Pastures often experience a pulse of phosphorus (P) when fertilised. We examined the role of arbuscular mycorrhizal fungi (AMF) in uptake of P from a pulse. Five legumes (Kennedia prostrata, Cullen australasicum, Bituminaria bituminosa, Medicago sativa, Trifolium subterraneum) were grown in a moderate P, sterilised field soil, either with (+AMF) or without (-AMF) addition of unsterilised field soil. After 9 to 10 weeks, half the pots received 15 mg P kg-1 of soil. One week later we measured: shoot and root dry weights; percentage of root length colonised by AMF; plant P, nitrogen and manganese (Mn) concentrations; and, rhizosphere carboxylates, pH and plant-available P. The P pulse raised root P concentration by a similar amount in uncolonised and colonised plants, but shoot P concentration increased by 143% in uncolonsied plants and 53% in colonised plants. Inoculation with AMF decreased the amount of rhizosphere carboxylates by 52%, raised rhizosphere pH by ∼0.2-0.7 pH units and lowered shoot Mn concentration by 38%. We conclude that AMF are not simply a means for plants to enhance P uptake when P is limiting, but also act to maintain shoot P within narrow boundaries and can affect nutrient uptake through their influence on rhizosphere chemistry.
Plant-Microbe Symbiosis
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Beneficial associations between plants and microbes
Curated by
Jean-Michel Ané
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Rhizobia establish a symbiotic relationship with their host legumes to induce the formation of nitrogen-fixing nodules. This process is regulated by many rhizobium regulators, including some two-component regulatory systems (TCSs). NtrY/NtrX, a TCS that was first identified in Azorhizobium caulinodans, is required for free-living nitrogen metabolism and symbiotic nodulation on Sesbania rostrata. However, its functions in a typical rhizobium such as Sinorhizobium meliloti remain unclear. Here we found that the S. meliloti response regulator NtrX but not the histidine kinase NtrY is involved in the regulation of exopolysaccharide production, motility, and symbiosis with alfalfa. A plasmid-insertion mutant of ntrX formed mucous colonies, which overproduced succinoglycan, an exopolysaccharide, by upregulating its biosynthesis genes. This mutant also exhibited motility defects due to reduced flagella and decreased expression of flagellins and regulatory genes. The regulation is independent of the known regulatory systems of ExoR/ExoS/ChvI, EmmABC and ExpR. Alfalfa plants inoculated with the ntrX mutant were small and displayed symptoms of nitrogen starvation. Interestingly, the deletion mutant of ntrY showed a phenotype similar to that of the parent strain. These findings demonstrate that the S. meliloti NtrX is a new regulator of succinoglycan production and motility that is not genetically coupled with NtrY. Wang D, Xue H, Wang Y, Yin R, Xie F, Luo L. (2013). Appl Environ Microbiol. Sep 13. [Epub ahead of print] Via IvanOresnik 
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And so the term begins, as does version 1.0 of my Coevolutionary Interactions course (bio3401). The idea behind the course is to use species interactions and the coevolutionary process to reinforce key theory/ideas in evolution and ecology.
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Professor Richard J. Morris, Institute Strategic Programme Leader at the John Innes Centre will give this seminar entitled "Cracking the symbiosis code" at the James Hutton Institute in Dundee on 22 October 2013.
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During calcium (Ca2+) signaling, decoding the stimulus-response coupling involves a set of Ca2+ sensor proteins or Ca2+-binding proteins (DeFalco et al., 2010a; Kudla et al., 2010). These proteins usually possess one or more classical helix-loop-helix elongation factor (EF) hand motifs. Three major types of Ca2+-sensor proteins in plants are calmodulin (CaM)/CaM-like proteins, calcium-dependent protein kinases (CDPKs), and calcineurin B-like proteins. As compared with animals, plant genomes encode more diversified Ca2+ sensors; with the exception of canonic CaM, all other types of Ca2+ sensors (CaM-like proteins,CDPKs, and calcineurin B-like proteins) are plant specific. The large population and unique structural composition of Ca2+-binding proteins and the diversity of the target proteins regulated by the Ca2+ sensors reflect the complexity of Ca2+signaling, which helps plants adapt to the changing environment. This update will be limited primarily to discussions on CaM and CaM-binding proteins and the recent advances in Ca2+/CaM-mediated signaling.
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A new movie looks at the lively life of healthy soil, and humans wouldn’t know it from a hole in the ground.
“It’s like Times Square on New Year’s Eve all the time,” Elaine Ingham says in the recent film “Symphony of the Soil” by Deborah Koons Garcia. Ingham, a well-known soil microbiologist, is excited about the teeming masses of creatures in the soil — bacteria, fungi and others — that make all life on Earth possible. And Koons Garcia manages to devote 101 minutes to the living soil without letting any of that excitement flag.
Jean-Michel Ané's insight:
Great trailer:
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Sustainable production of biomass for bioenergy relies on low-input crop production. Inoculation of bioenergy crops with plant growth-promoting endophytes has the potential to reduce fertilizer inputs through the enhancement of biological nitrogen fixation (BNF).Endophytes isolated from native poplar growing in nutrient-poor conditions were selected for a series of glasshouse and field trials designed to test the overall hypothesis that naturally occurring diazotrophic endophytes impart growth promotion of the host plants.Endophyte inoculations contributed to increased biomass over uninoculated control plants. This growth promotion was more pronounced with multi-strain consortia than with single-strain inocula. Biological nitrogen fixation was estimated through 15N isotope dilution to be 65% nitrogen derived from air (Ndfa).Phenotypic plasticity in biomass allocation and branch production observed as a result of endophyte inoculations may be useful in bioenergy crop breeding and engineering programs.
Jean-Michel Ané's insight:
Nitrogen-fixing poplars? What do you think about it?
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Leguminous plants establish symbiosis with nitrogen-fixing alpha- and betaproteobacteria, collectively called rhizobia, which provide combined nitrogen to support plant growth. Members of the inverted repeat-lacking clade of legumes impose terminal differentiation on their endosymbiotic bacterium partners with the help of the nodule-specific cysteine-rich (NCR) peptide family composed of close to 600 members. Among the few tested NCR peptides, cationic ones had antirhizobial activity measured by reduction or elimination of the CFU and uptake of the membrane-impermeable dye propidium iodide. Here, the antimicrobial spectrum of two of these peptides, NCR247 and NCR335, was investigated, and their effect on the transcriptome of the natural targetSinorhizobium meliloti was characterized. Both peptides were able to kill quickly a wide range of Gram-negative and Gram-positive bacteria; however, their spectra were only partially overlapping, and differences were found also in their efficacy on given strains, indicating that the actions of NCR247 and NCR335 might be similar though not identical. Treatment of S. meliloti cultures with either peptide resulted in a quick downregulation of genes involved in basic cellular functions, such as transcription-translation and energy production, as well as upregulation of genes involved in stress and oxidative stress responses and membrane transport. Similar changes provoked mainly in Gram-positive bacteria by antimicrobial agents were coupled with the destruction of membrane potential, indicating that it might also be a common step in the bactericidal actions of NCR247 and NCR335.
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Root nodule symbiosis (RNS) is one of the most efficient biological systems for nitrogen fixation and it occurs in 90% of genera in the Papilionoideae, the largest subfamily of legumes. Most papilionoid species show evidence of a polyploidy event occurred approximately 58 million years ago. Although polyploidy is considered to be an important evolutionary force in plants, the role of this papilionoid polyploidy event, especially its association with RNS, is not understood. In this study, we explored this role using an integrated comparative genomic approach and conducted gene expression comparisons and gene ontology enrichment analyses. The results show the following: (1) approximately a quarter of the papilionoid-polyploidy-derived duplicate genes are retained; (2) there is a striking divergence in the level of expression of gene duplicate pairs derived from the polyploidy event; and (3) the retained duplicates are frequently involved in the processes crucial for RNS establishment, such as symbiotic signalling, nodule organogenesis, rhizobial infection and nutrient exchange and transport. Thus, we conclude that the papilionoid polyploidy event might have further refined RNS and induced a more robust and enhanced symbiotic system. This conclusion partly explains the widespread occurrence of the Papilionoideae.
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Transcription factors (TFs) are thought to regulate many aspects of nodule and symbiosis development in legumes, although few TFs have been characterized functionally. Here, we describe REGULATOR OF SYMBIOSOME DIFFERENTIATION (RSD) of Medicago truncatula, a member of the Cysteine-2/Histidine-2 (C2H2) family of plant TFs that is required for normal symbiosome differentiation during nodule development. RSD is expressed in a nodule-specific manner, with maximal transcript levels in the bacterial invasion zone. A tobacco (Nicotiana tabacum) retrotransposon (Tnt1) insertion rsd mutant produced nodules that were unable to fix nitrogen and that contained incompletely differentiated symbiosomes and bacteroids. RSD protein was localized to the nucleus, consistent with a role of the protein in transcriptional regulation. RSD acted as a transcriptional repressor in a heterologous yeast assay. Transcriptome analysis of an rsd mutant identified 11 genes as potential targets of RSD repression. RSD interacted physically with the promoter of one of these genes, VAMP721a, which encodes vesicle-associated membrane protein 721a. Thus, RSD may influence symbiosome development in part by repressing transcription of VAMP721a and modifying vesicle trafficking in nodule cells. This establishes RSD as a TF implicated directly in symbiosome and bacteroid differentiation and a transcriptional regulator of secretory pathway genes in plants.
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Looking for some EnTErtainment? Play your way through the last 600 million years with Evolve or Perish, the newETE board game!
Are you adaptable enough to survive all the way to the present?
Download the game below and find out. You will need: 1) A printer to print out the two pages of the game. Color is nice, if not you can print it in black and white (and maybe color it in yourself?). 2) Scissors to trim one of the pages along the dotted line, and tape or glue to join the two pages. 3) A die (that's the singular of dice, in case you didn't know). 4) Each player needs to choose some small object to use as a token. Anything will do (for instance a jack, a miniature plastic animal, a pebble, or a penny) as long as each player's is different so you will know who is who. 5) One or two fellow tetrapods to play with, preferably of the species Homo sapiens.
Jean-Michel Ané's insight:
Great activity for your Saturday evenings!
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How are we going to feed a growing world without destroying the planet? Great video via The University of Minnesota's Institute on the Environment. Via Kamoun Lab @ TSL
Jean-Michel Ané's insight:
Really good video from colleagues in Minnesota highlighting the BIG problem that we are facing right now. How are we going to feed our growing human population in the next decades without destroying our planet and us ultimately? It is an exciting time to do research in plant sciences, microbiology, and agriculture to addess this issue but this is just one tool. We will need to have a serious collaborations, innovation and political courage at the international level to solve this issue.
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Members of the Glomeromycota form the arbuscular mycorrhiza (AM) symbiosis. They supply plants with inorganic nutrients, including nitrogen, from the soil. To gain insight into transporters potentially facilitating nitrogen transport processes, ammonium transporters (AMTs) of Geosiphon pyriformis, a glomeromycotan fungus forming symbiosis with cyanobacteria, were studied. Three AMT genes were identified, all three are expressed in the symbiotic stage. Localization and functional characterization of the proteins in a heterologous yeast system revealed distinct characteristics for each of them. GpyrAMT1 and GpyrAMT2 both were plasma membrane localized, but only GpyrAMT1 transported ammonium. Neither protein transported the ammonium analogue methylammonium. GpyrAMT3 unexpectedly was localized in the vacuolar membrane and has yet unknown transport characteristics. An unusual cysteine residue in the AMT signature of GpyrAMT2 and GpyrAMT3 was identified and the corresponding residue was demonstrated to play an important role in ammonium transport. Surprisingly, each of the three AMTs of G. pyriformis had very distinct features. The localization of an AMT in the yeast vacuolar membrane is novel, as well as the described amino acid residue that clearly influences ammonium transport. The AMT characteristics might reflect adaptations to the lifestyle of glomeromycotan fungi.
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Scientists from the United Arab Emirates [UAE] have isolated local salt- and drought-tolerant strains of Rhizobia, soil bacteria that fix nitrogen when they become established inside the root nodules of legumes.
Rhizobia bacteria establish a mutually beneficial relationship with their host plant in which they exchange nitrogen they fix for nutrients plants produce through photosynthesis, and could be integral to improving the quality and nitrogen content of soil.
The study was carried out by scientists from the Dubai-based International Center for Biosaline Agriculture (ICBA) in collaboration with the Dubai branch of the Birla Institute of Technology and Science.
"The project was conceived when I noticed, at a research farm in Dubai, some leguminous crops forming nodules to fix atmospheric nitrogen," lead author Nanduri Rao, a researcher at ICBA, tells SciDev.Net.
Rao's team began isolating several naturally moccurring Rhizobia strains from the root nodules of three leguminous plants: sesbania (Sesbania sesban), lablab (Lablab purpureus) and pigeon pea (Cajanus cajan). Rao explains that the team has also been studying the Rhizobiastrains'tolerance to environmental stresses such as high temperature, salinity, acidity and heavy metal concentrations in laboratory tests.
"The strains, native to the United Arab Emirates desert, were found to have a high tolerance to such stresses," Rao says, adding that a full length journal article based on this additional research is currently being prepared.
Daniele Daffonchio, professor of microbiology at the University of Milan, Italy, tells SciDev.Net that the identification of microbes capable of surviving in stressful conditions has important implications for agriculture in arid regions.
Philippe Normand, professor of microbial ecology at the University of Claude Bernard Lyon 1, France, tells SciDev.Net that the UAE project "has great potential for improving plant productivity under extreme environments," and is "an interesting approach for deserts". Via Dr. Stefan Gruenwald
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Bacteria present in the rhizosphere of plants often synthesize phytohormones, and these signals can consequently affect root system development. In legumes, plants adapt to nitrogen starvation by forming lateral roots as well as a new organ, the root nodule, following a symbiotic interaction with bacteria collectively referred to as rhizobia. As cytokinin (CK) phytohormones were shown to be necessary and sufficient to induce root nodule organogenesis, the relevance of CK production by symbiotic rhizobia was questioned. In this study, we analyzed quantitatively, by liquid chromatography-tandem mass spectrometry, the production of 25 forms of CK in nine rhizobia strains belonging to four different species. All bacterial strains were able to synthesize a mix of CK, and bioactive forms of CK, such as iP, were notably found to be secreted in bacterial culture supernatants. Use of a mutant affected in extracellular polysaccharide (EPS) production revealed a negative correlation of EPS production with the ability to secrete CK. In addition, analysis of a nonnodulating Sinorhizobium meliloti strain revealed a similar pattern of CK production and secretion when compared with a related nodulating strain. This indicates that bacterially produced CK are not sufficient to induce symbiotic nodulation.
Kisiala A, Laffont C, Emery RJ, Frugier F. (2013). Mol Plant Microbe Interact. Oct;26(10):1225-31. Via IvanOresnik
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ISIS Report 23/09/13. Nitrogen-fixing for All Crops Not the Answer. Patented and licensed technology based on a power point presentation and one paper
Jean-Michel Ané's insight:
I don't agree with everything but this opinion contains very good points.
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An astonishing variety of plant and animal cellular functions are coordinated by intracellular calcium (Ca2+; Berridge et al., 2000; White and Broadley, 2003). Although Ca2+ is a common second messenger, it is translated into specific developmental processes, implying specificity of recognition of individual Ca2+responses (Dolmetsch et al., 1997, 1998). Such specificity in Ca2+ signaling is a function of its mechanism of activation, the spatial nature of its release, and the developmental context within which the Ca2+ response occurs (Berridge et al., 2000; Evans et al., 2001; Ng and McAinsh, 2003; Di Capite et al., 2009). Together, the frequency, amplitude, and spatial location of the Ca2+ release can differ, and these lead to variations in Ca2+ responses, commonly known as the Ca2+signature (McAinsh and Pittman, 2009). The spatial differences across Ca2+responses derive from Ca2+ release from diverse stores, with differential activation of Ca2+ channels that occur in restricted locations within the cell (Berridge et al., 2000; Di Capite et al., 2009; McAinsh and Pittman, 2009).
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Legumes form a symbiotic interaction with Rhizobiaceae bacteria, which differentiate into nitrogen-fixing bacteroids within nodules. Here, we investigated in vivo the pH of the peribacteroid space (PBS) surrounding the bacteroid and pH variation throughout symbiosis. In vivoconfocal microscopy investigations, using acidotropic probes, demonstrated the acidic state of the PBS. In planta analysis of nodule senescence induced by distinct biological processes drastically increased PBS pH in the N2-fixing zone (zone III). Therefore, the PBS acidification observed in mature bacteroids can be considered as a marker of bacteroid N2 fixation. Using a pH-sensitive ratiometric probe, PBS pH was measured in vivo during the whole symbiotic process. We showed a progressive acidification of the PBS from the bacteroid release up to the onset of N2 fixation. Genetic and pharmacological approaches were conducted and led to disruption of the PBS acidification. Altogether, our findings shed light on the role of PBS pH of mature bacteroids in nodule functioning, providing new tools to monitor in vivo bacteroid physiology.
OLIVIER PIERRE, GILBERT ENGLER, JULIE HOPKINS, FRÉDÉRIC BRAU, ERIC BONCOMPAGNI, DIDIER HÉROUART (2013). Plant Cell and Environment 36: 2059-2070.
Via IvanOresnik
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Several plant species of the genus Psychotria (Rubiaceae) harborBurkholderia sp. bacteria within specialized leaf nodules. The bacteria are transmitted vertically between plant generations and have not yet been cultured outside of their host. This symbiosis is considered to be obligatory because plants devoid of symbionts fail to develop into mature individuals. The genome of ‘Candidatus Burkholderia kirkii’ has been sequenced recently and has revealed evidence of reductive genome evolution, as shown by the proliferation of insertion sequences and the presence of numerous pseudogenes. We employed shotgun proteomics to investigate the expression of ‘Ca. B. kirkii’ proteins in the leaf nodule. Drawing from this dataset and refined comparative genomics analyses, we designed a new pseudogene prediction algorithm and improved the genome annotation. We also found conclusive evidence that nodule bacteria allocate vast resources to synthesis of secondary metabolites, possibly of the C7N aminocyclitol family. Expression of a putative 2-epi-5-valiolone synthase, a key enzyme of the C7N aminocyclitol synthesis, is high in the nodule population but downregulated in bacteria residing in the shoot apex, suggesting that production of secondary metabolites is particularly important in the leaf nodule.
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On 4 July, good news arrived in the inbox of Ocorrafoo Cobange, a biologist at the Wassee Institute of Medicine in Asmara. It was the official letter of acceptance for a paper he had submitted 2 months earlier to the Journal of Natural Pharmaceuticals, describing the anticancer properties of a chemical that Cobange had extracted from a lichen. In fact, it should have been promptly rejected. Any reviewer with more than a high-school knowledge of chemistry and the ability to understand a basic data plot should have spotted the paper's short-comings immediately. Its experiments are so hopelessly flawed that the results are meaningless. I know because I wrote the paper. Ocorrafoo Cobange does not exist, nor does the Wassee Institute of Medicine. Over the past 10 months, I have submitted 304 versions of the wonder drug paper to open-access journals. More than half of the journals accepted the paper, failing to notice its fatal flaws. Beyond that headline result, the data from this sting operation reveal the contours of an emerging Wild West in academic publishing.
Jean-Michel Ané's insight:
Another inconvenient truth... "deceptive open-access journals parasitizing the scientific research community."
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Plants can develop an enhanced defensive capacity in response to infection by arbuscular mycorrhizal fungi (AMF). This ‘mycorrhiza-induced resistance’ (MIR) provides systemic protection against a wide range of attackers and shares characteristics with systemic acquired resistance (SAR) after pathogen infection and induced systemic resistance (ISR) following root colonisation by non-pathogenic rhizobacteria. It is commonly assumed that fungal stimulation of the plant immune system is solely responsible for MIR. In this opinion article, we present a novel model of MIR that integrates different aspects of the induced resistance phenomenon. We propose that MIR is a cumulative effect of direct plant responses to mycorrhizal infection and indirect immune responses to ISR-eliciting rhizobacteria in the mycorrhizosphere.
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Root–nodule symbiosis between leguminous plants and nitrogen-fixing bacteria (rhizobia) involves molecular communication between the two partners. Key components for the establishment of symbiosis are rhizobium-derived lipochitooligosaccharides (Nod factors; NFs) and their leguminous receptors (NFRs) that initiate nodule development and bacterial entry. Here we demonstrate that the soybean microsymbiontBradyrhizobium elkanii uses the type III secretion system (T3SS), which is known for its delivery of virulence factors by pathogenic bacteria, to promote symbiosis. Intriguingly, wild-type B. elkanii, but not the T3SS-deficient mutant, was able to form nitrogen-fixing nodules on soybean nfr mutant En1282. Furthermore, even the NF-deficient B. elkanii mutant induced nodules unless T3SS genes were mutated. Transcriptional analysis revealed that expression of the soybean nodulation-specific genes ENOD40 and NIN was increased in the roots of En1282 inoculated with B. elkanii but not with its T3SS mutant, suggesting that T3SS activates host nodulation signaling by bypassing NF recognition. Root-hair curling and infection threads were not observed in the roots of En1282 inoculated with B. elkanii, indicating that T3SS is involved in crack entry or intercellular infection. These findings suggest that B. elkanii has adopted a pathogenic system for activating host symbiosis signaling to promote its infection.
Steve Marek's curator insight,
October 2, 2013 11:44 AM
Changing the paradigm..."nod factors? we don't need no stinking nod factors'
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Nodule formation induced by nitrogen-fixing rhizobia depends on bacterial nodulation factors (NFs), modified chitin oligosaccharides with a fatty acid moiety. Certain NFs can be cleaved and inactivated by plant chitinases. However, the most abundant NF of Sinorhizobium meliloti, an O-acetylated and sulphated tetramer, is resistant to hydrolysis by all plant chitinases tested so far. Nevertheless, this NF is rapidly degraded in the host rhizosphere. Here, we identify and characterize MtNFH1 (Medicago truncatula Nod factor hydrolase 1), a legume enzyme structurally related to defense-related class V chitinases (glycoside hydrolase family 18). MtNFH1 lacks chitinase activity but efficiently hydrolyzes all tested NFs of S. meliloti. The enzyme shows a high cleavage preference, releasing exclusively lipo-disaccharides from NFs. Substrate specificity and kinetic properties of MtNFH1 were compared to those of class V chitinases from Arabidopsis thaliana and Nicotiana tabacum, which cannot hydrolyze tetrameric NFs of S. meliloti. The Michaelis-Menten constants of MtNFH1 for NFs are in the micromolar concentration range, whereas non-modified chitin oligosaccharides represent neither substrates nor inhibitors for MtNFH1. The three-dimensional structure of MtNFH1 was modeled on the basis of the known structure of class V chitinases. Docking simulation of NFs to MtNFH1 predicted a distinct binding cleft for the fatty acid moiety, which is absent in the class V chitinases. Point mutation analysis confirmed the modeled NF-MtNFH1 interaction. Silencing of MtNFH1 by RNA interference resulted in reduced NF degradation in the rhizosphere of M. truncatula. In conclusion, we have found a novel legume hydrolase that specifically inactivates NFs.
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The legume-rhizobia symbioses lead to the formation of a novel adaptive complex organ, termed the root nodule, which arises from cortical cell division and rhizobial infection in the root. Lipochitin oligosaccarides, Nod-Factors (NFs) secreted by rhizobia, are responsible for the onset of nodule development. Here we describe the characterization of Lotus japonicas, Nod factor Signaling Pathway2 (LjNSP2) protein that belongs to the plant GRAS family of transcription factors. Yeast two-hybrid analysis indicates that LjNSP2 alone has a transcription-stimulating ability and for this the SH2(src-homology2)-like domain, vital for function of STAT proteins is required. The ADG4 (the activation domain of GAL4)-LjNSP2 fusion coupled with BDG4 (the DNA binding domain of GAL4)-LjNSP2 increased the expression level a further 50%, whereas the ADG4-Ljnsp2–1 mutant fusion did not, indicating that LjNSP2 interacts with itself to form a homodimer and this depends on the SH2-like domain. Based on the evidence, we discuss the action of LjNSP2, compared with that of the family of animal-specific STAT transcription factors, which induce developmental programmes in response to external stimuli.
Jean-Michel Ané's insight:
Sorry to be a bit critical but... all this has been shown for the Medicago NSP2 protein in Hirsch et al (2009) already: http://www.plantcell.org/content/21/2/545.full What is the novelty here?
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Legumes, which develop a symbiosis with nitrogen-fixing bacteria, have an increased demand for iron. Iron is required for the synthesis of iron-containing proteins in the host, including the highly abundant leghemoglobin, and in bacteroids for nitrogenase and cytochromes of the electron transport chain. Deficiencies in iron can affect initiation and development of the nodule. Within root cells, iron is chelated with organic acids such as citrate and nicotianamine and distributed to other parts of the plant. Transport to the nitrogen-fixing bacteroids in infected cells of nodules is more complicated. Formation of the symbiosis results in bacteroids internalized within root cortical cells of the legume where they are surrounded by a plant-derived membrane termed the symbiosome membrane (SM). This membrane forms an interface that regulates nutrient supply to the bacteroid. Consequently, iron must cross this membrane before being supplied to the bacteroid. Iron is transported across the SM as both ferric and ferrous iron. However, uptake of Fe(II) by both the symbiosome and bacteroid is faster than Fe(III) uptake. Members of more than one protein family may be responsible for Fe(II) transport across the SM. The only Fe(II) transporter in nodules characterized to date is GmDMT1 (Glycine max divalent metal transporter 1), which is located on the SM in soybean. Like the root plasma membrane, the SM has ferric iron reductase activity. The protein responsible has not been identified but is predicted to reduce ferric iron accumulated in the symbiosome space prior to uptake by the bacteroid. With the recent publication of a number of legume genomes including Medicago truncatula and G. max, a large number of additional candidate transport proteins have been identified. Members of the NRAMP (natural resistance-associated macrophage protein), YSL (yellow stripe-like), VIT (vacuolar iron transporter), and ZIP (Zrt-, Irt-like protein) transport families show enhanced expression in nodules and are expected to play a role in the transport of iron and other metals across symbiotic membranes.
Brear EM, Day DA, Smith PM. (2013). Front Plant Sci. Sep 13;4:359. Via IvanOresnik
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