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The symbiosis between plants and nitrogen‐fixing bacteria is widespread among legumes and actinorhizal plants within the nitrogen‐fixing root nodule (NFN) clade. However, there are major differences, as well as similarities, in the symbioses between actinorhizal plants and Frankia and those of legumes and their associated rhizobia.This review provides an overview of NFN symbioses. We outline the evolution and biogeography of actinorhizal plants and legumes and compare and contrast their microsymbionts and symbiotic processes.Within the NFN clade, a far greater number of nodulated legumes exists, compared with actinorhizal plants, and legumes have a much wider biogeographical distribution. There are genetic and physiological differences between free‐living diazotrophic Frankia and the phylogenetically diverse rhizobia, most strains of which are unable to fix N2ex planta. Actinorhizal nodules are modified lateral roots with a central vascular system, whereas legume nodules are stem‐like organs with peripheral vascular systems. Most legumes contain their microsymbionts within symbiosomes, rather than the infection threads found in actinorhizal nodule cells. Legumes have greater control of their microsymbionts, and those within the Inverted Repeat Lacking Clade impose terminal differentiation on their bacteroids. Legumes also have effective processes for autoregulation of nodulation and downregulation of N2 fixation in response to high levels of soil N. These features of the legume‐rhizobia symbiosis have led to increased efficiencies in N2 fixation.Synthesis. We suggest that these characteristic features of the legume‐rhizobia symbiosis, specifically legumes' greater flexibility in the choice of microsymbiont partner and the evolution of increased efficiencies in N2 fixation, are factors that can explain why the majority of species within the Leguminosae have retained the ability to nodulate and how this has contributed to their evolutionary success. [ABSTRACT FROM AUTHOR]

Contents40I.40II.41III.44IV.48V.49VI.49VII.52VIII.5353References53 Summary: In the last decade, analyses of both molecular and morphological characters, including nodulation, have led to major changes in our understanding of legume taxonomy. In parallel there has been an explosion in the number of genera and species of rhizobia known to nodulate legumes. No attempt has been made to link these two sets of data or to consider them in a biogeographical context. This review aims to do this by relating the data to the evolution of the two partners: it highlights both longitudinal and latitudinal trends and considers these in relation to the location of major land masses over geological time. Australia is identified as being a special case and latitudes north of the equator as being pivotal in the evolution of highly specialized systems in which the differentiated rhizobia effectively become ammonia factories. However, there are still many gaps to be filled before legume nodulation is sufficiently understood to be managed for the benefit of a world in which climate change is rife. [ABSTRACT FROM AUTHOR]

Fraser Bergersen rose from humble beginnings in New Zealand to become a leading microbiologist who specialised in the physiology and biochemistry of legume nitrogen fixation. He and his family emigrated toAustralia in 1954.Virtually all of his careerwas spent in Canberra at CSIRO Plant Industry. In the 1970s, Bergersen and colleagues achieved world-wide prominence when they elucidated the role of leghaemoglobin in facilitating oxygen diffusion to the Bradyrhizobium bacteroids in soybean nodules and in the nitrogen fixation process itself. During the rest of his working life, Fraser Bergersen contributed greatly to understanding the role of oxygen, the mode of its delivery, and terminal oxidases in all forms of biological nitrogen fixation. [ABSTRACT FROM AUTHOR]

Arid and semi-arid areas occupy an increasing fraction of the Earth's surface. Legume floras exist for most of these areas, but there is little information as to whether the plants nodulate and fix nitrogen (N) in their native habitats, although many have been used over millennia for food, forage and medicinal and other uses. This review shows that, in those arid and semi-arid areas where data are available, the ability of legumes to nodulate is a significant attribute. It examines some host genera present and, where known, the bacteria that induce nodulation in them. With some exceptions all legumes from well-studied arid areas have the potential to nodulate. Semi-arid areas vary between continents in terms of legume genera present, the probable extent of N fixation and in the endosymbionts (rhizobia) that induce nodulation in them. With climate change and an increasing world population, there is an urgent need to develop the diverse range of nodulated legumes native to dry environments. With modern methods this goal is readily achievable. [ABSTRACT FROM AUTHOR]

Although nodulated legumes have been used by indigenous peoples in Africa for centuries, their full potential has never been realized. With modern technology there is scope for rapid improvement of both plant and microbial germplasm. This review gives examples of some recent developments in the form of case studies; these range from multipurpose human food crops, such as cowpea (Vigna unguiculata (L.) Walp.), through to beverages (teas) that are also income-generating such as rooibos (Aspalathus linearis (Burm. f.) R. Dahlgren, honeybush (Cyclopia Vent. spp.), and the widely used food additive gum arabic (Acacia senegal (L.) Willd.). These and other potential crops are well-adapted to the many different soil and climatic conditions of Africa, in particular, drought and low nutrients. All can nodulate and fix nitrogen, with varying degrees of effectiveness and using a range of bacterial symbionts. The further development of these and other species is essential, not only for African use, but also to retain the agricultural diversity that is essential for a changing world that is being increasingly dominated by a few crops such as soybean. [ABSTRACT FROM PUBLISHER]

Current evidence suggests that legumes evolved about 60 million years ago. Genetic material for nodulation was recruited from existing DNA, often following gene duplication. The initial process of infection probably did not involve either root hairs or infection threads. From this initial event, two branched pathways of nodule developmental processes evolved, one involving and one not involving the development of infection threads to ‘escort’ bacteria to young nodule cells. Extant legumes have a wide range of nodule structures and at least 25% of them do not have infection threads. The latter have uniform infected tissue whereas those that have infection threads have infected cells interspersed with uninfected (interstitial) cells. Each type of nodule may develop indeterminately, with an apical meristem, or show determinate growth. These nodule structures are host determined and are largely congruent with taxonomic position. In addition to variation on the plant side, the last 10 years have seen the recognition of many new types of ‘rhizobia’, bacteria that can induce nodulation and fix nitrogen. It is not yet possible to fit these into the emerging pattern of nodule evolution. [ABSTRACT FROM PUBLISHER]

The article discusses various studies on legume evolution. It describes when and where nodulation evolve in legumes, the key processes that led to nodule structures found in extant legumes, the rising number of nitrogen-fixing bacteria known to nodulate legumes and the role of ectomycorrhizas (ECMs) and endomycorrhizas in certain legume groups.

Contents Summary 11 I. When and where did legumes evolve? 12 II. Evolution of legumes and nodulation 12 III. Taxonomic assessment of nodulation 13 IV. What triggered the development of nodulation? 19 V. Nonclassical rhizobia 20 VI. Conclusions 21 Acknowledgements 23 References 23 Legumes evolved about 60 million years ago (Ma), and nodulation 58 Ma. Nonnodulation remains common in Caesalpinioideae, with smaller numbers in Mimosoideae and Papilionoideae. The first type of infection by bacteria may have been at junctions where lateral roots emerged, followed by formation of infection threads to confine bacteria and convey them to some cells in the developing nodule, where they were generally released into symbiosomes. Infection threads were a prerequisite for root-hair infection, a process better controlled by the host, leading to a higher degree of specificity between symbionts. An alternative process, dating from the same time and persisting in about 25% of legumes, did not involve infection threads, bacteria entering a few host cells, surrounded by an undefined matrix. These cells divided repeatedly to give uniform infected tissue, with bacteria released into symbiosomes. Such legumes may have less stringent control of nodulation processes, and are found mainly in tropical and warm temperate areas. In each type of nodule, meristems may or may not be retained, leading to indeterminate or determinate forms. Nodule morphology and structure are host-determined, but the effectiveness of nitrogen fixation is largely controlled by the bacterial symbionts, which vary greatly in genotypic and phenotypic characters. [ABSTRACT FROM AUTHOR]

This article comments on a paper by M. Diabate and colleagues on the role of nodulation and nitrogen fixation in West African legumes. The authors examined 156 species in six different forests and reported on their nodulation. Overall their data are consistent with the known distribution of nodulating behaviour in the Leguminosae. Some of their new reports are on species of genera widely reported as nodulating. Others are on new genera, from sections of the Leguminosae that include nonnodulating genera. Because the criteria on which their nodule identification was based were not given, it is difficult to evaluate these data. The family Leguminosae is generally subdivided into three subfamilies, Caesalpinioideae, Mimosoideae and Papilionoideae, and the extent of nodulation varies widely within these, from uncommon, through common to very common, within subfamilies, considerable correspondance with tribal affinities. Thus it is particularly important to be careful when reporting nodulation in groups where it is uncommon. The article also offered a longitudinal perspective on the significance of nodulation in African rainforest legumes. Clearly, as Diabate and colleagues stress, people need to know much more about the nodulation status of West African legumes if forests are to be properly understood and managed. This will require much careful work on the part of many people, particularly those who live in the countries concerned.

Presents developments in legume symbiosis research. Diversity in legumes and their rhizobia; Analysis of host genomes in legumes; Increase in biological nitrogen fixation by legumes; Nodule development and functioning; Features of legume symbioses; Location of symbiotic genes on different linkage groups of pea; Transformation of nonnodulating legumes into nodulating ones.

Deciduous, much branched shrub, 60-150 (to 250) cm in height. Main stems brown, twigs reddishbrown with scattered shiny yellowish glands, becoming dark, ascending at an angle. Young twigs downy. Roots nodulated with cluster roots; suckering habit. Buds small, ovoid, obtuse, reddish-brown, with several scales (Fl. Br. Isl.). Leaves alternate. Leaf laminae 2-6cm long, petiole 3-5mm long. Leaves oblanceolate or oblong-obovate in outline; subsessile. Base of blade narrowly wedge-shaped, tapering into the stalk. Margin coarsely toothed in upper third, entire below and undulate. Adaxial surface dull dark green with fine hairs. Abaxial side pale green with fine hairs on a prominent midrib. Conspicuous, scattered, shining yellowish resin glands on both surfaces. Glandular trichomes can be sessile or stalked. Stomata hypostomatous, anomocytic, venation camptodromous (leaf venation in which secondary veins bend forward and anastomose before reaching the margin of the leaf). Wood and leaves fragrant when bruised. [ABSTRACT FROM AUTHOR]

Seeds of the wetland legume, Lotus uliginosus, were germinated and grown in vermiculite which was either continuously flooded or well-drained. Plants from both treatments were infected by Mesorhizobium loti strain DUS341 via a classical' root hair pathway, although some flooded plants appeared to be infected via enlarged epidermal cells. Subsequent to infection by M. loti, nodule rneristems, which had developed within the root outer cortex, were penetrated by infection threads that released bacteria into the meristematic cells. The infection threads and infection droplets were immunogold labelled with monoclonal antibodies (MAC265 and MAC236) that recognize epitopes (at approx. 155/170 and 170/210 kI)a, respectively) on a glycoprotein component of the matrix that surrounded the bacteria within the threads or droplets. Although labelling of infection threads or infection droplets with MAC236 was stronger than that with MAC265, both antibodies strongly labelled material occluding intercellular spaces in the cortices of developing nodules that had not vet expressed nitrogenase (as determined by a lack of signal after immunogold labelling with an antibody raised against nitrogenase component II). After 60 d, nitrogenase activity, shoot and root dry weights, and nodule fresh weight per plant did not differ between the treatments. After a further 30 d submergence, the flooded stems developed extensive aerenchyma and there was profuse development of (nodulated) adventitious roots. Nodules also formed at the junction of adventitious roots and the subtending stem and these were connected vascularly to a small stalk of tissue which gave rise to both a nodule and an adventitious root. The flooded nodules had prominent lenticels, and possible air pathways from the atmosphere to the nitrogen-fixing bacteroids are discussed. [ABSTRACT FROM AUTHOR]

The growth and nitrogenase activity of stern nodules of Sesbania rostrata Brem were strongly influenced by the humidity surrounding the nodule. Nodules developed under high humidity showed the fastest rates of growth and the highest specific nitrogenase activity (determined by acetylene reduction). Development of stem nodules in both the light and dark was similar and showed a similar lowering of growth and activity with decreasing humidity. Stem nodules developed in the dark were pale and unlike the dark green nodules that developed in the light, they did not have chloroplasts in the nodule cortex. Adventitious root development required darkness and relative humidities approaching 100%. In stem nodules that were grown at high humidity and subsequently exposed to lower humidities, there was only a small loss of nitrogenase activity, and high rates of acetylene reduction were maintained for 6 d after treatments were begun. The area of nodule attachment to the stem or root was measured in nodules of many sizes and found to be consistently greater in stem nodules. This may have implications for the comparative water balance of nodules on these two organs. [ABSTRACT FROM AUTHOR]

The quantity of sand particles adhering to soybean roots is directly related to the degree of water stress imposed. Adherence is apparently by means of mucigel in the region of the root tip and is assisted by root hairs in older regions. [ABSTRACT FROM AUTHOR]

Three experiments were carried out to investigate the effects of shading and water supply on growth and nitrogen fixation by young plants of Lupinus arboreus (up to 4 months old). Firstly, two light treatments, 20,000 and 65oo lux, were given under controlled environment conditions, and plants sampled weekly for 7 weeks. Secondly, four shading treatments (100%, 37%, 19% and 10% full daylight) were given to plants grown outside and samples taken after4,8 and 10 weeks. Thirdly, response to and recovery from moisture stress were investigated under controlled-environment conditions. Results from the two shading experiments showed that L. arboreus responds to shade as a typical mesophyte. Growth (measured as dry weight) was proportional to the logarithm of relative irradiance. Roots were affected more than shoots and hence the shoot: root ratio rose with increased shading. Water content varied with age and shading, the responses differing amongst different parts of the plant. Photosynthetic efficiency increased with moderate shading but decreased, as did relative growth rate, at 10% full daylight. The compensation point was estimated to be 7-8% full daylight (approximately 70 J cm-1 day-1). Nodule number and weight were reduced by shading. There was a highly significant linear relationship between root and nodule dry weights, 1 g root tissue supporting 50 mg nodule tissue. Acetylene-reducing activity per plant was also proportional to log relative irradiance and this was largely accounted for by the smaller amounts of nodule tissue formed under shade. Activity per milligram fresh weight was only slightly reduced by moderate shading and this probably resulted from increased vacuolation of the infected cells. Only severe shading (10% full daylight) had a marked effect on activity per milligram fresh weight. [ABSTRACT FROM AUTHOR]

An intensive 3-month study of nitrogen fixation by Lupinus arboreus grown on open sand dunes and under different aged stands of Pinus radiata in New Zealand has been carried out. Nodule samples were obtained either by digging whole seedlings or by sieving 0.01 m³ soil cores. Nitrogen fixation as estimated by the acetylene reduction assay and 15N analysis agreed well with published data from biomass studies. On open dunes nodule growth and activity increased rapidly during spring and estimated nitrogen fixation reached a peak of 11.5 kg N ha-1 week-1 at about flowering. When fruits began to swell, both on open dunes and in forest sites, nitrogen fixation by lupins decreased. Light transmission in forest sites was not correlated with the time of pine thinning. Lupin growth, nodule biomass and nitrogen fixation in non-flowering lupins was closely related to the amount of light transmitted. When the sand became very dry nitrogen fixation was reduced and moisture stress was considered to be a factor likely to limit nitrogen fixation in drier parts of the year. Nitrogen fixation by lupin within forest stands on sand dunes is sufficient to make a significant contribution to the nitrogen economy of the forest. [ABSTRACT FROM AUTHOR]

Published data on, and hypotheses regarding the effect of NO3- on functioning of legume root nodules are reviewed. It is concluded that a short-term reversible effect of NO3- may act via an increased resistance to O2 diffusion in nodules; this is coupled to decreased bacteroid respiration. For longer exposures to NO3-, nodule activity is irreversibly lost, but how this relates to carbohydrate deprivation or NO2- accumulation is unclear. Complicating factors include denitrification reactions and the interaction of NO2- with leghaemoglobin. [ABSTRACT FROM AUTHOR]

The growth and nitrogen fixation of Trifolium ripens have been investigated during the imposition of and recovery from different levels of water stress. Growth (measured as fresh and dry weight of roots and shoots, shoot length and nitrogen content) and nitrogen fixation (per plant and per unit weight of nodule) were depressed by application of water stress. The ability of plants to recover and the time taken for recovery were both related to the duration of the stress period. Meristematic activity in nodules was resumed after watering, making clover nodules more resistant to stress than plants, such as soybean, with spherical nodules. Nitrogen fixation, estimated by the acetylene reduction technique, was compared with the total nitrogen content of plants. Differences between the estimated and actual nitrogen content varied with age and degree of water stress. [ABSTRACT FROM AUTHOR]

Water stress reduced nitrogen fixation of soybean plants grown in pots of sand. Provided that the nodules retained some activity, recovery on watering was normally complete in an hour. Using tritiated water (³HOH) it was confirmed that water could be withdrawn by nodules en route from the root to the shoot system. A field trial with Vicia faba showed a high degree of correlation between soil-water content and nitrogen-fixing activity. Slow natural drying over a 6-week period resulted in progressive reduction of activity, which was restored by irrigation. Maximum nitrogen fixation occurred at about field capacity; above this, activity was reduced due to waterlogging. Under the conditions of the experiment, water supply was the major environmental factor affecting nitrogen fixation. It is suggested that water stress affects nodule activity directly, but that this effect may be aggravated by reduced supplies of photosynthate from wilted leaves. [ABSTRACT FROM AUTHOR]

The fine structure of the non-infected (outer) region of a soybean root nodule is described. Apart from vascular traces, the cells are mainly vacuolate, with active cytoplasm. They are connected with each other and to cells of the infected region by numerous plasmodesmata. A network of air spaces runs throughout the nodule. Atmospherically applied water stress affects the outer cells of the nodule more quickly and more severely than the inner cells. In any region, vacuolate cells are more susceptible to stress than nonvacuolate cells. Loss of about 30% of their fresh weight results in breakdown of the cytoplasm into approximately spherical sub-units, some of which are coated with ribosomes. Organelles such as nuclei and mitochondria retain their structure longer than the rest of the cytoplasm. More severe stress is needed to affect infected and pericycle cells, which are nonvacuolate. It is concluded from the fine structure of stressed and unstressed nodules that the vacuolate cells of the cortex play an integral part in the nitrogen-fixing activities of the whole nodule. [ABSTRACT FROM AUTHOR]

The effects of osmotic stress on nitrogen fixation and respiration of soybean root nodules were investigated. Non-electrolytes, such as mannitol, depressed acetylene reduction when given to detached nodules in sufficient concentration to withdraw water from them. When given to the root systems of whole plants, hypotonic concentrations of mannitol had a depressing effect after 1-2 hours, the exact time depending on the size of the nodules. Equivalent concentrations of salts had a much more rapid effect on whole root systems and on detached nodules (less than 5 minutes); 100 m-equiv./l almost stopped acetylene reduction by small nodules. These effects could not be transmitted via the roots and needed contact between salt solution and nodules. They were accompanied by a decrease in respiratory activity and were fully reversible if the root systems were flushed with water within a few hours of treatment. High salt concentrations, as in sea water, had a rapid (1-2 minutes) effect, which after 20 minutes became irreversible. The effects of salt stress are interpreted as resulting from alterations in the metabolism of the cortical cells of the nodules. [ABSTRACT FROM AUTHOR]

When the fresh weight of detached soybean nodules is reduced below 80% of its maximum (fully turgid) value, irreversible changes occur. Acetylene and nitrogen reducing activities cease, respiratory rates become very low and there are gross structural changes. Such nodules are probably shed by the plant. Reduction of water content down to 80% of the maximum fresh weight, leads to a proportionate slowing down of acetylene reduction, with a concomitant reduction in respiratory activity. [ABSTRACT FROM AUTHOR]

Information on the 17th International Congress on Nitrogen Fixation (ICNF) held in Fremantle, Western Australia in November 2011 is presented. It mentions that legume researchers have showed that the distribution and evolution of different taxa are on-going process and closely related to biogeographical features, such as climate, soil, and isolation. Moreover, the process of the transformation of the landscape through the introduction of a suite of legumes was described by Neil Ballard.

Information about several papers discussed at the 16th International Congress on Nitrogen Fixation (ICNF) which was held in Big Sky, Montana in June 2009 is presented. Topics included genotypic interaction among legume hosts, anti-pathogenic adaptations of nitrogen-fixing symbionts, harsh environments, further development of legume production for food, and other wider benefits of legumes. It featured several nitrogen fixation researchers including J. Ardley, J. Howieson, and B. Boddey.

The authors describes two model legumes of Medicago truncatula and Lotus japonicus, that are being studied in order to elucidate the detailed interactions between host legume and rhizobial symbiont. They reveal that these two model legumes are characterized by their small genome and seed sizes and the sequencing of their rhizobia. They believe that the common bean, Phaseolus vulgaris, is an unreliable nodulator, because of the range of rhizobia with which it can associate.

Reports that bacterium-filled nodules found on legumes represent a mutually beneficial arrangement. Characteristics of soil bacteria; Significance of nitrogenase in nitrogen fixation.

The authors discuss the aspects of the nitrogen (N) fixation of cyanobacteria linked with feather moss. They cite a study which conducted literature reviews regarding the link between cyanobacteria and mosses. They say that the study showed the significance of Nostoc filaments in the mosses in boreal forests, noting that not all mosses form such asssociations. They add that the importance of N fixing organisms should be considered in many harsh environments.

An obituary of doctor H. D. L. Corby is presented.

Biological nitrogen fixation (BNF) by crop legumes reduces demand for industrial nitrogen fixation (INF). Nonetheless, rates of BNF in agriculture remain low, with strong negative feedback to BNF from reactive soil nitrogen (N) and drought. We show that breeding for yield has resulted in strong relationships between photosynthesis and leaf N in non-leguminous crops, whereas grain legumes show strong relations between leaf N and water use efficiency (WUE). We contrast these understandings with other studies that draw attention to the water costs of grain legume crops, and their potential for polluting the biosphere with N. We propose that breeding grain legumes for reduced stomatal conductance can increase WUE without compromising production or BNF. Legume crops remain a better bet than relying on INF. [ABSTRACT FROM AUTHOR]

Summary: Nitrogen‐fixing symbiosis is globally important in ecosystem functioning and agriculture, yet the evolutionary history of nodulation remains the focus of considerable debate. Recent evidence suggesting a single origin of nodulation followed by massive parallel evolutionary losses raises questions about why a few lineages in the N2‐fixing clade retained nodulation and diversified as stable nodulators, while most did not. Within legumes, nodulation is restricted to the two most diverse subfamilies, Papilionoideae and Caesalpinioideae, which show stable retention of nodulation across their core clades.We characterize two nodule anatomy types across 128 species in 56 of the 152 genera of the legume subfamily Caesalpinioideae: fixation thread nodules (FTs), where nitrogen‐fixing bacteroids are retained within the apoplast in modified infection threads, and symbiosomes, where rhizobia are symplastically internalized in the host cell cytoplasm within membrane‐bound symbiosomes (SYMs).Using a robust phylogenomic tree based on 997 genes from 147 Caesalpinioideae genera, we show that losses of nodulation are more prevalent in lineages with FTs than those with SYMs.We propose that evolution of the symbiosome allows for a more intimate and enduring symbiosis through tighter compartmentalization of their rhizobial microsymbionts, resulting in greater evolutionary stability of nodulation across this species‐rich pantropical legume clade. [ABSTRACT FROM AUTHOR]

More than 200 root-nodule bacterial strains were isolated from Leucaena leucocephala growing at 42 sampling sites across 12 states and three union territories of India. Genetic diversity was observed among 114 strains from various climatic zones; based on recA , these were identified as strains of Ensifer , Mesorhizobium , Rhizobium , and Bradyrhizobium. In multilocus sequence analysis (MLSA) strains clustered into several novel clades and lineages. Ensifer were predominant nodulating genotype isolated from majority of alkaline soils, while Mesorhizobium and Rhizobium strains were isolated from a limited sampling in North-Eastern states with acidic soils. Positive nodulation assays of selected Ensifer representing different genetic combinations of housekeeping and sym genes suggested their broad host range within the closely related mimosoid genera Vachellia , Senegalia , Mimosa , and Prosopis. Leucaena selected diverse strains of Ensifer and Mesorhizobium as symbionts depending on available soil pH, climatic, and other edaphic conditions in India. Lateral gene transfer seems to play a major role in genetic diversification of Ensifer exhibited in terms of Old World vs. Neotropical genetic make-up and mixed populations at several sites. Although Neotropical Ensifer strains were most symbiotically effective on Leucaena , the native Ensifer are promiscuous and particularly well-adapted to a wide range of sampling sites with varied climates and edaphic factors. [ABSTRACT FROM AUTHOR]

Using robust, pairwise comparisons and a global dataset, we show that nitrogen concentration per unit leaf mass for nitrogen-fixing plants (N2FP; mainly legumes plus some actinorhizal species) in nonagricultural ecosystems is universally greater (43-100%) than that for other plants (OP). This difference is maintained across Koppen climate zones and growth forms and strongest in the wet tropics and within deciduous angiosperms. N2FP mostly show a similar advantage over OP in nitrogen per leaf area (Narea), even in arid climates, despite diazotrophy being sensitive to drought. We also show that, for most N2FP, carbon fixation by photosynthesis (Asat) and stomatal conductance (gs) are not related to Narea--in distinct challenge to current theories that place the leaf nitrogen-Asat relationship at the center of explanations of plant fitness and competitive ability. Among N2FP, only forbs displayed an Narea-gs relationship similar to that for OP, whereas intrinsic water use efficiency (WUEi; Asat/gs) was positively related to Narea for woody N2FP. Enhanced foliar nitrogen (relative to OP) contributes strongly to other evolutionarily advantageous attributes of legumes, such as seed nitrogen and herbivore defense. These alternate explanations of clear differences in leaf N between N2FP and OP have significant implications (e.g., for global models of carbon fluxes based on relationships between leaf N and Asat). Combined, greater WUE and leaf nitrogen--in a variety of forms--enhance fitness and survival of genomes of N2FP, particularly in arid and semiarid climates. [ABSTRACT FROM AUTHOR]

Seed weight in Fabaceae ranges over seven orders of magnitude from 0.1 mg to 334 g. The frequency distribution is strongly right skewed with a single modal peak at 18 mg. The two sets of morphological characters, the presence or absence of endosperm and the anatomical form of the cotyledons, interact to define eight structurally and functionally distinct seed types. The predominant seed type is endospermic with foliar cotyledons. The other seed types have evidently arisen from it by transfer of storage function from endosperm to cotyledons, culminating in exendospermic seeds with storage cotyledons. This has probably happened repeatedly in disparate tribes and genera throughout the family. There is a highly significant allometric relationship between the logarithms of the weight of nitrogen in the seed and the total seed weight, with the datasets for nodulating and non-nodulating species yielding significantly different regression equations. The rate of increase in weight of nitrogen is lower than the rate of increase in total seed weight, and this discrepancy is greater in non-nodulating than in nodulating species. A consequence of the discrepancy is an overall inverse relationship between seed weight and nitrogen concentration. Seeds of the same type are significantly larger and have a higher absolute nitrogen content, but a lower nitrogen concentration in non-nodulating than in nodulating species. © 2011 The Linnean Society of London, Botanical Journal of the Linnean Society, 2011, 167, 251-280. [ABSTRACT FROM AUTHOR]

Background: Nitrogen fixation has been quantified for a range of crop legumes and actinorhizal plants under different agricultural/agroforestry conditions, but much less is known of legume and actinorhizal plant N2 fixation in natural ecosystems. Aims: To assess the proportion of total plant N derived from the atmosphere via the process of N2 fixation (%Ndfa) by actinorhizal and legume plants in natural ecosystems and their N input into these ecosystems as indicated by their 15N natural abundance. Methods: A comprehensive collation of published values of %Ndfa for legumes and actinorhizal plants in natural ecosystems and their N input into these ecosystems as estimated by their 15N natural abundance was carried out by searching the ISI Web of Science database using relevant key words. Results: The %Ndfa was consistently large for actinorhizal plants but very variable for legumes in natural ecosystems, and the average value for %Ndfa was substantially greater for actinorhizal plants. High soil N, in particular, but also low soil P and water content were correlated with low legume N2 fixation. N input into ecosystems from N2 fixation was very variable for actinorhizal and legume plants and greatly dependent on their biomass within the system. Conclusions: Measurement of 15N natural abundance has given greater understanding of where legume and actinorhizal plant N2 fixation is important in natural ecosystems. Across studies, the average value for %Ndfa was substantially greater for actinorhizal plants than for legumes, and the relative abilities of the two groups of plants to utilise mineral N requires further study. [ABSTRACT FROM PUBLISHER]

•When rhizobia differentiate inside legume host nodules to become nitrogen-fixing bacteroids, they undergo a physiological as well as a morphological transformation. These transformations are more extreme in some legume species than others, leading to fundamental differences in rhizobial life history and evolution. Here, we analysed the distribution of different bacteroid morphologies over a legume phylogeny to understand the evolutionary history of this host-influenced differentiation. •Using existing electron micrographs and new flow cytometric analyses, bacteroid morphologies were categorized as swollen or nonswollen for 40 legume species in the subfamily Papilionoideae. Maximum likelihood and Bayesian frameworks were used to reconstruct ancestral states at the bases of all major subclades within the papilionoids. •Extreme bacteroid differentiation leading to swelling was found in five out of the six major papilionoid subclades. The inferred ancestral state for the Papilionoideae was hosting nonswollen bacteroids, indicating at least five independent origins of host traits leading to swollen bacteroids. •Repeated evolution of host traits causing bacteroid swelling indicates a possible fitness benefit to the plant. Furthermore, as bacteroid swelling is often correlated with loss of reproductive viability, the evolution of bacteroid cooperation or cheating strategies could be fundamentally different between the two bacteroid morphologies. [ABSTRACT FROM AUTHOR]

Tree root pruning is a potential tool for managing belowground competition when trees and crops are grown together in agroforestry systems. We investigated the effects of tree root pruning on shoot growth and root distribution of Alnus acuminata (H.B. & K.), Casuarina equisetifolia L., Grevillea robusta A. Cunn. ex R. Br., Maesopsis eminii Engl. and Markhamia lutea (Benth.) K. Schum. and on yield of adjacent crops in sub-humid Uganda. The trees were 3 years old at the commencement of the study, and most species were competing strongly with crops. Tree roots were pruned 41 months after planting by cutting and back-filling a trench to a depth of 0.3 m, at a distance of 0.3 m from the trees, on one side of the tree row. The trench was reopened and roots recut at 50 and 62 months after planting. We assessed the effects on tree growth and root distribution over a 3 year period, and crop yield after the third root pruning at 62 months. Overall, root pruning had only a slight effect on aboveground tree growth: height growth was unaffected and diameter growth was reduced by only 4%. A substantial amount of root regrowth was observed by 11 months after pruning. Tree species varied in the number and distribution of roots, and C. equisetifolia and M. lutea had considerably more roots per unit of trunk volume than the other species, especially in the surface soil layers. Casuarina equisetifolia and M. eminii were the tree species most competitive with crops and G. robusta and M. lutea the least competitive. Crop yield data provided strong evidence of the redistribution of root activity following root pruning, with competition increasing on the unpruned side of tree rows. Thus, one-sided root pruning will be useful in only a few circumstances. [ABSTRACT FROM AUTHOR]

The cluster roots of Grevillea robusta A. Cunn. ex R. Br. are composed of determinate rootlets that stop growing, but remain physiologically active for several months. Their apical organization, both before and after maturation, was studied by light and transmission electron microscopy. Each cell layer forms a dome, with an initial cell at its end. Xylem elements form a complicated triarch array at the base of the rootlet, passing along the rootlet as two files, and then joining at the tip to form a single file, surrounded by six pericycle cells. At the base of the rootlet, shorter xylem cells and thick-walled support cells are visible. A root cap, present in rootlets grown in vermiculite, was eventually displaced by root hair growth. Rootlets grown in Hoagland's solution lacked root caps and were significantly shorter than those grown in vermiculite. Cell fate was analysed in terms of cell position and is discussed in terms of pattern and development. [ABSTRACT FROM AUTHOR]

Aims: To describe the structure of nodules of Vachellia ( Acacia) jacquemontii, and to characterise the rhizobia that occupy them. Methods: Light and electron microscopy were used to analyse nodules. Rhizobia were characterised using their 16S rRNA, housekeeping and symbiosis-related gene sequences. Results: Nodules of V. jacquemontii were typical of all other described mimosoid legumes. All 73 of the isolates were strains of Ensifer, and concatenated phylogenetic analysis of their housekeeping genes ( rrs, recA, atpD, glnII and dnaK) suggested that they are novel, forming separate lineages close to E. saheli. The phylogenies of the symbiosis-essential genes nodA and nifH were inconsistent with the housekeeping phylogenies. The nodA sequences of most isolates were close to that of E. arboris HAMBI 1552, but the nifH gene was found to be related to that of E. kostiensis HAMBI 1489. All the tested Ensifer strains, except for AJ24, were found to be capable of nodulating other species of Vachellia as well as native Indian Mimosa and Prosopis spp. Conclusions: Stressful conditions caused by the alkaline soil of the Thar Desert have resulted in V. jacquemontii being nodulated by diverse and promiscuous Ensifer species that are capable of nodulating other native members of the tribe Mimoseae. [ABSTRACT FROM AUTHOR]

Rhizobia of the genus Burkholderia have large-scale distribution ranges and are usually associated with South African papilionoid and South American mimosoid legumes, yet little is known about their genetic structuring at either local or global geographic scales. To understand variation at different spatial scales, from individual legumes in the fynbos (South Africa) to a global context, we analyzed chromosomal (16S rRNA, recA) and symbiosis (nifH, nodA, nodC) gene sequences. We showed that the global diversity of nodulation genes is generally grouped according to the South African papilionoid or South American mimosoid subfamilies, whereas chromosomal sequence data were unrelated to biogeography. While nodulation genes are structured on a continental scale, a geographic or host-specific distribution pattern was not detected in the fynbos region. In host range experiments, symbiotic promiscuity of Burkholderia tuberum STM678T and B. phymatum STM815T was discovered in selected fynbos species. Finally, a greenhouse experiment was undertaken to assess the ability of mimosoid (Mimosa pudica) and papilionoid (Dipogon lignosus, Indigofera filifolia, Macroptilium atropurpureum, and Podalyria calyptrata) species to nodulate in South African (fynbos) and Malawian (savanna) soils. While the Burkholderia-philous fynbos legumes (D. lignosus, I. filifolia, and P. calyptrata) nodulated only in their native soils, the invasive neotropical speciesM. pudica did not develop nodules in the African soils. The fynbos soil, notably rich in Burkholderia, seems to retain nodulation genes compatible with the local papilionoid legume flora but is incapable of nodulating mimosoid legumes that have their center of diversity in South America. [ABSTRACT FROM AUTHOR]

• The legume genus Mimosa has > 500 species, with two major centres of diversity, Brazil (c. 350 spp.) and Mexico (c. 100 spp.). In Brazil most species are nodulated by Burkholderia. Here we asked whether this is also true of native and endemic Mexican species. • We have tested this apparent affinity for betaproteobacteria by examining the symbionts of native and endemic species of Mimosa in Mexico, especially from the central highlands where Mimosa spp. have diversified. Nodules were tested for betaproteobacteria using in situ immunolocalization. Rhizobia isolated from the nodules were genetically characterized and tested for their ability to nodulate Mimosa spp. • Immunological analysis of 25 host taxa suggested that most (including all the highland endemics) were not nodulated by betaproteobacteria. Phylogenetic analyses of 16S rRNA, recA, nodA, nodC and nifH genes from 87 strains isolated from 20 taxa confirmed that the endemic Mexican Mimosa species favoured alphaproteobacteria in the genera Rhizobium and Ensifer: this was confirmed by nodulation tests. • Host phylogeny, geographic isolation and coevolution with symbionts derived from very different soils have potentially contributed to the striking difference in the choice of symbiotic partners by Mexican and Brazilian Mimosa species. [ABSTRACT FROM AUTHOR]

Rhizobial diversity and host preferences were assessed in 65 native Fynbos legumes of the papilionoid legume tribes Astragaleae, Crotalarieae, Genisteae, Indigofereae, Millettieae, Phaseoleae, Podalyrieae, Psoraleeae and Sesbanieae. Sequence analyses of chromosomal 16S rRNA, recA, atpD and symbiosis-related nodA, nifH genes in parallel with immunogold labelling assays identified the symbionts as alpha- (Azorhizobium, Bradyrhizobium, Ensifer, Mesorhizobium and Rhizobium) and beta-rhizobial (Burkholderia) lineages with the majority placed in the genera Mesorhizobium and Burkholderia showing a wide range of host interactions. Despite a degree of symbiotic promiscuity in the tribes Crotalarieae and Indigofereae nodulating with both alpha- and beta-rhizobia, Mesorhizobium symbionts appeared to exhibit a general host preference for the tribe Psoraleeae, whereas Burkholderia prevailed in the Podalyrieae. Although host genotype was the main factor determining rhizobial diversity, ecological factors such as soil acidity and site elevation were positively correlated with genetic variation within Mesorhizobium and Burkholderia, respectively, indicating an interplay of host and environmental factors on the distribution of Fynbos rhizobia. [ABSTRACT FROM AUTHOR]

The South African invasive legume Dipogon lignosus (Phaseoleae) produces nodules with both determinate and indeterminate characteristics in New Zealand (NZ) soils. Ten bacterial isolates produced functional nodules on D. lignosus. The 16S ribosomal RNA (rRNA) gene sequences identified one isolate as Bradyrhizobium sp., one isolate as Rhizobium sp. and eight isolates as Burkholderia sp. The Bradyrhizobium sp. and Rhizobium sp. 16S rRNA sequences were identical to those of strains previously isolated from crop plants and may have originated from inocula used on crops. Both 16S rRNA and DNA recombinase A ( recA) gene sequences placed the eight Burkholderia isolates separate from previously described Burkholderia rhizobial species. However, the isolates showed a very close relationship to Burkholderia rhizobial strains isolated from South African plants with respect to their nitrogenase iron protein ( nifH), N-acyltransferase nodulation protein A ( nodA) and N-acetylglucosaminyl transferase nodulation protein C ( nodC) gene sequences. Gene sequences and enterobacterial repetitive intergenic consensus (ERIC) PCR and repetitive element palindromic PCR (rep-PCR) banding patterns indicated that the eight Burkholderia isolates separated into five clones of one strain and three of another. One strain was tested and shown to produce functional nodules on a range of South African plants previously reported to be nodulated by Burkholderia tuberum STM678 which was isolated from the Cape Region. Thus, evidence is strong that the Burkholderia strains isolated here originated in South Africa and were somehow transported with the plants from their native habitat to NZ. It is possible that the strains are of a new species capable of nodulating legumes. [ABSTRACT FROM AUTHOR]

Background and Aims The large monophyletic genus Mimosa comprises approx. 500 species, most of which are native to the New World, with Central Brazil being the main centre of radiation. All Brazilian Mimosa spp. so far examined are nodulated by rhizobia in the betaproteobacterial genus Burkholderia. Approximately 10 Mya, transoceanic dispersal resulted in the Indian subcontinent hosting up to six endemic Mimosa spp. The nodulation ability and rhizobial symbionts of two of these, M. hamata and M. himalayana, both from north-west India, are here examined, and compared with those of M. pudica, an invasive species. Methods Nodules were collected from several locations, and examined by light and electron microscopy. Rhizobia isolated from them were characterized in terms of their abilities to nodulate the three Mimosa hosts. The molecular phylogenetic relationships of the rhizobia were determined by analysis of 16S rRNA, nifH and nodA gene sequences. Key Results Both native Indian Mimosa spp. nodulated effectively in their respective rhizosphere soils. Based on 16S rRNA, nifH and nodA sequences, their symbionts were identified as belonging to the alphaproteobacterial genus Ensifer, and were closest to the ‘Old World’ Ensifer saheli, E. kostiensis and E. arboris. In contrast, the invasive M. pudica was predominantly nodulated by Betaproteobacteria in the genera Cupriavidus and Burkholderia. All rhizobial strains tested effectively nodulated their original hosts, but the symbionts of the native species could not nodulate M. pudica. Conclusions The native Mimosa spp. in India are not nodulated by the Burkholderia symbionts of their South American relatives, but by a unique group of alpha-rhizobial microsymbionts that are closely related to the ‘local’ Old World Ensifer symbionts of other mimosoid legumes in north-west India. They appear not to share symbionts with the invasive M. pudica, symbionts of which are mostly beta-rhizobial. [ABSTRACT FROM PUBLISHER]

The threatened caesalpinioid legume Dimorphandra wilsonii, which is native to the Cerrado biome in Brazil, was examined for its nodulation and N2-fixing ability, and was compared with another, less-threatened species, D. jorgei. Nodulation and potential N2 fixation was shown on seedlings that had been inoculated singly with five bradyrhizobial isolates from mature D. wilsonii nodules. The infection of D. wilsonii by two of these strains (Dw10.1, Dw12.5) was followed in detail using light and transmission electron microscopy, and was compared with that of D. jorgei by Bradyrhizobium strain SEMIA6099. The roots of D. wilsonii were infected via small transient root hairs at 42 d after inoculation (dai), and nodules were sufficiently mature at 63 dai to express nitrogenase protein. Similar infection and nodule developmental processes were observed in D. jorgei. The bacteroids in mature Dimorphandra nodules were enclosed in plant cell wall material containing a homogalacturonan (pectic) epitope that was recognized by the monoclonal antibody JIM5. Analysis of sequences of their rrs (16S rRNA) genes and their ITS regions showed that the five D. wilsonii strains, although related to SEMIA6099, may constitute five undescribed species of genus Bradyrhizobium, whilst their nodD and nifH gene sequences showed that they formed clearly separated branches from other rhizobial strains. This is the first study to describe in full the N2-fixing symbiotic interaction between defined rhizobial strains and legumes in the sub-family Caesalpinioideae. This information will hopefully assist in the conservation of the threatened species D. wilsonii. [ABSTRACT FROM AUTHOR]

New Zealand became geographically isolated about 80 million years ago and this separation gave rise to a unique native flora including four genera of legume, Carmichaelia, Clianthus and Montigena in the Carmichaelinae clade, tribe Galegeae, and Sophora, tribe Sophoreae, sub-family Papilionoideae. Ten bacterial strains isolated from NZ Carmichaelinae growing in natural ecosystems grouped close to the Mesorhizobium huakuii type strain in relation to their 16S rRNA and nifH gene sequences. However, the ten strains separated into four groups on the basis of their recA and glnII sequences: all groups were clearly distinct from all Mesorhizobium type strains. The ten strains separated into two groups on the basis of their nodA sequences but grouped closely together in relation to nodC sequences; all nodA and nodC sequences were novel. Seven strains selected and the M. huakuii type strain (isolated from Astragalus sinicus) produced functional nodules on Carmichaelia spp., Clianthus puniceus and A. sinicus but did not nodulate two Sophora species. We conclude that rhizobia closely related to M. huakuii on the basis of 16S rRNA and nifH gene sequences, but with variable recA and glnII genes and novel nodA and nodC genes, are common symbionts of NZ Carmichaelinae. [ABSTRACT FROM AUTHOR]

Aims: To survey the occurrence of nodulated legumes in the arid and semi-arid areas of Western Rajasthan and to characterize their associated symbiotic bacteria. Methods: Herbaceous annual species were excavated whole, while tree species were studied as seedlings in the field or as trap plants in pot experiments. Nodules were examined by microscopy to confirm their effectiveness and to determine their internal structure. Bacteria isolated from the nodules were authenticated on their original hosts and were identified on the basis of 16S rRNA sequencing. Phylogenetic trees were inferred using the neighbour-joining method. Results: We studied 35 of more than 50 species of native legume reported from these areas. Legumes are drought escaping (annual species), drought tolerant perennials or trees possessing deep root systems and other adaptations to arid conditions. Nodulation was recorded in all members of the Papilionoideae and Mimosoideae, but only one species of Caesalpinioideae. Internal structure of nodules varied within these groups, especially with respect to the presence or absence of uninfected cells in the infected region. Full 16S rRNA gene sequencing revealed that the nodules harboured a range of nodulating bacteria belonging to the genera Sinorhizobium, Rhizobium and Bradyrhizobium, within which they formed separate sub clades. Conclusions: This study extends the range of legumes known to grow and nodulate in semi-arid regions, and provides information about their endosymbionts. [ABSTRACT FROM AUTHOR]

•An extensive survey of nodulation in the legume genus Mimosa was undertaken in two major biomes in Brazil, the Cerrado and the Caatinga, in both of which there are high degrees of endemicity of the genus. •Nodules were collected from 67 of the 70 Mimosa spp. found. Thirteen of the species were newly reported as nodulating. Nodules were examined by light and electron microscopy, and all except for M. gatesiae had a structure typical of effective Mimosa nodules. The endosymbiotic bacteria in nodules from all of the Mimosa spp. were identified as Burkholderia via immunolabelling with an antibody against Burkholderia phymatum STM815. •Twenty of the 23 Mimosa nodules tested were shown to contain nitrogenase by immunolabelling with an antibody to the nitrogenase Fe- ( nifH) protein, and using the δ15N (15N natural abundance) technique, contributions by biological N2 fixation of up to 60% of total plant N were calculated for Caatinga Mimosa spp. •It is concluded that nodulation in Mimosa is a generic character, and that the preferred symbionts of Brazilian species are Burkholderia. This is the first study to demonstrate N2 fixation by beta-rhizobial symbioses in the field. [ABSTRACT FROM AUTHOR]

Burkholderia has only recently been recognized as a potential nitrogen-fixing symbiont of legumes, but we find that the origins of symbiosis in Burkholderia are much deeper than previously suspected. We sampled 143 symbionts from 47 native species of Mimosa across 1800 km in central Brazil and found that 98% were Burkholderia. Gene sequences defined seven distinct and divergent species complexes within the genus Burkholderia. The symbiosis-related genes formed deep Burkholderia-specific clades, each specific to a species complex, implying that these genes diverged over a long period within Burkholderia without substantial horizontal gene transfer between species complexes. [ABSTRACT FROM AUTHOR]