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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]

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]

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]

•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]

• A total of 191 rhizobial isolates from the root nodules of three geographically separate populations of the invasive plant Mimosa pigra in Taiwan were examined using amplified rDNA restriction analysis, 16S rDNA sequences, protein profiles and ELISA. Of these, 96% were identified as Burkholderia and 4% as Cupriavidus taiwanensis. • The symbiosis-essential genes nodA and nifH were present in two strains of Burkholderia (PAS44 and PTK47), and in one of C. taiwanensis (PAS15). All three could nodulate M. pigra. • Light and electron microscopy studies with a green fluorescent protein transconjugant variant of strain PAS44 showed the presence of fluorescent bacteroids in M. pigra nodules. These bacteroids expressed the nifH protein, hence this is the first confirmation that Burkholderia is a genuine symbiont of legume nodules. • The predominance of Burkholderia in Taiwanese M. pigra suggests that this species may have brought its symbionts from its native South America, rather than entering into association with the Taiwanese Mimosa symbiont C. taiwanensis which so successfully nodulates Mimosa pudica and Mimosa diplotricha. [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 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]