Rhizobium lipo-chitooligosaccharide nodulation factors: signaling molecules mediating recognition and morphogenesis

Secretion systems and signal exchange between nitrogen-fixing rhizobia and legumes

The formation of symbiotic nitrogen-fixing nodules on the roots and/or stem of leguminous plants involves a complex signal exchange between both partners. Since many microorganisms are present in the soil, legumes and rhizobia must recognize and initiate communication with each other to establish symbioses. This results in the formation of nodules. Rhizobia within nodules exchange fixed nitrogen for carbon from the legume. Symbiotic relationships can become non-beneficial if one partner ceases to provide support to the other. As a result, complex signal exchange mechanisms have evolved to ensure continued, beneficial symbioses. Proper recognition and signal exchange is also the basis for host specificity. Nodule formation always provides a fitness benefit to rhizobia, but does not always provide a fitness benefit to legumes. Therefore, legumes have evolved a mechanism to regulate the number of nodules that are formed, this is called autoregulation of nodulation. Sequencing of many different rhizobia have revealed the presence of several secretion systems - and the Type III, Type IV, and Type VI secretion systems are known to be used by pathogens to transport effector proteins. These secretion systems are also known to have an effect on host specificity and are a determinant of overall nodule number on legumes. This review focuses on signal exchange between rhizobia and legumes, particularly focusing on the role of secretion systems involved in nodule formation and host specificity.

The NFP locus of Medicago truncatula controls an early step of Nod factor signal transduction upstream of a rapid calcium flux and root hair deformation

Establishment of the Rhizobium-legume symbiosis depends on a molecular dialogue, in which rhizobial nodulation (Nod) factors act as symbiotic signals, playing a key role in the control of specificity of infection and nodule formation. Using nodulation-defective (Nod-) mutants of Medicago truncatula to study the mechanisms controlling Nod factor perception and signalling, we have previously identified five genes that control components of a Nod factor-activated signal transduction pathway. Characterisation of a new M. truncatula Nod- mutant led to the identification of the Nod Factor Perception (NFP) locus. The nfp mutant has a novel phenotype among Nod- mutants of M. truncatula, as it does not respond to Nod factors by any of the responses tested. The nfp mutant thus shows no rapid calcium flux, the earliest detectable Nod factor response of wild-type plants, and no root hair deformation. The nfp mutant is also deficient in Nod factor-induced calcium spiking and early nodulin gene expression. While certain genes controlling Nod factor signal transduction also control the establishment of an arbuscular mycorrhizal symbiosis, the nfp mutant shows a wild-type mycorrhizal phenotype. These data indicate that the NFP locus controls an early step of Nod factor signal transduction, upstream of previously identified genes and specific to nodulation.

Structure of the Mesorhizobium huakuii and Rhizobium galegae Nod factors: a cluster of phylogenetically related legumes are nodulated by rhizobia producing Nod factors with alpha,beta-unsaturated N-acyl substitutions

Rhizobia are symbiotic bacteria that synthesize lipochitooligosaccharide Nod factors (NFs), which act as signal molecules in the nodulation of specific legume hosts. Based on the structure of their N-acyl chain, NFs can be classified into two categories: (i) those that are acylated with fatty acids from the general lipid metabolism; and (ii) those (= alphaU-NFs) that are acylated by specific alpha,beta-unsaturated fatty acids (containing carbonyl-conjugated unsaturation(s)). Previous work has described how rhizobia that nodulate legumes of the Trifolieae and Vicieae tribes produce alphaU-NFs. Here, we have studied the structure of NFs from two rhizobial species that nodulate important genera of the Galegeae tribe, related to Trifolieae and Vicieae. Three strains of Mesorhizobium huakuii, symbionts of Astragalus sinicus, produced as major NFs, pentameric lipochitooligosaccharides O-sulphated and partially N-glycolylated at the reducing end and N-acylated, at the non-reducing end, by a C18:4 fatty acid. Two strains of Rhizobium galegae, symbionts of Galega sp., produced as major NFs, tetrameric O-carbamoylated NFs that could be O-acetylated on the glucosamine residue next to the non-reducing terminal glucosamine and were N-acylated by C18 and C20 alpha,beta-unsaturated fatty acids. These results suggest that legumes nodulated by rhizobia synthesizing alphaU-NFs constitute a phylogenetic cluster in the Galegoid phylum.

Rhizobium nod factors induce increases in intracellular free calcium and extracellular calcium influxes in bean root hairs

Application of Nod factors to growing, responsive root hairs of the bean Phaseolus vulgaris induces marked changes in both the intracellular cytosolic free calcium (Ca2+) and in the influx of extracellular [Ca2+]. The intracellular [Ca2+], which has been measured by ratiometric imaging in cells microinjected with fura-2-dextran (70 kDa), elevates within 5 min from approximately 400 nM to 1500 nM in localised zones in the root hair apex. Of particular note is the observation that the elevated regions of [Ca2+] appear to shift position during short time intervals. Increases in and fluctuations of the intracellular [Ca2+] are also observed in the perinuclear region after 10-15 min treatment with Nod factors. The extracellular Ca2+ flux, detected with the non-invasive, calcium specific vibrating electrode, is inwardly directed and also increases quickly in response to Nod factors from 13 pmol cm-2 s-1 to 28 pmol cm-2 s-1. Chitin-oligomers, which are structurally similar but biologically inactive when compared to the active Nod factors, fail to elicit changes in either intracellular or extracellular Ca2+. The similar timing and location of the intracellular elevations and the increased extracellular influx provide support for the idea that Ca2+ participates in secretion and cell wall remodelling, which occur in anticipation of root hair deformation and curling.

High-resolution transcriptional analysis of the symbiotic plasmid of Rhizobium sp. NGR234

Most of the bacterial genes involved in nodulation of legumes (nod, nol and noe ) as well as nitrogen fixation (nif and fix ) are carried on pNGR234a, the 536 kb symbiotic plasmid (pSym) of the broad-host-range Rhizobium sp. NGR234. Putative transcription regulators comprise 24 of the predicted 416 open reading frames (ORFs) contained on this replicon. Computational analyses identified 19 nod boxes and 16 conserved NifA-sigma54 regulatory sequences, which are thought to co-ordinate the expression of nodulation and nitrogen fixation genes respectively. To analyse transcription of all putative ORFs, the nucleotide sequence of pNGR234a was divided into 441 segments designed to represent all coding and intergenic regions. Each of these segments was amplified by polymerase chain reactions, transferred to filters and probed with radioactively labelled RNA. RNA was extracted from bacterial cultures grown under various experimental conditions, as well as from bacteroids of determinate and indeterminate nodules. Generally, genes involved in the synthesis of Nod factors (e.g. the three hsn loci) were induced rapidly after the addition of flavonoids, whereas others thought to act within the plant (e.g. those encoding the type III secretion system) responded more slowly. Many insertion (IS) and transposon (Tn)-like sequences were expressed strongly under all conditions tested, while a number of loci other than those known to encode nod, noe, nol, nif and fix genes were also transcribed in nodules. Many more diverse transcripts were found in bacteroids of determinate as opposed to indeterminate nodules.

Chlorella virus PBCV-1 encodes functional glutamine: fructose-6-phosphate amidotransferase and UDP-glucose dehydrogenase enzymes

DNA sequence analysis of the 330-kb Chlorella virus PBCV-1 genome unexpectedly revealed several open reading frames which encode proteins that are homologous to sugar-manipulating enzymes including glutamine:fructose-6-phosphate amidotransferase (GFAT), UDP-glucose dehydrogenase (UDP-GlcDH), and hyaluronan synthase (HAS). PBCV-1 genes encoding the putative GFAT and UDP-GlcDH enzymes were expressed in Escherichia coli, and both recombinant proteins have the predicted enzyme activity in cell free extracts. These same two genes are transcribed early in PBCV-1 infection, and both genes are widespread among the Chlorella viruses. The products of the reactions catalyzed by these two enzymes are precursors in the biosynthesis of hyaluronan polysaccharide. Previous experiments established that, like the GFAT and UDP-GlcDH genes, the HAS gene is transcribed early and encodes a functional enzyme (DeAngelis, P. L., Jing. W., Graves, M. V., Burbank, D. E., and Van Etten, J. L. (1997) Science 278, 1800-1803). Interestingly, the predicted amino-acid sequences of the PBCV-1 GFAT and UDP-GlcDH enzymes are more similar to bacterial GFAT and UDP-GlcDH enzymes than to their eukaryotic counterparts. In contrast, the amino-acid sequence of the PBCV-1 HAS enzyme more closely resembles eukaryotic enzymes.

Expression of Rhizobium chitin oligosaccharide fucosyltransferase in zebrafish embryos disrupts normal development

In this report we present data about the effect of the Rhizobium NodZ enzyme on zebrafish development. We injected zebrafish embryos with a plasmid expressing NodZ protein, and we confirmed that the enzyme is active and has chitin oligosaccharide fucosyltransferase (NodZ) activity in vitro. In addition, the embryos injected with the NodZ-expressing plasmid, but not with a control plasmid, showed malformations or bends in the tail, and in some cases shunted tail structures and fused somites. These results clearly indicate that the likely substrates for this enzyme, chitin oligosaccharides and free N-glycans, have essential functions during early vertebrate embryogenesis.

Substrate specificities of tobacco chitinases

Ten tobacco chitinases (1,4-N-acetyl-beta-D-glucosaminide glycanhydrolase, EC 3.2.1.14) were purified from tobacco leaves hypersensitively reacting to tobacco mosaic virus. The 10 enzymes, which belong to five distinct structural classes of plant chitinases, were incubated with several potential substrates such as chitin, a beta-1,4 N-acetyl-D-glucosamine (GlcNAc) polymer, chitosan (partially deacetylated chitin), chitin oligomers of variable length and bacterial cell wall. Tobacco chitinases are all endotype enzymes that liberate oligomers from chitin and are capable of processing the chito-oligomers further at differential rates. Chitin reaction products were separated and quantified by HPLC and differential kinetics of oligomer accumulation and degradation were observed with the distinct classes of chitinases. Depending on the substrate to be hydrolysed, each isoform displayed a different spectrum of activity. For example, class I isoforms were the most active on chitin and (GlcNAc)4-6 whereas class III basic isoforms were the most efficient in inducing bacterial lysis. Class V and class VI chitinases were shown to more readily hydrolyse chitin oligomers than the chitin polymer itself. Together, these data indicate that the 10 tobacco chitinases represent complementary enzymes which may have synergistic effects on their substrates. This paper discusses their implication in plant defense by attacking pathogen's structural components and in plant development by maturing signal molecules.