MOLECULAR AND CELLULAR ASPECTS OF THE ARBUSCULAR MYCORRHIZAL SYMBIOSIS

Phytoremediation of organic contaminants in soil and groundwater

Phytoremediation is an emerging technology for the clean-up of sites contaminated with hazardous chemicals. The term phytoremediation refers to a number of technologies that use photoautotrophic vascular plants for the remediation of sites contaminated with inorganic and organic contaminants. Phytoremediation of organic contaminants can be organized by considering 1) the green liver concept, which elucidates the metabolism of contaminants in planta versus that of contaminants ex planta (e.g. rhizosphere), 2) processes that lead to complete degradation (mineralization) of contaminants as opposed to those that only lead to partial degradation or transformation, and 3) active plant uptake versus passive processes (e.g. sorption). Understanding of these processes needs an interdisciplinary approach involving chemists, biologists, soil scientists, and environmentalists. This Review presents the basic concepts of phytoremediation of organic contaminants in soil and groundwater using selected contaminants as examples.

Metabolite profiling of mycorrhizal roots of Medicago truncatula

Metabolite profiling of soluble primary and secondary metabolites, as well as cell wall-bound phenolic compounds from roots of barrel medic (Medicago truncatula) was carried out by GC-MS, HPLC and LC-MS. These analyses revealed a number of metabolic characteristics over 56 days of symbiotic interaction with the arbuscular mycorrhizal (AM) fungus Glomus intraradices, when compared to the controls, i.e. nonmycorrhizal roots supplied with low and high amounts of phosphate. During the most active stages of overall root mycorrhization, elevated levels of certain amino acids (Glu, Asp, Asn) were observed accompanied by increases in amounts of some fatty acids (palmitic and oleic acids), indicating a mycorrhiza-specific activation of plastidial metabolism. In addition, some accumulating fungus-specific fatty acids (palmitvaccenic and vaccenic acids) were assigned that may be used as markers of fungal root colonization. Stimulation of the biosynthesis of some constitutive isoflavonoids (daidzein, ononin and malonylononin) occurred, however, only at late stages of root mycorrhization. Increase of the levels of saponins correlated AM-independently with plant growth. Only in AM roots was the accumulation of apocarotenoids (cyclohexenone and mycorradicin derivatives) observed. The structures of the unknown cyclohexenone derivatives were identified by spectroscopic methods as glucosides of blumenol C and 13-hydroxyblumenol C and their corresponding malonyl conjugates. During mycorrhization, the levels of typical cell wall-bound phenolics (e.g. 4-hydroxybenzaldehyde, vanillin, ferulic acid) did not change; however, high amounts of cell wall-bound tyrosol were exclusively detected in AM roots. Principal component analyses of nonpolar primary and secondary metabolites clearly separated AM roots from those of the controls, which was confirmed by an hierarchical cluster analysis. Circular networks of primary nonpolar metabolites showed stronger and more frequent correlations between metabolites in the mycorrhizal roots. The same trend, but to a lesser extent, was observed in nonmycorrhizal roots supplied with high amounts of phosphate. These results indicate a tighter control of primary metabolism in AM roots compared to control plants. Network correlation analyses revealed distinct clusters of amino acids and sugars/aliphatic acids with strong metabolic correlations among one another in all plants analyzed; however, mycorrhizal symbiosis reduced the cluster separation and enlarged the sugar cluster size. The amino acid clusters represent groups of metabolites with strong correlations among one another (cliques) that are differently composed in mycorrhizal and nonmycorrhizal roots. In conclusion, the present work shows for the first time that there are clear differences in development- and symbiosis-dependent primary and secondary metabolism of M. truncatula roots.

Structural implications of mutations in the pea SYM8 symbiosis gene, the DMI1 ortholog, encoding a predicted ion channel

The Pisum sativum SYM8 gene plays an essential part in both rhizobial and mycorrhizal symbioses. Mutation of sym8 in the original type line R25 blocks nodulation, mycorrhization, and Nod-factor-induced calcium spiking, an early component of the nodulation signaling pathway. We describe four new sym8 alleles of pea, which fall into the same complementation group as R25. The sym8 mutants are phenotypically similar to Medicago truncatula dmi1 mutants and map to a syntenic location. We used sequence homology to isolate the pea ortholog of M. truncatula DMI1 and have shown that the cloned pea ortholog can complement a M. truncatula dmi1 mutant for nodulation. Each of the five pea sym8 mutants carries a mutation in the DMI1 ortholog, confirming that the pea SYM8 is the DMI1 ortholog. Based on predicted structural similarities with an archaebacterial ion channel, we propose that SYM8 forms a tetrameric calcium-gated channel of a predicted structure similar to the archaebacterial potassium channel but containing a filter region that is different. The predicted structure identifies four aspartate residues (one from each subunit) forming the channel opening. We made a mutation changing the aspartate to valine and identified a missense mutation (changing alanine to valine adjacent to the aspartate residues) in this predicted filter region; both mutations caused a loss of function. We also identified a loss-of-function missense mutation (changing arginine to isoleucine) in a domain proposed to link the predicted channel and the gating ring domains, indicating that this mutation may block function by preventing a protein conformational change being transmitted from the gating-ring domain to the pore domain.

Laser microdissection reveals that transcripts for five plant and one fungal phosphate transporter genes are contemporaneously present in arbusculated cells

The establishment of a symbiotic interaction between plant roots and arbuscular mycorrhizal (AM) fungi requires both partners to undergo significant morphological and physiological modifications which eventually lead to reciprocal beneficial effects. Extensive changes in gene expression profiles recently have been described in transcriptomic studies that have analyzed the whole mycorrhizal root. However, because root colonization by AM fungi involves different cell types, a cell-specific gene expression pattern is likely to occur. We have applied the laser microdissection (LMD) technology to investigate expression profiles of both plant and fungal genes in Lycopersicon esculentum roots colonized by Glomus mosseae. A protocol to harvest arbuscule-containing cells from paraffin sections of mycorrhizal roots has been developed using a Leica AS LMD system. RNA of satisfactory quantity and quality has been extracted for molecular analysis. Transcripts for plant phosphate transporters (LePTs), selected as molecular markers for a functional symbiosis, have been detected by reverse-transcriptase polymerase chain reaction assays and associated to distinct cell types, leading to novel insights into the distribution of LePT mRNAs. In fact, the transcripts of the five phosphate transporters (PTs) have been detected contemporaneously in the same arbusculated cell population, unlike from the neighboring noncolonized cells. In addition, fungal H(+)ATPase (GmHA5) and phosphate transporter (GmosPT) mRNAs were found exclusively in arbusculated cells. The discovery that five plant and one fungal PT genes are consistently expressed inside the arbusculated cells provides a new scenario for plant-fungus nutrient exchanges.

Cooperation of plants and microorganisms: getting closer to the genetic construction of sustainable agro-systems

The molecular research into two types of beneficial plant-microbe symbioses is reviewed: nutritional (with N(2)-fixing bacteria or mycorrhizal fungi) and defensive (with endo- and epiphytic microbes suppressing pathogens and phytophagans). These symbioses are based on the signaling interactions that result in the development of novel tissue/cellular structures and of extended metabolic capacities in the partners, which greatly improve the adaptive potential of plants due to a decrease in their sensitivity to biotic and abiotic stresses. The molecular, genetic and ecological knowledge on plant-microbe interactions provides a strategy for the organization of sustainable crop production based on substituting the agrochemicals (mineral fertilizers, pesticides) by microbial inoculants. An improvement of plant-microbe symbioses should involve the coordinated modifications in the partners' genotypes resulting in highly complementary combinations. These modifications should be based on the broad utilization of genetic resources from natural symbiotic systems aimed at: (i) increased competitiveness of the introduced (effective) with respect to local (ineffective) microbial strains, and (ii) overcoming the limiting steps in the metabolic machineries of the symbiotic systems.

Regulation of arbuscular mycorrhization by carbon. The symbiotic interaction cannot be improved by increased carbon availability accomplished by root-specifically enhanced invertase activity

The mutualistic interaction in arbuscular mycorrhiza (AM) is characterized by an exchange of mineral nutrients and carbon. The major benefit of AM, which is the supply of phosphate to the plant, and the stimulation of mycorrhization by low phosphate fertilization has been well studied. However, less is known about the regulatory function of carbon availability on AM formation. Here the effect of enhanced levels of hexoses in the root, the main form of carbohydrate used by the fungus, on AM formation was analyzed. Modulation of the root carbohydrate status was performed by expressing genes encoding a yeast (Saccharomyces cerevisiae)-derived invertase, which was directed to different subcellular locations. Using tobacco (Nicotiana tabacum) alcc::wINV plants, the yeast invertase was induced in the whole root system or in root parts. Despite increased hexose levels in these roots, we did not detect any effect on the colonization with Glomus intraradices analyzed by assessment of fungal structures and the level of fungus-specific palmitvaccenic acid, indicative for the fungal carbon supply, or the plant phosphate content. Roots of Medicago truncatula, transformed to express genes encoding an apoplast-, cytosol-, or vacuolar-located yeast-derived invertase, had increased hexose-to-sucrose ratios compared to beta-glucuronidase-transformed roots. However, transformations with the invertase genes did not affect mycorrhization. These data suggest the carbohydrate supply in AM cannot be improved by root-specifically increased hexose levels, implying that under normal conditions sufficient carbon is available in mycorrhizal roots. In contrast, tobacco rolC::ppa plants with defective phloem loading and tobacco pyk10::InvInh plants with decreased acid invertase activity in roots exhibited a diminished mycorrhization.

A leucine-rich repeat protein is required for growth promotion and enhanced seed production mediated by the endophytic fungus Piriformospora indica in Arabidopsis thaliana

Piriformospora indica, a basidiomycete of the Sebacinaceae family, promotes the growth, development and seed production of a variety of plant species. Arabidopsis plants colonized with the fungus produce 22% more seeds than uncolonized plants. Deactivating the Arabidopsis single-copy gene DMI-1, which encodes an ion carrier required for mycorrihiza formation in legumes, does not affect the beneficial interaction between the two symbiotic partners. We used cellular and molecular responses initiated during the establishment of the interaction between P. indica and Arabidopsis roots to isolate mutants that fail to respond to the fungus. An ethylmethane sulfonate mutant (Piriformospora indica-insensitive-2; pii-2), and a corresponding insertion line, are impaired in a leucine-rich repeat protein (At1g13230). The protein pii-2, which contains a putative endoplasmic reticulum retention signal, is also found in Triton X-100-insoluble plasma membrane microdomains, suggesting that it is present in the endoplasmic reticulum/plasma membrane continuum in Arabidopsis roots. The microdomains also contain an atypical receptor protein (At5g16590) containing leucine-rich repeats, the message of which is transiently upregulated in Arabidopsis roots in response to P. indica. This response is not detectable in At1g13230 mutants, and the protein is not detectable in the At1g13230 mutant microdomains. Partial deactivation of a gene for a sphingosine kinase, which is required for the biosynthesis of sphingolipid found in plasma membrane microdomains, also affects the Arabidopsis/P. indica interaction. Thus, pii-2, and presumably also At5g16590, two proteins present in plasma membrane microdomains, appear to be involved in P. indica-induced growth promotion and enhanced seed production in Arabidopsis thaliana.

Mutations in DMI3 and SUNN modify the appressorium-responsive root proteome in arbuscular mycorrhiza

Modification of the Medicago truncatula root proteome during the early stage of arbuscular mycorrhizal symbiosis was investigated by comparing, using two-dimensional electrophoresis, the protein patterns obtained from non-inoculated roots and roots synchronized for Glomus intraradices appressorium formation. This approach was conducted in wild-type (J5), mycorrhiza-defective (TRV25, dmi3), and autoregulation-defective (TR122, sunn) M. truncatula genotypes. The groups of proteins that responded to appressorium formation were further compared between wild-type and mutant genotypes; few overlaps and major differences were recorded, demonstrating that mutations in DMI3 and SUNN modified the appressorium-responsive root proteome. Except for a chalcone reductase, none of the differentially displayed proteins that could be identified using matrix-assisted laser desorption ionization time-of-flight mass spectrometry previously was known as appressorium responsive. A DMI3-dependent increased accumulation of signal transduction-related proteins (dehydroascorbate reductase, cyclophilin, and actin depolymerization factor) was found to precede mycorrhiza establishment. Differences in the accumulation of proteins related to plant defense reactions, cytoskeleton rearrangements, and auxin signaling upon symbiont contact were recorded between wild-type and hypermycorrhizal genotypes, pointing to some putative pathways by which SUNN may regulate very early arbuscule formation.

Transcriptional snapshots provide insights into the molecular basis of arbuscular mycorrhiza in the model legume Medicago truncatula

The arbuscular mycorrhizal (AM) association between terrestrial plants and soil fungi of the phylum Glomeromycota is the most widespread beneficial plant-microbe interaction on earth. In the course of the symbiosis, fungal hyphae colonise plant roots and supply limiting nutrients, in particular phosphorus, in exchange for carbon compounds. Owing to the obligate biotrophy of mycorrhizal fungi and the lack of genetic systems to study them, targeted molecular studies on AM symbioses proved to be difficult. With the emergence of plant genomics and the selection of suitable models, an application of untargeted expression profiling experiments became possible. In the model legume Medicago truncatula, high-throughput expressed sequence tag (EST)-sequencing in conjunction with in silico and experimental transcriptome profiling provided transcriptional snapshots that together defined the global genetic program activated during AM. Owing to an asynchronous development of the symbiosis, several hundred genes found to be activated during the symbiosis cannot be easily correlated with symbiotic structures, but the expression of selected genes has been extended to the cellular level to correlate gene expression with specific stages of AM development. These approaches identified marker genes for the AM symbiosis and provided the first insights into the molecular basis of gene expression regulation during AM.

Mutualism and parasitism: the yin and yang of plant symbioses

Plants are solar-powered sugar factories that feed a multitude of other organisms. Many of these organisms associate directly with host plants to gain access to the plant's photosynthates. Such symbioses encompass a wide collection of styles ranging from mutualistic to commensal and parasitic. Among these, the mutualistic arbuscular mycorrhizal (AM) symbiosis is one of the evolutionarily oldest symbioses of plants, relying on the formation of an intimate relationship between fungi of the Glomeromycota and roots of the majority of vascular flowering plants. In this symbiosis, the fungus intracellularly colonizes living root cells, implying the existence of an extreme form of compatibility. Interestingly, molecular events that happen in the plant in response to mycorrhizal colonization also occur in other beneficial and, as recently shown, even antagonistic plant symbioses. Thus, basic 'compatibility modules' appear to be partially conserved between mutualism and parasitism.

pho2, a phosphate overaccumulator, is caused by a nonsense mutation in a microRNA399 target gene

We recently demonstrated that microRNA399 (miR399) controls inorganic phosphate (Pi) homeostasis by regulating the expression of UBC24 encoding a ubiquitin-conjugating E2 enzyme in Arabidopsis (Arabidopsis thaliana). Transgenic plants overexpressing miR399 accumulated excessive Pi in the shoots and displayed Pi toxic symptoms. In this study, we revealed that a previously identified Pi overaccumulator, pho2, is caused by a single nucleotide mutation resulting in early termination within the UBC24 gene. The level of full-length UBC24 mRNA was reduced and no UBC24 protein was detected in the pho2 mutant, whereas up-regulation of miR399 by Pi deficiency was not affected. Several characteristics of Pi toxicity in the pho2 mutant were similar to those in the miR399-overexpressing and UBC24 T-DNA knockout plants: both Pi uptake and translocation of Pi from roots to shoots increased and Pi remobilization within leaves was impaired. These phenotypes of the pho2 mutation could be rescued by introduction of a wild-type copy of UBC24. Kinetic analyses revealed that greater Pi uptake in the pho2 and miR399-overexpressing plants is due to increased Vmax. The transcript level of most PHT1 Pi transporter genes was not significantly altered, except PHT1;8 whose expression was enhanced in Pi-sufficient roots of pho2 and miR399-overexpressing compared with wild-type plants. In addition, changes in the expression of several organelle-specific Pi transporters were noticed, which may be associated with the redistribution of intracellular Pi under excess Pi. Furthermore, miR399 and UBC24 were colocalized in the vascular cylinder. This observation not only provides important insight into the interaction between miR399 and UBC24 mRNA, but also supports their systemic function in Pi translocation and remobilization.

Phylogenetic distribution and evolution of mycorrhizas in land plants

A survey of 659 papers mostly published since 1987 was conducted to compile a checklist of mycorrhizal occurrence among 3,617 species (263 families) of land plants. A plant phylogeny was then used to map the mycorrhizal information to examine evolutionary patterns. Several findings from this survey enhance our understanding of the roles of mycorrhizas in the origin and subsequent diversification of land plants. First, 80 and 92% of surveyed land plant species and families are mycorrhizal. Second, arbuscular mycorrhiza (AM) is the predominant and ancestral type of mycorrhiza in land plants. Its occurrence in a vast majority of land plants and early-diverging lineages of liverworts suggests that the origin of AM probably coincided with the origin of land plants. Third, ectomycorrhiza (ECM) and its derived types independently evolved from AM many times through parallel evolution. Coevolution between plant and fungal partners in ECM and its derived types has probably contributed to diversification of both plant hosts and fungal symbionts. Fourth, mycoheterotrophy and loss of the mycorrhizal condition also evolved many times independently in land plants through parallel evolution.

Contribution of the arbuscular mycorrhizal symbiosis to heavy metal phytoremediation

High concentrations of heavy metals (HM) in the soil have detrimental effects on ecosystems and are a risk to human health as they can enter the food chain via agricultural products or contaminated drinking water. Phytoremediation, a sustainable and inexpensive technology based on the removal of pollutants from the environment by plants, is becoming an increasingly important objective in plant research. However, as phytoremediation is a slow process, improvement of efficiency and thus increased stabilization or removal of HMs from soils is an important goal. Arbuscular mycorrhizal (AM) fungi provide an attractive system to advance plant-based environmental clean-up. During symbiotic interaction the hyphal network functionally extends the root system of their hosts. Thus, plants in symbiosis with AM fungi have the potential to take up HM from an enlarged soil volume. In this review, we summarize current knowledge about the contribution of the AM symbiosis to phytoremediation of heavy metals.

Signaling in the arbuscular mycorrhizal symbiosis

Many microorganisms form symbioses with plants that range, on a continuous scale, from parasitic to mutualistic. Among these, the most widespread mutualistic symbiosis is the arbuscular mycorrhiza, formed between arbuscular mycorrhizal (AM) fungi and vascular flowering plants. These associations occur in terrestrial ecosystems throughout the world and have a global impact on plant phosphorus nutrition. The arbuscular mycorrhiza is an endosymbiosis in which the fungus inhabits the root cortical cells and obtains carbon provided by the plant while it transfers mineral nutrients from the soil to the cortical cells. Development of the symbiosis involves the differentiation of both symbionts to create novel symbiotic interfaces within the root cells. The aim of this review is to explore the current understanding of the signals and signaling pathways used by the symbionts for the development of the AM symbiosis. Although the signal molecules used for initial communication are not yet known, recent studies point to their existence. Within the plant, there is evidence of arbuscular mycorrhiza-specific signals and of systemic signaling that influences phosphate-starvation responses and root development. The landmark cloning of three plant signaling proteins required for the development of the symbiosis has provided the first insights into a signaling pathway that is used by AM fungi and by rhizobia for their symbiotic associations with legumes.

A nucleoporin is required for induction of Ca2+ spiking in legume nodule development and essential for rhizobial and fungal symbiosis

Nuclear-cytoplasmic partitioning and traffic between cytoplasmic and nuclear compartments are fundamental processes in eukaryotic cells. Nuclear pore complexes mediate transport of proteins, RNAs and ribonucleoprotein particles in and out of the nucleus. Here we present positional cloning of a plant nucleoporin gene, Nup133, essential for a symbiotic signal transduction pathway shared by Rhizobium bacteria and mycorrhizal fungi. Mutation of Nup133 results in a temperature sensitive nodulation deficient phenotype and absence of mycorrhizal colonization. Root nodules developing with reduced frequency at permissive temperatures are ineffective and electron microscopy show that Rhizobium bacteria are not released from infection threads. Measurement of ion fluxes using a calcium-sensitive dye show that Nup133 is required for the Ca2+ spiking normally detectable within minutes after application of purified rhizobial Nod-factor signal molecules to root hairs. Localization of NUP133 in the nuclear envelope of root cells and root hair cells shown with enhanced yellow fluorescent protein fusion proteins suggests a novel role for NUP133 nucleoporins in a rapid nuclear-cytoplasmic communication after host-plant recognition of symbiotic microbes. Our results identify a component of an intriguing signal process requiring interaction at the cell plasma membrane and at intracellular nuclear and plastid organelle-membranes to induce a second messenger.

Isoprenoid metabolism and plastid reorganization in arbuscular mycorrhizal roots

Plant root-colonizing arbuscular mycorrhizal (AM) fungi activate the methylerythritol phosphate pathway, carotenoid biosynthesis and oxidative carotenoid cleavage in roots, leading to C13 and C14 apocarotenoids, that is, cyclohexenone and mycorradicin derivatives. Mycorradicin causes the characteristic yellow coloration of many AM roots accumulating within a complex mixture of unknown components. The accumulating C13 cyclohexenones exhibit various ring substitutions and different glycosyl moieties. Transcript levels of the first two enzymes of the MEP pathway, 1-deoxy-D-xylulose 5-phosphate synthase and 1-deoxy-D-xylulose 5-phosphate reductoisomerase, and of the carotenoid pathway, phytoene desaturase and zeta-carotene desaturase, along with a carotenoid-cleaving dioxygenase, are markedly increased in AM roots. This correlates with proliferation and reorganization of root plastids. These results allow at this point only speculation about the significance of apocarotenoid accumulation: participation in the production of signaling molecules and control of fungal colonization or protection against soil-borne pathogens; protection of root cells against oxidative damage of membranes by reactive oxygen species; and promotion of the symbiotic interactions between plant roots and AM fungi.

The role of root exudates in rhizosphere interactions with plants and other organisms

The rhizosphere encompasses the millimeters of soil surrounding a plant root where complex biological and ecological processes occur. This review describes recent advances in elucidating the role of root exudates in interactions between plant roots and other plants, microbes, and nematodes present in the rhizosphere. Evidence indicating that root exudates may take part in the signaling events that initiate the execution of these interactions is also presented. Various positive and negative plant-plant and plant-microbe interactions are highlighted and described from the molecular to the ecosystem scale. Furthermore, methodologies to address these interactions under laboratory conditions are presented.

A journey through signaling in arbuscular mycorrhizal symbioses 2006

Recent years have seen fascinating contributions to our understanding of the molecular dialogue between fungi and plants entering into arbuscular mycorrhizal (AM) symbioses. Attention has shifted from descriptions of physiological and cellular events to molecular genetics and modern chemical diagnostics. Genes, signal transduction pathways and the chemical structures of components relevant to the symbiosis have been defined. This review examines our current knowledge of signals and mechanisms involved in the establishment of AM symbioses.

Auxins in the development of an arbuscular mycorrhizal symbiosis in maize

While the levels of free auxins in maize (Zea mays L.) roots during arbuscular mycorrhiza formation have been previously described in detail, conjugates of indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) with amino acids and sugars were neglected. In this study, we have therefore determined free, ester and amide bound auxins in roots of maize inoculated with Glomus intraradices during early stages of the colonization process. Ester conjugates of IAA and IBA were found only in low amounts and they did not increase in AM colonized roots. The Levels of IAA and IBA amide conjugates increased 20 and 30 days past inoculation (dpi). The formation of free and conjugated IBA but not IAA was systemically induced during AM colonization in leaves of maize plants. This implicated a role for auxin conjugate synthesis and hydrolysis during AM. We have therefore investigated the in vivo metabolism of 3H-labeled IBA by TLC but only slight differences between control and AM-inoculated roots were observed. The activity of auxin conjugate hydrolase activity measured with three different putative substrates showed a decrease in infected roots compared to controls. The fluorinated IBA analog TFIBA inhibited IBA formation in leaves after application to the root system, but was not transported from roots to shoots. AM hyphae were also not able to transport TFIBA. Our results indicate complex control mechanisms to regulate the levels of free and conjugated auxins, which are locally and systemically induced during early stages of the formation of an arbuscular mycorrhizal symbiosis.

Suppression of allene oxide cyclase in hairy roots of Medicago truncatula reduces jasmonate levels and the degree of mycorrhization with Glomus intraradices

During the symbiotic interaction between Medicago truncatula and the arbuscular mycorrhizal (AM) fungus Glomus intraradices, an endogenous increase in jasmonic acid (JA) occurs. Two full-length cDNAs coding for the JA-biosynthetic enzyme allene oxide cyclase (AOC) from M. truncatula, designated as MtAOC1 and MtAOC2, were cloned and characterized. The AOC protein was localized in plastids and found to occur constitutively in all vascular tissues of M. truncatula. In leaves and roots, MtAOCs are expressed upon JA application. Enhanced expression was also observed during mycorrhization with G. intraradices. A partial suppression of MtAOC expression was achieved in roots following transformation with Agrobacterium rhizogenes harboring the MtAOC1 cDNA in the antisense direction under control of the cauliflower mosaic virus 35S promoter. In comparison to samples transformed with 35SuidA, roots with suppressed MtAOC1 expression exhibited lower JA levels and a remarkable delay in the process of colonization with G. intraradices. Both the mycorrhization rate, quantified by fungal rRNA, and the arbuscule formation, analyzed by the expression level of the AM-specific gene MtPT4, were affected. Staining of fungal material in roots with suppressed MtAOC1 revealed a decreased number of arbuscules, but these did not exhibit an altered structure. Our results indicate a crucial role for JA in the establishment of AM symbiosis.

RNA interference identifies a calcium-dependent protein kinase involved in Medicago truncatula root development

Changes in cellular or subcellular Ca2+ concentrations play essential roles in plant development and in the responses of plants to their environment. However, the mechanisms through which Ca2+ acts, the downstream signaling components, as well as the relationships among the various Ca2+-dependent processes remain largely unknown. Using an RNA interference-based screen for gene function in Medicago truncatula, we identified a gene that is involved in root development. Silencing Ca2+-dependent protein kinase1 (CDPK1), which is predicted to encode a Ca2+-dependent protein kinase, resulted in significantly reduced root hair and root cell lengths. Inactivation of CDPK1 is also associated with significant diminution of both rhizobial and mycorrhizal symbiotic colonization. Additionally, microarray analysis revealed that silencing CDPK1 alters cell wall and defense-related gene expression. We propose that M. truncatula CDPK1 is a key component of one or more signaling pathways that directly or indirectly modulates cell expansion or cell wall synthesis, possibly altering defense gene expression and symbiotic interactions.

Enhancing plant phosphorus use efficiency for sustainable cropping

Phosphorus (P) is one of the least available mineral nutrients to the plants in many cropping environments. Sub-optimal P nutrition can lead to yield losses in the range of 10% to 15% of the maximal yields. P deficiency is more critical in highly withered soils as well as in calcareous and alkaline soils. Amelioration attempts by addition of phosphatic fertilizers are economically and ecologically unsound as the efficiency of added phosphatic fertilizers is very low. Inoculation with the mineral phosphate solubilizing microbes has not helped much due to inconsistent performance of the inoculants under field conditions. These factors have led to examine the opportunities for developing genetically enhanced plants with better P use efficiency (PUE) through efficient P absorption, transportation and internal utilization. In order to improve the PUE in crop plants, it is important to explore genetic variation for all its associated traits. Inter- and intra-specific variations for these traits are known to exist and are shown to be under genetic and physiological controls, but modified by the plant-environment interactions. A more comprehensive understanding of the molecular and physiological basis of P uptake, transportation and utilization is leading to formulation of strategies aimed at developing better P efficient cultivars suited for sustainable cropping with less P fertilizer inputs. Issues relating to enhancing PUE through genetic manipulations of crop cultivar parameters are discussed.

Identification of membrane-associated proteins regulated by the arbuscular mycorrhizal symbiosis

A sub-cellular proteomic approach was carried out to monitor membrane-associated protein modifications in response to the arbuscular mycorrhizal (AM) symbiosis. Membrane proteins were extracted from Medicago truncatula roots either inoculated or not with the AM fungus Glomus intraradices. Comparative two-dimensional electrophoresis revealed that 36 spots were differentially displayed in response to the fungal colonization including 15 proteins induced, 3 up-regulated and 18 down-regulated. Among them, seven proteins were found to be commonly down-regulated in AM-colonized and phosphate-fertilized roots. Twenty-five spots out of the 36 of interest could be identified by matrix assisted laser desorption/ionisation-time of flight and/or tandem mass spectrometry analyses. Excepting an acid phosphatase and a lectin, none of them was previously reported as being regulated during AM symbiosis. In addition, this proteomic approach allowed us for the first time to identify AM fungal proteins in planta.

Chemotropism in the arbuscular mycorrhizal fungus Glomus mosseae

In this work, we report the occurrence of chemotropism in the arbuscular mycorrhizal (AM) fungus Glomus mosseae. Fungal hyphae were able to respond to host-derived signals by reorienting their growth towards roots and to perceive chemotropic signals at a distance of at least 910 microm from roots. In order to reach the source of chemotropic signals, hyphal tips crossed interposed membranes emerging within 1 mm from roots, eventually establishing mycorrhizal symbiosis. The specificity of chemotropic growth was evidenced by hyphal growth reorientation and membrane penetration occurring only in experimental systems set up with host plants. Since pre-symbiotic growth is a critical stage in the life cycle of obligate AM fungal symbionts, chemotropic guidance may represent an important mechanism functional to host root location, appressorium formation and symbiosis establishment.

Combined transcriptome profiling reveals a novel family of arbuscular mycorrhizal-specific Medicago truncatula lectin genes

The large majority of plants are capable of undergoing a tight symbiosis with arbuscular mycorrhizal (AM) fungi. During this symbiosis, highly specialized new structures called arbuscules are formed within the host cells, indicating that, during interaction with AM fungi, plants express AM-specific genetic programs. Despite increasing efforts, the number of genes known to be induced in the AM symbiosis is still low. In order to identify novel AM-induced genes which have not been listed before, 5,646 expressed sequence tags (ESTs) were generated from two Medicago truncatula cDNA libraries: a random cDNA library (MtAmp) and a suppression subtractive hybridization (SSH) library (MtGim), the latter being designed to enhance the cloning of mycorrhiza-upregulated genes. In silico expression analysis was applied to identify those tentative consensus sequences (TCs) of The Institute for Genomic Research M. truncatula gene index (MtGI) that are composed exclusively of ESTs deriving from the MtGim or MtAmp library, but not from any other cDNA library of the MtGI. This search revealed 115 MtAmp- or MTGim-specific TCs. For the majority of these TCs with sequence similarities to plant genes, the AM-specific expression was verified by quantitative reverse-transcription polymerase chain reaction. Annotation of the novel genes induced in mycorrhizal roots suggested their involvement in different transport as well as signaling processes and revealed a novel family of AM-specific lectin genes. The expression of reporter gene fusions in transgenic roots revealed an arbuscule-related expression of two members of the lectin gene family, indicating a role for AM-specific lectins during arbuscule formation or functioning.

Expression of a receptor kinase in Arabidopsis roots is stimulated by the basidiomycete Piriformospora indica and the protein accumulates in Triton X-100 insoluble plasma membrane microdomains

Piriformospora indica, an endophytic fungus of the Sebacinaceae family, colonises the roots of a wide variety of plant species and promotes their growth, in a manner similar to mycorrhizal fungi. We demonstrate that the fungus also interacts with the non-mycorrhizal host Arabidopsis thaliana. Promotion of root growth was detectable even before noticeable root colonization, and was accompanied by a massive transfer of phosphate from the media to the aerial parts of the seedlings. During the recognition period of both organisms, the message for a receptor kinase with leucine-rich repeats is transiently upregulated. The kinase is located in Triton X-100-insoluble plasma membrane microdomains. Thus, this is one of the earliest events of a plant root in response to a fungus reported to date.

Evaluation of the role of genes encoding for dehydrin proteins (LEA D-11) during drought stress in arbuscular mycorrhizal Glycine max and Lactuca sativa plants

In this study, it has been determined whether the arbuscular mycorrhizal (AM) symbiosis is able to alter the pattern of dehydrin (LEA D-11 group) transcript accumulation under drought stress, and whether such a possible alteration functions in the protection of the host plants against drought. Two dehydrin-encoding genes have been cloned from Glycine max (gmlea 8 and gmlea 10) and one from Lactuca sativa (lslea 1) and they have been analysed for their contribution to the response against drought in mycorrhizal soybean and lettuce plants. Results with soybean plants showed that most of the treatments did not show LEA gene expression under well-watered conditions. The higher gene expression was found in non-inoculated plants subjected to drought. Only plants singly inoculated with Bradyrhizobium japonicum showed an important level of LEA gene expression under well-watered conditions and a reduced level under drought-stress conditions. The same results were confirmed in subsequent experiments and at the latest stage of a time-course experiment. In lettuce, the lslea 1 gene was also induced by drought stress in all treatments. However, the level of induction was clearly higher in roots from non-inoculated plants than in roots from the two AM treatments assayed. The overall results demonstrated that the levels of lea transcript accumulation in mycorrhizal treatments subjected to drought were considerably lower than in the corresponding non-mycorrhizal plants, indicating that the accumulation of LEA proteins is not a mechanism by which the AM symbiosis protects their host plant.

Comparative transcriptomics of rice reveals an ancient pattern of response to microbial colonization

Glomalean fungi induce and colonize symbiotic tissue called arbuscular mycorrhiza on the roots of most land plants. Other fungi also colonize plants but cause disease not symbiosis. Whole-transcriptome analysis using a custom-designed Affymetrix Gene-Chip and confirmation with real-time RT-PCR revealed 224 genes affected during arbuscular mycorrhizal symbiosis. We compared these transcription profiles with those from rice roots that were colonized by pathogens (Magnaporthe grisea and Fusarium moniliforme). Over 40% of genes showed differential regulation caused by both the symbiotic and at least one of the pathogenic interactions. A set of genes was similarly expressed in all three associations, revealing a conserved response to fungal colonization. The responses that were shared between pathogen and symbiont infection may play a role in compatibility. Likewise, the responses that are different may cause disease. Some of the genes that respond to mycorrhizal colonization may be involved in the uptake of phosphate. Indeed, phosphate addition mimicked the effect of mycorrhiza on 8% of the tested genes. We found that 34% of the mycorrhiza-associated rice genes were also associated with mycorrhiza in dicots, revealing a conserved pattern of response between the two angiosperm classes.

Molecular and cell biology of arbuscular mycorrhizal symbiosis

The roots of most extant plants are able to become engaged in an interaction with a small group of fungi of the fungal order Glomales (Glomeromycota). This interaction-arbuscular mycorrhizal (AM) symbiosis-is the evolutionary precursor of most other mutualistic root-microbe associations. The molecular analysis of this interaction can elucidate basic principles regarding such associations. This review summarizes our present knowledge about cellular and molecular aspects of AM. Emphasis is placed on morphological changes in colonized cells, transfer of nutrients between both interacting partners, and plant defence responses. Similarities to and differences from other associations of plant and microorganisms are highlighted regarding defence reactions and signal perception.

Differential location of alpha-expansin proteins during the accommodation of root cells to an arbuscular mycorrhizal fungus

alpha-Expansins are extracellular proteins that increase plant cell-wall extensibility. We analysed their pattern of expression in cucumber roots in the presence and in the absence of the mycorrhizal fungus, Glomus versiforme. The distribution of alpha-expansins was investigated by use of two polyclonal antibodies (anti-EXPA1 and anti-EXPA2, prepared against two different cucumber alpha-expansins) in immunoblotting, immunofluorescence, and immunogold experiments. Immunoblot results indicate the presence of a 30-kDa band specific for mycorrhizal roots. The two antibodies identify antigens with a different distribution in mycorrhizal roots: anti-EXPA1 labels the interface zone, but the plant cell walls only weakly. By contrast, the anti-EXPA2 labels only the plant cell walls. In order to understand the potential role of alpha-expansins during the accommodation of the fungus inside root cells, we prepared semi-thin sections to measure the size of cortical cells and the thickness of cortical cell walls in mycorrhizal and non-mycorrhizal root. Mycorrhizal cortical cells were significantly larger than non-mycorrhizal cells and had thicker cell walls. In double-labelling experiments with cellobiohydrolase-gold complex, we observed that cellulose was co-localized with alpha-expansins. Taken together, the results demonstrate that alpha-expansins are more abundant in the cucumber cell walls upon mycorrhizal infection; we propose that these wall-loosening proteins are directly involved in the accommodation of the fungus by infected cortical cells.

The mycorrhizal fungus Gigaspora margarita possesses a CuZn superoxide dismutase that is up-regulated during symbiosis with legume hosts

A full-length cDNA showing high similarity to previously described CuZn superoxide dismutases (SODs) was identified in an expressed sequence tag collection from germinated spores of the arbuscular mycorrhizal fungus Gigaspora margarita (BEG 34). The corresponding gene sequence, named GmarCuZnSOD, is composed of four exons. As revealed by heterologous complementation assays in a yeast mutant, GmarCuZnSOD encodes a functional polypeptide able to confer increased tolerance to oxidative stress. The GmarCuZnSOD RNA was differentially expressed during the fungal life cycle; highest transcript levels were found in fungal structures inside the roots as observed on two host plants, Lotus japonicus and Medicago truncatula. These structures also reacted positively to 3,3'-diaminobenzidine, used to localize H2O2 accumulation. This H2O2 is likely to be produced by CuZnSOD activity since treatment with a chelator of copper ions, generally used to inhibit CuZnSODs, strongly reduced the 3,3'-diaminobenzidine deposits. A slight induction of GmarCuZnSOD gene expression was also observed in germinated spores exposed to L. japonicus root exudates, although the response showed variation in independent samples. These results provide evidence of the occurrence, in an arbuscular mycorrhizal fungus, of a functional SOD gene that is modulated during the life cycle and may offer protection as a reactive oxygen species-inactivating system against localized host defense responses raised in arbuscule-containing cells.

Two genes encoding different truncated hemoglobins are regulated during root nodule and arbuscular mycorrhiza symbioses of Medicago truncatula

The MtTrHb1 and MtTrHb2 genes of the model legume Medicago truncatula Gaertn. encode proteins homologous to truncated hemoglobins (TrHb) from plants and a range of different microorganisms. Induction of MtTrHb1 in root nodules and expression of MtTrHb2 in root nodules, as well as in mycorrhizal roots, were shown by quantitative real-time reverse transcription-polymerase chain reaction (RT-PCR). The promoters of both genes were PCR-amplified and fused to the gusAint coding region. By analysing these gusAint-fusions in transgenic root tissues, we were able to localize their activity in root nodules and in roots colonized by arbuscular mycorrhizal (AM) fungi. Whereas the promoter of MtTrHb1 was activated in the infected cells of the nitrogen-fixing zone of root nodules, the MtTrHb2 promoter was predominantly active in the nodule vascular tissue. This expression pattern correlates with the presence of an 'organ-specific element' (OSE)-like sequence in the MtTrHb1 promoter, which is not present in the MtTrHb2 regulatory unit. Concerning the AM symbiosis, only the MtTrHb2 promoter mediated an expression in arbuscule-containing cells and in the root vascular tissue of mycorrhizal root segments colonized by the fungus Glomus intraradices.

Fungal elicitation of signal transduction-related plant genes precedes mycorrhiza establishment and requires the dmi3 gene in Medicago truncatula

Suppressive subtractive hybridization and expressed sequence tag sequencing identified 29 plant genes which are upregulated during the appressorium stage of mycorrhiza establishment between Medicago truncatula J5 (Myc+) and Glomus mosseae. Eleven genes coding plant proteins with predicted functions in signal transduction, transcription, and translation were investigated in more detail for their relation to early events of symbiotic interactions. Expression profiling showed that the genes are activated not only from the appressorium stage up to the fully established symbiosis in the Myc+ genotype of M. truncatula, but also when the symbionts are not in direct cell contact, suggesting that diffusible fungal molecules (Myc factors) play a, role in the induction of a signal-transduction pathway. Transcript accumulation in roots of a mycorrhiza-defective Myc- dmi3 mutant of M. truncatula is not modified by appressorium formation or diffusible fungal molecules, indicating that the signal transduction pathway is required for a successful G. mosseae-M. truncatula interaction leading to symbiosis development. The symbiotic nodulating bacterium Sinorhizobium meliloti does not activate the 11 genes, which supposes early discrimination by plant roots between the microbial symbionts.

Transcriptome profiling in root nodules and arbuscular mycorrhiza identifies a collection of novel genes induced during Medicago truncatula root endosymbioses

Transcriptome profiling based on cDNA array hybridizations and in silico screening was used to identify Medicago truncatula genes induced in both root nodules and arbuscular mycorrhiza (AM). By array hybridizations, we detected several hundred genes that were upregulated in the root nodule and the AM symbiosis, respectively, with a total of 75 genes being induced during both interactions. The second approach based on in silico data mining yielded several hundred additional candidate genes with a predicted symbiosis-enhanced expression. A subset of the genes identified by either expression profiling tool was subjected to quantitative real-time reverse-transcription polymerase chain reaction for a verification of their symbiosis-induced expression. That way, induction in root nodules and AM was confirmed for 26 genes, most of them being reported as symbiosis-induced for the first time. In addition to delivering a number of novel symbiosis-induced genes, our approach identified several genes that were induced in only one of the two root endosymbioses. The spatial expression patterns of two symbiosis-induced genes encoding an annexin and a beta-tubulin were characterized in transgenic roots using promoter-reporter gene fusions.

Expression patterns of defense-related genes in different types of arbuscular mycorrhizal development in wild-type and mycorrhiza-defective mutant tomato

The expression of defense-related genes was analyzed in the interactions of six arbuscular mycorrhizal (AM) fungi with the roots of wild-type tomato (Lycopersicon esculentum Mill.) cv. 76R and of the near-isogenic mycorrhiza-defective mutant rmc. Depending on the fungal species, wild-type tomato forms both major morphological AM types, Arum and Paris. The mutant rmc blocks the penetration of the root surface or invasion of the root cortex by most species of AM fungi, but one fungus has been shown to develop normal mycorrhizas. In the wild-type tomato, accumulation of mRNA representing a number of defense-related genes was low in Arum-type interactions, consistent with findings for this AM morphotype in other plant species. In contrast, Paris-type colonization, particularly by members of the family Gigasporaceae, was accompanied by a substantial transient increase in expression of some defense-related genes. However, the extent of root colonization did not differ significantly in the two wild-type AM morphotypes, suggesting that accumulation of defense gene products per se does not limit mycorrhiza development. In the mutant, interactions in which the fungus failed to penetrate the root lacked significant accumulation of defense gene mRNAs. However, phenotypes in which the fungus penetrated epidermal or hypodermal cells were associated with an enhanced and more prolonged gene expression. These results are discussed in relation to the mechanisms that may underlie the specificity of the interactions between AM fungi and the rmc mutant.

Functional genomics of arbuscular mycorrhiza: decoding the symbiotic cell programme

More extensive insight into plant genes involved in the symbiotic programme of arbuscular mycorrhiza is presently being achieved by global approaches that aim to discover novel genes or subsets of genes that are essential to cell programmes in the different steps of plant–fungal interactions. The strategy of functional genomics based on large-scale differential RNA expression analyses (differential-display reverse transcriptase - PCR), electronic Northerns, suppressive subtractive hybridization, DNA chips) is presented, with a focus on arbuscular mycorrhiza in Pisum sativum and Medicago truncatula. The most recent knowledge about gene networks that are modulated in roots during arbuscular establishment and functioning is discussed.Key words: arbuscular mycorrhiza, symbiotic programme, gene expression, pea, annual alfalfa.

Expression profiling of up-regulated plant and fungal genes in early and late stages of Medicago truncatula-Glomus mosseae interactions

Suppression subtractive hybridization (SSH), expression profiling and EST sequencing identified 12 plant genes and six fungal genes that are expressed in the arbuscular mycorrhizal symbiosis between Medicago truncatula and Glomus mosseae. All the plant genes and three of the fungal genes were up-regulated in symbiotic tissues. Expression of 15 of the genes is described for the first time in mycorrhizal roots and two are novel sequences. Six M. truncatula genes were also activated during appressorium formation at the root surface, suggesting a role in this early stage of mycorrhiza establishment, whilst the other six plant genes were only induced in the late stages of mycorrhization and could be involved in the development or functioning of the symbiosis. Phosphate fertilization had no significant influence on expression of any of the plant genes. Expression profiling of G. mosseae genes indicated that two of them may be associated with appressorium development on roots and one with arbuscule formation or function. The other three fungal genes were expressed throughout the life-cycle of G. mosseae.

Regulatory mechanisms during the plant – arbuscular mycorrhizal fungus interaction

Abundant data are available on some aspects of the arbuscular mycorrhizal symbiosis, for example, plant nutrition, but because of difficulties immanent to arbuscular mycorrhizal fungi, such as the inability to culture them axenically, the relatively long time it takes to achieve root colonization, and the simultaneous presence of different morphologic stages of the fungus in the root, less information is accumulated on other aspects such as the regulation of mycorrhization. Regulatory processes in the plant – arbuscular mycorrhizal fungus interaction start before root colonization by the fungus and even before a direct physical contact between the host and the fungal symbiont. Some of the signals exchanged are still a matter of debate and will be discussed further on. After the penetration of the root by the fungus, depending on the developmental stage of the arbuscular mycorrhizal association (e.g., early or mature), a range of plant responses is activated. The possible function of several plant responses in the regulation of mycorrhization is discussed.Key words: arbuscular mycorrhiza, Glomales, autoregulation, flavonoid, recognition, root exudates.

Exploring structural definitions of mycorrhizas, with emphasis on nutrient-exchange interfaces

The roots or other subterranean organs of most plants develop symbioses, mycorrhizas, with fungal symbionts. Historically, mycorrhizas have been placed into seven categories based primarily on structural characteristics. A new category has been proposed for symbiotic associations of some leafy liverworts. An important feature of mycorrhizas is the interface involved in nutrient exchange between the symbionts. With the exception of ectomycorrhizas, in which fungal hyphae remain external to plant cell walls, all mycorrhizas are characterized by fungal hyphae breaching cell walls but remaining separated from the cell cytoplasm by a plant-derived membrane and an interfacial matrix that forms an apoplastic compartment. The chemical composition of the interfacial matrix varies in complexity. In arbuscular mycorrhizas (both Arum-type and Paris-type), molecules typical of plant primary cell walls (i.e., cellulose, pectins, β-1,3-glucans, hydroxyproline-rich glycoproteins) are present. In ericoid mycorrhizas, only rhamnogalacturonans occur in the interfacial matrix surrounding intracellular hyphal complexes. The matrix around intracellular hyphal complexes in orchid mycorrhizas lacks plant cell wall compounds until hyphae begin to senesce, then molecules similar to those found in primary cell walls are deposited. The interfacial matrix has not been studied in arbutoid mycorrhizas and ectendomycorrhizas. In ectomycorrhizas, the apoplastic interface consists of plant cell wall and fungal cell wall; alterations in these may enhance nutrient transfer. In all mycorrhizas, nutrients must pass into the symplast of both partners at some point, and therefore current research is exploring the nature of the opposing membranes, particularly in relation to phosphorus and sugar transporters.Key words: interface, apoplastic compartment, Hartig net, arbuscule, intracellular complex, nutrient exchange.

Phosphate transport in Arabidopsis: Pht1;1 and Pht1;4 play a major role in phosphate acquisition from both low- and high-phosphate environments

Of the mineral nutrients essential for plant growth, phosphorus plays the widest diversity of roles and a lack of phosphorus has profound effects on cellular metabolism. At least eight members of the Arabidopsis Pht1 phosphate (Pi) transporter family are expressed in roots and Pht1;1 and Pht1;4 show the highest transcript levels. The spatial and temporal expression patterns of these two genes show extensive overlap. To elucidate the in planta roles of Pht1;1 and Pht1;4, we identified loss-of-function mutants and also created a double mutant, lacking both Pht1;1 and Pht1;4. Consistent with their spatial expression patterns, membrane location and designation as high-affinity Pi transporters, Pht1;1 and Pht1;4 contribute to Pi transport in roots during growth under low-Pi conditions. In addition, during growth under high-Pi conditions, the double mutant shows a 75% reduction in Pi uptake capacity relative to wildtype. Thus, Pht1;1 and Pht1;4 play significant roles in Pi acquisition from both low- and high-Pi environments.

cDNA arrays as a tool to identify mycorrhiza-regulated genes: identification of mycorrhiza-induced genes that encode or generate signaling molecules implicated in the control of root growth

Arbuscular mycorrhizas (AM) are symbiotic associations formed by fungi from the Glomeromycota and most angiosperms. Despite the widespread occurrence of the association, its ecological significance, and its potential importance in agriculture, relatively little is known at the molecular level about the development, functioning, and regulation of the symbiosis. We have selected Medicago truncatula Gaertn. 'Jemalong' and an AM fungus, Glomus versiforme (Karsten) Berch, for molecular genetic analyses of the AM symbiosis. Here we used macroarrays as a screening tool to enable the rapid identification of genes that show differential expression in mycorrhizal roots. Forty-three genes showing increased transcript levels and 18 genes showing decreased transcripts in mycorrhizal roots were identified. This set contained several genes predicted to encode regulatory proteins including an alpha-fucosidase implicated in the generation of signaling molecules that modulate plant growth and a gene encoding a putative peptide also implicated in the control of plant growth.Key words: legume, symbiosis, arbuscular mycorrhizal fungi.

Construction and validation of cDNA-based Mt6k-RIT macro- and microarrays to explore root endosymbioses in the model legume Medicago truncatula

To construct macro- and microarray tools suitable for expression profiling in root endosymbioses of the model legume Medicago truncatula, we PCR-amplified a total of 6048 cDNA probes representing genes expressed in uninfected roots, mycorrhizal roots and young root nodules [Nucleic Acids Res. 30 (2002) 5579]. Including additional probes for either tissue-specific or constitutively expressed control genes, 5651 successfully amplified gene-specific probes were used to grid macro- and to spot microarrays designated Mt6k-RIT (M. truncatula 6k root interaction transcriptome). Subsequent to a technical validation of microarray printing, we performed two pilot expression profiling experiments using Cy-labeled targets from Sinorhizobium meliloti-induced root nodules and Glomus intraradices-colonized arbuscular mycorrhizal roots. These targets detected marker genes for nodule and arbuscular mycorrhiza development, amongst them different nodule-specific leghemoglobin and nodulin genes as well as a mycorrhiza-specific phosphate transporter gene. In addition, we identified several dozens of genes that have so far not been reported to be differentially expressed in nodules or arbuscular mycorrhiza thus demonstrating that Mt6k-RIT arrays serve as useful tools for an identification of genes relevant for legume root endosymbioses. A comprehensive profiling of such candidate genes will be very helpful to the development of breeding strategies and for the improvement of cultivation management targeted at increasing legume use in sustainable agricultural systems.

Arabidopsis pdr2 reveals a phosphate-sensitive checkpoint in root development

Plants have evolved complex strategies to maintain phosphate (Pi) homeostasis and to maximize Pi acquisition when the macronutrient is limiting. Adjustment of root system architecture via changes in meristem initiation and activity is integral to the acclimation process. However, the mechanisms that monitor external Pi status and interpret the nutritional signal remain to be elucidated. Here, we present evidence that the Pi deficiency response, pdr2, mutation disrupts local Pi sensing. The sensitivity and amplitude of metabolic Pi-starvation responses, such as Pi-responsive gene expression or accumulation of anthocyanins and starch, are enhanced in pdr2 seedlings. However, the most conspicuous alteration of pdr2 is a conditional short-root phenotype that is specific for Pi deficiency and caused by selective inhibition of root cell division followed by cell death below a threshold concentration of about 0.1 mm external Pi. Measurements of general Pi uptake and of total phosphorus (P) in root tips exclude a defect in high-affinity Pi acquisition. Rescue of root meristem activity in Pi-starved pdr2 by phosphite (Phi), a non-metabolizable Pi analog, and divided-root experiments suggest that pdr2 disrupts sensing of low external Pi availability. Thus, PDR2 is proposed to function at a Pi-sensitive checkpoint in root development, which monitors environmental Pi status, maintains and fine-tunes meristematic activity, and finally adjusts root system architecture to maximize Pi acquisition.

Common gene expression in Medicago truncatula roots in response to Pseudomonas fluorescens colonization, mycorrhiza development and nodulation

•  Beneficial rhizosphere microorganisms may share similar molecular steps during root colonization. To test this hypothesis, we compared Medicago truncatula Gaertn. gene expression in roots colonized, or not colonized, by Glomus mosseae BEG12, Pseudomonas fluorescens C7R12 or Sinorhizobium meliloti 2011. •  Pseudomonas fluorescens C7R12 formed colonies on the surface of M. truncatula roots and colonized root tissues intercellularly and intracellularly in a way similar to that previously described for other plants. •  Semiquantitative reverse transcriptase polymerase chain reaction of a set of 12 mycorrhiza upregulated M. truncatula genes revealed different expression profiles in roots 3 weeks after inoculation with P. fluorescens or S. meliloti. Pseudomonas fluorescens colonization activated seven of the plant genes while nodulated root systems showed increased expression in only three genes and five appeared to be downregulated. •  This first report of similar gene induction by a fluorescent pseudomonad and a mycorrhizal fungus in roots supports the hypothesis that some plant cell programmes may be shared during root colonization by these beneficial microorganisms. Less similarity existed in expression of the gene set with nodulation by S. meliloti.