ fusion when it was present in the soybean-nodulating bacteria B. japonicum and Rhizobium fredii, but not when it was present in R. trifolii. MATERIALS AND METHODSStrains and Plasmids. Standard procedures (16) were used for DNA manipulations. A HindIII fragment containing the nod region of B. japonicum USDA 123 was cloned in both orientations into the single HindIII site of plC19R (17). BamHI-Bgl II fragments containing the nod genes and flanking polylinker sequences were then cloned into the BamHI site of the broad-host-range vector pGD926 (18) resulting in pEA2-21 and pEA4-10 (Fig. 1) (19) agar and germinated for 3 days in the d...

“…homology (40) (9)(10)(11)(12)(13)18), which are involved in host-range specificity at the legume genus level.…”

Section: Resultsmentioning

confidence: 99%

“…Host-specific nodulation (hsn) determinants have also been reported in Rhizobium melioti (10), Rhizobium leguminosarum biovars trifolii (11) and viceae (12), in the broad host-range Rhizobium sp. strain MPIK3030 (13), and in B. japonicum (7,14,15).…”

mentioning

confidence: 95%

The Bradyrhizobium japonicum nolA gene and its involvement in the genotype-specific nodulation of soybeans.

Sadowsky

Cregan

Göttfert

et al. 1991

139

115

Several soybean genotypes have been identified which specifically exclude modulation by members of Bradyrhizobiumjaponicum serocluster 123. We have identified and sequenced a DNA region from B. japonicum strain USDA 110 which is involved in genotype-specifc modulation of soybeans. This 2.3-kilobase region, cloned in pMJS12, allows B. japonicum serocluster 123 isolates to form nodules on plants of serogroup 123-restricting genotypes. The nodules, however, were ineffective for symbiotic nitrogen fixation. The nodulation-complementing region is located approximately 590 base pairs transcriptionally downstream from nodD2. The 5' end of pMJS12 contains a putative open reading frame (ORF) of 710 base pairs, termed nowl. Transposon Tn3-HoHo mutations only within the ORF abolished modulation complementation. The N terminus of the predicted noL4 gene product has strong similarity with the N terminus of MerR, the regulator of mercury resistance genes. Translational IacZ fusion experiments indicated that nolA was moderately induced by soybean seed extract and the isoflavone genistein. Restriction fragments that hybridize to pMJS12 were detected in genomic DNAs from both nodulation-restricted and -unrestricted strains.