Lupin, a Unique Legume That Is Nodulated by Multiple Microsymbionts: The Role of Horizontal Gene Transfer

Lupin is a high-protein legume crop that grows in a wide range of edaphoclimatic conditions where other crops are not viable. Its unique seed nutrient profile can promote health benefits, and it has been proposed as a phytoremediation plant. Most rhizobia nodulating Lupinus species belong to the genus Bradyrhizobium, comprising strains that are phylogenetically related to B. cytisi, B. hipponenese, B. rifense, B. iriomotense/B. stylosanthis, B. diazoefficiens, B. japonicum, B. canariense/B. lupini, and B. retamae/B. valentinum. Lupins are also nodulated by fast-growing bacteria within the genera Microvirga, Ochrobactrum, Devosia, Phyllobacterium, Agrobacterium, Rhizobium, and Neorhizobium. Phylogenetic analyses of the nod and nif genes, involved in microbial colonization and symbiotic nitrogen fixation, respectively, suggest that fast-growing lupin-nodulating bacteria have acquired their symbiotic genes from rhizobial genera other than Bradyrhizobium. Horizontal transfer represents a key mechanism allowing lupin to form symbioses with bacteria that were previously considered as non-symbiotic or unable to nodulate lupin, which might favor lupin’s adaptation to specific habitats. The characterization of yet-unstudied Lupinus species, including microsymbiont whole genome analyses, will most likely expand and modify the current lupin microsymbiont taxonomy, and provide additional knowledge that might help to further increase lupin’s adaptability to marginal soils and climates.

Flavonoid Accumulation Varies in Medicago truncatula in Response to Mercury Stress

Mercury (Hg) contamination is increasing worldwide in both wild ecosystems and agricultural soils due to natural processes, but mostly to anthropic activities. The molecular mechanisms involved in Hg toxicity and tolerance in plants have been extensively studied; however, the role of flavonoids in response to Hg stress remains to be investigated. We conducted a metabolomic study to analyze the changes induced at the secondary metabolite level in three Hg-tolerant and one Hg-sensitive Medicago truncatula cultivars. A total of 46 flavonoid compounds, classified into five different flavonoid families: anthocyanidins, flavones, isoflavones, pterocarpan flavonoids, and flavanones, along with their respective glycoconjugate derivatives, were identified in leaf and root tissues. The synthesis of free isoflavones, followed by monoglycosylation and further malonylation was shown to be characteristic of root samples, whereas higher glycosylation, followed by further acylation with coumaric and ferulic acid was characteristic of leaf tissues. While minor changes were observed in leaves, significant quantitative changes could be observed in roots upon Hg treatment. Some flavonoids were strongly upregulated in roots, including malonylglucosides of biochanin A, formononetin and medicarpin, and aglycones biochanin, daidzein, and irisolidone. Hg tolerance appeared to be mainly associated to the accumulation of formononetin MalGlc, tricin GlcAGlcA, and afrormosin Glc II in leaves, whereas aglycone accumulation was associated with tolerance to Hg stress in roots. The results evidence the alteration of the flavonoid metabolic profile and their glycosylation processes in response to Hg stress. However, notable differences existed between varieties, both in the basal metabolic profile and in the response to treatment with Hg. Overall, we observed an increase in flavonoid production in response to Hg stress, and Hg tolerance appeared to be associated to a characteristic glycosylation pattern in roots, associated with the accumulation of aglycones and monoglycosylated flavonoids. The findings are discussed in the context of the flavonoid biosynthetic pathway to provide a better understanding of the role of these secondary metabolites in the response and tolerance to Hg stress in M. truncatula.

Adaptive Mechanisms Make Lupin a Choice Crop for Acidic Soils Affected by Aluminum Toxicity

Almost half of the world's agricultural soils are acidic, and most of them present significant levels of aluminum (Al) contamination, with Al³⁺ as the prevailing phytotoxic species. Lupin is a protein crop that is considered as an optimal alternative to soybean cultivation in cold climates. Lupins establish symbiosis with certain soil bacteria, collectively known as rhizobia, which are capable of fixing atmospheric nitrogen. Moreover, some lupin species, especially white lupin, form cluster roots, bottlebrush-like structures specialized in the mobilization and uptake of nutrients in poor soils. Cluster roots are also induced by Al toxicity. They exude phenolic compounds and organic acids that chelate Al to form non-phytotoxic complexes in the rhizosphere and inside the root cells, where Al complexes are accumulated in the vacuole. Lupins flourish in highly acidic soils where most crops, including other legumes, are unable to grow. Some lupin response mechanisms to Al toxicity are common to other plants, but lupin presents specific tolerance mechanisms, partly as a result of the formation of cluster roots. Al-induced lupin organic acid secretion differs from P-induced secretion, and organic acid transporters functions differ from those in other legumes. Additionally, symbiotic rhizobia can contribute to Al detoxification. After revising the existing knowledge on lupin distinct Al tolerance mechanisms, we conclude that further research is required to elucidate the specific organic acid secretion and Al accumulation mechanisms in this unique legume, but definitely, white lupin arises as a choice crop for cultivation in Al-rich acidic soils in temperate climate regions.

Transgenic Medicago truncatula Plants That Accumulate Proline Display Enhanced Tolerance to Cadmium Stress

Cadmium (Cd) accumulation in agricultural soils constitutes a serious problem for crop yields and food safety. It is known that proline (Pro) can rapidly accumulate in plant tissues in response to abiotic stress. To analyze the potential protective effect of Pro accumulation against Cd toxicity, we compared the response to Cd stress of wild-type (WT) Medicago truncatula and a transgenic line that we had previously obtained and characterized (p18), which expressed the Δ ¹-pyrroline-5-carboxylate synthetase gene from Vigna aconitifolia (VaP5CS), and accumulated high Pro levels. Cadmium significantly reduced germination of WT seeds compared to p18 seeds, and seedling relative root growth, a valid indicator of metal tolerance, was significantly higher for p18 than WT seedlings. We analyzed the relative expression of genes related to Pro metabolism, phytochelatin biosynthesis. antioxidant machinery, and NADPH recycling, which are relevant mechanisms in the response to Cd stress. They presented differential expression in the seedlings of both genotypes both under control conditions and under Cd stress, suggesting that the Cd response mechanisms might be constitutively activated in the transgenic line. Pro accumulation promoted higher survival, enhanced growth performance, and minor nutrient imbalance in transgenic p18 plants compared to WT plants. These facts, together with the recorded gluthatione levels, lipid peroxidation and antioxidant enzyme activities strongly suggested that VaP5CS expression and Pro accumulation conferred enhanced Cd tolerance to M. truncatula p18 plants, which was likely mediated by changes in Pro metabolism, increased phytochelatin biosynthesis and a more efficient antioxidant response. Moreover, p18 roots accumulated significantly higher Cd amounts than WT roots, while Cd translocation to the aerial part was similar to WT plants, thus suggesting that high Pro levels increased not only Cd tolerance, but also Cd phytostabilization by rhizosequestration.

Cadmium-Tolerant and -Sensitive Cultivars Identified by Screening of Medicago truncatula Germplasm Display Contrasting Responses to Cadmium Stress

Cadmium (Cd) pollution in soils is an increasing problem worldwide, and it affects crop production and safety. We identified Cd-tolerant and -sensitive cultivars by testing 258 accessions of Medicago truncatula at seedling stage, using the relative root growth (RRG) as an indicator of Cd tolerance. The factorial analysis (principal component analysis method) of the different growth parameters analyzed revealed a clear differentiation between accessions depending on the trait (tolerant or sensitive). We obtained a normalized index of Cd tolerance, which further supported the suitability of RRG to assess Cd tolerance at seedling stage. Cd and elements contents were analyzed, but no correlations with the tolerance trait were found. The responses to Cd stress of two accessions which had similar growth in the absence of Cd, different sensitivity to the metal but similar Cd accumulation capacity, were analyzed during germination, seedling stage, and in mature plants. The results showed that the Cd-tolerant accession (CdT) displayed a higher tolerance than the sensitive cultivar (CdS) in all the studied stages. The increased gene expression of the three main NADPH recycling enzymes in CdT might be key for this tolerance. In CdS, Cd stress produced strong expression of most of the genes that encode enzymes involved in glutathione and phytochelatin biosynthesis (MtCYS, MtγECS, and MtGSHS), as well as GR, but it was not enough to avoid a redox status imbalance and oxidative damages. Our results on gene expression, enzyme activity, antioxidant content, and lipid peroxidation indicate different strategies to cope with Cd stress between CdS and CdT, and provide new insights on Cd tolerance and Cd toxicity mechanisms in M. truncatula.

Nitrogen and Phosphorus Interplay in Lupin Root Nodules and Cluster Roots

Nitrogen (N) and phosphorus (P) are two major plant nutrients, and their deficiencies often limit plant growth and crop yield. The uptakes of N or P affect each other, and consequently, understanding N-P interactions is fundamental. Their signaling mechanisms have been studied mostly separately, and integrating N-P interactive regulation is becoming the aim of some recent works. Lupins are singular plants, as, under N and P deficiencies, they are capable to develop new organs, the N2-fixing symbiotic nodules, and some species can also transform their root architecture to form cluster roots, hundreds of short rootlets that alter their metabolism to induce a high-affinity P transport system and enhance synthesis and secretion of organic acids, flavonoids, proteases, acid phosphatases, and proton efflux. These modifications lead to mobilization in the soil of, otherwise unavailable, P. White lupin (Lupinus albus) represents a model plant to study cluster roots and for understanding plant acclimation to nutrient deficiency. It tolerates simultaneous P and N deficiencies and also enhances uptake of additional nutrients. Here, we present the structural and functional modifications that occur in conditions of P and N deficiencies and lead to the organogenesis and altered metabolism of nodules and cluster roots. Some known N and P signaling mechanisms include different factors, including phytohormones and miRNAs. The combination of the individual N and P mechanisms uncovers interactive regulation pathways that concur in nodules and cluster roots. L. albus interlinks N and P recycling processes both in the plant itself and in nature.

Potassium content diminishes in infected cells of Medicago truncatula nodules due to the mislocation of channels MtAKT1 and MtSKOR/GORK

Rhizobia establish a symbiotic relationship with legumes that results in the formation of root nodules, where bacteria encapsulated by a membrane of plant origin (symbiosomes), convert atmospheric nitrogen into ammonia. Nodules are more sensitive to ionic stresses than the host plant itself. We hypothesize that such a high vulnerability might be due to defects in ion balance in the infected tissue. Low temperature SEM (LTSEM) and X-ray microanalysis of Medicago truncatula nodules revealed a potassium (K⁺) decrease in symbiosomes and vacuoles during the life span of infected cells. To clarify K⁺ homeostasis in the nodule, we performed phylogenetic and gene expression analyses, and confocal and electron microscopy localization of two key plant Shaker K⁺ channels, AKT1 and SKOR/GORK. Phylogenetic analyses showed that the genome of some legume species, including the Medicago genus, contained one SKOR/GORK and one AKT1 gene copy, while other species contained more than one copy of each gene. Localization studies revealed mistargeting and partial depletion of both channels from the plasma membrane of M. truncatula mature nodule-infected cells that might compromise ion transport. We propose that root nodule-infected cells have defects in K⁺ balance due to mislocation of some plant ion channels, as compared with non-infected cells. The putative consequences are discussed.

Mercury-Tolerant Ensifer medicae Strains Display High Mercuric Reductase Activity and a Protective Effect on Nitrogen Fixation in Medicago truncatula Nodules Under Mercury Stress

Mercury (Hg) is extremely toxic for all living organisms. Hg-tolerant symbiotic rhizobia have the potential to increase legume tolerance, and to our knowledge, the mechanisms underlying Hg tolerance in rhizobia have not been investigated to date. Rhizobial strains of Ensifer medicae, Rhizobium leguminosarum bv. trifolii and Bradyrhizobium canariense previously isolated from severely Hg-contaminated soils showed different levels of Hg tolerance. The ability of the strains to reduce mercury Hg²⁺ to Hg⁰, a volatile and less toxic form of mercury, was assessed using a Hg volatilization assay. In general, tolerant strains displayed high mercuric reductase activity, which appeared to be inducible in some strains when grown at a sub-lethal HgCl₂ concentration. A strong correlation between Hg tolerance and mercuric reductase activity was observed for E. medicae strains, whereas this was not the case for the B. canariense strains, suggesting that additional Hg tolerance mechanisms could be playing a role in B. canariense. Transcript abundance from merA, the gene that encodes mercuric reductase, was quantified in tolerant and sensitive E. medicae and R. leguminosarum strains. Tolerant strains presented higher merA expression than sensitive ones, and an increase in transcript abundance was observed for some strains when bacteria were grown in the presence of a sub-lethal HgCl₂ concentration. These results suggest a regulation of mercuric reductase in rhizobia. Expression of merA genes and mercuric reductase activity were confirmed in Medicago truncatula nodules formed by a sensitive or a tolerant E. medicae strain. Transcript accumulation in nodules formed by the tolerant strain increased when Hg stress was applied, while a significant decrease in expression occurred upon stress application in nodules formed by the Hg-sensitive strain. The effect of Hg stress on nitrogen fixation was evaluated, and in our experimental conditions, nitrogenase activity was not affected in nodules formed by the tolerant strain, while a significant decrease in activity was observed in nodules elicited by the Hg-sensitive bacteria. Our results suggest that the combination of tolerant legumes with tolerant rhizobia constitutes a potentially powerful tool in the bioremediation of Hg-contaminated soils.

Genome-Wide Association Study Reveals Complex Genetic Architecture of Cadmium and Mercury Accumulation and Tolerance Traits in Medicago truncatula

Heavy metals are an increasing problem due to contamination from human sources that and can enter the food chain by being taken up by plants. Understanding the genetic basis of accumulation and tolerance in plants is important for reducing the uptake of toxic metals in crops and crop relatives, as well as for removing heavy metals from soils by means of phytoremediation. Following exposure of Medicago truncatula seedlings to cadmium (Cd) and mercury (Hg), we conducted a genome-wide association study using relative root growth (RRG) and leaf accumulation measurements. Cd and Hg accumulation and RRG had heritability ranging 0.44 - 0.72 indicating high genetic diversity for these traits. The Cd and Hg trait associations were broadly distributed throughout the genome, indicated the traits are polygenic and involve several quantitative loci. For all traits, candidate genes included several membrane associated ATP-binding cassette transporters, P-type ATPase transporters, oxidative stress response genes, and stress related UDP-glycosyltransferases. The P-type ATPase transporters and ATP-binding cassette protein-families have roles in vacuole transport of heavy metals, and our findings support their wide use in physiological plant responses to heavy metals and abiotic stresses. We also found associations between Cd RRG with the genes CAX3 and PDR3, two linked adjacent genes, and leaf accumulation of Hg associated with the genes NRAMP6 and CAX9. When plant genotypes with the most extreme phenotypes were compared, we found significant divergence in genomic regions using population genomics methods that contained metal transport and stress response gene ontologies. Several of these genomic regions show high linkage disequilibrium (LD) among candidate genes suggesting they have evolved together. Minor allele frequency (MAF) and effect size of the most significant SNPs was negatively correlated with large effect alleles being most rare. This is consistent with purifying selection against alleles that increase toxicity and abiotic stress. Conversely, the alleles with large affect that had higher frequencies that were associated with the exclusion of Cd and Hg. Overall, macroevolutionary conservation of heavy metal and stress response genes is important for improvement of forage crops by harnessing wild genetic variants in gene banks such as the Medicago HapMap collection.

The Symbiosome: Legume and Rhizobia Co-evolution toward a Nitrogen-Fixing Organelle?

In legume nodules, symbiosomes containing endosymbiotic rhizobial bacteria act as temporary plant organelles that are responsible for nitrogen fixation, these bacteria develop mutual metabolic dependence with the host legume. In most legumes, the rhizobia infect post-mitotic cells that have lost their ability to divide, although in some nodules cells do maintain their mitotic capacity after infection. Here, we review what is currently known about legume symbiosomes from an evolutionary and developmental perspective, and in the context of the different interactions between diazotroph bacteria and eukaryotes. As a result, it can be concluded that the symbiosome possesses organelle-like characteristics due to its metabolic behavior, the composite origin and differentiation of its membrane, the retargeting of host cell proteins, the control of microsymbiont proliferation and differentiation by the host legume, and the cytoskeletal dynamics and symbiosome segregation during the division of rhizobia-infected cells. Different degrees of symbiosome evolution can be defined, specifically in relation to rhizobial infection and to the different types of nodule. Thus, our current understanding of the symbiosome suggests that it might be considered a nitrogen-fixing link in organelle evolution and that the distinct types of legume symbiosomes could represent different evolutionary stages toward the generation of a nitrogen-fixing organelle.

Medicago truncatula natural resistance-associated macrophage Protein1 is required for iron uptake by rhizobia-infected nodule cells

Iron is critical for symbiotic nitrogen fixation (SNF) as a key component of multiple ferroproteins involved in this biological process. In the model legume Medicago truncatula, iron is delivered by the vasculature to the infection/maturation zone (zone II) of the nodule, where it is released to the apoplast. From there, plasma membrane iron transporters move it into rhizobia-containing cells, where iron is used as the cofactor of multiple plant and rhizobial proteins (e.g. plant leghemoglobin and bacterial nitrogenase). MtNramp1 (Medtr3g088460) is the M. truncatula Natural Resistance-Associated Macrophage Protein family member, with the highest expression levels in roots and nodules. Immunolocalization studies indicate that MtNramp1 is mainly targeted to the plasma membrane. A loss-of-function nramp1 mutant exhibited reduced growth compared with the wild type under symbiotic conditions, but not when fertilized with mineral nitrogen. Nitrogenase activity was low in the mutant, whereas exogenous iron and expression of wild-type MtNramp1 in mutant nodules increased nitrogen fixation to normal levels. These data are consistent with a model in which MtNramp1 is the main transporter responsible for apoplastic iron uptake by rhizobia-infected cells in zone II.

The future of lupin as a protein crop in Europe

Europe has become heavily dependent on soya bean imports, entailing trade agreements and quality standards that do not satisfy the European citizen's expectations. White, yellow, and narrow-leafed lupins are native European legumes that can become true alternatives to soya bean, given their elevated and high-quality protein content, potential health benefits, suitability for sustainable production, and acceptability to consumers. Nevertheless, lupin cultivation in Europe remains largely insufficient to guarantee a steady supply to the food industry, which in turn must innovate to produce attractive lupin-based protein-rich foods. Here, we address different aspects of the food supply chain that should be considered for lupin exploitation as a high-value protein source. Advanced breeding techniques are needed to provide new lupin varieties for socio-economically and environmentally sustainable cultivation. Novel processes should be optimized to obtain high-quality, safe lupin protein ingredients, and marketable foods need to be developed and offered to consumers. With such an integrated strategy, lupins can be established as an alternative protein crop, capable of promoting socio-economic growth and environmental benefits in Europe.

The Sinorhizobium fredii HH103 lipopolysaccharide is not only relevant at early soybean nodulation stages but also for symbiosome stability in mature nodules

In this work we have characterised the Sinorhizobium fredii HH103 greA lpsB lpsCDE genetic region and analysed for the first time the symbiotic performance of Sinorhizobium fredii lps mutants on soybean. The organization of the S. fredii HH103 greA, lpsB, and lpsCDE genes was equal to that of Sinorhizobium meliloti 1021. S. fredii HH103 greA, lpsB, and lpsE mutant derivatives produced altered LPS profiles that were characteristic of the gene mutated. In addition, S. fredii HH103 greA mutants showed a reduction in bacterial mobility and an increase of auto-agglutination in liquid cultures. RT-PCR and qPCR experiments demonstrated that the HH103 greA gene has a positive effect on the transcription of lpsB. Soybean plants inoculated with HH103 greA, lpsB or lpsE mutants formed numerous ineffective pseudonodules and showed severe symptoms of nitrogen starvation. However, HH103 greA and lps mutants were also able to induce the formation of a reduced number of soybean nodules of normal external morphology, allowing the possibility of studying the importance of bacterial LPS in later stages of the S. fredii HH103-soybean symbiosis. The infected cells of these nodules showed signs of early termination of symbiosis and lytical clearance of bacteroids. These cells also had very thick walls and accumulation of phenolic-like compounds, pointing to induced defense reactions. Our results show the importance of bacterial LPS in later stages of the S. fredii HH103-soybean symbiosis and their role in preventing host cell defense reactions. S. fredii HH103 lpsB mutants also showed reduced nodulation with Vigna unguiculata, although the symbiotic impairment was less pronounced than in soybean.

Flavodoxin overexpression confers tolerance to oxidative stress in beneficial soil bacteria and improves survival in the presence of the herbicides paraquat and atrazine

AIM

To determine whether expression of a cyanobacterial flavodoxin in soil bacteria of agronomic interest confers protection against the widely used herbicides paraquat and atrazine.

METHODS AND RESULTS

The model bacterium Escherichia coli, the symbiotic nitrogen-fixing bacterium Ensifer meliloti and the plant growth-promoting rhizobacterium Pseudomonas fluorescens Aur6 were transformed with expression vectors containing the flavodoxin gene of Anabaena variabilis. Expression of the cyanobacterial protein was confirmed by Western blot. Bacterial tolerance to oxidative stress was tested in solid medium supplemented with hydrogen peroxide, paraquat or atrazine. In all three bacterial strains, flavodoxin expression enhanced tolerance to the oxidative stress provoked by hydrogen peroxide and by the reactive oxygen species-inducing herbicides, witnessed by the enhanced survival of the transformed bacteria in the presence of these oxidizing agents.

CONCLUSIONS

Flavodoxin overexpression in beneficial soil bacteria confers tolerance to oxidative stress and improves their survival in the presence of the herbicides paraquat and atrazine. Flavodoxin could be considered as a general antioxidant resource to face oxidative challenges in different micro-organisms.

SIGNIFICANCE AND IMPACT OF THE STUDY

The use of plant growth-promoting rhizobacteria or nitrogen-fixing bacteria with enhanced tolerance to oxidative stress in contaminated soils is of significant agronomic interest. The enhanced tolerance of flavodoxin-expressing bacteria to atrazine and paraquat points to potential applications in herbicide-treated soils.

Effects of salt stress and rhizobial inoculation on growth and nitrogen fixation of three peanut cultivars

Increasing soil salinity represents a major constraint for agriculture in arid and semi-arid lands, where mineral nitrogen (N) deficiency is also a frequent characteristic of soils. Biological N fixation by legumes may constitute a sustainable alternative to chemical fertilisation in salinity-affected areas, provided that adapted cultivars and inoculants are available. Here, the performance of three peanut cultivars nodulated with two different rhizobial strains that differ in their salt tolerance was evaluated under moderately saline water irrigation and compared with that of N-fertilised plants. Shoot weight was used as an indicator of yield. Under non-saline conditions, higher yields were obtained using N fertilisation rather than inoculation for all the varieties tested. However, under salt stress, the yield of inoculated plants became comparable to that of N-fertilised plants, with minor differences depending on the peanut cultivar and rhizobial strain. Our results indicate that N fixation might represent an economical, competitive and environmentally friendly choice with respect to mineral N fertilisation for peanut cultivation under moderate saline conditions.

Alfalfa nodules elicited by a flavodoxin-overexpressing Ensifer meliloti strain display nitrogen-fixing activity with enhanced tolerance to salinity stress

Nitrogen fixation by legumes is very sensitive to salinity stress, which can severely reduce the productivity of legume crops and their soil-enriching capacity. Salinity is known to cause oxidative stress in the nodule by generating reactive oxygen species (ROS). Flavodoxins are involved in the response to oxidative stress in bacteria and cyanobacteria. Prevention of ROS production by flavodoxin overexpression in bacteroids might lead to a protective effect on nodule functioning under salinity stress. Tolerance to salinity stress was evaluated in alfalfa nodules elicited by an Ensifer meliloti strain that overexpressed a cyanobacterial flavodoxin compared with nodules produced by the wild-type bacteria. Nitrogen fixation, antioxidant and carbon metabolism enzyme activities were determined. The decline in nitrogenase activity associated to salinity stress was significantly less in flavodoxin-expressing than in wild-type nodules. We detected small but significant changes in nodule antioxidant metabolism involving the ascorbate-glutathione cycle enzymes and metabolites, as well as differences in activity of the carbon metabolism enzyme sucrose synthase, and an atypical starch accumulation pattern in flavodoxin-containing nodules. Salt-induced structural and ultrastructural alterations were examined in detail in alfalfa wild-type nodules by light and electron microscopy and compared to flavodoxin-containing nodules. Flavodoxin reduced salt-induced structural damage, which primarily affected young infected tissues and not fully differentiated bacteroids. The results indicate that overexpression of flavodoxin in bacteroids has a protective effect on the function and structure of alfalfa nodules subjected to salinity stress conditions. Putative protection mechanisms are discussed.

Immunohistochemical observation of indole-3-acetic acid at the IAA synthetic maize coleoptile tips

To investigate the distribution of IAA (indole-3-acetic acid) and the IAA synthetic cells in maize coleoptiles, we established immunohistochemistry of IAA using an anti-IAA-C-monoclonal antibody. We first confirmed the specificity of the antibody by comparing the amounts of endogenous free and conjugated IAA to the IAA signal obtained from the IAA antibody. Depletion of endogenous IAA showed a corresponding decrease in immuno-signal intensity and negligible cross-reactivity against IAA-related compounds, including tryptophan, indole-3-acetamide, and conjugated-IAA was observed. Immunolocalization showed that the IAA signal was intense in the approximately 1 mm region and the outer epidermis at the approximately 0.5 mm region from the top of coleoptiles treated with 1-N-naphthylphthalamic acid. By contrast, the IAA immuno-signal in the outer epidermis almost disappeared after 5-methyl-tryptophan treatment. Immunogold labeling of IAA with an anti-IAA-N-polyclonal antibody in the outer-epidermal cells showed cytoplasmic localization of free-IAA, but none in cell walls or vacuoles. These findings indicated that IAA is synthesized in the 0–2.0 mm region of maize coleoptile tips from Trp, in which the outer-epidermal cells of the 0.5 mm tip are the most active IAA synthetic cells.

Nitrogen fixation persists under conditions of salt stress in transgenic Medicago truncatula plants expressing a cyanobacterial flavodoxin

Several recent studies have demonstrated that the expression of a cyanobacterial flavodoxin in plants can provide tolerance to a wide range of environmental stresses. Indeed, this strategy has been proposed as a potentially powerful biotechnological tool to generate multiple-tolerant crops. To determine whether flavodoxin expression specifically increased tolerance to salt stress and whether it might also preserve legume nitrogen fixation under saline conditions, the flavodoxin gene was introduced into the model legume Medicago truncatula. Expression of flavodoxin did not confer saline tolerance to the whole plant, although the sensitive nitrogen-fixing activity was maintained under salt stress in flavodoxin-expressing plants. Our results indicate that flavodoxin induced small but significant changes in the enzymatic activities involved in the nodule redox balance that might be responsible for the positive effect on nitrogen fixation. Expression of flavodoxin can be regarded as a potential tool to improve legume symbiotic performance under salt stress, and possibly other environmental stresses.

Starch granule initiation in Arabidopsis requires the presence of either class IV or class III starch synthases

The mechanisms underlying starch granule initiation remain unknown. We have recently reported that mutation of soluble starch synthase IV (SSIV) in Arabidopsis thaliana results in restriction of the number of starch granules to a single, large, particle per plastid, thereby defining an important component of the starch priming machinery. In this work, we provide further evidence for the function of SSIV in the priming process of starch granule formation and show that SSIV is necessary and sufficient to establish the correct number of starch granules observed in wild-type chloroplasts. The role of SSIV in granule seeding can be replaced, in part, by the phylogenetically related SSIII. Indeed, the simultaneous elimination of both proteins prevents Arabidopsis from synthesizing starch, thus demonstrating that other starch synthases cannot support starch synthesis despite remaining enzymatically active. Herein, we describe the substrate specificity and kinetic properties of SSIV and its subchloroplastic localization in specific regions associated with the edges of starch granules. The data presented in this work point to a complex mechanism for starch granule formation and to the different abilities of SSIV and SSIII to support this process in Arabidopsis leaves.

Overexpression of flavodoxin in bacteroids induces changes in antioxidant metabolism leading to delayed senescence and starch accumulation in alfalfa root nodules

Sinorhizobium meliloti cells were engineered to overexpress Anabaena variabilis flavodoxin, a protein that is involved in the response to oxidative stress. Nodule natural senescence was characterized in alfalfa (Medicago sativa) plants nodulated by the flavodoxin-overexpressing rhizobia or the corresponding control bacteria. The decline of nitrogenase activity and the nodule structural and ultrastructural alterations that are associated with nodule senescence were significantly delayed in flavodoxin-expressing nodules. Substantial changes in nodule antioxidant metabolism, involving antioxidant enzymes and ascorbate-glutathione cycle enzymes and metabolites, were detected in flavodoxin-containing nodules. Lipid peroxidation was also significantly lower in flavodoxin-expressing nodules than in control nodules. The observed amelioration of the oxidative balance suggests that the delay in nodule senescence was most likely due to a role of the protein in reactive oxygen species detoxification. Flavodoxin overexpression also led to high starch accumulation in nodules, without reduction of the nitrogen-fixing activity.

Multiple roles for cytokinin receptors and cross-talk of signaling pathways

Cytokinin receptors (CRs) are hybrid-type histidine kinases, membrane proteins with a cytokinin-binding extracellular domain. CRs initiate and propagate cytokinin signaling by means of phosphorylation and phosphotransfer to downstream proteins. In legumes, some members of the CR multigenic family are essential for nodulation. In two recent works, we investigated the involvement of two new CRs, MsHK1 from Medicago sativa, and LaHK1 from Lupinus albus, in nodule morphogenesis, senescence and stress response. LaHK1 expression increased during the first stages of lupin nodule development, while MsHK1 expression was localized in the meristem and the invasion zone of alfalfa nodules pointing to a role for CRs in nodule cell proliferation and differentiation. Both CRs were also induced during nodule senescence. MsHK1 expression increased under osmotic stress and both genes were induced following dark stress, indicating that CRs are also likely to play a significant role in the response to stress. We propose multiple roles for CRs which, when analyzed jointly with recent results from other authors, suggest coordinated cross-talk of different signaling systems.

Diversity of rhizobial bacteria isolated from nodules of the Gypsophyte Ononis tridentata L. growing in Spanish soils

The aim of this work is to describe the diversity and phylogeny of rhizobial bacteria associated to nodules of Ononis tridentata L. in different geographical regions of Spain. Twenty-two bacterial isolates were characterized using several molecular techniques (16S amplified ribosomal deoxyribonucleic acid restriction analysis, fingerprinting, and sequencing) and phylogenies were inferred from their 16S and nodC gene sequences. Phylogenetically, the isolates grouped with the genera Rhizobium, Mesorhizobium, Phylobacterium, and Bosea. The nodC gene, essential for nodulation, was detected for the first time in isolates close to the genera Bosea and Phyllobacterium. The bacteria isolated showed a high diversity at the genus, species, and strain level regardless of the geographical origin of the host plant. This is the first report describing bacteria associated to nodules of O. tridentata. This shrub legume is highly prized for the revegetation of gypsum soils in semiarid Mediterranean areas. Our molecular description of bacteria associated to this legume improves the current understanding of the ecology of this plant species. Our findings have implications for formulating suitable bacterial inocula to recover gypsum ecosystems.

A salt stress-responsive cytokinin receptor homologue isolated from Medicago sativa nodules

A new cytokinin receptor homologue, MsHK1, was isolated from Medicago sativa root nodules. MsHK1 expression was induced in alfalfa seedlings by exogenous application of the cytokinin trans-zeatin. Transcript accumulation was detected in different plant organs. MsHK1 expression was induced by salt stress in alfalfa roots, leaves and nodules, and transcript accumulation in the vascular bundles pointed to a putative role in osmosensing for MsHK1 and/or other close cytokinin receptor homologues. Expression in the meristem and the invasion zone of the nodule suggest a role for cytokinin receptors in cytokinin sensing during nodule cell division and differentiation.

A cytokinin receptor homologue is induced during root nodule organogenesis and senescence in Lupinus albus L

Here we report the isolation of a new cytokinin receptor homologue, LaHK1, from lupin (Lupinus albus) root nodules. LaHK1 transcript accumulation was detected in different plant organs, and expression was analyzed throughout nodule development. We observed notably higher expression in nodule primordia and young nodules compared to the root or to mature nodules. We also detected elevated transcript accumulation in naturally senescent nodules and in senescent nodules subjected to foliar dark stress. The results could be an indication of a putative role of this cytokinin receptor homologue in nodule development, from morphogenesis through senescence.

A light and electron microscopy analysis of the events leading to male sterility in Ogu-INRA CMS of rapeseed (Brassica napus)

Ogura cytoplasmic male sterility (CMS) occurs naturally in radish and has been introduced into rapeseed (Brassica napus) by protoplast fusion. As with all CMS systems, it involves a constitutively expressed mitochondrial gene which induces male sterility to otherwise hermaphroditic plants (so they become females) and a nuclear gene named restorer of fertility that restores pollen production in plants carrying a sterility-inducing cytoplasm. A correlative approach using light and electron microscopy was applied to define what stages throughout development were affected and the subcellular events leading to the abortion of the developing pollen grains upon the expression of the mitochondrial protein. Three central stages of development (tetrad, mid-microspore and vacuolate microspore) were compared between fertile, restored, and sterile plants. At each stage observed, the pollen in fertile and restored plants had similar cellular structures and organization. The deleterious effect of the sterility protein expression started as early as the tetrad stage. No typical mitochondria were identified in the tapetum at any developmental stage and in the vacuolate microspores of the sterile plants. In addition, some striking ultrastructural alterations of the cell's organization were also observed compared with the normal pattern of development. The results showed that Ogu-INRA CMS was due to premature cell death events of the tapetal cells, presumably by an autolysis process rather than a normal PCD, which impairs pollen development at the vacuolate microspore stage, in the absence of functional mitochondria.

The phenotype of soluble starch synthase IV defective mutants of Arabidopsis thaliana suggests a novel function of elongation enzymes in the control of starch granule formation

All plants and green algae synthesize starch through the action of the same five classes of elongation enzymes: the starch synthases. Arabidopsis mutants defective for the synthesis of the soluble starch synthase IV (SSIV) type of elongation enzyme have now been characterized. The mutant plants displayed a severe growth defect but nonetheless accumulated near to normal levels of polysaccharide storage. Detailed structural analysis has failed to yield any change in starch granule structure. However, the number of granules per plastid has dramatically decreased leading to a large increase in their size. These results, which distinguish the SSIV mutants from all other mutants reported to date, suggest a specific function of this enzyme class in the control of granule numbers. We speculate therefore that SSIV could be selectively involved in the priming of starch granule formation.

Conformation of cytoskeletal elements during the division of infected Lupinus albus L. nodule cells

Lupin nodule cells maintain their ability to divide for several cycles after being infected by endosymbiotic rhizobia. The conformation of the cytoskeletal elements of nodule cells was studied by fluorescence labelling, immunocytochemistry, and laser confocal and transmission electron microscopy. The dividing infected cells showed the normal microtubule and actin patterns of dividing plant cells. The clustered symbiosomes were tethered to the spindle-pole regions and moved to the cell poles during spindle elongation. In metaphase, anaphase, and early telophase, the symbiosomes were found at opposite cell poles where they did not interfere with the spindle filaments or phragmoplast. This symbiosome positioning was comparable with that of the organelles (which ensures organelle inheritance during plant cell mitosis). Tubulin microtubules and actin microfilaments appeared to be in contact with the symbiosomes. The possible presence of actin molecular motor myosin in nodules was analysed using a monoclonal antibody against the myosin light chain. The antigen was detected in protein extracts of nodule and root cytosol as bands of approximately 20 kDa (the size expected). In the nodules, an additional polypeptide of 65 kDa was found. Immunogold techniques revealed the antigen to be localized over thin microfilaments linked to the cell wall, as well as over the thicker microfilament bundles and surrounding the symbiosomes. The pattern of cytoskeleton rearrangement in dividing infected cells, along with the presence of myosin antigen, suggests that the positioning of symbiosomes in lupin nodule cells might depend on the same mechanisms used to partition genuine plant cell organelles during mitosis.

Transgenic Medicago truncatula plants that accumulate proline display nitrogen-fixing activity with enhanced tolerance to osmotic stress

Legume root nodule nitrogen-fixing activity is severely affected by osmotic stress. Proline accumulation has been shown to induce tolerance to salt stress, and transgenic plants over-expressing Delta(1)-pyrroline-5-carboxylate synthetase (P5CS), which accumulates high levels of proline, display enhanced osmotolerance. Here, we transformed the model legume Medicago truncatula with the P5CS gene from Vigna aconitifolia, and nodule activity was evaluated under osmotic stress in transgenic plants that showed high proline accumulation levels. Nitrogen fixation was significantly less affected by salt treatment compared to wild-type (WT) plants. To our knowledge, this is the first time that transgenic legumes have been produced that display nitrogen-fixing activity with enhanced tolerance to osmotic stress. We studied the expression of M. truncatula proline-related endogenous genes M. truncatulaDelta(1)-pyrroline-5-carboxylate synthetase 1 (MtP5CS1), M. truncatulaDelta(1)-pyrroline-5-carboxylate synthetase 2 (MtP5CS2), M. truncatula ornithine delta-aminotransferase (MtOAT), M. truncatula proline dehydrogenase (MtProDH) and a proline transporter gene in both WT and transgenic plants. Our results indicate that proline metabolism is finely regulated in response to osmotic stress in an organ-specific manner. The transgenic model allowed us to analyse some of the biochemical and molecular mechanisms that are activated in the nodule in response to high salt conditions, and to ascertain the essential role of proline in the maintenance of nitrogen-fixing activity under osmotic stress.

Nuclear DNA endoreduplication and expression of the mitotic inhibitor Ccs52 associated to determinate and lupinoid nodule organogenesis

Lotus japonicus determinate nodules differ greatly from indeterminate nodules in their organogenesis and morphological characteristics, whereas Lupinus albus lupinoid nodules share features of determinate and indeterminate nodules. The mitotic inhibitor Ccs52A is essential for endoreduplication and ploidy-dependent cell enlargement during symbiotic cell differentiation in Medicago truncatula indeterminate nodules. ccs52A homolog genes were isolated from lupin and lotus nodules; the deduced Ccs52A proteins showed high sequence similarity with other Cdh-1-type activators of the anaphase-promoting complex and were grouped with A-type Ccs52 proteins from different plants. In lupin, ccs52A expression was restricted to the earlier stages of nodule development, whereas ccs52A transcripts accumulated in lotus nodule primordia and, to a lesser extent, in mature nodules. Nodule development in Lupinus albus involved a progressive increase in nuclear and cellular size and ploidy level; similarly, Lotus japonicus nodules contained polyploid nuclei and enlarged cells in the infected zone. Nevertheless, in situ hybridization experiments showed the highest ccs52A expression in the inner cortex cells of the lupin nodule primordium, probably associated to the increased size of these cells in mature nodules. In view of our results, Ccs52A-mediated endoreduplication appears to be a universal mechanism required for nodule cell differentiation during the establishment of nitrogen-fixing symbioses.

Involvement of mitogen-activated protein kinases in the symbiosis Bradyrhizobium-Lupinus

In plants, mitogen-activated protein kinases (MAPKs) are involved in signalling to hormones, cell cycle regulation, stresses, and plant defence responses. In this work, several MAPKs were detected by immunobloting in roots and nodules of Lupinus albus produced by inoculation with Bradyrhizobium sp. (Lupinus). In vitro kinase assays showed that inoculation of seedling roots with B. sp. (Lupinus) activates salt stress-inducible and stress-activated MAPKs after 5 min of incubation. By contrast, inoculation with dead B. sp. (Lupinus) or the heterologous bacteria Sinorhizobium meliloti did not induce salt stress-inducible and stress-activated MAPK activities. In vivo experiments showed that inoculation with B. sp. (Lupinus) induced the activation of MAPKs in roots. The maximal activation was in the region of the root tip with emerging hairs, which corresponds to the infection zone. The p38 MAPK inhibitors SB 202190 and SB 203580 blocked these kinase activities. Experiments with SB 202190 and the MAPKK inhibitor UO 126 altered the pattern of nodulation in the main root, decreasing the number and weight of nodules produced in the upper sites while increasing the nodule number in the younger lower root zone. These data suggest that MAPK inhibition blocks early events in the susceptible root zone to rhizobial infection, delaying nodulation, and support a role for MAPKs in the infection and nodulation of L. albus by B. sp. (Lupinus).

Aldehyde oxidase (AO) in the root nodules of Lupinus albus and Medicago truncatula: identification of AO in meristematic and infection zones

Phytohormones are involved in the organogenesis of legume root nodules. The source of the auxin indole-3-acetic acid (IAA) in nodules has not been clearly determined. We studied the enzyme aldehyde oxidase (AO; EC 1.2.3.1), that catalyzes the last step of IAA biosynthesis in plants, in the nodules of Lupinus albus and Medicago truncatula. Primordia and young lupin nodules and mature M. truncatula nodules showed AO activity bands after native polyacrylamide gel electrophoresis. Gel activity analyses using indole-3-aldehyde as substrate indicated that the nodules of white lupin and M. truncatula have the capability to synthesize IAA via the indole-3-pyruvic acid pathway. Immunolocalization and in situ hybridization experiments revealed that AO is preferentially expressed in the meristematic and the invasion zones in Medicago nodules and in the lateral meristematic zone of Lupinus nodules. High IAA immunolabeling was also detected in the meristematic and invasion zones. Low expression levels and no AO activity were detected in lupin Fix- nodules that displayed restricted growth and early senescence. We propose that local synthesis of IAA in the root nodule meristem and modulation of AO expression and activity are involved in regulation of nodule development.

Legume nodule senescence: roles for redox and hormone signalling in the orchestration of the natural aging process

Research on legume nodule development has contributed greatly to our current understanding of plant-microbe interactions. However, the factors that orchestrate root nodule senescence have received relatively little attention. Accumulating evidence suggests that redox signals contribute to the establishment of symbiosis and senescence. Although degenerative in nature, nodule senescence is an active process programmed in development in which reactive oxygen species (ROS), antioxidants, hormones and proteinases have key roles. Nodules have high levels of the redox buffers, ascorbate and glutathione, which are important in the nodulation process and in senescence. These metabolites decline with N-fixation as the nodule ages but the resultant decrease in redox buffering capacity does not necessarily lead to enhanced ROS or oxidative stress. We propose models by which ROS and antioxidants interact with hormones such as abscisic acid in the orchestration of nodule senescence.

An unusual infection mechanism and nodule morphogenesis in white lupin (Lupinus albus)

•  The infection of white lupin (Lupinus albus) roots and the early stages in organogenesis of the lupinoid nodule are characterized in detail in this work. •  Immunolabelling of Bradyrhizobium sp. (Lupinus) ISLU16 and green fluorescent protein labelling of Mesorhizobium loti NZP2037, two strains that induce nodulation in L. albus, allowed us to monitor the infection and morphogenesis process. Light and transmission electron microscopy, low-temperature scanning electron microscopy, fluorescence and confocal microscopy were employed. •  Rhizobia penetrated the root intercellularly at the junction between the root hair base and an adjacent epidermal cell. Bacteria invaded the subepidermal cortical cell immediately beneath the root hair through structurally altered cell wall regions. The newly infected cell divided repeatedly to form the central infected zone of the young nodule. Bacteria seemed to be equally distributed between the daughter cells. •  A new mode of direct epidermal infection and an unusual morphogenesis for indeterminate nodules lead to the formation of the lupinoid nodule with unique characteristics.

Immunolocalization of alkaloids and X-ray microanalysis of elements in lupin seeds

Immunolocalization of alkaloids in lupin seeds (Lupinus spp.) has been performed by cryofixation and conventional methods. Alkaloids were localized in the protein bodies of the cotyledon cells. Some immunogold particles in the walls of these cells were also observed. There were no differences in the sites of localization between the two mentioned methods. X-ray microanalysis of elements showed the presence of P, Mg, S, and K in the protein bodies of cotyledon cells in lupin seeds. The role of K+ in alkaloids transport is discussed.

Axillary block in children: single or multiple injection?

The goal of this double-blind prospective study was to compare the effect of a single injection versus multiple fractionated doses on the onset time and quality of motor and sensory block, obtained in 70 children anaesthetized with axillary block alone. The brachial plexus was identified with a peripheral nerve stimulator, and blocked with 0.5 ml.kg-1 of 1.5% lignocaine with adrenaline. In Group S (single injection), the total volume was injected after location of one nerve. In Group M (multiple fractionated doses), two nerves were located, including necessarily one nerve implicated in the surgical territory. Motor and sensory blocks were assessed according to Lanz's scale before surgery by a blinded observer. A block was considered complete if there was no feeling in at least three nerve territories at 30 min. No difference was found between groups for motor and sensory block quality. However the onset time of the block was faster after multiple fractionated doses (Group M, 25+/-7 min vs Group S, 29+/-4 min) and was faster in younger children (5-9 years: M=23+/-7 min vs S=28+/-5 min, 10-15 years: no difference). There was a significant difference in the quality of the sensory blockade of the musculocutaneous nerve: 18 versus 8 complete blocks, 10 versus 14 incomplete blocks, respectively for Group M versus Group S. No adverse effect was observed and analgesia was prolonged for more than 4 h. We can conclude that, unlike adults, fractionated doses in chilren bring no benefit to the quality of sensory and motor block. Selective block of the musculocutaneous nerve is recommended when a surgical procedure takes place in this territory.

Inhibition of the activity of a neuronal kappaB-binding factor by glutamate

Activation of transcription factors with affinity for kappaB enhancers is generally correlated with enhanced survival of neurons. In an apparent exception, excitotoxic concentrations of glutamate have been reported to elevate the activity of one such factor, nuclear factor-kappaB (NF-kappaB). Our data indicate that the constitutive neuronal kappaB-binding factor (NKBF) is distinct from bona fide NF-kappaB (RelA/p50 heterodimer). Therefore, we analyzed glutamate's effects on KB-binding activity in highly enriched primary neuronal cultures and in mixed neuron/glia cocultures. Electrophoretic mobility shift assays indicated that a 30-60-min exposure to 50-500 microM glutamate reduced NKBF activity by as much as 70%. Subtoxic doses of glutamate had little or no effect on this DNA-binding activity. Selective antagonists of either NMDA or AMPA [(RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate]/kai nat e receptors inhibited the influence of glutamate on NKBF activity. The effect of glutamate was mimicked by calcium ionophore, and it was blocked by lowering extracellular calcium concentrations or by cyclosporin A. Bona fide NF-kappaB was found only in cocultures containing significant numbers of glia, where it could be activated by glutamate. These data suggest that the primary influence of excitatory amino acids on neuronal KB-binding activity is an inhibitory one, strengthening the correlation between this transcriptional parameter and neuronal survival.

Characterization of a neuronal kappaB-binding factor distinct from NF-kappaB

Transcription factors that bind kappaB enhancer elements have begun to garner wide attention in neurobiology. Data suggest that activation of kappaB-binding factors in neurons can be protective against various neurotoxins, but other data have connected NF-kappaB to cell death. In electrophoretic mobility shift assays of kappaB-binding activity, we have found that the predominant activity in rat brain tissue, in primary neurons, and in neuronal cell lines has a mobility inconsistent with that of bona fide NF-kappaB (RelA-p50 heterodimer). We have tentatively termed this activity neuronal kappaB-binding factor (NKBF). Competition assays with various DNA probes distinguished NKBF from NF-kappaB. Probes that efficiently bind the p50 homodimer were able to compete with a conventional NF-kappaB probe for NKBF binding, but NKBF did not react with antibodies to p50 (or any other known Rel family members). Furthermore, UV-crosslinking indicated that NKBF is composed of two polypeptides of 82 kDa and 27 kDa. Although NKBF activity can be elevated in a manner independent of new macromolecular synthesis, it does not appear to be modulated by IkappaB. Finally, no NF-kappaB was induced by glutamate in highly enriched neuronal cultures, although it was induced in neuron-glia cocultures. These data suggest that the primary kappaB-binding transcription factor in neurons is a novel protein complex distinct from NF-kappaB.

[Severe streptococcal group A infection complicating varicella]

Varicella is a common viral infection which is generally benign in infancy and has a good outcome. It may sometimes be complicated by severe group A streptococcal superinfection.

CASE REPORT

Three days after the beginning of varicella, a previously healthy 2-year-old girl presented with left leg pain, lameness and edema of all four limbs. Toxic shock syndrome occurred, due to beta-hemolytic group A Streptococcus grown from blood culture. Computerized tomography (CT) scan showed a mild effusion involving both hips. Cefotaxim was administered, but the week after magnetic resonance imaging (MRI) showed a necrotizing fasciitis and a lesion of the left leg leading to a patchy femoral diaphysis consistent with osteomyelitis. Joint aspirate culture did not grow. The left leg was immobilized in plaster for 6 weeks and the child was given cefotaxim and fosfomycin parenterally during 30 days, then followed by 45 days of oral amoxicillin. She recovered without sequelae.

CONCLUSION

Group A Streptococcus infection is a dangerous complication of varicella. It must be considered in case of any joint pain occurring during or just after this disease. The choice of the best treatment needs full collaboration between surgeons, radiologists and pediatricians.

Comparative hemodynamic depression of sevoflurane versus halothane in infants: an echocardiographic study

BACKGROUND

The cardiovascular side effects of volatile anesthetics are one of the chief causes of postoperative complications in children, and infants seem to be at the greatest risk for this. This study compared cardiovascular changes at equipotent concentrations of sevoflurane and halothane in infants.

METHODS

Thirty infants classified as American Society of Anesthesiologists physical status I or II who required elective surgery were randomized to receive either halothane or sevoflurane for inhalation induction. Cardiovascular and echocardiographic data were recorded in both groups at baseline and at end-tidal concentrations of 1 and 1.5 minimum alveolar concentration (MAC).

RESULTS

Sevoflurane did not alter heart rate or cardiac index at all concentrations compared with awake values. Sevoflurane significantly decreased blood pressure and systemic vascular resistance compared with awake values at all concentrations. Shortening fraction and rate-corrected velocity of circumferential fiber shortening decreased at 1.5 but not at 1 MAC. Myocardial contractility assessed by stress-velocity index and stress-shortening index decreased significantly at all concentrations, but did not fall into the abnormal range at any concentration. Halothane caused a greater decrease in heart rate, shortening fraction, stress-shortening index, velocity of circumferential fiber shortening, stress-velocity index, and cardiac index at all concentrations than did sevoflurane.

CONCLUSION

Sevoflurane causes a lesser decrease in cardiac output than does halothane in infants.

Isolation of monoclonal antibodies reacting with the core component of lipopolysaccharide from Rhizobium leguminosarum strain 3841 and mutant derivatives

Monoclonal antibodies reacting with the core oligosaccharide or lipid A component of Rhizobium lipopolysaccharide (LPS) could be useful for the elucidation of the structure and biosynthesis of this group of macromolecules. Mutant derivatives of Rhizobium leguminosarum 3841 with LPS structures lacking the major O-antigen moiety were used as immunogens, and eight antibodies were selected for further study. All the antibodies reacted with the fast-migrating species known as LPS-2 following gel electrophoresis of Rhizobium cell extracts. For four of these antibodies, reactivity with affinity-purified LPS was lost after mild acid hydrolysis, indicating that they probably recognized the core oligosaccharide component. The four other antibodies still reacted with acid-treated LPS and may recognize the lipid A moiety, which is stable to mild acid hydrolysis. The pattern of antibody staining after gel electrophoresis revealed differences in LPS-2 epitope structure between each of the mutants and the wild type. Furthermore, for each of the mutants the antibodies crossreacted with a minor band that migrated more slowly than LPS-2; we have termed this more slowly migrating form LPS-3. The majority of the antibodies also reacted with LPS from strain CE109, a derivative of Rhizobium etli CE3, confirming that the LPS core antigens can be relatively conserved between strains of different Rhizobium species. One of the antibodies isolated in this study (JIM 32) was unusual because it appeared to react with all forms of LPS from strain 3841 (namely, LPS-1, LPS-2, and LPS-3). Furthermore, JIM 32 reacted positively with the LPS from many strains of Rhizobium tested (excluding the Rhizobium meliloti subgroup). JIM 32 did not react with representative strains from Bradyrhizobium, Azorhizobium or other related bacterial species.

[The use of serological tests for detecting HIV infection in general clinical consultations and hospital consultations]

AIMS

To determine the number and type of HIV tests requested in general practice (GP) and in hospital outpatient clinics, to identify principle risk behaviors, to determine the percentage of HIV positive tests and to identify possible factors associated with the request of HIV tests.

METHODS

Cross-sectional, analytical and observational study, involving 80 GPs and 45 hospital specialists in the region of Lisbon. All requests for HIV tests were analysed during a 12 month period.

RESULTS

936 HIV tests were requested, with a mean of 12.47 in GP and 0.69 in the hospital. Risk behaviors observed were mainly heterosexual contacts and intravenous drug abuse (IVDA). The motives of the requests mainly were pregnancy, risk behaviors in GP and the presence of symptoms suggesting HIV infection in the hospital. The initiative of the request came from doctors in 70% of the cases. The percentage of HIV positive tests (ELISA + Western blot) was 4.2% in GP and 32% in the hospital. According to risk behaviors, the percentage of seropositivity was 33% in homo/bisexuals, 13% in IVDA, 7% in heterosexuals with risk behaviors and 0.2% in individuals with unidentified risk behaviors.

CONCLUSIONS

The patterns of request of HIV tests differ in hospital and in GP. In a significant percentage of cases, no informed consent was obtained prior to HIV testing, both in hospital and GP. This study may serve as an indicator of the need for information and education programs concerning HIV testing directed to health professionals and the general population.

Ionic Stress and Osmotic Pressure Induce Different Alterations in the Lipopolysaccharide of a Rhizobium meliloti Strain

A halotolerant strain of Rhizobium meliloti was isolated from nodules of a Melilotus plant growing in a salt marsh in Donana National Park (southwest Spain). This strain, EFB1, is able to grow at NaCl concentrations of up to 500 mM, and no effect on growth is produced by 300 mM NaCl. EFB1 showed alterations on its lipopolysaccharide (LPS) structure that can be related to salt stress: (i) silver-stained electrophoretic profiles showed a different mobility that was dependent on ionic stress but not on osmotic pressure, and (ii) a monoclonal antibody, JIM 40, recognized changes in LPS that were dependent on osmotic stress. Both modifications on LPS may form part of the adaptive mechanism of this bacterium for saline environments.

Levels of representation in the interpretation of anaphoric reference and instrument inference

The level of representation accessed when inferences are made during sentence comprehension was examined. The inferences investigated included antecedent assignment for both definite noun phrase anaphors and pronouns and also instrument inferences. In making these inferences, a listener must access the inferred element, whether an antecedent or an instrument, in either a linguistic form representation or a discourse model. The level of representation involved in these inferences was determined by exploiting differences in the lexical decision and naming tasks, which were argued to exhibit differential sensitivity to representational levels. In three experiments, the priming of antecedent and instrument targets in the lexical decision task was compared with priming of the same targets in the naming task. Differences in the patterns of activation across the two tasks indicated that all three types of inferences required accessing elements in a discourse model. Three control experiments ruled out simple context or congruity checking as an explanation for our results. The following conclusions were also supported by these studies: (1) Antecedent assignment occurs immediately after processing an anaphor; (2) antecedent assignment involves inhibition for the inappropriate antecedent rather than facilitation for the appropriate antecedent; (3) although subjects do not make instrument inferences when they hear isolated sentences containing verbs that strongly imply certain instruments, the inferences are made when sentences are preceded by a context that mentions the instrument.