Evolutionary relationships among Ascochyta species infecting wild and cultivated hosts in the legume tribes Cicereae and Vicieae

Evolutionary relationships were inferred among a worldwide sample of Ascochyta fungi from wild and cultivated legume hosts based on phylogenetic analyses of DNA sequences from the ribosomal internal transcribed spacer regions (ITS), as well as portions of three protein-coding genes: glyceraldehyde-3-phosphate-dehydrogenase (G3PD), translation elongation factor 1-alpha (EF) and chitin synthase 1 (CHS). All legume-associated Ascochyta species had nearly identical ITS sequences and clustered with other Ascochyta, Phoma and Didymella species from legume and nonlegume hosts. Ascochyta pinodes (teleomorph: Mycosphaerella pinodes [Berk. & Blox.] Vestergen) clustered with Didymella species and not with well characterized Mycosphaerella species from other hosts and we propose that the name Didymella pinodes (Berk. & Blox.) Petrak (anamorph: Ascochyta pinodes L.K. Jones) be used to describe this fungus. Analysis of G3PD revealed two major clades among legume-associated Ascochyta fungi with members of both clades infecting pea ("Ascochyta complex"). Analysis of the combined CHS, EF and G3PD datasets revealed that isolates from cultivated pea (P. sativum), lentil (Lens culinaris), faba bean (Vicia faba) and chickpea (Cicer arietinum) from diverse geographic locations each had identical or similar sequences at all loci. Isolates from these hosts clustered in well supported clades specific for each host, suggesting a co-evolutionary history between pathogen and cultivated host. A. pisi, A. lentis, A. fabae and A. rabiei represent phylogenetic species infecting pea, lentil, faba bean and chickpea, respectively. Ascochyta spp. from wild relatives of pea and chickpea clustered with isolates from related cultivated hosts. Isolates sampled from big-flower vetch (Vicia grandiflora) were polyphyletic suggesting that either this host is colonized by phylogenetically distinct lineages of Ascochyta or that the hosts are polyphyletic and infected by distinct evolutionary lineages of the pathogen. Phylogenetic species identified among legume-associated Ascochyta spp. were fully concordant with previously described morphological and biological species.     

Recent changes to the classification of symbiotic,nitrogen-fixing,legume-associating bacteria: a review

The Rhizobia are collectively comprised of gram negative soil bacteria that have the ability to form symbiotic nitrogen-fixing root and/or stem nodules in association with leguminous plants. The taxonomy of these bacteria is continually in a state of flux, in large part due to rapid development of refined molecular biology techniques. The isolation and characterization of new, and often different, legumes-nodulating bacteria on a variety of plant hosts has resulted in the naming of many new rhizobial species. Here we update the taxonomy of the legume-nodulating bacteria and describe newly identified rhizobia capable of nodulating edible legumes and legume trees. In 1990, there was only one bacterial species that was known to nodulate common bean worldwide (Rhizobium leguminosarum sv. phaseoli), one species that nodulated faba bean (Rhizobium leguminosarum sv. viciae), and two species that nodulated soybean (Bradyrhizobium japonicum and Rhizobium fredii). Today, nearly 14, 11, 6, 5, 5, 4, 3 and 2 species have been defined that are capable of nodulating common bean, soybean, cowpea, chickpea, peanut, lentils, faba bean and pea, respectively. The recent use of whole genome based taxonomy (genomotaxonomy) will surely change how we define this important group of bacteria. The identification of several rhizobial species that are able to nodulate and fix nitrogen with edible legumes may enhance the production of these crops and can compensate for worldwide deficiencies in human nutritional needs in the future.     

Salt-tolerant rhizobia isolated from a Tunisian oasis that are highly effective for symbiotic N2-fixation with Phaseolus vulgaris constitute a novel biovar (bv. mediterranense) of Sinorhizobium meliloti

Nodulation of common bean was explored in six oases in the south of Tunisia. Nineteen isolates were characterized by PCR–RFLP of 16S rDNA. Three species of rhizobia were identified, Rhizobium etli, Rhizobium gallicum and Sinorhizobium meliloti. The diversity of the symbiotic genes was then assessed by PCR–RFLP of nodC and nifH genes. The majority of the symbiotic genotypes were conserved between oases and other soils of the north of the country. Sinorhizobia isolated from bean were then compared with isolates from Medicago truncatula plants grown in the oases soils. All the nodC types except for nodC type p that was specific to common bean isolates were shared by both hosts. The four isolates with nodC type p induced N₂-fixing effective nodules on common bean but did not nodulate M. truncatula and Medicago sativa. The phylogenetic analysis of nifH and nodC genes showed that these isolates carry symbiotic genes different from those previously characterized among Medicago and bean symbionts, but closely related to those of S. fredii Spanish and Tunisian isolates effective in symbiosis with common bean but unable to nodulate soybean. The creation of a novel biovar shared by S. meliloti and S. fredii, bv. mediterranense, was proposed.     

Phosphorus benefits of different legume crops to subsequent wheat grown in different soils of Western Australia

Certain legume crops, including white lupin (Lupinus albus L.), mobilise soil-bound phosphorus (P) through root exudates. The changes in the rhizosphere enhance P availability to these crops, and possibly to subsequent crops growing in the same soil. We conducted a pot experiment to compare phosphorus acquisition of three legume species with that of wheat, and to determine whether the legume crops influence growth and P uptake of a subsequent wheat crop. Field pea (Pisum sativum L.), faba bean (Vicia faba L.), white lupin (Lupinus albus L.) and wheat (Triticum aestivum L.) were grown in three different soils to which we added no or 20 mg P kg^–1 soil (P0, P20). Growth, P content and rhizosphere carboxylates varied significantly amongst crops, soils and P levels. Total P content of the plants was increased with applied phosphorus. Phosphorus content of faba bean was 3.9 and 8.8 mg/pot, at P0 and P20, respectively, which was about double that of all other species at the respective P levels. Field pea and white lupin had large amounts of rhizosphere carboxylates, whereas wheat and faba bean had negligible amounts in all three soils at both P levels. Wheat grew better after legumes than after wheat in all three soils. The effect of the previous plant species was greater when these previous species had received P fertiliser. All the legumes increased plant biomass of subsequent wheat significantly over the unplanted pots in all the soils. Faba bean was unparalleled in promoting subsequent wheat growth on all fertilised soils. This experiment clearly demonstrated a residual benefit of the legume crops on the growth of the subsequent wheat crop due to enhanced P uptake. Faba bean appeared to be a suitable P-mobilising legume crop plant for use in rotations with wheat.     

Is nitrogen accumulation in grain legumes responsive to growth or ontogeny?

Nitrogen (N) accumulation in legumes is one of the main determinants of crop yield. Although N accumulation from symbiotic nitrogen fixation or N absorption from the soil has been widely investigated, there is no clear consensus on timing of the beginning of N accumulation and the termination of N accumulation and the physiological events that may be associated with these two events. The analyses conducted in this study aimed at identifying the determinant of N accumulation in two grain legume species. Nitrogen accumulation dynamics and mass accumulation and development stages were recorded in the field for several genotypes of common bean (Phaseolus vulgaris) and faba bean (Vicia faba) under different growing conditions. This study showed that during the vegetative stages, N accumulation rate was correlated with mass accumulation rate. However, the maximum accumulation of N did not correspond to the time of the maximum mass accumulation. In fact, for both species, N accumulation was found to persist in seed growth. This challenges a common hypothesis that seed growth causes a decrease in N accumulation because of a shift of the photosynthate supply to support the seed growth. Even more surprising was the shift of the active accumulation of N in faba bean to late in the growing season as compared with common bean. N accumulation by faba bean only was initiated at high rates very late in vegetative growth and persisted at high rates well into seed fill.     

Diversity in symbiotic specificity of cowpea rhizobia indigenous to Zimbabwean soils

Tropical cowpea rhizobia are often presumed to be generally promiscuous but poor N fixers. This study was conducted to evaluate symbiotic interactions of 59 indigenous rhizobia isolates (49 of them from cowpea (Vigna unguiculata)), with up to 13 other (mostly tropical) legume species. Host ranges averaged 2.4 and 2.3 legume species each for fast- and slow-growing isolates respectively compared to 4.3 for slow-growing reference cowpea strains. An average of 22% and 19% of fast- and slow-growing cowpea isolates respectively were effective on each of 12 legume species tested. We conclude that the indigenous cowpea rhizobia studied have relatively narrow host ranges. The ready nodulation of different legumes in tropical soils appears due to the diversity of indigenous symbiotic genotypes, each consisting of subgroups compatible with a limited number of legume species.     

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

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

Identification of fast-growing rhizobia nodulating tropical legumes from Puerto Rico as Rhizobium gallicum and Rhizobium tropici

Fifteen isolates from several nodulated tropical legumes from Puerto Rico (USA) were characterised by their phenotypic, molecular and symbiotic features. The identification of isolates was based on a polyphasic approach, including phenotypic characteristics, 16S rRNA sequencing, Low molecular weight (LMW) RNA profiles, Two Primers-RAPD patterns, and restriction patterns from 16S rDNA molecules. Despite of the variety of hosts included in this study the 15 isolates were separated into only two groups that corresponded to Rhizobium gallicum and Rhizobium tropici. This work shows that R. gallicum and R. tropici nodulate legume plants, such as Sesbania, Caliandra, Poitea, Piptadenia, Neptunia and Mimosa species, that were not previously considered as hosts for these rhizobia. Moreover, some of these host plants can be nodulated by both species. The results confirm the great promiscuity of R. tropici and also support the hypothesis that the species R. gallicum may be native from America or cosmopolitan and worldwide spread.     

Detection and Partial Characterization of Milk vetch dwarf virus Isolates from Faba Bean (Vicia faba L.) in Yunnan Province, China

A virus disease of faba bean ( Vicia faba L.) in China, characterized by leaf yellowing and rolling and plant stunting, was shown to be caused by a virus of the genus Nanovirus based on serological reactions to nanovirus-specific monoclonal antibodies and the generation of polymerase chain reaction amplicons using nanovirus-specific primers. To identify the faba bean-infecting nanovirus, regions of the DNA components encoding the master replication initiator protein and capsid protein of two nanovirus isolates from China were cloned, sequenced and compared with those of other members of the genus Nanovirus . The two Chinese virus isolates shared nucleotide sequence identities ranging from 95 to 98% with the type isolate of Milk vetch dwarf virus (MDV) from Japan. They were thus identified as isolates of MDV, a virus so far known to cause important diseases of legumes in Japan. This is the first record of MDV-infecting faba bean in China.     

A new species of Devosia that forms a unique nitrogen-fixing root-nodule symbiosis with the aquatic legume Neptunia natans (L.f.) druce

Rhizobia are the common bacterial symbionts that form nitrogen-fixing root nodules in legumes. However, recently other bacteria have been shown to nodulate and fix nitrogen symbiotically with these plants. Neptunia natans is an aquatic legume indigenous to tropical and subtropical regions and in African soils is nodulated by Allorhizobium undicola. This legume develops an unusual root-nodule symbiosis on floating stems in aquatic environments through a unique infection process. Here, we analyzed the low-molecular-weight RNA and 16S ribosomal DNA (rDNA) sequence of the same fast-growing isolates from India that were previously used to define the developmental morphology of the unique infection process in this symbiosis with N. natans and found that they are phylogenetically located in the genus Devosia, not Allorhizobium or RHIZOBIUM: The 16S rDNA sequences of these two Neptunia-nodulating Devosia strains differ from the only species currently described in that genus, Devosia riboflavina. From the same isolated colonies, we also located their nodD and nifH genes involved in nodulation and nitrogen fixation on a plasmid of approximately 170 kb. Sequence analysis showed that their nodD and nifH genes are most closely related to nodD and nifH of Rhizobium tropici, suggesting that this newly described Neptunia-nodulating Devosia species may have acquired these symbiotic genes by horizontal transfer.     

Phenotypic and genetic diversity of Moroccan rhizobia isolated from Vicia faba and study of genes that are likely to be involved in their osmotolerance

Rhizobia are symbiotic nitrogen-fixing bacteria in root nodules of legumes. In Morocco, faba bean (Vicia faba L.), which is the main legume crop cultivated in the country, is often grown in marginal soils of arid and semi-arid regions. This study examines the phenotypic diversity of rhizobia nodulating V. faba isolated from different regions in Morocco for tolerance to some abiotic stresses. A total of 106 rhizobia strains isolated from nodules were identified at the species level by analysing 16S rDNA. Additionally, for selected strains recA, otsA, kup and nodA fragments were sequenced. 102 isolates are likely to belong to Rhizobium leguminosarum or R. laguerreae and 4 isolates to Ensifer meliloti. All strains tolerating salt concentrations of 428 or 342 mM NaCl as well as 127 or 99 mM Na2SO4 were highly resistant to alkaline conditions (pH 10) and high temperature (44 °C). Three strains: RhOF4 and RhOF53 (both are salt-tolerant) and RhOF6 (salt-sensitive) were selected to compare the influence of different levels of salt stress induced by NaCl on growth and on trehalose and potassium accumulation. We find a direct correlation between the trehalose contents of the rhizobial strains and their osmotolerance.     

Subgroups of the Cowpea Miscellany: Symbiotic Specificity within Bradyrhizobium spp. for Vigna unguiculata, Phaseolus lunatus, Arachis hypogaea, and Macroptilium atropurpureum

Rhizobia classified as Bradyrhizobium spp. comprise a highly heterogeneous group of bacteria that exhibit differential symbiotic characteristics on hosts in the cowpea miscellany cross-inoculation group. To delineate the degree of specificity exhibited by four legumes in the cowpea miscellany, we tested the symbiotic characteristics of indigenous cowpea bradyrhizobia on cowpea (Vigna unguiculata), siratro (Macroptilium atropurpureum), lima bean (Phaseolus lunatus), and peanut (Arachis hypogaea). The most-probable-number counts of indigenous bradyrhizobia at three sites on Maui, Hawaii, were substantially different on the four hosts: highest on siratro, intermediate on cowpea, and significantly lower on both lima bean and peanut. Bradyrhizobia from single cowpea nodules from the most-probable-number assays were inoculated onto the four hosts. Effectiveness patterns of these rhizobia on cowpea followed a normal distribution but were strikingly different on the other legumes. The effectiveness profiles on siratro and cowpea were similar but not identical. The indigenous cowpea-derived bradyrhizobia were of only moderate effectiveness on siratro and were in all cases lower than the inoculant-quality reference strain. Between 5 and 51% of the bradyrhizobia, depending on site, failed to nodulate peanut, whereas 0 to 32% failed to nodulate lima bean. No significant correlation was observed between the relative effectiveness of the bradyrhizobia on cowpea and their corresponding effectiveness on either lima bean or peanut. At all sites, bradyrhizobia that were ineffective on cowpea but that effectively nodulated lima bean, peanut, or both were found. Eighteen percent or fewer of the bradyrhizobia were as effective on lima bean as the reference inoculant strain; 44% or fewer were as effective on peanut as the reference strain. Only 18% of all cowpea-derived bradyrhizobia tested were able to form N(2)-fixing nodules on both lima bean and peanut. These results indicate the need to measure indigenous bradyrhizobial population characteristics directly with the crop of interest to obtain an accurate assessment of the need to inoculate.     

Genetic diversity of Mimosa pudica rhizobial symbionts in soils of French Guiana: investigating the origin and diversity of Burkholderia phymatum and other beta-rhizobia

The genetic diversity of 221 Mimosa pudica bacterial symbionts trapped from eight soils from diverse environments in French Guiana was assessed by 16S rRNA PCR-RFLP, REP-PCR fingerprints, as well as by phylogenies of their 16S rRNA and recA housekeeping genes, and by their nifH, nodA and nodC symbiotic genes. Interestingly, we found a large diversity of beta-rhizobia, with Burkholderia phymatum and Burkholderia tuberum being the most frequent and diverse symbiotic species. Other species were also found, such as Burkholderia mimosarum, an unnamed Burkholderia species and, for the first time in South America, Cupriavidus taiwanensis. The sampling site had a strong influence on the diversity of the symbionts sampled, and the specific distributions of symbiotic populations between the soils were related to soil composition in some cases. Some alpha-rhizobial strains taxonomically close to Rhizobium endophyticum were also trapped in one soil, and these carried two copies of the nodA gene, a feature not previously reported. Phylogenies of nodA, nodC and nifH genes showed a monophyly of symbiotic genes for beta-rhizobia isolated from Mimosa spp., indicative of a long history of interaction between beta-rhizobia and Mimosa species. Based on their symbiotic gene phylogenies and legume hosts, B. tuberum was shown to contain two large biovars: one specific to the mimosoid genus Mimosa and one to South African papilionoid legumes.     

Rhizobium sophorae is the dominant rhizobial symbiont of Vicia faba L. In North China

Faba bean (Vicia faba L.) is a major introduced grain-legume crop cultivated in China. In this study, rhizobia that nodulated faba bean grown in soils from three sites in North China (Hebei Province) were isolated and characterized. Firstly, isolates were categorized into genotypes by ribosomal IGS PCR-RFLP analysis, then representatives of the different IGS genotypes were further identified by phylogenetic analyses of 16S rRNA, housekeeping (atpD, recA) and nodulation (nodC) gene sequences. Rhizobial distribution based on the IGS genotype was related to the different soil physicochemical features by redundancy analysis. IGS typing and phylogenetic analyses of 16S rRNA and concatenated housekeeping gene sequences affiliated the 103 rhizobial strains isolated into four Rhizobium species/genospecies. A total of 69 strains of 3 IGS types were assigned to R. sophorae, 20 isolates of 5 IGS types to R. changzhiense and 9 isolates of 3 IGS types to R. indicum. The representative strain of the five remaining isolates (1 IGS type) was clearly separated from all Rhizobium type strains and was most closely related to defined genospecies according to the recently described R. leguminosarum species complex. Rhizobium sophorae strains (67% of total isolates) were common in all sites and shared an identical nodC sequence typical of faba bean symbionts belonging to symbiovar viciae. In this first study of rhizobia nodulating faba bean in Hebei Province, China, R. sophorae was found to be the dominant symbiont in contrast to other countries.     

Polyphasic characterization of rhizobia microsymbionts of common bean [<Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> (L.)] isolated in Mato Grosso do Sul,a hotspot of Brazilian biodiversity

Common bean [Phaseolus vulgaris (Linnaeus)] is the key source of protein, carbohydrates and micronutrients for over 300 million people in the tropics. Like many legumes, P. vulgaris can fix atmospheric nitrogen in symbiosis with rhizobia, alleviating the need for the expensive and polluting N-fertilizers. The crop is known to nodulate with a wide range of rhizobia and, although Brazil is not a center of genetic origin/domestication of P. vulgaris, a variety of rhizobial species have been found as symbionts of the legume. Mato Grosso do Sul (MS) is one of the largest common bean producer states in Brazil, with reports of high yields and abundant natural nodulation. The objective of this study was to evaluate the diversity of 73 indigenous rhizobia isolated from common bean grown in 22 municipalities of MS. Great morphophysiological and genetic diversity was found, as indicated by the six and 35 clusters formed, considering the similarity level of 75 and 70%, respectively, for the phenotypic and rep-PCR dendrograms. Eleven representative isolates were selected for detailed genetic characterization using 16S rRNA and three protein-coding housekeeping genes, glnII, gyrB and recA. We identified species originated from the centers of origin/domestication of the legume, R. etli and R. phaseoli, species probably indigenous of Brazil, R. leucaenae and others of the Rhizobium/Agrobacterium clade, in addition to putative new species. The results highlight the great rhizobial diversity of the region.     

Genetic approaches for breeding heat stress tolerance in faba bean (<Emphasis Type="Italic">Vicia faba</Emphasis> L.)

Vicia faba L. (faba bean) is an important legume and is cultivated essentially as a cool-season crop. Changes in sowing dates and lack of precipitation expose faba bean crop to drought and heat stresses. The gradual rise in global temperatures owing to climate change is likely to exacerbate the detrimental effects of hot and dry climatic conditions on faba bean cultivation. High temperature stress is particularly damaging to faba bean during the flowering period, when the viability of pollen is critical for successful reproduction. Recent studies have shown that maintenance of protein homeostasis through synthesis of heat shock proteins plays a key role in the heat response of plants. To date, there has been no significant work linking the heat response of faba bean to the repertoire of its heat shock proteins. While quantitative trait loci have been identified for resistance against biotic stresses in faba bean, there is no parallel success with abiotic stresses in this species. Programs aiming at genetic improvement of the heat/drought resistance of this crop by both conventional breeding and molecular breeding methods are hampered because of the large and majorly ill-analyzed genome of faba bean plants. Likewise, molecular and biotechnology-related tools are poorly developed for faba bean; as a result, the fruits of transgenic research developed with model plant species are not reaching this crop. While specifically discussing the prospects for the genetic improvement of faba bean against heat and drought stresses, we highlight the areas of research which need to be strengthened on faba bean.     

Divergent Bradyrhizobium symbionts on Tachigali versicolor from Barro Colorado Island, Panama

Relationships of root-nodule bacteria from the tree Tachigali versicolor (legume subfamily Caesalpinioideae) were analyzed for 20 isolates sampled from juvenile plants growing on Barro Colorado Island (BCI), Panama. Bacterial genetic diversity appeared to be low. In the highly polymorphic 5' intervening sequence region of 23S rRNA, all isolates had the same length variant. A 472 bp segment spanning this region was sequenced in four isolates, and all proved to be identical at every nucleotide position. RFLP analysis of a 868 bp fragment of the nitrogenase alpha-subunit gene likewise indicated that all 20 isolates shared an identical set of restriction sites. Phylogenetic analysis of both partial 23S rRNA and nearly full-length 16S rRNA sequences showed that bacterial symbionts of T. versicolor fall into the genus Bradyrhizobium. However, they are divergent from the bradyrhizobia associated with other BCI legumes, as well as from other currently known bacteria in this genus. Inoculation tests with two promiscuously-nodulating legumes showed that bacteria from T. versicolor were unable to form nodules on Vigna unguiculata, but did nodulate Macroptilium atropurpureum, although the nodules lacked nitrogen fixation activity. The association of Tachigali with a divergent lineage of Bradyrhizobium is noteworthy in view of this plant's position within a clade of the mostly non-nodulating "primitive" legume subfamily Caesalpinioideae that gave rise to the predominantly nodulating subfamily Mimosoideae.     

Diversity of rhizobia and interactions among the host legumes and rhizobial genotypes in an agricultural-forestry ecosystem

To investigate the diversity of rhizobia and interactions among the host legumes and rhizobial genotypes in the same habitat, a total of 97 rhizobial strains isolated from nine legume species grown in an agricultural-forestry ecosystem were identified into seven genomic species and 12 symbiotic genotypes within the genera Bradyrhizobium, Mesorhizobium, Rhizobium and Sinorhizobium based upon analyses of genomic DNA regions and symbiotic genes. The results evidenced that the symbiotic genotypes of rhizobia were consistent with their hosts of origin; revealed that vertical transfer was the main mechanism in rhizobia to maintain the symbiotic genes but lateral transfer of symbiotic genes might have happened between the closely related rhizobial species; suggested the existence of co-distribution and co-evolution among the legume hosts and compatible rhizobia. All of these data demonstrated that the biogeography of rhizobia was a result of interactions among the host legumes, bacterial genomic backgrounds and environments.     

Interspecific and intraspecific variation in the performance of three pest aphid species on five grain legume hosts

The suitability of five grain legume species (narrow-leafed lupin, chickpea, faba bean, field pea, lentil) as hosts for three aphid species (green peach aphid, cowpea aphid, bluegreen aphid) was evaluated by measuring the mean relative growth rate (MRGR) and survivorship of nymphs over a 5 day period. For each aphid species, intraspecific (interclonal) variation was also determined by independently measuring the performance of 30 clones collected from a variety of hosts and from different parts of the Western Australia (WA) wheatbelt. The suitability of the grain legumes varied among aphid species. Chickpea was not a suitable host for any of the aphids tested. Averaged over all clones, lentil and faba bean were the most suitable hosts for cowpea aphid, and narrow-leafed lupin was the most suitable host for green peach aphid. Field pea was a suitable host for all three species, but only at a suboptimal level. Cowpea aphid showed the greatest amount of intraspecific variation, with significant variation in MRGR among clones on all hosts except chickpea and significant variation in survivorship on chickpea and lupin. For green peach aphid, there was significant variation in MRGR among clones on field pea and lupin, but in survivorship on lupin only. Bluegreen aphid clones showed significant variation only for MRGR on faba bean and lupin. There were positive correlations in performance of green peach aphid clones on faba bean and lentil, and of cowpea aphid clones on faba bean and lentil. Bluegreen aphid clones showed a negative correlation in performance on field pea and faba bean. These results show the importance of screening cultivars against a wide variety of aphid clones when assessing aphid susceptibility in breeding programmes. The implications of these results on the adaptability of parthenogenetic aphids are discussed.     

Phylogenetic relationship of Lotus uliginosus symbionts with bradyrhizobia nodulating genistoid legumes

Lotus species are legumes with potential for pastures in soils with low-fertility and environmental constraints. The aim of this work was to characterize bacteria that establish efficient nitrogen-fixing symbiosis with the forage species Lotus uliginosus. A total of 39 isolates were obtained from nodules of L. uliginosus naturally growing in two different locations of Portugal. Molecular identification of the isolates plus the commercial inoculant strain NZP2039 was performed by REP-PCR, 16S rRNA RFLP, and 16S rRNA, glnII and recA sequence analyses. Limited genetic diversity was found among the L. uliginosus symbionts, which showed a close phylogenetic relationship with the species Bradyrhizobium japonicum. The symbiotic nifH, nodA and nodC gene sequences were closely related with the corresponding genes of various Bradyrhizobium strains isolated from Lupinus and other genistoid legumes and therefore were phylogenetically separated from other Lotus spp. rhizobia. The L. uliginosus bradyrhizobia were able to nodulate and fix nitrogen in association with L. uliginosus, could nodulate Lotus corniculatus with generally poor nitrogen-fixing efficiency, formed nonfixing nodules in Lotus tenuis and Lupinus luteus roots and were unable to nodulate Glycine soja or Glycine max. Thus, L. uliginosus rhizobia seem closely related to B. japonicum biovar genistearum strains.