Abortion of infection during the Rhizobium meliloti—alfalfa symbiotic interaction is accompanied by a hypersensitive reaction

Nodulation, the organogenetic process resulting from the symbiotic interaction between Rhizobium and legumes, is under the feedback control of the plant. However, the autoregulatory mechanisms controlling root nodule formation are poorly understood. In this paper it is shown that alfalfa can react to infection by its symbiont Rhizobium meliloti by eliciting a defence mechanism similar to the hypersensitive reaction (HR) observed in incompatible plant-pathogen interactions. After the first nodule primordia have been induced, an increasing proportion of infection threads abort in a single or a few root cortical cells in which both symbionts simultaneously undergo necrosis. Autofluorescent, cytochemical and immunolocalization assays revealed that phenolic compounds and proteins associated with defence mechanisms in plants have accumulated in the necrotic cells. These results lead to the proposition that the elicitation of a HR is part of the mechanism by which the plant controls infection and, therefore, regulates nodulation.     

Crystal structure of NC1 domains. Structural basis for type IV collagen assembly in basement membranes

Type IV collagen, which is present in all metazoan, exists as a family of six homologous alpha(IV) chains, alpha1-alpha6, in mammals. The six chains assemble into three different triple helical protomers and self-associate as three distinct networks. The network underlies all epithelia as a component of basement membranes, which play important roles in cell adhesion, growth, differentiation, tissue repair and molecular ultrafiltration. The specificity of both protomer and network assembly is governed by amino acid sequences of the C-terminal noncollagenous (NC1) domain of each chain. In this study, the structural basis for protomer and network assembly was investigated by determining the crystal structure of the ubiquitous [(alpha1)(2).alpha2](2) NC1 hexamer of bovine lens capsule basement membrane at 2.0 A resolution. The NC1 monomer folds into a novel tertiary structure. The (alpha1)(2).alpha2 trimer is organized through the unique three-dimensional domain swapping interactions. The differences in the primary sequences of the hypervariable region manifest in different secondary structures, which determine the chain specificity at the monomer-monomer interfaces. The trimer-trimer interface is stabilized by the extensive hydrophobic and hydrophilic interactions without a need for disulfide cross-linking.     

Crystal structure of RNA 3'-terminal phosphate cyclase, a ubiquitous enzyme with unusual topology

BACKGROUND: RNA cyclases are a family of RNA-modifying enzymes that are conserved in eucarya, bacteria and archaea. They catalyze the ATP-dependent conversion of the 3'-phosphate to the 2',3'-cyclic phosphodiester at the end of RNA, in a reaction involving formation of the covalent AMP-cyclase intermediate. These enzymes might be responsible for production of the cyclic phosphate RNA ends that are known to be required by many RNA ligases in both prokaryotes and eukaryotes. RESULTS: The high-resolution structure of the Escherichia coli RNA 3'-terminal phosphate cyclase was determined using multiwavelength anomalous diffraction. Two orthorhombic crystal forms of E. coli cyclase (space group P2(1)2(1)2(1) and P2(1)2(1)2) were used to solve and refine the structure to 2.1 A resolution (R factor 20.4%; R(free) 27.6%). Each molecule of RNA cyclase consists of two domains. The larger domain contains three repeats of a folding unit comprising two parallel alpha helices and a four-stranded beta sheet; this fold was previously identified in translation initiation factor 3 (IF3). The large domain is similar to one of the two domains of 5-enolpyruvylshikimate-3-phosphate synthase and UDP-N-acetylglucosamine enolpyruvyl transferase. The smaller domain uses a similar secondary structure element with different topology, observed in many other proteins such as thioredoxin. CONCLUSIONS: The fold of RNA cyclase consists of known elements connected in a new and unique manner. Although the active site of this enzyme could not be unambiguously assigned, it can be mapped to a region surrounding His309, an adenylate acceptor, in which a number of amino acids are highly conserved in the enzyme from different sources. The structure of E. coli cyclase will be useful for interpretation of structural and mechanistic features of this and other related enzymes.     

Fragmentation and Genetic Diversity in Romnalda (Dasypogonaceae), a Rare Rain forest Herbaceous Genus from New Guinea and Australia

Rain forests are expected to be amongst the ecosystem types most affected by fragmentation due to their high species diversity, high endemism, complexity of interactions, and contrast with surrounding altered matrix. Due to their shorter life cycles and dependence on canopy cover, rain forest understory herbs are expected to indicate the effects of recent fragmentation more rapidly than canopy trees. This study investigated all four known species of the genus Romnalda , all of which are rare rain forest herbaceous species, to investigate the possible effects of habitat fragmentation and isolation on genetic diversity and gene flow. Allozymes were used as genetic markers and regional remnant vegetation maps were used to compare landscape fragmentation. We found that R. strobilacea populations in a highly fragmented landscape were genetically depauperate compared with those of its congeneric species that are found within continuous rain forest habitats and that allelic diversity decreased with decreasing population size but not geographic distance in R. strobilacea . Given the similarity among the species, our results indicate that all Romnalda species are potentially susceptible to loss of genetic diversity due to habitat fragmentation within relatively short timeframes. The results indicate that populations are not highly genetically differentiated and there is little evidence of genetic provenance where the species have restricted geographic ranges. Thus, species recovery programs would be better to focus on maintaining population size and genetic diversity rather than population differentiation.     

Multilocus phylogeography reveals nested endemism in a gecko across the monsoonal tropics of Australia

Multilocus phylogeography can uncover taxonomically unrecognized lineage diversity across complex biomes. The Australian monsoonal tropics include vast, ecologically intact savanna‐woodland plains interspersed with ancient sandstone uplands. Although recognized in general for its high species richness and endemism, the biodiversity of the region remains underexplored due to its remoteness. This is despite a high rate of ongoing species discovery, especially in wetter regions and for rock‐restricted taxa. To provide a baseline for ongoing comparative analyses, we tested for phylogeographic structure in an ecologically generalized and widespread taxon, the gecko Heteronotia binoei. We apply coalescent analyses to multilocus sequence data (mitochondrial DNA and eight nuclear DNA introns) from individuals sampled extensively and at fine scale across the region. The results demonstrate surprisingly deep and geographically nested lineage diversity. Several intra‐specific clades previously shown to be endemic to the region were themselves found to contain multiple, short‐range lineages. To infer landscapes with concentrations of unique phylogeographic diversity, we probabilistically estimate the ranges of lineages from point data and then, combining these estimates with the nDNA species tree, estimate phyloendemism across the region. Highest levels of phyloendemism occur in northern Top End, especially on islands, across the topographically complex Arnhem escarpment, and across the sandstone ranges of the western Gulf region. These results drive home that deep phylogeographic structure is prevalent in tropical low‐dispersal taxa, even ones that are ubiquitous across geography and habitats.     

Identification of gangliosides GD1b and GT1b as receptors for BK virus

Gangliosides have been shown to be plasma membrane receptors for both murine polyomavirus and SV40, while JC virus uses serotonin receptors. In contrast, little is known of the membrane receptor and entry pathway for BK virus (BKV), which can cause severe disease in immunosuppressed bone marrow and renal transplant patients. Using sucrose flotation assays, we investigated BKV binding to and interaction with human erythrocyte membranes and determined that this interaction was dependent on a neuraminidase-sensitive, proteinase K-resistant molecule. BKV was found to interact with the gangliosides GT1b and GD1b. The terminal alpha2-8-linked disialic acid motif, present in both of these gangliosides, is likely to be important for this interaction. We also determined that the addition of GD1b and GT1b to LNCaP cells, which are normally resistant to BKV infection, made them susceptible to the virus. In addition, BKV interacted with membranes extracted from the endoplasmic reticulum (ER) and infection was blocked by the addition of brefeldin A, which interferes with transport from the ER to the Golgi apparatus. These data demonstrate that BKV uses the gangliosides GT1b and GD1b as receptors and passes through the ER on the way to the nucleus.     

THE INTERACTION OF AUXIN AND ETHYLENE IN THE MAINTENANCE OF LEAF BLADE FORM IN PHASEOLUS VULGARIS L. VAR. PINTO

Auxin treatment results in hyponastic curvature of the primary leaves of Phaseolus vulgaris L. var pinto. Ethylene production by hyponastic leaves is detected within 1 hr after treatment with IAA in concentrations at or above 1 μm. The amount of ethylene detected is proportional to the concentration of auxin applied. Untreated control leaves and leaves treated with 2,3,5-tri-iodobenzoic acid or gibberellic acid did not produce ethylene detectable by our equipment. The hyponastic curvature induced by auxin treatment can be inhibited by exogenous application of ethylene or ethylene-generating compounds, and these treatments produce epinasty in auxin-treated leaves. Treatment with inhibitors of ethylene synthesis or action, such as aminoethoxy-vinylglycine, carbon dioxide, or heat treatment, prolong hyponasty. The planar form, therefore, appears to be affected by both hyponastic auxin effect and an epinastic ethylene effect.     

The consequences of Lactobacillus vini and Dekkera bruxellensis as contaminants of the sugarcane-based ethanol fermentation

This work describes the effects of the presence of the yeast Dekkera bruxellensis and the bacterium Lactobacillus vini on the industrial production of ethanol from sugarcane fermentation. Both contaminants were quantified in industrial samples, and their presence was correlated to a decrease in ethanol concentration and accumulation of sugar. Then, laboratory mixed-cell fermentations were carried out to evaluate the effects of these presumed contaminants on the viability of Saccharomyces cerevisiae and the overall ethanol yield. The results showed that high residual sugar seemed the most significant factor arising from the presence of D. bruxellensis in the industrial process when compared to pure S. cerevisiae cultures. Moreover, when L. vini was added to S. cerevisiae cultures it did not appear to affect the yeast cells by any kind of antagonistic effect under stable fermentations. In addition, when L. vini was added to D. bruxellensis cultures, it showed signs of being able to stimulate the fermentative activity of the yeast cells in a way that led to an increase in the ethanol yield.