Flavonoid-induced calcium signalling in Rhizobium leguminosarum bv. viciae

Mucoromycotina 'fine root endophytes': a new molecular model for plant-fungal mutualisms?

The most studied plant-fungal symbioses to date are the interactions between plants and arbuscular mycorrhizal (AM) fungi of the Glomeromycotina clade. Advancements in phylogenetics and microbial community profiling have distinguished a group of symbiosis-forming fungi that resemble AM fungi as belonging instead to the Mucoromycotina. These enigmatic fungi are now known as Mucoromycotina 'fine root endophytes' and could provide a means to understand the origins of plant-fungal symbioses. Most of our knowledge of the mechanisms of fungal symbiosis comes from investigations using AM fungi. Here, we argue that inclusion of Mucoromycotina fine root endophytes in future studies will expand our understanding of the mechanisms, evolution, and ecology of plant-fungal symbioses.

Fulvic Acid Promotes Legume-Rhizobium Symbiosis by Stimulating Endogenous Flavonoids Synthesis and Secretion

Fulvic acid (FA) promotes symbiosis between legumes and rhizobia. To elucidate from the aspect of symbiosis, the effects of root irrigation of water-soluble humic materials (WSHM) or foliar spraying of its highly active component, FA, on soybean root exudates and on rhizosphere microorganisms were investigated. As a result, WSHM/FA treatments significantly altered root exudate metabolite composition, and isoflavonoids were identified as key contributors in both treatments compared to the control. Increased expression of genes related to the isoflavonoid biosynthesis were validated by RT-qPCR in both treatments, which notably elevated the synthesis of symbiotic signals genistein, daidzin, coumestrol, and biochanin A. Moreover, the WSHM/FA treatments induced a change in rhizosphere microbial community, coupled with an increase in the relative abundance of rhizobia. Our findings showed that WSHM/FA promotes symbiosis by stimulating the endogenous flavonoid synthesis and leads to rhizobia accumulation in the rhizosphere. This study provides new insights into mechanisms underlying the FA-mediated promotion of symbiosis.

Strigolactone GR24-mediated mitigation of phosphorus deficiency through mycorrhization in aerobic rice

Strigolactones (SLs) are a new class of plant hormones that play a significant role in regulating various aspects of plant growth promotion, stress tolerance and influence the rhizospheric microbiome. GR24 is a synthetic SL analog used in scientific research to understand the effects of SL on plants and to act as a plant growth promoter. This study aimed to conduct hormonal seed priming at different concentrations of GR24 (0.1, 0.5, 1.0, 5.0 and 10.0 µM with and without arbuscular mycorrhizal fungi (AMF) inoculation in selected aerobic rice varieties (CR Dhan 201, CR Dhan 204, CR Dhan 205, and CR Dhan 207), Kasalath-IC459373 (P-tolerant check), and IR-36 (P-susceptible check) under phosphorus (P)-deficient conditions to understand the enhancement of growth and priming effects in mycorrhization. Our findings showed that seed priming with 5.0 µM SL GR24 enhanced the performance of mycorrhization in CR Dhan 205 (88.91 %), followed by CR Dhan 204 and 207, and AMF sporulation in CR Dhan 201 (31.98 spores / 10 gm soil) and CR Dhan 207 (30.29 spores / 10 g soil), as well as rice growth. The study showed that the highly responsive variety CR Dhan 207 followed by CR Dhan 204, 205, 201, and Kasalath IC459373 showed higher P uptake than the control, and AMF treated with 5.0 µM SL GR24 varieties CR Dhan 205 followed by CR Dhan 207 and 204 showed the best performance in plant growth, chlorophyll content, and soil functional properties, such as acid and alkaline phosphatase activity, soil microbial biomass carbon (MBC), dehydrogenase activity (DHA), and fluorescein diacetate activity (FDA). Overall, AMF intervention with SL GR24 significantly increased plant growth, soil enzyme activity, and uptake of P compared to the control. Under P-deficient conditions, seed priming with 5.0 µM strigolactone GR24 and AMF inoculum significantly increased selected aerobic rice growth, P uptake, and soil enzyme activities. Application of SLs formulations with AMF inoculum in selected aerobic rice varieties, CR Dhan 207, CR Dhan 204, and CR Dhan 205, will play an important role in mycorrhization, growth, and enhancement of P utilization under P- nutrient deficient conditions.

Calcium in the Life Cycle of Legume Root Nodules

The present review highlights both the fundamental questions of calcium localization, compartmentation, and its participation in symbiosome signaling cascades during nodule formation and functioning. Apparently, the main link of such signaling is the calmodulin…calcium- and calmodulin-dependent protein kinases…CYCLOPS…NIN…target genes cascade. The minimum threshold level of calcium as a signaling agent in the presence of intracellular reserves determines the possibility of oligotrophy and ultraoligotrophy in relation to this element. During the functioning of root nodules, the Ca2+-ATPases activity maintains homeostasis of low calcium concentrations in the cytosol of nodule parenchyma cells. Disturbation of this homeostasis can trigger the root nodule senescence. The same reasons determine the increase in the effectiveness of symbiosis with the help of seed priming with sources of calcium. Examples of calcium response polymorphism in components of nitrogen fixing simbiosis important in practical terms are shown.

Comparative physiological and transcriptome analysis provide insights into the inhibitory effect of 6-pentyl-2H-pyran-2-one on Clarireedia jacksonii

Clarireedia spp. is a destructive phytopathogenic fungus that causes turf dollar spot of bent-grass, leading to widespread lawn death. In this study, we explored the antifungal capability of 6-pentyl-2H-pyran-2-one (6PP), a natural metabolite volatilized by microorganisms, which plays an important role in the biological control of turfgrass dollar spot. However, the mechanisms by which 6PP inhibits Clarireedia jacksonii remain unknown. In the present study, C. jacksonii mycelial growth was inhibited by the 6PP treatment and the 6PP treatment damaged cell membrane integrity, causing an increase in relative conduc-tivity. Furthermore, physiological and biochemistry assay showed that 6PP treatment can enhance reactive oxygen species (ROS) levels, malondialdehyde (MDA) content obviously increased with 6PP exposure, increased alchohol dehydrogenase (ADH) and depleted acetalde-hyde dehydrogenase (ALDH), and activated the activities of many antioxidant enzymes in C. jacksonii. Gen Ontology and Kyoto Encyclopedia of Genes and Genomes analysis revealed that some genes in C. jacksonii after 6PP treatment related to integrity of the cell wall and membrane, and oxidative stress were significantly downregulated. It is worth mentioning that the fatty acid degradation pathway is significantly upregulated, with an increase in ATP content and ATP synthase activity, which may promote fungal cell apoptosis. Moreover, we found that the expression of ABC transporters, and glutathione metabolism encoding genes were increased to respond to external stimuli. Taken together, these findings revealed the potential antifungal mechanism of 6PP against Clarireedia spp., which also provides a theoretical basis for the commercial utilization of 6PP as a green pesticide in the future.

Calcium/calmodulin-mediated microbial symbiotic interactions in plants

Cytoplasmic calcium (Ca²⁺) transients and nuclear Ca²⁺ oscillations act as hubs during root nodulation and arbuscular mycorrhizal symbioses. Plants perceive bacterial Nod factors or fungal signals to induce the Ca²⁺ oscillation in the nucleus of root hair cells, and subsequently activate calmodulin (CaM) and Ca²⁺/CaM-dependent protein kinase (CCaMK). Ca²⁺ and CaM-bound CCaMK phosphorylate transcription factors then initiate down-stream signaling events. In addition, distinct Ca²⁺ signatures are activated at different symbiotic stages: microbial colonization and infection; nodule formation; and mycorrhizal development. Ca²⁺ acts as a key signal that regulates a complex interplay of downstream responses in many biological processes. This short review focuses on advances in Ca²⁺ signaling-regulated symbiotic events. It is meant to be an introduction to readers in and outside the field of bacterial and fungal symbioses. We summarize the molecular mechanisms underlying Ca²⁺/CaM-mediated signaling in fine-tuning both local and systemic symbiotic events.

Revealing potential functions of hypothetical proteins induced by genistein in the symbiosis island of Bradyrhizobium japonicum commercial strain SEMIA 5079 (= CPAC 15)

BACKGROUND

Bradyrhizobium japonicum strain SEMIA 5079 (= CPAC 15) is a nitrogen-fixing symbiont of soybean broadly used in commercial inoculants in Brazil. Its genome has about 50% of hypothetical (HP) protein-coding genes, many in the symbiosis island, raising questions about their putative role on the biological nitrogen fixation (BNF) process. This study aimed to infer functional roles to 15 HP genes localized in the symbiosis island of SEMIA 5079, and to analyze their expression in the presence of a nod-gene inducer.

RESULTS

A workflow of bioinformatics tools/databases was established and allowed the functional annotation of the HP genes. Most were enzymes, including transferases in the biosynthetic pathways of cobalamin, amino acids and secondary metabolites that may help in saprophytic ability and stress tolerance, and hydrolases, that may be important for competitiveness, plant infection, and stress tolerance. Putative roles for other enzymes and transporters identified are discussed. Some HP proteins were specific to the genus Bradyrhizobium, others to specific host legumes, and the analysis of orthologues helped to predict roles in BNF.

CONCLUSIONS

All 15 HP genes were induced by genistein and high induction was confirmed in five of them, suggesting major roles in the BNF process.

Antifungal activity of liquiritin in Phytophthora capsici comprises not only membrane-damage-mediated autophagy, apoptosis, and Ca2+ reduction but also an induced defense responses in pepper

Phytophthora capsici causes a severe soil-borne disease in a wide variety of vegetables; to date, no effective strategies to control P. capsici have been developed. Liquiritin (LQ) is a natural flavonoid found in licorice (Glycyrrhiza spp.) root, and it is used in pharmaceuticals. However, the antifungal activity of LQ against P. capsici remains unknown. In the present study, we demonstrated that LQ inhibits P. capsici mycelial growth and sporangial development. In addition, the EC₅₀ of LQ was 658.4 mg/L and LQ caused P. capsici sporangia to shrink and collapse. Next, LQ severely damaged the cell membrane integrity, leading to a 2.0-2.5-fold increase in relative electrical conductivity and malondialdehyde concentration, and a 65-70% decrease in sugar content. Additionally, the H₂O₂ content was increased about 2.0-2.5 fold, but the total antioxidant activity, catalase activity and laccase activity were attenuated by 40-45%, 30-35% and 70-75%. LQ also induced autophagy, apoptosis, and reduction of intracellular Ca²⁺ content. Furthermore, LQ inhibited P. capsici pathogenicity by reducing the expression of virulence genes PcCRN4 and Pc76RTF, and stimulating the plant defense (including the activated transcriptional expression of defense-related genes CaPR1, CaDEF1, and CaSAR82, and the increased antioxidant enzyme activity). Our results not only elucidate the antifungal mechanism of LQ but also suggest a promising alternative to commercial fungicides or a key compound in the development of new fungicides for the control of the Phytophthora disease.

Application of Strigolactones to Plant Roots to Influence Formation of Symbioses

Strigolactones play a potent role in the rhizosphere as a signal to symbiotic microbes including arbuscular mycorrhizal fungi and rhizobial bacteria. This chapter outlines guidelines for application of strigolactones to pea roots to influence symbiotic relationships, and includes careful consideration of type of strigolactones applied, solvent use, frequency of application and nutrient regime to optimize experimental conditions.

The Significance of Flavonoids in the Process of Biological Nitrogen Fixation

Nitrogen is essential for the growth of plants. The ability of some plant species to obtain all or part of their requirement for nitrogen by interacting with microbial symbionts has conferred a major competitive advantage over those plants unable to do so. The function of certain flavonoids (a group of secondary metabolites produced by the plant phenylpropanoid pathway) within the process of biological nitrogen fixation carried out by Rhizobium spp. has been thoroughly researched. However, their significance to biological nitrogen fixation carried out during the actinorhizal and arbuscular mycorrhiza-Rhizobium-legume interaction remains unclear. This review catalogs and contextualizes the role of flavonoids in the three major types of root endosymbiosis responsible for biological nitrogen fixation. The importance of gaining an understanding of the molecular basis of endosymbiosis signaling, as well as the potential of and challenges facing modifying flavonoids either quantitatively and/or qualitatively are discussed, along with proposed strategies for both optimizing the process of nodulation and widening the plant species base, which can support nodulation.

Hormone modulation of legume-rhizobial symbiosis

Leguminous plants can establish symbiotic associations with diazotropic rhizobia to form nitrogen-fixating nodules, which are classified as determinate or indeterminate based on the persistence of nodule meristem. The formation of nitrogen-fixing nodules requires coordinating rhizobial infection and root nodule organogenesis. The formation of an infection thread and the extent of nodule formation are largely under plant control, but vary with environmental conditions and the physiological state of the host plants. Many achievements in these two areas have been made in recent decades. Phytohormone signaling pathways have gradually emerged as important regulators of root nodule symbiosis. Cytokinin, strigolactones (SLs) and local accumulation of auxin can promote nodule development. Ethylene, jasmonic acid (JA), abscisic acid (ABA) and gibberellic acid (GA) all negatively regulate infection thread formation and nodule development. However, salicylic acid (SA) and brassinosteroids (BRs) have different effects on the formation of these two nodule types. Some peptide hormones are also involved in nodulation. This review summarizes recent findings on the roles of these plant hormones in legume-rhizobial symbiosis, and we propose that DELLA proteins may function as a node to integrate plant hormones to regulate nodulation.

Transcriptional analysis of genes involved in competitive nodulation in Bradyrhizobium diazoefficiens at the presence of soybean root exudates

Nodulation competition is a key factor that limits symbiotic nitrogen fixation between rhizobia and their host legumes. Soybean root exudates (SREs) are thought to act as signals that influence Bradyrhizobium ability to colonize roots and to survive in the rhizosphere, and thus they act as a key determinant of nodulation competitiveness. In order to find the competitiveness-related genes in B. diazoefficiens, the transcriptome of two SREs treated B. diazoefficiens with completely different nodulation abilities (B. diazoefficiens 4534 and B. diazoefficiens 4222) were sequenced and compared. In SREs treated strain 4534 (SREs-4534), 253 unigenes were up-regulated and 204 unigenes were down-regulated. In SREs treated strain 4534 (SREs-4222), the numbers of up- and down-regulated unigenes were 108 and 185, respectively. There were considerable differences between the SREs-4534 and SREs-4222 gene expression profiles. Some differentially expressed genes are associated with a two-component system (i.g., nodW, phyR-σEcfG), bacterial chemotaxis (i.g., cheA, unigene04832), ABC transport proteins (i.g., unigene02212), IAA (indole-3-acetic acid) metabolism (i.g., nthA, nthB), and metabolic fitness (i.g., put.), which may explain the higher nodulation competitiveness of B. diazoefficiens in the rhizosphere. Our results provide a comprehensive transcriptomic resource for SREs treated B. diazoefficiens and will facilitate further studies on competitiveness-related genes in B. diazoefficiens.

Strigolactones in the Rhizosphere: Friend or Foe?

Strigolactones are well-known endogenous plant hormones that play a major role in planta by influencing different physiological processes. Moreover, ex planta, strigolactones are important signaling molecules in root exudates and function as host detection cues to launch mutualistic interactions with arbuscular mycorrhizal fungi in the rhizosphere. However, parasitic plants belonging to the Orobanchaceae family hijacked this communication system to stimulate their seed germination when in close proximity to the roots of a suitable host. As a result, the secretion of strigolactones by the plant can have both favorable and detrimental outcomes. Here, we discuss these dual positive and negative effects of strigolactones and we provide a detailed overview on the role of these molecules in the complex dialogs between plants and different organisms in the rhizosphere.

Strigolactones in Plant Interactions with Beneficial and Detrimental Organisms: The Yin and Yang

Strigolactones (SLs) are plant hormones that have important roles as modulators of plant development. They were originally described as ex planta signaling molecules in the rhizosphere that induce the germination of parasitic plants, a role that was later linked to encouraging the beneficial symbiosis with arbuscular mycorrhizal (AM) fungi. Recently, the focus has shifted to examining the role of SLs in plant-microbe interactions, and has revealed roles for SLs in the association of legumes with nitrogen-fixing rhizobacteria and in interactions with disease-causing pathogens. Here, we examine the role of SLs in plant interactions with beneficial and detrimental organisms, and propose possible future biotechnological applications.

The Rhizobium tropici CIAT 899 NodD2 protein regulates the production of Nod factors under salt stress in a flavonoid-independent manner

In the symbiotic associations between rhizobia and legumes, NodD promotes the expression of the nodulation genes in the presence of appropriate flavonoids. This set of genes is implied in the synthesis of Nodulation factors, which are responsible for launching the nodulation process. Rhizobium tropici CIAT 899 is the most successful symbiont of Phaseolus vulgaris and can nodulate a variety of legumes. This strain produces Nodulation factors under abiotic stress such as acidity or high concentration of salt. Genome sequencing of CIAT 899 allowed the identification of five nodD genes. Whereas NodD1 is essential to nodulate Leucaena leucocephala, Lotus japonicus and Macroptilium atropurpureum, symbiosis with P. vulgaris and Lotus burtii decreased the nodule number but did not abolish the symbiotic process when NodD1 is absent. Nodulation factor synthesis under salt stress is not regulated by NodD1. Here we confirmed that NodD2 is responsible for the activation of the CIAT 899 symbiotic genes under salt stress. We have demonstrated that NodD1 and NodD2 control the synthesis of the Nod factor necessary for a successful symbiosis with P. vulgaris and L. burtii. This is the first time that NodD is directly implied in the activation of the symbiotic genes under an abiotic stress.

Early signaling, synthesis, transport and metabolism of ureides

The symbiosis between α nitrogen (N2)-fixing Proteobacteria (family Rhizobiaceae) and legumes belonging to the Fabaceae (a single phylogenetic group comprising three subfamilies: Caesalpinioideae, Mimosoideae and Papilionoideae) results in the formation of a novel root structure called a nodule, where atmospheric N2 is fixed into NH3(+). In the determinate type of nodules harbored by Rhizobium-nodulated Fabaceae species, newly synthesized NH3(+) is finally converted into allantoin (C4H6N4O3) and allantoic acid (C4H8N4O4) (ureides) through complex pathways involving at least 20 different enzymes that act synchronously in two types of nodule cells with contrasting ultrastructure, including the tree nodule cell organelles. Newly synthesized ureides are loaded into the network of nodule-root xylem vessels and transported to aerial organs by the transpirational water current. Once inside the leaves, ureides undergo an enzymatically driven reverse process to yield NH4(+) that is used for growth. This supports the role of ureides as key nitrogen (N)-compounds for the growth and yield of legumes nodulated by Rhizobium that grow in soils with a low N content. Thus, a concrete understanding of the mechanisms underlying ureide biogenesis and catabolism in legumes may help agrobiologists to achieve greater agricultural discoveries. In this review we focus on the transmembranal and transorganellar symplastic and apoplastic movement of N-precursors within the nodules, as well as on the occurrence, localization and properties of enzymes and genes involved in the biogenesis and catabolism of ureides. The synthesis and transport of ureides are not unique events in Rhizobium-nodulated N2-fixing legumes. Thus, a brief description of the synthesis and catabolism of ureides in non-legumes was included for comparison. The establishment of the symbiosis, nodule organogenesis and the plant's control of nodule number, synthesis and translocation of ureides via feed-back inhibition mechanisms are also reviewed.

Symbiosis with an endobacterium increases the fitness of a mycorrhizal fungus, raising its bioenergetic potential

Arbuscular mycorrhizal fungi (AMF) occur in the rhizosphere and in plant tissues as obligate symbionts, having key roles in plant evolution and nutrition. AMF possess endobacteria, and genome sequencing of the endobacterium Candidatus Glomeribacter gigasporarum revealed a reduced genome and a dependence on the fungal host. To understand the effect of bacteria on fungal fitness, we used next-generation sequencing to analyse the transcriptional profile of Gigaspora margarita in the presence and in the absence of its endobacterium. Genomic data on AMF are limited; therefore, we first generated a gene catalogue for G. margarita. Transcriptome analysis revealed that the endobacterium has a stronger effect on the pre-symbiotic phase of the fungus. Coupling transcriptomics with cell biology and physiological approaches, we demonstrate that the bacterium increases the fungal sporulation success, raises the fungal bioenergetic capacity, increasing ATP production, and eliciting mechanisms to detoxify reactive oxygen species. By using TAT peptide to translocate the bioluminescent calcium reporter aequorin, we demonstrated that the line with endobacteria had a lower basal intracellular calcium concentration than the cured line. Lastly, the bacteria seem to enhance the fungal responsiveness to strigolactones, the plant molecules that AMF perceive as branching factors. Although the endobacterium exacts a nutritional cost on the AMF, endobacterial symbiosis improves the fungal ecological fitness by priming mitochondrial metabolic pathways and giving the AMF more tools to face environmental stresses. Thus, we hypothesise that, as described for the human microbiota, endobacteria may increase AMF innate immunity.

Identification of ferredoxin II as a major calcium binding protein in the nitrogen-fixing symbiotic bacterium Mesorhizobium loti

BACKGROUND

Legumes establish with rhizobial bacteria a nitrogen-fixing symbiosis which is of the utmost importance for both plant nutrition and a sustainable agriculture. Calcium is known to act as a key intracellular messenger in the perception of symbiotic signals by both the host plant and the microbial partner. Regulation of intracellular free Ca(2+) concentration, which is a fundamental prerequisite for any Ca(2+)-based signalling system, is accomplished by complex mechanisms including Ca(2+) binding proteins acting as Ca(2+) buffers. In this work we investigated the occurrence of Ca(2+) binding proteins in Mesorhizobium loti, the specific symbiotic partner of the model legume Lotus japonicus.

RESULTS

A soluble, low molecular weight protein was found to share several biochemical features with the eukaryotic Ca(2+)-binding proteins calsequestrin and calreticulin, such as Stains-all blue staining on SDS-PAGE, an acidic isoelectric point and a Ca(2+)-dependent shift of electrophoretic mobility. The protein was purified to homogeneity by an ammonium sulfate precipitation procedure followed by anion-exchange chromatography on DEAE-Cellulose and electroendosmotic preparative electrophoresis. The Ca(2+) binding ability of the M. loti protein was demonstrated by (45)Ca(2+)-overlay assays. ESI-Q-TOF MS/MS analyses of the peptides generated after digestion with either trypsin or endoproteinase AspN identified the rhizobial protein as ferredoxin II and confirmed the presence of Ca(2+) adducts.

CONCLUSIONS

The present data indicate that ferredoxin II is a major Ca(2+) binding protein in M. loti that may participate in Ca(2+) homeostasis and suggest an evolutionarily ancient origin for protein-based Ca(2+) regulatory systems.

From lateral root density to nodule number, the strigolactone analogue GR24 shapes the root architecture of Medicago truncatula

In the rhizosphere, strigolactones not only act as crucial signalling molecules in the communication of plants with parasitic weeds and arbuscular mycorrhiza, but they also play a key role in regulating different aspects of the root system. Here we investigated how strigolactones influence the root architecture of Medicago truncatula. We provide evidence that addition of the synthetic strigolactone analogue GR24 has an inhibitory effect on the lateral root density. Moreover, treatment with GR24 of Sinorhizobium meliloti-inoculated M. truncatula plants affects the nodule number both positively and negatively, depending on the concentration. Plants treated with 0.1 µM GR24 had a slightly increased number of nodules, whereas concentrations of 2 and 5 µM strongly reduced it. This effect was independent of the autoregulation of nodulation mechanism that is controlled by SUPER NUMERIC NODULE. Furthermore, we demonstrate that GR24 controls the nodule number through crosstalk with SICKLE-dependent ethylene signalling. Additionally, because the expression of the nodulation marker EARLY NODULATION11 was strongly reduced in GR24-treated plants, we concluded that strigolactones influence nodulation at a very early stage of the symbiotic interaction.

Strigolactone involvement in root development, response to abiotic stress, and interactions with the biotic soil environment

Strigolactones, recently discovered as plant hormones, regulate the development of different plant parts. In the root, they regulate root architecture and affect root hair length and density. Their biosynthesis and exudation increase under low phosphate levels, and they are associated with root responses to these conditions. Their signaling pathway in the plant includes protein interactions and ubiquitin-dependent repressor degradation. In the root, they lead to changes in actin architecture and dynamics as well as localization of the PIN-FORMED auxin transporter in the plasma membrane. Strigolactones are also involved with communication in the rhizosphere. They are necessary for germination of parasitic plant seeds, they enhance hyphal branching of arbuscular mycorrhizal fungi of the Glomus and Gigaspora spp., and they promote rhizobial symbiosis. This review focuses on the role played by strigolactones in root development, their response to nutrient deficiency, and their involvement with plant interactions in the rhizosphere.

The intracellular delivery of TAT-aequorin reveals calcium-mediated sensing of environmental and symbiotic signals by the arbuscular mycorrhizal fungus Gigaspora margarita

Arbuscular mycorrhiza (AM) is an ecologically relevant symbiosis between most land plants and Glomeromycota fungi. The peculiar traits of AM fungi have so far limited traditional approaches such as genetic transformation. The aim of this work was to investigate whether the protein transduction domain of the HIV-1 transactivator of transcription (TAT) protein, previously shown to act as a potent nanocarrier for macromolecule delivery in both animal and plant cells, may translocate protein cargoes into AM fungi. We evaluated the internalization into germinated spores of Gigaspora margarita of two recombinant TAT fusion proteins consisting of either a fluorescent (GFP) or a luminescent (aequorin) reporter linked to the TAT peptide. Both TAT-fused proteins were found to enter AM fungal mycelia after a short incubation period (5-10 min). Ca2+ measurements in G. margarita mycelia pre-incubated with TAT-aequorin demonstrated the occurrence of changes in the intracellular free Ca2+ concentration in response to relevant stimuli, such as touch, cold, salinity, and strigolactones, symbiosis-related plant signals. These data indicate that the cell-penetrating properties of the TAT peptide can be used as an effective strategy for intracellularly delivering proteins of interest and shed new light on Ca2+ homeostasis and signalling in AM fungi.

Calcium signaling in plant endosymbiotic organelles: mechanism and role in physiology

Recent studies have demonstrated that chloroplasts and mitochondria evoke specific Ca(2+) signals in response to biotic and abiotic stresses in a stress-dependent manner. The identification of Ca(2+) transporters and Ca(2+) signaling molecules in chloroplasts and mitochondria implies that they play roles in controlling not only intra-organellar functions, but also extra-organellar processes such as plant immunity and stress responses. It appears that organellar Ca(2+) signaling might be more important to plant cell functions than previously thought. This review briefly summarizes what is known about the molecular basis of Ca(2+) signaling in plant mitochondria and chloroplasts.

Evaluation of the ecotoxicity of pollutants with bioluminescent microorganisms

This chapter deals with the use of bioluminescent microorganisms in environmental monitoring, particularly in the assessment of the ecotoxicity of pollutants. Toxicity bioassays based on bioluminescent microorganisms are an interesting complement to classical toxicity assays, providing easiness of use, rapid response, mass production, and cost effectiveness. A description of the characteristics and main environmental applications in ecotoxicity testing of naturally bioluminescent microorganisms, covering bacteria and eukaryotes such as fungi and dinoglagellates, is reported in this chapter. The main features and applications of a wide variety of recombinant bioluminescent microorganisms, both prokaryotic and eukaryotic, are also summarized and critically considered. Quantitative structure-activity relationship models and hormesis are two important concepts in ecotoxicology; bioluminescent microorganisms have played a pivotal role in their development. As pollutants usually occur in complex mixtures in the environment, the use of both natural and recombinant bioluminescent microorganisms to assess mixture toxicity has been discussed. The main information has been summarized in tables, allowing quick consultation of the variety of luminescent organisms, bioluminescence gene systems, commercially available bioluminescent tests, environmental applications, and relevant references.

Strigolactones fine-tune the root system

Strigolactones were originally discovered to be involved in parasitic weed germination, in mycorrhizal association and in the control of shoot architecture. Despite their clear role in rhizosphere signaling, comparatively less attention has been given to the belowground function of strigolactones on plant development. However, research has revealed that strigolactones play a key role in the regulation of the root system including adventitious roots, primary root length, lateral roots, root hairs and nodulation. Here, we review the recent progress regarding strigolactone regulation of the root system and the antagonism and interplay with other hormones.

Calcium-dependent regulation of genes for plant nodulation in Rhizobium leguminosarum detected by iTRAQ quantitative proteomic analysis

Rhizobia, the nitrogen-fixing bacterial symbionts of legumes, represent an agricultural application of primary relevance and a model of plant-microbe molecular dialogues. We recently described rhizobium proteome alterations induced by plant flavonoids using iTRAQ. Herein, we further extend that experimentation, proving that the transient elevation in cytosolic calcium is a key signaling event necessary for the expression of the nodulation (nod) genes. Ca(2+) involvement in nodulation is a novel issue that we recently flagged with genetic and physiological approaches and that hereby we demonstrate also by proteomics. Exploiting the multiple combinations of 4-plex iTRAQ, we analyzed Rhizobium leguminosarum cultures grown with or without the nod gene-inducing plant flavonoid naringenin and in the presence or absence of the extracellular Ca(2+) chelator EGTA. We quantified over a thousand proteins, 189 of which significantly altered upon naringenin and/or EGTA stimulation. The expression of NodA, highly induced by naringenin, is strongly reduced when calcium availability is limited by EGTA. This confirms, from a proteomic perspective, that a Ca(2+) influx is a necessary early step in flavonoid-mediated legume nodulation by rhizobia. We also observed other proteins affected by the different treatments, whose identities and roles in nodulation and rhizobium physiology are likewise discussed.

Quantitative analysis of the naringenin-inducible proteome in Rhizobium leguminosarum by isobaric tagging and mass spectrometry

The rhizobium-legume interaction is a critical cornerstone of crop productivity and environmental sustainability. Its potential improvement relies on elucidation of the complex molecular dialogue between its two partners. In the present study, the proteomic patterns of gnotobiotic cultures of Rhizobium leguminosarum bv. viciae 3841 grown for 6 h in presence or absence of the nod gene-inducing plant flavonoid naringenin (10 μM) were analyzed using the iTRAQ approach. A total of 1334 proteins were identified corresponding to 18.67% of the protein-coding genes annotated in the sequenced genome of bv. viciae 3841. The abundance levels of 47 proteins were increased upon naringenin treatment showing fold change ratios ranging from 1.5 to 25 in two biological replicates. Besides the nod units, naringenin enhanced the expression of a number of other genes, many of which organized in operons, including β(1-2) glucan production and secretion, succinoglycan export, the RopA outer membrane protein with homology to an oligogalacturonide-specific porin motif, other enzymes for carbohydrate and amino acid metabolism, and proteins involved in the translation machinery. Data were validated at the transcriptional and phenotypic levels by RT-PCR and an assay of secreted sugars in culture supernatants, respectively. The current approach provides not only a high-resolution analysis of the prokaryotic proteome but also unravels the rhizobium molecular dialogue with legumes by detecting the enhanced expression of several symbiosis-associated proteins, whose flavonoid-dependency had not yet been reported.

Strigolactones: Internal and external signals in plant symbioses?

As the newest plant hormone, strigolactone research is undergoing an exciting expansion. In less than five years, roles for strigolactones have been defined in shoot branching, secondary growth, root growth and nodulation, to add to the growing understanding of their role in arbuscular mycorrhizae and parasitic weed interactions. Strigolactones are particularly fascinating as signaling molecules as they can act both inside the plant as an endogenous hormone and in the soil as a rhizosphere signal. Our recent research has highlighted such a dual role for strigolactones, potentially acting as both an endogenous and exogenous signal for arbuscular mycorrhizal development. There is also significant interest in examining strigolactones as putative regulators of responses to environmental stimuli, especially the response to nutrient availability, given the strong regulation of strigolactone production by nitrate and phosphate observed in many species. In particular, the potential for strigolactones to mediate the ecologically important response of mycorrhizal colonization to phosphate has been widely discussed. However, using a mutant approach we found that strigolactones are not essential for phosphate regulation of mycorrhizal colonization or nodulation. This is consistent with the relatively mild impairment of phosphate control of seedling root growth observed in Arabidopsis strigolactone mutants. This contrasts with the major role for strigolactones in phosphate control of shoot branching of rice and Arabidopsis and indicates that the integration of strigolactones into our understanding of nutrient response will be complex. New data presented here, along with the recent discovery of phosphate specific CLE peptides, indicates a potential role for PsNARK, a component of the autoregulation of nodulation pathway, in phosphate control of nodulation.

Strigolactones and the regulation of pea symbioses in response to nitrate and phosphate deficiency

New roles for the recently identified group of plant hormones, the strigolactones, are currently under active investigation. One of their key roles is to regulate plant symbioses. These compounds act as a rhizosphere signal in arbuscular mycorrhizal symbioses and as a positive regulator of nodulation in legumes. The phosphorous and nitrogen status of the soil has emerged as a powerful regulator of strigolactone production. However, until now, the potential role of strigolactones in regulating mycorrhizal development and nodulation in response to nutrient deficiency has not been proven. In this paper, the role of strigolactone synthesis and response in regulating these symbioses is examined in pea (Pisum sativum L.). Pea is well suited to this study, since there is a range of well-characterized strigolactone biosynthesis and response mutants that is unique amongst legumes. Evidence is provided for a novel endogenous role for strigolactone response within the root during mycorrhizal development, in addition to the action of strigolactones on the fungal partner. The strigolactone response pathway that regulates mycorrhizal development also appears to differ somewhat from the response pathway that regulates nodulation. Finally, studies with strigolactone-deficient pea mutants indicate that, despite strong regulation of strigolactone production by both nitrogen and phosphate, strigolactones are not required to regulate these symbioses in response to nutrient deficiency.

Oligogalacturonides: novel signaling molecules in Rhizobium-legume communications

Oligogalacturonides are pectic fragments of the plant cell wall, whose signaling role has been described thus far during plant development and plant-pathogen interactions. In the present work, we evaluated the potential involvement of oligogalacturonides in the molecular communications between legumes and rhizobia during the establishment of nitrogen-fixing symbiosis. Oligogalacturonides with a degree of polymerization of 10 to 15 were found to trigger a rapid intracellular production of reactive oxygen species in Rhizobium leguminosarum bv. viciae 3841. Accumulation of H(2)O(2), detected by both 2',7'-dichlorodihydrofluorescein diacetate-based fluorescence and electron-dense deposits of cerium perhydroxides, was transient and did not affect bacterial cell viability, due to the prompt activation of the katG gene encoding a catalase. Calcium measurements carried out in R. leguminosarum transformed with the bioluminescent Ca(2+) reporter aequorin demonstrated the induction of a rapid and remarkable intracellular Ca(2+) increase in response to oligogalacturonides. When applied jointly with naringenin, oligogalacturonides effectively inhibited flavonoid-induced nod gene expression, indicating an antagonistic interplay between oligogalacturonides and inducing flavonoids in the early stages of plant root colonization. The above data suggest a novel role for oligogalacturonides as signaling molecules released in the rhizosphere in the initial rhizobium-legume interaction.

The role of flavonoids in root-rhizosphere signalling: opportunities and challenges for improving plant-microbe interactions

The flavonoid pathway produces a diverse array of plant compounds with functions in UV protection, as antioxidants, pigments, auxin transport regulators, defence compounds against pathogens and during signalling in symbiosis. This review highlights some of the known function of flavonoids in the rhizosphere, in particular for the interaction of roots with microorganisms. Depending on their structure, flavonoids have been shown to stimulate or inhibit rhizobial nod gene expression, cause chemoattraction of rhizobia towards the root, inhibit root pathogens, stimulate mycorrhizal spore germination and hyphal branching, mediate allelopathic interactions between plants, affect quorum sensing, and chelate soil nutrients. Therefore, the manipulation of the flavonoid pathway to synthesize specifically certain products has been suggested as an avenue to improve root-rhizosphere interactions. Possible strategies to alter flavonoid exudation to the rhizosphere are discussed. Possible challenges in that endeavour include limited knowledge of the mechanisms that regulate flavonoid transport and exudation, unforeseen effects of altering parts of the flavonoid synthesis pathway on fluxes elsewhere in the pathway, spatial heterogeneity of flavonoid exudation along the root, as well as alteration of flavonoid products by microorganisms in the soil. In addition, the overlapping functions of many flavonoids as stimulators of functions in one organism and inhibitors of another suggests caution in attempts to manipulate flavonoid rhizosphere signals.

Plant-bacteria association and symbiosis: are there common genomic traits in alphaproteobacteria?

Alphaproteobacteria show a great versatility in adapting to a broad range of environments and lifestyles, with the association between bacteria and plants as one of the most intriguing, spanning from relatively unspecific nonsymbiotic association (as rhizospheric or endophytic strains) to the highly species-specific interaction of rhizobia. To shed some light on possible common genetic features in such a heterogeneous set of plant associations, the genomes of 92 Alphaproteobacteria strains were analyzed with a fuzzy orthologs-species detection approach. This showed that the different habitats and lifestyles of plant-associated bacteria (soil, plant colonizers, symbiont) are partially reflected by the trend to have larger genomes with respect to nonplant-associated species. A relatively large set of genes specific to symbiotic bacteria (73 orthologous groups) was found, with a remarkable presence of regulators, sugar transporters, metabolic enzymes, nodulation genes and several genes with unknown function that could be good candidates for further characterization. Interestingly, 15 orthologous groupspresent in all plant-associated bacteria (symbiotic and nonsymbiotic), but absent in nonplant-associated bacteria, were also found, whose functions were mainly related to regulation of gene expression and electron transport. Two of these orthologous groups were also detected in fully sequenced plant-associated Betaproteobacteria and Gammaproteobacteria. Overall these results lead us to hypothesize that plant-bacteria associations, though quite variable, are partially supported by a conserved set of unsuspected gene functions.

Strigolactones promote nodulation in pea

Strigolactones are recently defined plant hormones with roles in mycorrhizal symbiosis and shoot and root architecture. Their potential role in controlling nodulation, the related symbiosis between legumes and Rhizobium bacteria, was explored using the strigolactone-deficient rms1 mutant in pea (Pisum sativum L.). This work indicates that endogenous strigolactones are positive regulators of nodulation in pea, required for optimal nodule number but not for nodule formation per se. rms1 mutant root exudates and root tissue are almost completely deficient in strigolactones, and rms1 mutant plants have approximately 40% fewer nodules than wild-type plants. Treatment with the synthetic strigolactone GR24 elevated nodule number in wild-type pea plants and also elevated nodule number in rms1 mutant plants to a level similar to that seen in untreated wild-type plants. Grafting studies revealed that nodule number and strigolactone levels in root tissue of rms1 roots were unaffected by grafting to wild-type scions indicating that strigolactones in the root, but not shoot-derived factors, regulate nodule number and provide the first direct evidence that the shoot does not make a major contribution to root strigolactone levels.