The role of Nod signal structures in the determination of host specificity in the Rhizobium-legume symbiosis

Rhizobial acyl carrier proteins and their roles in the formation of bacterial cell-surface components that are required for the development of nitrogen-fixing root nodules on legume hosts

Acyl carrier protein (ACP) of Escherichia coli is a small acidic protein which functions as carrier of growing acyl chains during their biosynthesis and as donor of acyl chains during transfer to target molecules. This unique ACP of E. coli is expressed constitutively. In more complex bacteria, multiple ACPs are present, indicating a channeling of pools of multi-carbon units into different biosynthetic routes. In rhizobia, for example, besides the constitutive ACP (AcpP) involved in the biosynthesis and transfer of common fatty acids, three specialized ACPs have been reported: (1) the flavonoid-inducible nodulation protein NodF, (2) AcpXL that transfers 27-hydroxyoctacosanoic acid to a sugar backbone during lipid A biosynthesis, and (3) the RkpF protein which is required for the biosynthesis of rhizobial capsular polysaccharides. All three of those specialized rhizobial ACPs are required for the biosynthesis of cell-surface molecules that play a role in establishing the symbiotic relationship between rhizobia and their legume hosts. Surprisingly, the recently sequenced genomes from Mesorhizobium loti and Sinorhizobium meliloti suggest even more candidates for ACPs in rhizobia.

Effect of mutations in Pisum sativum L. genes blocking different stages of nodule development on the expression of late symbiotic genes in Rhizobium leguminosarum bv. viciae

In this report, the expression of late symbiotic genes (fnrN, fixN, and nifA) of Rhizobium leguminosarum bv. viciae was studied in nodules of mutant pea lines blocked at four successive stages of nodule development. Bacterial gene expression was analyzed in situ with transcriptional gusA reporter gene fusions. As a control, a constitutively expressed gusA gene was included. In the nodules of Nop(nodule persistence) mutants (mutant in gene sym13), which had not yet exhibited signs of premature senescence, the expression patterns observed were identical to those in wild-type nodules. Normal expression of fusions also occurred in nodules defective at the infection droplet differentiation stage (mutant in gene sym40) in which bacteria are endocytosed, but infection threads and infection droplets are hypertrophied. In contrast, in Itn- (infection thread formation inside the nodule tissue) mutants (mutant gene sym33), in which there is no endocytosis of bacteria, expression of the constitutive fusion was only in infection threads and no activity was shown for the other fusions. From this it can be concluded that functionality of the plant gene Sym33, i.e., bacterial endocytosis, is a prerequisite for the expression of late symbiotic genes in the microsymbiont. No morphologically distinct interzone II-III could be detected in nodules blocked at the bacteroid differentiation stage (mutants in gene sym31). The constitutive fusion was expressed equally throughout the nodule tissue (except for the meristem), and the activity of fusions to late symbiotic genes increased gradually with a maximal expression level at the base of the nodule. This is consistent with an altered oxygen barrier previously reported for these nodules. By including double mutants, earlier results on sequential functioning of gene pairs sym33-sym40 and sym31-sym13 could be confirmed and it could be demonstrated that the developmental epistasis found at the morphological level also is reflected in the expression pattern of late symbiotic genes in the microsymbiont.

Function of chitin oligosaccharides in plant and animal development

In plant development chitin oligosaccharides have been studied intensively as part of the communication between leguminous plants and Rhizobium bacteria. The Rhizobium bacteria synthesize and secrete lipochitin oligosaccharides (LCOs) to induce the development of a root nodule, in which the bacteria will infiltrate to start a symbiotic relation with the plant. Here we will give an overview of the biosynthetic route used by the bacteria to synthesize these LCOs. Perception by the plant will also be discussed as well as early responses to the LCOs. By working with the genes from the biosynthetic route, other genes were identified that share homology with the chitin synthase genes from Rhizobium. These genes are now isolated from human, mouse, chick, Xenopus and zebrafish and can be divided into three classes. They are mainly expressed during early development at the same stage as chitin oligosaccharide synthase activity can be detected. A controversy has been risen about their biochemical activity and will be further discussed here.

Nod factor requirements for efficient stem and root nodulation of the tropical legume Sesbania rostrata

Azorhizobium caulinodans ORS571 synthesizes mainly pentameric Nod factors with a household fatty acid, an N-methyl, and a 6-O-carbamoyl group at the nonreducing-terminal residue and with a d-arabinosyl, an l-fucosyl group, or both at the reducing-terminal residue. Nodulation on Sesbania rostrata was carried out with a set of bacterial mutants that produce well characterized Nod factor populations. Purified Nod factors were tested for their capacity to induce root hair formation and for their stability in an in vitro degradation assay with extracts of uninfected adventitious rootlets. The glycosylations increased synergistically the nodulation efficiency and the capacity to induce root hairs, and they protected the Nod factor against degradation. The d-arabinosyl group was more important than the l-fucosyl group for nodulation efficiency. Replacement of the 6-O-l-fucosyl group by a 6-O-sulfate ester did not affect Nod factor stability, but reduced nodulation efficiency, indicating that the l-fucosyl group may play a role in recognition. The 6-O-carbamoyl group contributes to nodulation efficiency, biological activity, and protection, but could be replaced by a 6-O-acetyl group for root nodulation. The results demonstrate that none of the studied substitutions is strictly required for triggering normal nodule formation. However, the nodulation efficiency was greatly determined by the synergistic presence of substitutions. Within the range tested, fluctuations of Nod factor amounts had little impact on the symbiotic phenotype.

Heterologous rhizobial lipochitin oligosaccharides and chitin oligomers induce cortical cell divisions in red clover roots, transformed with the pea lectin gene

Division of cortical cells in roots of leguminous plants is triggered by lipochitin oligosaccharides (LCOs) secreted by the rhizobial microsymbiont. Previously, we have shown that presence of pea lectin in transgenic white clover hairy roots renders these roots susceptible to induction of root nodule formation by pea-specific rhizobia (C. L. Díaz, L. S. Melchers, P. J. J. Hooykaas, B. J. J. Lugtenberg, and J. W. Kijne, Nature 338:579-581, 1989). Here, we report that pea lectin-transformed red clover hairy roots form nodule primordium-like structures after inoculation with pea-, alfalfa-, and Lotus-specific rhizobia, which normally do not nodulate red clover. External application of a broad range of purified LCOs showed all of them to be active in induction of cortical cell divisions and cell expansion in a radial direction, resulting in formation of structures that resemble nodule primordia induced by clover-specific rhizobia. This activity was obvious in about 50% of the red clover plants carrying hairy roots transformed with the pea lectin gene. Also, chitopentaose, chitotetraose, chitotriose, and chitobiose were able to induce cortical cell divisions and cell expansion in a radial direction in transgenic roots, but not in control roots. Sugar-binding activity of pea lectin was essential for its effect. These results show that transformation of red clover roots with pea lectin results in a broadened response of legume root cortical cells to externally applied potentially mitogenic oligochitin signals.

Genes and signal molecules involved in the rhizobia-leguminoseae symbiosis

The symbiosis between Rhizobium bacteria and their host plants is dependent on the specific recognition of signal molecules produced by each partner. Many players in the signal exchange have been identified. Among them are signal molecules such as flavonoids, LCOs, auxin, cytokinin, ethylene and uridine and genes such as Enod40, Enod2 and Enod12. Their interconnection, however, is only starting to be understood. The most recent insights into their interconnection include: advances in the use of transgenic leguminous plants containing reporter gene constructs for studying the effect of the signal molecules; novel methods for delivery of signal molecules using ballistic microtargeting; and the discovery of the role of chitin oligosaccharides in animal embryogenesis.

Bacterial nodulation protein NodZ is a chitin oligosaccharide fucosyltransferase which can also recognize related substrates of animal origin

The nodZ gene, which is present in various soil bacteria such as Bradyrhizobium japonicum, Azorhizobium caulinodans, and Rhizobium loti, is involved in the addition of a fucosyl residue to the reducing N-acetylglucosamine residue of lipochitin oligosaccharide (LCO) signal molecules. Using an Escherichia coli strain that produces large quantities of the NodZ protein of B. japonicum, we have purified the NodZ protein to homogeneity. The purified NodZ protein appears to be active in an in vitro transfucosylation assay in which GDP-beta-fucose and LCOs or chitin oligosaccharides are used as substrates. The products of the in vitro reaction using chitin oligosaccharides as substrate were studied by using mass spectrometry, linkage analysis, and composition analysis. The data show that one fucose residue is added to C6 of the reducing-terminal N-acetylglucosamine residue. The substrate specificity of NodZ protein was analyzed in further detail, using radiolabeled GDP-beta-fucose as the donor. The results show that chitin oligosaccharides are much better substrates than LCOs, suggesting that in Rhizobium NodZ fucosylates chitin oligosaccharides prior to their acylation. The free glycan core pentasaccharides of N-linked glycoproteins are also substrates for NodZ. Therefore, the NodZ enzyme seems to have an activity equivalent to that of the enzyme involved in the addition of the C6-linked fucosyl substituent in the glycan core of N-linked glycoproteins in eukaryotes. Oligosaccharides that contain only one N-acetylglucosamine at the reducing terminus are also substrates for NodZ, although in this case very high concentrations of such oligosaccharides are needed. An example is the leukocyte antigen Lewis-X, which can be converted by NodZ to a novel fucosylated derivative that could be used for binding studies with E-selectin.

Rhizobium nodulation protein NodC is an important determinant of chitin oligosaccharide chain length in Nod factor biosynthesis

Synthesis of chitin oligosaccharides by NodC is the first committed step in the biosynthesis of rhizobial lipochitin oligosaccharides (LCOs). The distribution of oligosaccharide chain lengths in LCOs differs between various Rhizobium species. We expressed the cloned nodC genes of Rhizobium meliloti, R. leguminosarum bv. viciae, and R. loti in Escherichia coli. The in vivo activities of the various NodC proteins differed with respect to the length of the major chitin oligosaccharide produced. The clearest difference was observed between strains with R. meliloti and R. loti NodC, producing chitintetraose and chitinpentaose, respectively. In vitro experiments, using UDP-[14C]GlcNAc as a precursor, show that this difference reflects intrinsic properties of these NodC proteins and that it is not influenced by the UDP-GlcNAc concentration. Analysis of oligosaccharide chain lengths in LCOs produced by a R. leguminosarum bv. viciae nodC mutant, expressing the three cloned nodC genes mentioned above, shows that the difference in oligosaccharide chain length in LCOs of R. meliloti and R. leguminosarum bv. viciae is due only to nodC. The exclusive production of LCOs which contain a chitinpentaose backbone by R. loti strains is not due to NodC but to end product selection by Nod proteins involved in further modification of the chitin oligosaccharide. These results indicate that nodC contributes to the host specificity of R. meliloti, a conclusion consistent with the results of several studies which have shown that the lengths of the oligosaccharide backbones of LCOs can strongly influence their activities on host plants.

The role of lipochitooligosaccharides in root nodule organogenesis and plant cell growth

Lipochitooligosaccharides (Nod signals) excreted by rhizobia induce the formation of symbiotic root nodules in leguminous plants. This process is host plant specific, depending on the structural modifications of Nod signals. Rapid responses of plant roots in single cell assays have provided powerful tools in dissecting Nod signal transduction pathways and in elucidating the molecular basis of host specificity. Recent findings indicate that lipochitooligosaccharides, as well as symbiosis-related genes, also function in non legumes, pointing to a general role for these elements in plant morphogenesis.