Molecular Biology of Metal Homeostasis and Detoxification: From Microbes to Man (Topics in Current G
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The presence of these metal-chelator transporters does not necessarily indicate the ability of that particular bacteria of synthesizing the chelator. Plants benefit from this scenario. As bacteria acidify the surrounding soil, some of the solubilized metal can be used by the plant Zhang et al. It can also use bacterial metallophores as sources of metals Bar-Ness et al. Rhizospheric bacteria can also directly affect plant metal uptake mechanism by means of volatile organic compounds Zhang et al.
These molecules are able to induce the iron deficiency response in a FIT1 -dependent manner Zhang et al. These volatiles are detected by the shoots, triggering the transmission of systemic iron deficiency signals Zamioudis et al. The relationship between plant and the microorganisms in its rhizosphere is dynamic. The plant microbiome can be altered to adapt to different developmental stages or to biotic and abiotic stresses Carvalhais et al. This is achieved by changing the composition of root exudates Chaparro et al.
These changes could also be responsible for the variations observed in the plant microbiome of plants grown in metal deficient soils Pii et al. Plants have developed a number of mechanisms to obtain these nutrients directly from soil. This has been best studied in the case of iron uptake, where two approaches Strategies I and II are followed Kobayashi and Nishizawa, Figure 2.
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This process is very tightly controlled to prevent iron overload of the plant using at least three levels of regulation: At the transcriptional level, a number of bHLH factors control the expression of these genes Colangelo and Guerinot, ; Sivitz et al. Ubiquitinization plays a role in controlling the protein levels of some of these transcription factors, as well as membrane recycling of IRT1 Barberon et al. Furthermore, upon reduction of non-iron metal substrates of IRT1, this transporter polarly localizes to the soil-facing side of the epidermal plasma membrane Barberon et al. Strategy I seems to be the most ancient one, since it is used by dicots and some monocots rice; Ricachenevsky and Sperotto, Subsequently the complex is introduced into the plant by YSL transporters Curie et al.
Strategy II is carried out by all monocots Ricachenevsky and Sperotto, However, the separation of the two strategies reductive vs. These include phenolics, coumarins carboxylates, and flavins Cesco et al. Copper uptake from soil very likely follows similar strategies as for iron. Molybdenum is incorporated by plants as molybdate, instead of a cationic form. In green algae C. Some of these fungi are basidiomycetes and ascomycetes that can establish a plethora of different types of mycorrhizal relationships: Arbuscular mycorrhiza is formed through a complex and very regulated process Gutjahr and Parniske, After spore germination, and detection of strigolactones, the germinating hyphae branches to maximize the chances of contacting the root epidermis.
There, the fungus forms an appresorium that will allow penetration into the root cortex. Once in the cortex, the hyphae disperse though the intercellular spaces and at regular intervals, they penetrate into the cells, branching multiple times, and constituting an arbuscule. The arbuscules do not cross the plant cell plasma membrane, but establish a very close interface with a differentiated host plasma membrane cell, the periarbuscular membrane Gutjahr and Parniske, Across these two membranes arbuscular plasma membrane and periarbuscular membrane nutrients are exchanged Rausch et al.
In addition, as the fungal intracellular mycelium and arbuscules develop, the fungal extracellular mycelium grows, producing branched absorbing structures BAS at regular intervals, which, when the conditions are right, they develop spores Bago et al. In spite of sometimes reaching an area measured in square kilometers, the fungal colony is just one protoplasm, with millions of nuclei distributed at regular intervals in the coenocytic mycelium Smith and Read, In natural environments plants greatly rely on arbuscular mycorrhizal fungi AMF to feed themselves Smith and Read, This nutrient exchange is critical for the symbiosis; otherwise the arbuscules are aborted Javot et al.
The connection of AMF with plant transition metal nutrition has been known from very early on. Mosse showed that iron and copper content in apple seedlings increased upon mycorrhization.
Further studies have shown a role of AMF in improving uptake of additional metals in several different plant species Caris et al. Experiments using radio-labeled metals and a mesh that created a fungal compartment, so that labeled metals could only be reached by fungal hyphae, proved that the host plant is able to recover metal from the soil through the mycorrhizal fungi Caris et al. Further support for the existence of a mycorrhizal metal delivery pathway is that mycorrhizal plants diminish the expression levels of some root metal transporters compared to non-mycorrhizal roots.
Metal transfer from AMF to their hosts is controlled to prevent metal overload of the host. For instance, when metal concentrations in soil are toxic, mycorrhizal plants accumulate less metals in their shoots than non-mycorrhizal ones Diaz et al. Therefore, AMF act as metal buffers, increasing metal delivery to the plant under low metal levels, but decreasing metal uptake when toxic levels of metals are present.
Metal uptake by mycorrhizal fungi should be quite similar to free living fungi. Detailed expression and localization analyses of these genes is required to confirm this possibility. The mechanism of nutrient delivery from AMF to the host is hypothesized to be mediated by vacuoles that act as carriers Jin et al. Vacuoles can be directed to different locations in an active way through the cytoskeleton Allaway and Ashford, Phosphate that is incorporated by the BAS is delivered to the vacuoles where it is polymerized into polyphosphate fibers Ezawa et al.
Other nutrients are loaded into the negatively charged vacuoles, helping to stabilize the charges Jin et al. Vacuoles would also play a role in metal detoxification. Vacuolar metal uploading has to be mediated by metal transporters. Two candidates are likely participating in this process: All these genes could be responsible for metal loading of vacuoles. Once the metal-loaded vacuole reaches the arbuscule, polyphosphate is hydrolized and then transferred to the host.
As a result, the associated metals would be released into the cytosol by yet-to-be-determined metal transporters. Recently Tamayo et al. This protein has the closest homology to a yeast vacuolar Ctr protein doing a similar function. However, further evidences subcellular localization of the transporter, gene silencing by Host Induced Gene Silencing are required to conclusively proof this role. Nutrient recovery from the periarbuscular space is mediated by transporters specific of infected cells for phosphate and ammonia Javot et al.
By analogy, it can be expected that metal transporters specific of the colonized cells are also mediating metal uptake. Transcriptomic analyses of these cells indicate that at least they specifically express a Ctr gene, and they up-regulate members of other metal uptake families ZIP and Nramp; Gaude et al. In maize, OPT8 could putatively be involved in iron recovery from the periarbuscular space, given that its expression is highly induced in mycorrhizal roots Kobae et al.
The role of this transporter is also supported by the co-upregulation of NAS genes Kobae et al. Further detailed analyses of these proteins still remain to confirm this putative involvement, however, the already available data reflect an increased metal uptake at the cortical layers of mycorrhizal plants, consistent with a role of AMF in delivery metals to their host.
Once inside the root, within the hyphae of the Hartig net, metals and other nutrients will be delivered by a poorly characterized process. Once the metals are incorporated from soil they are transported to sink organs where the demand is higher. Arguably, photosynthesis is the plant physiological process with the highest transition metal requirements, at least during vegetative growth Yruela, Consequently, an important part of the transition metals recovered from soil are directed to leaves. In legumes there are also additional metal sinks, the root nodules.
In these organs, symbiotic nitrogen fixation is carried out, which also has relatively large transition metal requirements Brear et al. Once the metals are into the root cortex, either through the epidermis or through AMF, they symplastically and apoplastically reach the endodermis Figure 2.
Markus Tamas
Therefore, a number of metal transporters must exist to mediate metal release to the apoplast and subsequent apoplastic metal uptake and, at the pericycle, other metal transporters mediate their release into the xylem. However, there is not much information on transition metal transporters uploading or unloading metals to the apoplast. This is likely due to the lack of phenotype of mutants in genes that would play a role in endodermal transporters required for apoplast metal uptake.
This seems to indicate that the symplastic metal translocation pathway is the most predominant or at least sufficient to satisfy most of the plant metal demands. Nramp transporters could play a similar role. When the encoding gene is knocked-out, neither growth alteration nor iron-deficiency phenotypes is observed, indicating that the apoplastic pathway for metal delivery to leaves is not essential.
However, apolastic iron precipitates were observed in the root cortex, consistent with a role in apoplastic iron uptake. More is known about how metals are released into the xylem. P-type ATPases play a role in metal extrusion into the xylem. As a result, a number of metal binding molecules are present to allow for metal trafficking along the sap.
FRD3 mutation results in iron deficiency in shoots and an accumulation of this metal in the roots, consistent with a major reduction of root to shoot transport. This transporter is not only involved in xylem citrate loading, but more generally in iron transport across symplastically disconnected tissues Roschzttardtz et al.
Metal release from the xylem is not very well characterized. It has to go from the xylem into the mesophyll cells Figure 3. In addition, YSL transporters would also be involved in metal unloading from the xylem Conte and Walker, , as indicated by the study of the tomato mutant chloronerva.
This mutant shows intervenal chlorosis, the result of the mutation of gene encoding for a NA synthase gene Ling et al. Due to this mutation, iron unloading from the xylem is impaired causing the chlorosis. Consequently, at this level iron speciation must change from iron-citrate to iron-NA, suggesting that YSL transporters would be mediating iron transfer to the xylem. Given the abundance of NA in these tissues Stephan et al. Metal delivery to the mesophyll cells.
X is used to indicate an unknown metal chelator. Most legumes, in addition to the shoot, have other metal sinks: These are differentiated root organs that develop as the result of the interaction with specific soil bacteria species generally known as rhizobia Rhizobium, Bradyrhizobium ,… Elkan, ; Oldroyd, ; Downie, Rhizobia, upon detecting specific flavonoids released by the host plant, produce specific nod factors, that are detected by proteins of the LysM-receptor kinase family in the root epidermis Oldroyd, ; Downie, This triggers a signaling cascade that results in curling of a root hair, invagination of the plasma membrane, that guides the rhizobia trapped by the root hair into the plant.
As this is occurring, cells dedifferentiate in the root pericycle and develop a nodule primordia, in a process with many common aspects to lateral root development Xiao et al. Rhizobia reach the nodule cortex, and are released into the host cell cytosol, in an endocytic-like process Huisman et al. There, in the appropriate biochemical conditions, rhizobia differentiate into bacteroids Kondorosi et al. Nodulation is a process that shares many common elements with mycorrhization the chemical nature of nod and myc factors, or the common signaling pathway , as it very likely evolved from it Parniske, ; Gutjahr and Parniske, ; Oldroyd, This process requires large amounts of energy and is very sensitive to O 2 , which poisons the enzyme.
However, rhizobia are strict aerobes and cannot produce energy in anaerobic conditions. This high affinity also requires the existence of high affinity cytochrome oxidases that can use O 2 as electron acceptor when present at very low concentrations. This is cytochrome oxidase cbb3 that has a iron and copper cofactors Preisig et al. Other metalloenzymes are also critical for symbiotic nitrogen fixation, such as many of the detoxifiers of the free radicals that are produced in the nodule Becana et al.
Heavy Metals: Biological Importance and Detoxification Strategies
The variety of metalloproteins and the high concentrations required of them, make the nodule one of the main metal sinks in legumes. In fact, metal bioavailability limits nodule appearance and development Tang et al. To avoid this, nodulated plants typically induce their metal deficiency responses to ensure an adequate supply of metals to the nodule Terry et al. This response also indicates that all these metals have to be provided by the host plant, rather than using any type of rhizobial metal reservoir. Metal delivery to the nodule could, theoretically be through the epidermal layer as in roots , delivered by the vasculature as the shoot , or use pre-existing metal reserves.
Studies of metal visualization using synchrotron based X-ray fluorescence in M. Indeterminate nodules are a type of nodule that has an apical meristem, and is, in theory, able to grow indefinitely. As it does, different developmental zones appear. By studying metal distribution along an indeterminate nodule, S-XRF analyses could study the different stages of nodule development.
The results showed no metal storages either in the meristematic region or in the epidermis.
In Zone IV, the metal concentration in the infected cells diminished and they were relocalized to the vasculature. Ferroreductase activity in nodules is also increased Slatni et al. No S-XRF analyses have been carried out in determinate-type nodules as in soybean, without the meristematic region, and consequently, no clear zonation. However, the mutation of a nodule-specific citrate transporter LjMATE1 results in iron accumulation in the vasculature and reduced nitrogen fixation rates Takanashi et al.
Metal transport in indeterminate nodules. A General overview of metal delivery and recovery to legume nodules. B Detail of transport process to deliver metals to symbiosomes in the nodule Zone II. Mt is used as a prefix to indicate M. X or MP are used to indicate an unkown metal chelator or a general metallophore, respectively. This transporter is localized in the plasma membrane of these cells, and nramp1 mutant plants have reduced nitrogen fixation capabilities which are restored when the mutated gene is reintroduced or when the plants are watered with iron-fortified solutions.
Zone II is the only region of the nodule where MtNramp1 is expressed, supporting the hypothesis that this area is where metals are released form the vessels. Nramp transporters can also transport other divalent metals, and consequently MtNramp1 could conceivably introduce other elements in the cells. However, no changes in the concentration of other elements where observed in either nodules or roots of nramp1 plants. In addition to citrate, NA should also be playing an important role in metal speciation in the nodule, since mutation in a NA synthase gene in M.
This result indicates that during nodule maturation, metal speciation changes with an important effect on the functioning of the symbiosomes. It could be due to several different possibilities, such as NA being the intracellular metal carrier, NA being the metal donor for a specific intracellular transporter required for nitrogen fixation in the nodule, or NA mediating metal delivery to the nodule. More information on nodule metal speciation and in the specific localization of each metal species is required to draw further conclusions.
After being introduced into the rhizobia-infected cells, cytosolic metals have to be delivered to the same organelle as in a regular plant cell, and to symbiosomes. There is very little information on how this is done. Other possibility is a more directed way, either by establishing different pools accessible to only some acceptor, or by using different metal carrying proteins depending on its final destination.
Identifying this mechanism is critical toward the current efforts to develop nitrogen fixing capabilities in non-legumes, since providing the metal cofactor in a timely manner is essential for nitrogenase assembly and function. These efforts will be greatly helped by the unequivocal identification of the proteins required for metal transport across the symbiosome. Seminal work by Moreau et al. However, the transporters belonging to these families that have been biochemically characterized transport metals toward the cytosol Figure 1 Nevo and Nelson, ; Lin et al.
More recently, Hakoyama et al. This would make SEN1 a good candidate to directly provide metal through the symbiosome membrane, although information of its subcellular localization has not been provided yet. It would also be expected a MATE transporter collaborating in iron transport across the symbiosome membrane. If this is also valid for the nodule, we would expect a citrate-exporter, similar to A.
Zinc transport into the symbiosome could also potentially be carried out by proteins carrying he PLAC8 superfamily motif recently identified in the proteome of G. In the case of copper, it would be expected that a P 1b -ATPases would play this role. However, in the available transcriptomic data there is no ATPase upregulated in the nodule Benedito et al.
Similarly to copper, no molybdate transporter has been identified in the symbiosome membrane in spite of the importance of molybdenum in symbiotic nitrogen fixation. However, some sulfate transporters can also mediate molybdate transfer across membranes Fitzpatrick et al. Consequently, symbiosome-specific sulfate transporter SST1 Krussell et al. Once they cross the symbiosome membrane, metals are accumulated in the peribacteroideal space, as indicated by radiotracer studies of iron LeVier et al.
From there, metals have to cross the rhizobial outer membrane. Very little information is available on the identity of these metal transporters. Based on the study of metal uptake by free-living rhizobia or by pathogenic bacteria, it can be speculated that a metallophore-mediated system could be used LeVier et al. Many of these complexes are substrate of the TonB-dependent transporters TBDTs; Postle and Larsen, , that can mediate the uptake of iron, zinc, cobalt, or nickel complexes Chakraborty et al.
Further support for metallophore role in metal transport across bacteroideal outer membrane is that R. However, other systems are probably in place, since mutation of these transport systems does not show any major effect on symbiotic nitrogen fixation capabilities Yeoman et al. From the bacteroideal periplasm, metals have to be transported into the cytosol.
This transport complex is essential for nitrogenase maturation.
The identity of the transporters of other essential metals iron, copper, zinc,… has not been determined, what indicated that this might be a robust process where the mutation of single transport genes might not be enough to substantially reduce their transport. In some cases, metal delivery by the plant systems could theoretically overload the bacterial metal homeostatic mechanisms, as seems to be indicated by the presence of a metal pool in the peribacteroideal space LeVier et al.
Markus Tamas - University of Gothenburg, Sweden
These systems are not critical for symbiotic nitrogen fixation, although, they are expressed in symbiotic conditions and they confer metal tolerance under free-living conditions. In some rhizobia, there has been a duplication of fixI , resulting in differential roles along the nodule Patel et al. Similar mechanisms of metal transport must be in place for other endosymbiotic interactions in which the microsymbiont is isolated from soil, as is the case of actinorhiza.
In fact, control of nutrient delivery would be a possible mechanism of controlling the microbiont proliferation, since by limiting access to essential nutrients or encouraging it, growth rates can be modulated. However, very little is known on the specifics of modulation of metal transfer to other beneficial endosymbiotic bacteria. Metal delivery to seeds and to younger leaves seem to be carried out through the phloem Curie et al. At nodes in the stem and minor veins metal are very likely transferred from the xylem to the phloem Andriunas et al.
In the case of iron and A. This transporter is localized in companion cells in minor veins and stem nodes, and its mutation results in iron accumulation in the vicinity of these veins, as well as in the xylem sap. As expected of a OPT transporter, it does not use iron as substrate but rather an iron-chelator complex to be determined. No information is available for how other transition elements are transferred from the xylem to the phloem. In addition to transfer from the xylem, phloem also obtains metals from senescent organs. As the plant flowers, sink organs such as leaves and nodules in the case of legumes senesce and their nutrients are recycled.
Alterations in NA levels results in reproductive abnormalities, indicating that this molecule participates in metal delivery to the flowers Stephan et al. Consistent with this is the role of YSL transporters in metal seed loading. Moreover, double mutant ysl1ysl3 plants had higher zinc and copper content in leaves, while reduced levels of iron, zinc, and copper where detected in their seeds.
Legumes also recycle their metals from the nodules Burton et al. Overall, these and other data suggest that senescent organs are an important source of metals for flowering and embryo development Hocking and Pate, ; Burton et al. Metals are also required for gametogenesis. Copper is needed for pollen tube development, as indicated by pollen abnormalities detected in copt1 A. Iron, delivered as iron-citrate with the help of FRD3, is also essential for pollen development Roschzttardtz et al.
FRD3 could also play a role in embryogenesis, as citrate would solubilize the iron in the nutritive solution around the developing embryo Roschzttardtz et al. Iron delivered to the embryos is directed to vascular tissues were it will be stored in vacuoles in endodermal cells Roschzttardtz et al. Upon germination, Nramp3 and Nramp4 could be responsible for remobilization of these iron reserves, as iron is stored in the vacuoles of endospermal cells to be later used as the seed germinates Lanquar et al.
In the last three decades, we have gained a deep insight on what transporters are involved in root metal uptake and translocation to the shoot. We have also identified many of the metal-carrier molecules, as well as unveiled many of the complex regulatory pathways. More recently, as technology improved, the role of microbes in plant metal homeostasis is being better understood, as are the mechanisms mediating metal exchange with the endosymbionts. However, several other aspects have been insufficiently addressed.
For instance, information on metal homeostasis in other, less studied, endosymbiosis is still lacking, very probably due to the difficulties of obtaining axenic cultures for some of them. More information is also required of the microbiome of plants growing under different levels of metal nutrition. In addition, although we know of multiple different metal transporters and some carrier proteins, their final destination, the identity of the metalloproteins that will use these metals, remains elusive.
Consequently, we are missing a key element to better understand intracellular metal trafficking and use. At a systemic level we still need to determine which are the metal sensors, the signals that determine the plant metal nutritional levels, as well as to determine how the plant controls the shoot to root metal fluxes. This later aspect is especially important in legumes, since symbiotic nitrogen fixation is also an important metal sink and metal partitioning with leaves is critical to correctly balancing carbon and nitrogen fixation rates. Steps are being taken to tackle this question in the coming years.
Improved methods for metalloproteomics are being developed, and elements involved in shoot-to-root metal transport in legumes are being unveiled to have a better understanding of metal partitioning in legumes, which together with improved metal imaging and metal speciation methods point toward obtaining a very clear picture on how plants use metals and the role that microorganisms have on plant metal homeostasis. This will allow us to select inoculants which will improve plant metal uptake, as well as cultivars with enhanced metal recovery capabilities from AMF or from senescent nodules, as well as increased delivery for symbiotic nitrogen fixation.
MG-G outlined the manuscript, wrote the remaining sections, and put together the manuscript. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. We would also like to apologize to those colleagues whose work, due to space limitations, we have not cited.
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Topics in Current Genetics. Free Preview. © Molecular Biology of Metal Homeostasis and Detoxification. From Microbes to Man This book brings together current knowledge of the molecular basis of metal homeostasis and. Molecular Biology of Metal Homeostasis and Detoxification: From Microbes to Man (Topics in Current Genetics) [Markus J. Tamás, Enrico Martinoia] on.
Molecular characterization of a copper transport protein in S. Tryptophan scanning analysis of the membrane domain of CTR-copper transporters. Read more Read less. From the Back Cover One of the challenges faced by every cell as well as by whole organisms is to maintain appropriate concentrations of essential nutrient metals while excluding nonessential toxic metals. Topics in Current Genetics Book 14 Hardcover: Springer; edition February 10, Language: Be the first to review this item Amazon Best Sellers Rank: Related Video Shorts 0 Upload your video.
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