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

Lipo-chitooligosaccharide and thuricin 17 act as plant growth promoters and alleviate drought stress in Arabidopsis thaliana

Lipo-chito-oligosaccharide (LCO-from Bradyrhizobium japonicum) and thuricin 17 (Th17-from Bacillus thuringiensis) are bacterial signal compounds from the rhizosphere of soybean that have been shown to enhance plant growth in a range of legumes and non-legumes. In this study, an attempt to quantify phytohormones involved in the initial hours after exposure of Arabidopsis thaliana to these compounds was conducted using UPLC-ESI-MS/MS. A petri-plate assay was conducted to screen for drought stress tolerance to PEG 8000 infusion and plant growth was studied 21-days post-stress. Arabidopsis thaliana plants grown in trays with drought stress imposed by water withhold were used for free proline determination, elemental analysis, and untargeted proteomics using LC-MS/MS studies. At 24 h post-exposure to the signal compounds under optimal growth conditions, Arabidopsis thaliana rosettes varied in their responses to the two signals. While LCO-treated rosettes showed a decrease in total IAA, cytokinins, gibberellins, and jasmonic acid, increases in ABA and SA was very clear. Th17-treated rosettes, on the other hand, showed an increase in IAA and SA. Both treatments resulted in decreased JA levels. Under severe drought stress imposed by PEG 8000 infusion, LCO and Th17 treatments were found to significantly increase fresh and dry weight over drought-stressed control plates, indicating that the presence of the signaling compounds decreased the negative effects experienced by the plants. Free proline content increased in LCO- and Th17-treated plants after water-withhold drought stress. Elemental analysis showed a significant increase in carbon percentage at the lower concentration of Th17. Untargeted proteomics revealed changes in the levels of drought-specific ribosomal proteins, glutathione S-transferase, late embryogenesis proteins, vegetative storage proteins 1 and 2, thaumatin-like proteins, and those related to chloroplast and carbon metabolism. The roles of some of these significantly affected proteins detected under drought stress are discussed.

Priming of defense-related genes in Brassica oleracea var. capitata using concentrated metabolites produced by Rhizobium tropici CIAT 899

To verify the potential of metabolites extracted from Rhizobium tropici to trigger the priming of defense responses in cruciferous plants, we analyzed the expression of defense-related genes by qRT-PCR. Brassica oleracea var. capitata, susceptible to Xanthomonas campestris pv. campestris, were grown in greenhouse conditions. At 18 days after sowing, plants were inoculated with 1 mL of 1% concentrated metabolites produced by R. tropici (CM-RT) in the root. In a second experiment, leaves were sprayed with 1 mL of a solution containing 1% CM-RT. Aerial and root tissue were collected separately at 0 (non-treated control condition), 24, and 48 h after application, submitted to RNA extraction and gene expression analysis by qRT-PCR. The results showed that, after root treatment with CM-RT, most evaluated genes were upregulated at 24 h after application and downregulated at 48 h after application in roots, while in leaves, genes were downregulated both at 24 and 48 h after application. On the other hand, leaf treatment with CM-RT showed that most evaluated genes in leaves and roots were upregulated at 24 and 48 h after application. These results indicate that the effect of CM-RT applied in roots seems restricted to the applied region and is not sustained, while the application in leaves results in a more systemic response and maintenance of the effect of CM-RT for a longer period. The results obtained in this study emphasize the biotechnological potential of using metabolites of R. tropici as an elicitor of active defense responses in plants.

Microbial Derived Compounds, a Step Toward Enhancing Microbial Inoculants Technology for Sustainable Agriculture

Sustainable agriculture remains a focus for many researchers, in an effort to minimize environmental degradation and climate change. The use of plant growth promoting microorganisms (PGPM) is a hopeful approach for enhancing plant growth and yield. However, the technology faces a number of challenges, especially inconsistencies in the field. The discovery, that microbial derived compounds can independently enhance plant growth, could be a step toward minimizing shortfalls related to PGPM technology. This has led many researchers to engage in research activities involving such compounds. So far, the findings are promising as compounds have been reported to enhance plant growth under stressed and non-stressed conditions in a wide range of plant species. This review compiles current knowledge on microbial derived compounds, taking a reader through a summarized protocol of their isolation and identification, their relevance in present agricultural trends, current use and limitations, with a view to giving the reader a picture of where the technology has come from, and an insight into where it could head, with some suggestions regarding the probable best ways forward.

Bioconversion of Chitin to Bioactive Chitooligosaccharides: Amelioration and Coastal Pollution Reduction by Microbial Resources

Chitin-metabolizing products are of high industrial relevance in current scenario due to their wide biological applications, relatively lower cost, greater abundance, and sustainable supply. Chitooligosaccharides have remarkably wide spectrum of applications in therapeutics such as antitumor agents, immunomodulators, drug delivery, gene therapy, wound dressings, as chitinase inhibitors to prevent malaria. Hypocholesterolemic and antimicrobial activities of chitooligosaccharides make them a molecule of choice for food industry, and their functional profile depends on the physicochemical characteristics. Recently, chitin-based nanomaterials are also gaining tremendous importance in biomedical and agricultural applications. Crystallinity and insolubility of chitin imposes a major hurdle in the way of polymer utilization. Chemical production processes are known to produce chitooligosaccharides with variable degree of polymerization and properties along with ecological concerns. Biological production routes mainly involve chitinases, chitosanases, and chitin-binding proteins. Development of bio-catalytic production routes for chitin will not only enhance the production of commercially viable chitooligosaccharides with defined molecular properties but will also provide a means to combat marine pollution with value addition.

Fast induction of biosynthetic polysaccharide genes lpxA, lpxE, and rkpI of Rhizobium sp. strain PRF 81 by common bean seed exudates is indicative of a key role in symbiosis

Rhizobial surface polysaccharides (SPS) are, together with nodulation (Nod) factors, recognized as key molecules for establishment of rhizobia-legume symbiosis. In Rhizobium tropici, an important nitrogen-fixing symbiont of common bean (Phaseolus vulgaris L.), molecular structures and symbiotic roles of the SPS are poorly understood. In this study, Rhizobium sp. strain PRF 81 genes, belonging to the R. tropici group, were investigated: lpxA and lpxE, involved in biosynthesis and modification of the lipid-A anchor of lipopolysaccharide (LPS), and rkpI, involved in synthesis of a lipid carrier required for production of capsular polysaccharides (KPS). Reverse transcription quantitative PCR (RT-qPCR) analysis revealed, for the first time, that inducers released from common bean seeds strongly stimulated expression of all three SPS genes. When PRF 81 cells were grown for 48 h in the presence of seed exudates, twofold increases (p < 0.05) in the transcription levels of lpxE, lpxA, and rkpI genes were observed. However, higher increases (p < 0.05) in transcription rates, about 50-fold for lpxE and about 30-fold for lpxA and rkpI, were observed after only 5 min of incubation with common bean seed exudates. Evolutionary analyses revealed that lpxA and lpxE of PRF81 and of the type strain of R. tropici CIAT899(T)clustered with orthologous Rhizobium radiobacter and were more related to R. etli and Rhizobium leguminosarum, while rkpI was closer to the Sinorhizobium sp. group. Upregulation of lpxE, lpxA, and rkpI genes suggests that seed exudates can modulate production of SPS of Rhizobium sp. PRF81, leading to cell wall changes necessary for symbiosis establishment.

Biotechnological potential of rhizobial metabolites to enhance the performance of Bradyrhizobium spp. and Azospirillum brasilense inoculants with soybean and maize

Agricultural sustainability may represent the greatest encumbrance to increasing food production. On the other hand, as a component of sustainability, replacement of chemical fertilizers by bio-fertilizers has the potential to lower costs for farmers, to increase yields, and to mitigate greenhouse-gas emissions and pollution of water and soil. Rhizobia and plant-growth-promoting rhizobacteria (PGPR) have been broadly used in agriculture, and advances in our understanding of plant-bacteria interactions have been achieved; however, the use of signaling molecules to enhance crop performance is still modest. In this study, we evaluated the effects of concentrated metabolites (CM) from two strains of rhizobia-Bradyrhizobium diazoefficiens USDA 110(T) (BD1) and Rhizobium tropici CIAT 899(T) (RT1)-at two concentrations of active compounds (10(-8) and 10(-9) M)-on the performances of two major plant-microbe interactions, of Bradyrhizobium spp.-soybean (Glycine max (L.) Merr.) and Azospirillum brasilense-maize (Zea mays L.). For soybean, one greenhouse and two field experiments were performed and effects of addition of CM from the homologous and heterologous strains, and of the flavonoid genistein were investigated. For maize, three field experiments were performed to examine the effects of CM from RT1. For soybean, compared to the treatment inoculated exclusively with Bradyrhizobium, benefits were achieved with the addition of CM-BD1; at 10(-9) M, grain yield was increased by an average of 4.8%. For maize, the best result was obtained with the addition of CM-RT1, also at 10(-9) M, increasing grain yield by an average of 11.4%. These benefits might be related to a combination of effects attributed to secondary compounds produced by the rhizobial strains, including exopolysaccharides (EPSs), plant hormones and lipo-chitooligosaccharides (LCOs). The results emphasize the biotechnological potential of using secondary metabolites of rhizobia together with inoculants containing both rhizobia and PGPR to improve the growth and yield of grain crops.

Cytology, biochemistry and molecular changes during coffee fruit development

In commercial coffee species (Coffea arabica and Coffea canephora), fruit development is a lengthy process, characterized by tissue changes and evolutions. For example, soon after fecundation and up to mid development, the fruit is mainly constituted of the pericarp and perisperm tissue. Thereafter, the perisperm gradually disappears and is progressively replaced by the endosperm (true seed). Initially present in a "liquid" state, the endosperm hardens as it ripens during the maturation phase, as a result of accumulation of storage proteins, sucrose and complex polysaccharides representing the main reserves of the seed. The last step of maturation is characterized by the dehydration of the endosperm and the color change of the pericarp. Important quantitative and qualitative changes accompany fruit growth, highlighting the importance of its study to better understand the final characteristics of coffee beans. Following a description of the coffee fruit tissues, this review presents some data concerning biochemical, enzymatic and gene expression variations observed during the coffee fruit development. The latter will also be analyzed in the light of recent data (electronic expression profiles) arising from the Brazilian Coffee Genome Project.

Protein glycosylation in bacterial mucosal pathogens

In eukaryotes, glycosylated proteins are ubiquitous components of extracellular matrices and cellular surfaces. Their oligosaccharide moieties are implicated in a wide range of cell-cell and cell-matrix recognition events that are required for biological processes ranging from immune recognition to cancer development. Glycosylation was previously considered to be restricted to eukaryotes; however, through advances in analytical methods and genome sequencing, there have been increasing reports of both O-linked and N-linked protein glycosylation pathways in bacteria, particularly amongst mucosal-associated pathogens. Studying glycosylation in relatively less-complicated bacterial systems provides the opportunity to elucidate and exploit glycoprotein biosynthetic pathways. We will review the genetic organization, glycan structures and function of glycosylation systems in mucosal bacterial pathogens, and speculate on how this knowledge may help us to understand glycosylation processes in more complex eukaryotic systems and how it can be used for glycoengineering.

Developmental downregulation of rhizobial genes as a function of symbiosome differentiation in symbiotic root nodules of Pisum sativum

•  The expression of nodA and dctA genes of Rhizobium leguminosarum bv. viciae has been studied in mutant nodules of pea (Pisum sativum L.), blocked at the following developmental stages: infection thread development inside the nodule (Itn); infection droplet differentiation (Idd); bacteroid differentiation after endocytosis (Bad); and nodule persistence (Nop). •  With the use of reporter fusions to these symbiotic bacterial genes it was shown that both nodA and dctA were expressed at all developmental stages, with a pattern similar to that of constitutive, symbiosis-unrelated genes. •  As well as two constitutively expressed genes, both nodA and dctA genes seemed to be subjected to gradual downregulation in nodule bacteria, correlating with the stage of bacteroid differentiation reached. No such effect was observed for the symbiotic, oxygen-regulated fixN gene. The bacteroid development stage also appeared to be related to the ability of bacteria that have been subjected to endocytosis to resume free-living vegetative growth. •  The results support the suggestion that bacteroid differentiation into a nitrogen-fixing, organelle-like form, is a gradual process involving several stages, each controlled by different plant genes.

Promiscuity of hosting nitrogen fixation in rice: an overview from the legume perspective

The subject area of this review provides extraordinary challenges and opportunities. The challenges relate to the fact that the integration of various fields such as microbiology, biochemistry, plant physiology, eukaryotic as well as bacterial genetics, and applied plant sciences are required to assess the disposition of rice, an alien host, for establishing such a unique phenomenon as biological nitrogen fixation. The opportunities signify that, if successful, the breakthrough will have a significant impact on the global economy and will help improve the environment. This review highlights the literature related to the area of legume-rhizobia interactions, particularly those aspects whose understanding is of particular interest in the perspective of rice. This review also discusses the progress achieved so far in this area of rice research and the possibility of built-in nitrogen fixation in rice in the future. However, it is to be borne in mind that such research does not ensure any success at this point. It provides a unique opportunity to broaden our knowledge and understanding about many aspects of plant growth regulation in general.

Gene expression in nematode feeding sites

The feeding sites induced by sedentary root-endoparasitic nematodes have long fascinated researchers. Nematode feeding sites are constructed from plant cells, modified by the nematode to feed itself. Powerful new techniques are allowing us to begin to elucidate the molecular mechanisms that produce the ultrastructural features in nematode feeding cells. Many plant genes that are expressed in feeding sites produced by different nematodes have been identified in several plant species. Nematode-responsive plant genes can now be grouped in categories related to plant developmental pathways and their roles in the making of a feeding site can be illuminated. The black box of how nematodes bring about such elaborate cell differentiation in the plant is also starting to open. Although the information is far from complete, the groundwork is set so that the functions of the plant and nematode genes in feeding site development can begin to be assessed.

Role of programmed cell death in development

Programmed cell death (PCD) is an integral part of both animal and plant development. In animals, model systems such as Caenorhabditis elegans, Drosophila melanogaster, and mice have shown a general cell death profile of induction, caspase mediation, cell death, and phagocytosis. Tremendous strides have been made in cell death research in animals in the past decade. The ordering of the C. elegans genes Ced-3, 4 and 9, identification of caspase-activated DNase that degrades nuclear DNA during PCD, identification of signal transduction modules involving caspases as well as the caspase-independent pathway, and the involvement of mitochondria are some of the findings of immense value in understanding animal PCDs. Similarly, the caspase inactivation mechanisms of infecting viruses to stall host cell death give a new dimension to the viral infection process. However, plant cell death profiles provide an entirely different scenario. The presence of a cell wall that cannot be phagocytosed, absence of the hallmarks of animal PCDs such as DNA laddering, formation of apoptotic bodies, a cell-death-specific nuclease, a biochemical machinery of killer enzymes such as caspases all point to novel ways of cell elimination. Large gaps in our understanding of plant cell death have prompted speculative inferences and comparisons with animal cell death mechanisms. This paper deals with both animals and plants for a holistic view on cell death in eukaryotes.

Oxidative burst in alfalfa-Sinorhizobium meliloti symbiotic interaction

Reactive oxygen species are produced as an early event in plant defense response against avirulent pathogens. We show here that alfalfa responds to infection with Sinorhizobium meliloti by production of superoxide and hydrogen peroxide. This similarity in the early response to infection by pathogenic and symbiotic bacteria addresses the question of which mechanism rhizobia use to counteract the plant defense response.

Critical protective role of bacterial superoxide dismutase in rhizobium-legume symbiosis

In nitrogen-poor soils, rhizobia elicit nodule formation on legume roots, within which they differentiate into bacteroids that fix atmospheric nitrogen. Protection against reactive oxygen species (ROS) was anticipated to play an important role in Rhizobium-legume symbiosis because nitrogenase is extremely oxygen sensitive. We deleted the sodA gene encoding the sole cytoplasmic superoxide dismutase (SOD) of Sinorhizobium meliloti. The resulting mutant, deficient in superoxide dismutase, grew almost normally and was only moderately sensitive to oxidative stress when free living. In contrast, its symbiotic properties in alfalfa were drastically affected. Nitrogen-fixing ability was severely impaired. More strikingly, most SOD-deficient bacteria did not reach the differentiation stage of nitrogen-fixing bacteroids. The SOD-deficient mutant nodulated poorly and displayed abnormal infection. After release into plant cells, a large number of bacteria failed to differentiate into bacteroids and rapidly underwent senescence. Thus, bacterial SOD plays a key protective role in the symbiotic process.

The early nodulin gene MtN6 is a novel marker for events preceding infection of Medicago truncatula roots by Sinorhizobium meliloti

MtN6 belongs to a series of cDNA clones representing Medicago truncatula genes transcriptionally activated during nodulation by Sinorhizobium meliloti (P. Gamas, F. de Carvalho Niebel, N. Lescure, and J. V. Cullimore, Mol. Plant-Microbe Interact. 9:233-242, 1996). We show here by in situ hybridization that MtN6 transcripts specifically accumulate first at very localized regions in the outer root cell layers, corresponding to outer cortical cells containing preinfection threads. At later stages, MtN6 expression is observed ahead of growing infection threads, including in the infection zone of mature root nodules. Interestingly, regulation of MtN6 is clearly distinct from that of other early nodulins expressed in the same region of the nodule, in terms of response to bacterial symbiotic mutants and to purified Nod factors. We thus suggest that MtN6 represents the first specific marker of a pathway involved in preparation to infection, which is at least partly controlled by Nod factors. Finally, we discuss the intriguing sequence homology shown by MtN6 to a protein from Emericella (Aspergillus) nidulans, FluG, that plays a key role in controlling the organogenesis of conidiophores (B. N. Lee and T. H. Adams, Genes Dev. 8:641-651, 1994).

In vitro sulfotransferase activity of NoeE, a nodulation protein of Rhizobium sp. NGR234

Soil bacteria of the genera Azorhizobium, Bradyrhizobium, and Rhizobium liberate morphogenetic lipochitin-oligosaccharides (Nod factors) into legume rhizospheres. Nod factors, which are synthesized by the products of rhizobial nodulation (nod) genes, vary in core length as well as in the number and type of substitutions. In Rhizobium sp. NGR234, the N-acylated pentamers of N-acetyl-D-glucosamine carry an O-methylfucose group on the reducing terminus that is substituted, on a mutually exclusive basis, with either an acetyl or a sulfuryl group. A sulfotransferase encoded by noeE is required for adjunction of activated sulfate donated by 3'-phosphoadenosine 5'-phosphosulfate (PAPS). Here we show that when expressed in NGR234 cured of its symbiotic plasmid (= ANU265) or when purified as a fusion protein (MBP-NoeE), NoeE transfers sulfate from PAPS to fucosylated lipochitin-oligosaccharides. Enzyme assays showed that sulfotransferase activity is dependent on the presence of an acyl group (stearic and vaccenic acids were tested) since no activity was detected when fucosylated oligochitins (oligomers of two to six N-acetyl-D-glucosamine units) were used as substrates. Thus, NoeE is unique in that it is the only characterized sulfotransferase that is specific for fucosylated Nod factors. It probably acts after NodA, which acylates the amino-sugar backbone.

The Medicago truncatula MtAnn1 gene encoding an annexin is induced by Nod factors and during the symbiotic interaction with Rhizobium meliloti

Here we report the characterization of a new Nod factor-induced gene from Medicago truncatula identified by mRNA differential display. This gene, designated MtAnn1, encodes a protein homologous to the annexin family of calcium- and phospholipid-binding proteins. We further show that the MtAnn1 gene is also induced during symbiotic associations with Rhizobium meliloti, both at early stages in bacterial-inoculated roots and in nodule structures. By in situ hybridization, we demonstrate that MtAnn1 expression in nodules is mainly associated with the distal region of invasion zone II not containing infection threads, revealing MtAnn1 as a new marker gene of the pre-infection zone. Moreover, analyses of MtAnn1 expression in response to bacterial symbiotic mutants suggest that the expression of MtAnn1 during nodulation requires biologically active Nod factors and is independent of the infection process.

Purification and characterization of a novel chitinase-lysozyme, of another chitinase, both hydrolysing Rhizobium meliloti Nod factors, and of a pathogenesis-related protein from Medicago sativa roots

The symbiosis between Rhizobium meliloti and Medicago sativa (Leguminosae) involves the interaction of lipochito-oligosaccharides (Nod factors) excreted by bacteria with specific proteins of the host plant. The cleavage of Nod factors can be used as an enzymic assay to identify novel hydrolytic enzymes. Here a soluble extract of 3-day-old roots was fractionated by anion exchange, affinity chromatography, gel filtration and native electrophoresis. Two acidic chitinases (pI 4.6-5.4), CHIT24 and CHIT36, designated in accordance with their molecular mass in kDa, were separated. CHIT24 cleaves all tested Nod factors to produce lipotrisaccharides with the preference NodRm-V(S)>NodRm-IV >NodRm-IV(S)>=NodRm-IV(Ac,S); it also hydrolyses colloidal 3H-chitin and has lysozyme activity. The kinetics of Nod factor degradation by CHIT24 depends on substrate structural parameters, namely the length of the oligosaccharide chain and sulphation (S) at the reducing end, but not much on acetylation (Ac) at the non-reducing end. The 25-residue N-terminal sequence of CHIT24 has no similarity with known chitinases or lysozymes, indicating that it is a novel type of hydrolase. CHIT36 also hydrolyses NodRm-V(S) into NodRm-III, but it is inactive towards NodRm-IV(S) and NodRm-IV(Ac,S) formed by R. meliloti. Finally, a 17 kDa protein, P17, was co-purified with CHIT24. It neither degrades Nod factors nor exhibits lysozyme activity and shows complete identity, at the 15-residue N-terminal sequence, with a class 10 pathogenesis-related protein, PR-10.

nolO and noeI (HsnIII) of Rhizobium sp. NGR234 are involved in 3-O-carbamoylation and 2-O-methylation of Nod factors

Loci unique to specific rhizobia direct the adjunction of special groups to the core lipo-oligosaccharide Nod factors. Host-specificity of nodulation (Hsn) genes are thus essential for interaction with certain legumes. Rhizobium sp. NGR234, which can nodulate >110 genera of legumes, possesses three hsn loci and secretes a large family of Nod factors carrying specific substituents. Among them are 3-O (or 4-O)- and 6-O-carbamoyl groups, an N-methyl group, and a 2-O-methylfucose residue which may bear either 3-O-sulfate or 4-O (and 3-O)-acetyl substituents. The hsnIII locus comprises a nod box promoter followed by the genes nodABCIJnolOnoeI. Complementation and mutation analyses show that the disruption of any one of nodIJ, nolO, or noeI has no effect on nodulation. Conjugation of nolO into Rhizobium fredii extends the host range of the recipient to the non-hosts Calopogonium caeruleum and Lablab purpureus, however. Chemical analyses of the Nod factors produced by the NodI, NolO, and NoeI mutants show that the nolO and noeI gene products are required for 3 (or 4)-O-carbamoylation of the nonreducing terminus and for 2-O-methylation of the fucosyl group, respectively. Confirmation that NolO is a carbamoyltransferase was obtained from analysis of the Nod factors produced by R. fredii containing nolO; all are carbamoylated at O-3 (or O-4) on the nonreducing terminus. Since mutation of both nolO and nodU fails to completely abolish production of monocarbamoylated NodNGR factors, it is clear that a third carbamoyltransferase must exist. Nevertheless, the specificities of the two known enzymes are clearly different. NodU is only able to transfer carbamate to O-6 while NolO is specific for O-3 (or O-4) of NodNGR factors.

Symbiosis-specific expression of two Medicago truncatula nodulin genes, MtN1 and MtN13, encoding products homologous to plant defense proteins

Two Medicago truncatula nodulin genes putatively encoding proteins structurally related to two classes of proteins commonly associated with plant defense reactions have been characterized. MtN1 is homologous to two small, cysteine-rich, pathogen-inducible proteins from pea (pI39 and pI230), whereas MtN13 is closely related to the PR10 family of pathogenesis-related proteins. We show that neither MtN1 nor MtN13 is induced in leaves in response to pathogenic bacteria, and that both are exclusively expressed during nodulation. In situ hybridization experiments as well as Northern (RNA) studies of interactions between M. truncatula and either wild-type Rhizobium meliloti or mutants deficient in infection establish that MtN1 is associated with the infection process, while MtN13 represents the first specific marker described for the nodule outer cortex. Possible roles for MtN1 and MtN13 are discussed. We also present the identification of another member of the PR10 family, designated as MtPR10-1, whose regulation is strikingly different from that observed for MtN13, being constitutively expressed in roots and pathogen-inducible in leaves.

Nod factor internalization and microtubular cytoskeleton changes occur concomitantly during nodule differentiation in alfalfa

Reorganization of the plant cytoskeleton is thought to play an important role during nodule ontogeny. In situ immunolocalisation of tubulin reveals that important cytoskeletal changes, implying a transient disorganization followed by a newly patterned reorganization, occur in indeterminate and determinate nodules. In alfalfa nodules, cytoskeletal changes closely parallel the symbiotic differentiation features related to cell infection, bacterial release, endopolyploidization, cell enlargement, cell spatial organization and organelle ultrastructure and positioning. Moreover, the fact that microtubule disorganization can be correlated with Nod factor internalization in central infected cells suggests that Nod factors are possibly involved in the control of cytoskeletal changes which direct the differentiation of bacteria-containing cells.

Characterization and molecular evolution of a vertebrate hyaluronan synthase gene family

The three mammalian hyaluronan synthase (HAS) genes and the related Xenopus laevis gene, DG42, belong to a larger evolutionarily conserved vertebrate HAS gene family. We have characterized additional vertebrate HAS genes from chicken (chas2 and chas3) and Xenopus (xhas2, xhas3, and a unique Xenopus HAS-related sequence, xHAS-rs). Genomic structure analyses demonstrated that all vertebrate HAS genes share at least one exon-intron boundary, suggesting that they evolved from a common ancestral gene. Furthermore, the Has2 and Has3 genes are identical in structure, suggesting that they arose by a gene duplication event early in vertebrate evolution. Significantly, similarities in the genomic structures of the mouse Has1 and Xenopus DG42 genes strongly suggest that they are orthologues. Northern analyses revealed a similar temporal expression pattern of HAS genes in developing mouse and Xenopus embryos. Expression of mouse Has2, Has3, and Xenopus Has1 (DG42) led to hyaluronan biosynthesis in transfected mammalian cells. However, only mouse Has2 and Has3 expressing cells formed significant hyaluronan-dependent pericellular coats in culture, implying both functional similarities and differences among vertebrate HAS enzymes. We propose that vertebrate hyaluronan biosynthesis is regulated by a comparatively ancient gene family that has arisen by sequential gene duplication and divergence.