Calcium, kinases and nodulation signalling in legumes

Lotus japonicus nodulation requires two GRAS domain regulators, one of which is functionally conserved in a non-legume

A new nodulation-defective mutant of Lotus japonicus does not initiate nodule cortical cell division in response to Mesorhizobium loti, but induces root hair deformation, Nod factor-induced calcium spiking, and mycorrhization. This phenotype, together with mapping data, suggested that the mutation could be in the ortholog of the Medicago truncatula NSP1 gene (MtNSP1). The sequence of the orthologous gene (LjNSP1) in the L. japonicus mutant (Ljnsp1-1) revealed a mutation causing a premature stop resulting in loss of the C-terminal 23 amino acids. We also sequenced the NSP2 gene from L. japonicus (LjNSP2). A mutant (Ljnsp2-3) with a premature stop codon was identified by TILLING showing a similar phenotype to Ljnsp1-1. Both LjNSP1 and LjNSP2 are predicted GRAS (GAI, RGA, SCR) domain transcriptional regulators. Transcript steady-state levels of LjNSP1 and LjNSP2 initially decreased and then increased following infection by M. loti. In hairy root transformations, LjNSP1 and MtNSP1 complemented both Mtnsp1-1 and Ljnsp1-1 mutants, demonstrating that these orthologous proteins have a conserved biochemical function. A Nicotiana benthamiana NSP1-like gene (NbNSP1) was shown to restore nodule formation in both Ljnsp1-1 and Mtnsp1-1 mutants, indicating that NSP1 regulators from legumes and non-legumes can propagate the Nod factor-induced signal, activating appropriate downstream targets. The L. japonicus nodules complemented with NbNSP1 contained some cells with abnormal bacteroids and could fix nitrogen. However, the NbNSP1-complemented M. truncatula nodules did not fix nitrogen and contained very few bacteria released from infection threads. These observations suggest that NSP1 is also involved in infection, bacterial release, and normal bacteroid formation in nodule cells.

Identification of symbiotically defective mutants of Lotus japonicus affected in infection thread growth

During the symbiotic interaction between legumes and rhizobia, the host cell plasma membrane and associated plant cell wall invaginate to form a tunnel-like infection thread, a structure in which bacteria divide to reach the plant root cortex. We isolated four Lotus japonicus mutants that make infection pockets in root hairs but form very few infection threads after inoculation with Mesorhizobium loti. The few infection threads that did initiate in the mutants usually did not progress further than the root hair cell. These infection-thread deficient (itd) mutants were unaffected for early symbiotic responses such as calcium spiking, root hair deformation, and curling, as well as for the induction of cortical cell division and the arbuscular mycorrhizal symbiosis. Complementation tests and genetic mapping indicate that itd2 is allelic to Ljsym7, whereas the itdl, itd3, and itd4 mutations identified novel loci. Bacterial release into host cells did occur occasionally in the itdl, itd2, and itd3 mutants suggesting that some infections may succeed after a long period and that infection of nodule cells could occur normally if the few abnormal infection threads that were formed reached the appropriate nodule cells.

Analysis of calcium spiking using a cameleon calcium sensor reveals that nodulation gene expression is regulated by calcium spike number and the developmental status of the cell

Rhizobium-made Nod factors induce rapid changes in both Ca(2+) and gene expression. Mutations and inhibitors that abolish Nod-factor-induced Ca(2+) spiking block gene induction, indicating a specific role for Ca(2+) spiking in signal transduction. We used transgenic Medicago truncatula expressing a "cameleon" Ca(2+) sensor to assess the relationship between Nod-factor-induced Ca(2+) spiking and the activation of downstream gene expression. In contrast to ENOD11 induction, Ca(2+) spiking is activated in all root-hair cells and in epidermal or pre-emergent root hairs cells in the root tip region. Furthermore, cortical cells immediately below the epidermal layer also show slow Ca(2+) spiking and these cells lack Nod-factor-induced ENOD11 expression. This indicates a specialization in nodulation gene induction downstream of Nod-factor perception and signal transduction. There was a gradient in the frequency of Ca(2+) spiking along the root, with younger root-hair cells having a longer period between spikes than older root hairs. Using a Ca(2+)-pump inhibitor to block Ca(2+) spiking at various times after addition of Nod factor, we conclude that about 36 consecutive Ca(2+) spikes are sufficient to induce ENOD11-GUS expression in root hairs. To determine if the length of time of Ca(2+) spiking or the number of Ca(2+) spikes is more critical for Nod-factor-induced ENOD11 expression, jasmonic acid (JA) was added to reduce the rate of Nod-factor-induced Ca(2+) spiking. This revealed that even when the period between Ca(2+) spikes was extended, an equivalent number of Ca(2+) spikes were required for the induction of ENOD11. However, this JA treatment did not affect the spatial patterning of ENOD11-GUS expression suggesting that although a minimal number of Ca(2+) spikes are required for Nod-factor-induced gene expression, other factors restrict the expression of ENOD11 to a subset of responding cells.

Extracellular ATP in plants. Visualization, localization, and analysis of physiological significance in growth and signaling

Extracellular ATP (eATP) in animals is well documented and known to play an important role in cellular signaling (e.g. at the nerve synapse). The existence of eATP has been postulated in plants; however, there is no definitive experimental evidence for its presence or an explanation as to how such a polar molecule could exit the plant cell and what physiological role it may play in plant growth and development. The presence of eATP in plants (Medicago truncatula) was detected by constructing a novel reporter; i.e. fusing a cellulose-binding domain peptide to the ATP-requiring enzyme luciferase. Application of this reporter to plant roots allowed visualization of eATP in the presence of the substrate luciferin. Luciferase activity could be detected in the interstitial spaces between plant epidermal cells and predominantly at the regions of actively growing cells. The levels of eATP were closely correlated with regions of active growth and cell expansion. Pharmacological compounds known to alter cytoplasmic calcium levels revealed that ATP release is a calcium-dependent process and may occur through vesicular fusion, an important step in the polar growth of actively growing root hairs. Reactive oxygen species (ROS) activity at the root hair tip is not only essential for root hair growth, but also dependent on the cytoplasmic calcium levels. Whereas application of exogenous ATP and a chitin mixture increased ROS activity in root hairs, no changes were observed in response to adenosine, AMP, ADP, and nonhydrolyzable ATP (betagammameATP). However, application of exogenous potato (Solanum tuberosum) apyrase (ATPase) decreased ROS activity, suggesting that cytoplasmic calcium gradients and ROS activity are closely associated with eATP release.

Long-distance signaling to control root nodule number

Symbiotic nitrogen fixation is beneficial to legumes. Excessive nodule development, however, disturbs the host growth by over-consuming energy from the plant. To keep a balance, legumes possess a systemic negative feedback regulatory system called 'autoregulation of nodulation', which controls the nodule number and the nodulation zone through long-distance signaling. Plants that are deficient in autoregulation display a hypernodulating phenotype. Recently, genes encoding a CLAVATA1-like receptor-like kinase that mediates autoregulation of nodulation have been identified from several legumes, such as Lotus japonicus and soybean. Other hypernodulation mutants that are regulated by shoot or root genotypes have also been isolated.

Analysis of Nod-factor-induced calcium signaling in root hairs of symbiotically defective mutants of Lotus japonicus

Nodulation (Nod)-factor signaling molecules are essential for rhizobia to initiate the nitrogen-fixing symbiotic interaction with legumes. Using a dual dye ratiometric calcium imaging technique, we have shown that 10 nM Nod factor added to roots of Lotus japonicus seedlings induces an intracellular calcium increase (calcium flux) that precedes oscillations in intracellular calcium (calcium spiking). The calcium flux was not observed with 1 or 0.1 nM Nod factor, which did induce calcium spiking. The calcium flux was variable in timing of initiation and duration and was observed in approximately half of the root hairs examined. Representatives from 11 complementation groups of symbiotically defective mutants were analyzed for the calcium flux. Mutants from four groups (sym6, ccamk, sym35, and nin) which retained calcium spiking all showed a normal calcium flux. Two classes of mutants (nfr1 and nfr5) lacked both calcium influx and calcium spiking, whereas five classes of mutants (symRK, castor, pollux, nup133, and sym24) defective for calcium spiking retained a calcium flux. There was no correlation between calcium spiking and induction of root hair deformation by Nod factor. We propose that increased bacterial numbers within infection foci in root hairs leads to accumulation of Nod factor to sufficient levels to activate the calcium flux, and this may drive infection thread growth.

Bacterial endophytes and their interactions with hosts

Recent molecular studies on endophytic bacterial diversity have revealed a large richness of species. Endophytes promote plant growth and yield, suppress pathogens, may help to remove contaminants, solubilize phosphate, or contribute assimilable nitrogen to plants. Some endophytes are seedborne, but others have mechanisms to colonize the plants that are being studied. Bacterial mutants unable to produce secreted proteins are impaired in the colonization process. Plant genes expressed in the presence of endophytes provide clues as to the effects of endophytes in plants. Molecular analysis showed that plant defense responses limit bacterial populations inside plants. Some human pathogens, such as Salmonella spp., have been found as endophytes, and these bacteria are not removed by disinfection procedures that eliminate superficially occurring bacteria. Delivery of endophytes to the environment or agricultural fields should be carefully evaluated to avoid introducing pathogens.

Nodulation independent of rhizobia induced by a calcium-activated kinase lacking autoinhibition

Legumes, such as Medicago truncatula, form mutualistic symbiotic relationships with nitrogen-fixing rhizobial bacteria. This occurs within specialized root organs--nodules--that provide the conditions required for nitrogen fixation. A rhizobium-derived signalling molecule, Nod factor, is required to establish the symbiosis. Perception of Nod factor in the plant leads to the induction of Ca2+ oscillations, and the transduction of this Ca2+ signal requires DMI3 (refs 2, 3), which encodes the protein kinase Ca2+/calmodulin-dependent protein kinase (CCaMK). Central to the regulation of CCaMK is an autoinhibitory domain that negatively regulates kinase activity. Here we show that the specific removal of the autoinhibition domain leads to the autoactivation of the nodulation signalling pathway in the plant, with the resultant induction of nodules and nodulation gene expression in the absence of bacterial elicitation. This autoactivation requires nodulation-specific transcriptional regulators in the GRAS family. This work demonstrates that the release of autoinhibition from CCaMK after calmodulin binding is a central switch that is sufficient to activate nodule morphogenesis. The fact that a single regulation event is sufficient to induce nodulation highlights the possibility of transferring this process to non-legumes.

Signals and Responses: Choreographing the Complex Interaction between Legumes and alpha- and beta-Rhizobia

The nitrogen-fixing symbiosis between bacteria in the family Rhizobiaceae and members of the legume family (Fabaceae) has been well studied, particularly from the perspective of the early signaling and recognition events. Recent studies of non-nodulating legume mutants have resulted in the identification of a number of genes that are responsive to signal molecules from the bacteria. However, a second group of nodule-forming bacteria, completely unrelated to the Rhizobiaceae, which are alpha-Proteobacteria, has been discovered. These bacteria belong to the beta-Proteobacteria and have been designated beta-rhizobia to distinguish them from the better-known alpha-rhizobia. Here, we review what is known in this economically important symbiosis about the interaction between legumes and alpha-rhizobia, and we incorporate information, where known, about the beta-rhizobia.

New nodulation mutants responsible for infection thread development in Lotus japonicus

Legume plants develop specialized root organs, the nodules, through a symbiotic interaction with rhizobia. The developmental process of nodulation is triggered by the bacterial microsymbiont but regulated systemically by the host legume plants. Using ethylmethane sulfonate mutagenesis as a tool to identify plant genes involved in symbiotic nodule development, we have isolated and analyzed five nodulation mutants, Ljsym74-3, Ljsym79-2, Ljsym79-3, Ljsym80, and Ljsym82, from the model legume Lotus japonicus. These mutants are defective in developing functional nodules and exhibit nitrogen starvation symptoms after inoculation with Mesorhizobium loti. Detailed observation revealed that infection thread development was aborted in these mutants and the nodules formed were devoid of infected cells. Mapping and complementation tests showed that Ljsym74-3, and Ljsym79-2 and Ljsym79-3, were allelic with reported mutants of L. japonicus, alb1 and crinkle, respectively. The Ljsym82 mutant is unique among the mutants because the infection thread was aborted early in its development. Ljsym74-3 and Ljsym80 were characterized as mutants with thick infection threads in short root hairs. Map-based cloning and molecular characterization of these genes will help us understand the genetic mechanism of infection thread development in L. japonicus.

Crosstalk between jasmonic acid, ethylene and Nod factor signaling allows integration of diverse inputs for regulation of nodulation

Plant hormones interact at many different levels to form a network of signaling pathways connected by antagonistic and synergistic interactions. Ethylene and jasmonic acid both act to regulate the plant's responsiveness to a common set of biotic stimuli. In addition ethylene has been shown to negatively regulate the plant's response to the rhizobial bacterial signal, Nod factor. This regulation occurs at an early step in the Nod factor signal transduction pathway, at or above Nod factor-induced calcium spiking. Here we show that jasmonic acid also inhibits the plant's responses to rhizobial bacteria, with direct effects on Nod factor-induced calcium spiking. However, unlike ethylene, jasmonic acid not only inhibits spiking but also suppresses the frequency of calcium oscillations when applied at lower concentrations. This effect of jasmonic acid is amplified in the ethylene-insensitive mutant skl, indicating an antagonistic interaction between these two hormones for regulation of Nod factor signaling. The rapidity of the effects of ethylene and jasmonic acid on Nod factor signaling suggests direct crosstalk between these three signal transduction pathways. This work provides a model by which crosstalk between signaling pathways can rapidly integrate environmental, developmental and biotic stimuli to coordinate diverse plant responses.

Aging in legume symbiosis. A molecular view on nodule senescence in Medicago truncatula

Rhizobia reside as symbiosomes in the infected cells of legume nodules to fix atmospheric nitrogen. The symbiotic relation is strictly controlled, lasts for some time, but eventually leads to nodule senescence. We present a comprehensive transcriptomics study to understand the onset of nodule senescence in the legume Medicago truncatula. Distinct developmental stages with characteristic gene expression were delineated during which the two symbiotic partners were degraded consecutively, marking the switch in nodule tissue status from carbon sink to general nutrient source. Cluster analysis discriminated an early expression group that harbored regulatory genes that might be primary tools to interfere with pod filling-related or stress-induced nodule senescence, ultimately causing prolonged nitrogen fixation. Interestingly, the transcriptomes of nodule and leaf senescence had a high degree of overlap, arguing for the recruitment of similar pathways.

A rice calcium- and calmodulin-dependent protein kinase restores nodulation to a legume mutant

The Medicago truncatula DMI3 gene encodes a calcium- and calmodulin-dependent protein kinase (CCaMK) that is necessary for the establishment of both rhizobial and mycorrhizal symbioses. The two symbiotic signaling pathways diverge downstream of DMI3; therefore, it has been proposed that legumes have evolved a particular form of CCaMK, acting like a switch able both to discriminate between rhizobial and mycorrhizal calcium signatures and to trigger the appropriate downstream signaling pathway. To test this hypothesis, we examined whether a CCaMK gene from a nonlegume species was able to restore the rhizobial symbiotic properties of a M. truncatula dmi3 mutant. Our results show that a CCaMK gene from rice can restore nodule formation, indicating that CCaMKs from nonlegumes can interpret the calcium signature elicited by rhizobial Nod factors and activate the appropriate downstream target. The nodules did not contain bacteria, which suggests that DMI3 is also involved in the control of the infection process.

Plant calmodulins and calmodulin-related proteins: multifaceted relays to decode calcium signals

The calmodulin (CaM) family is a major class of calcium sensor proteins which collectively play a crucial role in cellular signaling cascades through the regulation of numerous target proteins. Although CaM is one of the most conserved proteins in all eukaryotes, several features of CaM and its downstream effector proteins are unique to plants. The continuously growing repertoire of CaM-binding proteins includes several plant-specific proteins. Plants also possess a particular set of CaM isoforms and CaM-like proteins (CMLs) whose functions have just begun to be elucidated. This review summarizes recent insights that help to understand the role of this multigene family in plant development and adaptation to environmental stimuli.

Tracing nonlegume orthologs of legume genes required for nodulation and arbuscular mycorrhizal symbioses

Most land plants can form a root symbiosis with arbuscular mycorrhizal (AM) fungi for assimilation of inorganic phosphate from the soil. In contrast, the nitrogen-fixing root nodule symbiosis is almost completely restricted to the legumes. The finding that the two symbioses share common signaling components in legumes suggests that the evolutionarily younger nitrogen-fixing symbiosis has recruited functions from the more ancient AM symbiosis. The recent advances in cloning of the genes required for nodulation and AM symbioses from the two model legumes, Medicago truncatula and Lotus japonicus, provide a unique opportunity to address biological questions pertaining to the evolution of root symbioses in plants. Here, we report that nearly all cloned legume genes required for nodulation and AM symbioses have their putative orthologs in nonlegumes. The orthologous relationship can be clearly defined on the basis of both sequence similarity and microsyntenic relationship. The results presented here serve as a prelude to the comparative analysis of orthologous gene function between legumes and nonlegumes and facilitate our understanding of how gene functions and signaling pathways have evolved to generate species- or family-specific phenotypes.

Reactive oxygen species play a role in regulating a fungus-perennial ryegrass mutualistic interaction

Although much is known about the signals and mechanisms that lead to pathogenic interactions between plants and fungi, comparatively little is known about fungus-plant mutualistic symbioses. We describe a novel role for reactive oxygen species (ROS) in regulating the mutualistic interaction between a clavicipitaceous fungal endophyte, Epichloë festucae, and its grass host, Lolium perenne. In wild-type associations, E. festucae grows systemically in intercellular spaces of leaves as infrequently branched hyphae parallel to the leaf axis. A screen to identify symbiotic genes isolated a fungal mutant that altered the interaction from mutualistic to antagonistic. This mutant has a single-copy plasmid insertion in the coding region of a NADPH oxidase gene, noxA. Plants infected with the noxA mutant lose apical dominance, become severely stunted, show precocious senescence, and eventually die. The fungal biomass in these associations is increased dramatically, with hyphae showing increased vacuolation. Deletion of a second NADPH oxidase gene, noxB, had no effect on the E. festucae-perennial ryegrass symbiosis. ROS accumulation was detected cytochemically in the endophyte extracellular matrix and at the interface between the extracellular matrix and host cell walls of meristematic tissue in wild-type but not in noxA mutant associations. These results demonstrate that fungal ROS production is critical in maintaining a mutualistic fungus-plant interaction.

Early responses to Nod factors and mycorrhizal colonization in a non-nodulating Phaseolus vulgaris mutant

Legumes can acquire nitrogen through a symbiotic interaction with rhizobial bacteria. The initiation of this process is determined by a molecular dialogue between the two partners. Legume roots exude flavonoids that induce the expression of the bacterial nodulation genes, which encode proteins involved in the synthesis and secretion of signals called Nod factors (NFs). NFs signal back to the plant root and trigger several responses, leading to bacterial invasion and nodule formation. Here, we describe the molecular and cellular characterization of a Phaseolus vulgaris non-nodulating mutant (NN-mutant). Root hair cells of the NN-mutant plant respond with swelling and branching when inoculated with Rhizobium etli, albeit without curling induction. Furthermore, neither initiation of cell division in the outer cortex, nor entrapment of bacteria nor infection thread formation was observed. Both the bean wild-type and the NN-mutant responded with elevated intracellular calcium changes in the root hairs. Although the NN-mutant is deficient in early nodulin gene expression when inoculated with R. etli, it can be effectively colonized by arbuscular mycorrhizal fungi (Glomus intraradices). Our data indicate that the P. vulgaris NN-mutant is not blocked at the NFs early perception stage, but at later downstream stages between Ca(2+) signaling and early nodulin induction. This supports the idea that both microsymbionts are perceived and trigger different downstream pathways in the host plant.

Nitrogen fixation mutants of Medicago truncatula fail to support plant and bacterial symbiotic gene expression

The Rhizobium-legume symbiosis culminates in the exchange of nutrients in the root nodule. Bacteria within the nodule reduce molecular nitrogen for plant use and plants provide bacteria with carbon-containing compounds. Following the initial signaling events that lead to plant infection, little is known about the plant requirements for establishment and maintenance of the symbiosis. We screened 44,000 M2 plants from fast neutron-irradiated Medicago truncatula seeds and isolated eight independent mutant lines that are defective in nitrogen fixation. The eight mutants are monogenic and represent seven complementation groups. To monitor bacterial status in mutant nodules, we assayed Sinorhizobium meliloti symbiosis gene promoters (nodF, exoY, bacA, and nifH) in the defective in nitrogen fixation mutants. Additionally, we used an Affymetrix oligonucleotide microarray to monitor gene expression changes in wild-type and three mutant plants during the nodulation process. These analyses suggest the mutants can be separated into three classes: one class that supports little to no nitrogen fixation and minimal bacterial expression of nifH; another class that supports no nitrogen fixation and minimal bacterial expression of nodF, bacA, and nifH; and a final class that supports low levels of both nitrogen fixation and bacterial nifH expression.

Pharmacological evidence for activation of phospholipid and small GTP binding protein signalling cascades by Nod factors

The effects of lipo-chitin oligosaccharide Nod factors (NodNGR[S] from Rhizobium sp. NGR234) on root hair deformation in Vigna unguiculata (L.) Walp. were studied using pharmacological agents to mimic and/or inhibit their action. It was hypothesised that the rearrangement of the cytoskeleton seen during Nod factor induced root hair deformation is modulated by protein kinase C, monomeric G proteins of the Rho superfamily and the location and amount of phosphatidylinositol 3-phosphates (PI3Ps). This hypothesis is supported by the following observations. The protein kinase C activators, 12-deoxyphorbol 13-acetate (DPA) and diacylglycerol kinase inhibitor 1, stimulated root hair deformation to a level similar to that seen with Nod factors or mastoparan, whereas the inhibitor Gö 6976 inhibited root hair deformations induced by NodNGR[S], mastoparan, DPA and diacylglycerol kinase inhibitor 1. The Ras antagonists mevastatin and sulindac sulphide, and the Rho antagonist exoenzyme C3 toxin from Clostridium botulinum all inhibited Nod factor stimulated root hair deformation. Pasteurella multocida toxin activates Rho and stimulated root hair deformation, this stimulation was inhibited by both neomycin and exoenzyme C3 toxin. The PI3 kinase inhibitors, wortmannin and LY-294002 attenuated Nod factor induced root hair deformation. These studies were complemented with actin immunoprecipitations of root hair enriched microsomal membrane preparations from V. unguiculata which pulled down small GTP binding proteins. Root hair deformation is an important early stage in the formation of nitrogen fixing nodules and this study highlights that these processes may depend on signalling cascades involving phospholipids and small GTP binding proteins.

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.

Calcium in plant defence-signalling pathways

In plant cells, the calcium ion is a ubiquitous intracellular second messenger involved in numerous signalling pathways. Variations in the cytosolic concentration of Ca2+ ([Ca2+]cyt) couple a large array of signals and responses. Here we concentrate on calcium signalling in plant defence responses, particularly on the generation of the calcium signal and downstream calcium-dependent events participating in the establishment of defence responses with special reference to calcium-binding proteins.

Arbuscular mycorrhizal fungi elicit a novel intracellular apparatus in Medicago truncatula root epidermal cells before infection

The penetration of arbuscular mycorrhizal (AM) fungi through the outermost root tissues of the host plant is a critical step in root colonization, ultimately leading to the establishment of this ecologically important endosymbiotic association. To evaluate the role played by the host plant during AM infection, we have studied in vivo cellular dynamics within Medicago truncatula root epidermal cells using green fluorescent protein labeling of both the plant cytoskeleton and the endoplasmic reticulum. Targeting roots with Gigaspora hyphae has revealed that, before infection, the epidermal cell assembles a transient intracellular structure with a novel cytoskeletal organization. Real-time monitoring suggests that this structure, designated the prepenetration apparatus (PPA), plays a central role in the elaboration of the apoplastic interface compartment through which the fungus grows when it penetrates the cell lumen. The importance of the PPA is underlined by the fact that M. truncatula dmi (for doesn't make infections) mutants fail to assemble this structure. Furthermore, PPA formation in the epidermis can be correlated with DMI-dependent transcriptional activation of the Medicago early nodulin gene ENOD11. These findings demonstrate how the host plant prepares and organizes AM infection of the root, and both the plant-fungal signaling mechanisms involved and the mechanistic parallels with Rhizobium infection in legume root hairs are discussed.

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.

Gibberellins are involved in nodulation of Sesbania rostrata

Upon submergence, Azorhizobium caulinodans infects the semiaquatic legume Sesbania rostrata via the intercellular crack entry process, resulting in lateral root-based nodules. A gene encoding a gibberellin (GA) 20-oxidase, SrGA20ox1, involved in GA biosynthesis, was transiently up-regulated during lateral root base nodulation. Two SrGA20ox1 expression patterns were identified, one related to intercellular infection and a second observed in nodule meristem descendants. The infection-related expression pattern depended on bacterially produced nodulation (Nod) factors. Pharmacological studies demonstrated that GAs were involved in infection pocket and infection thread formation, two Nod factor-dependent events that initiate lateral root base nodulation, and that they were also needed for nodule primordium development. Moreover, GAs inhibited the root hair curling process. These results show that GAs are Nod factor downstream signals for nodulation in hydroponic growth.

Transcript enrichment of Nod factor-elicited early nodulin genes in purified root hair fractions of the model legume Medicago truncatula

This article describes an efficient procedure to study Nod factor-induced gene expression in root hairs of the model legume Medicago truncatula. By developing an improved method of fracturing frozen root hairs, it has been possible to obtain a highly purified root hair fraction from M. truncatula seedlings yielding sufficient RNA for real-time quantitative RT-PCR expression analysis. After Nod factor treatment it was possible to detect up to 100-fold increases of MtENOD11 and pMtENOD11-gus transcript levels in root hair RNA. This corresponds to 5-7-fold higher induction levels than for entire root tissue preparations. Furthermore, the use of these enriched RNA samples has revealed for the first time a very significant induction (30-fold) of the MtENOD40 gene in root hairs in response to Nod factors. It is concluded that the rapid and convenient procedure described here will be particularly useful for detecting tissue-specific low-level gene expression in root hairs responding to Rhizobium Nod factors or other exogenous signals.

Cochleata: getting to the root of legume nodules

The homeotic mutant of Pisum sativum, cochleata, has stipules replaced by alternative leaf components, abnormal flowers and reduced fertility. Although the root system dry weight, root lengths and nodule numbers of cochleata are similar to those of its wild type, the nodulation phenotype of the mutant is unique. The nodules typically dichotomously branch and multiple callus and root structures emerge from their meristems. These nodule-roots incorporate a peripheral vascular bundle of the nodule into their own central vascular cylinder. Both the nodules and roots of the hybrid structures appear functional. Roles for COCHLEATA in development are discussed.

Plants, symbiosis and parasites: a calcium signalling connection

A unique family of protein kinases has evolved with regulatory domains containing sequences that are related to Ca(2+)-binding EF-hands. In this family, the archetypal Ca(2+)-dependent protein kinases (CDPKs) have been found in plants and some protists, including the malarial parasite, Plasmodium falciparum. Recent genetic evidence has revealed isoform-specific functions for a CDPK that is essential for Plasmodium berghei gametogenesis, and for a related chimeric Ca(2+) and calmodulin-dependent protein kinase (CCaMK) that is essential to the formation of symbiotic nitrogen-fixing nodules in plants. In Arabidopsis thaliana, the analysis of 42 isoforms of CDPK and related kinases is expected to delineate Ca(2+) signalling pathways in all aspects of plant biology.

Expression of the Medicago truncatula DM12 gene suggests roles of the symbiotic nodulation receptor kinase in nodules and during early nodule development

The Medicago truncatula DMI2 gene encodes a receptorlike kinase required for establishing root endosymbioses. The DMI2 gene was shown to be expressed much more highly in roots and nodules than in leaves and stems. In roots, its expression was not altered by nitrogen starvation or treatment with lipochitooligosaccharidic Nod factors. Moreover, the DMI2 mRNA abundance in roots of the nfp, dmil, dmi3, nsp1, nsp2, and hcl symbiotic mutants was similar to the wild type, whereas lower levels in some dmi2 mutants could be explained by regulation by the nonsense-mediated decay, RNA surveillance mechanism. Using pDMI2::GUS fusions, the expression of DMI2 in roots appeared to be localized primarily in the cortical and epidermal cells of the younger, lateral roots and was not observed in the root apices. Following inoculation with Sinorhizobium meliloti, the DMI2 gene was induced in the nodule primordia, before penetration by the infection threads. No increased expression was seen in lateral-root primordia. In nodules, expression was observed primarily in a few cell layers of the pre-infection zone. These results are consistent with the DMI2 gene mediating Nod factor perception and transduction leading to rhizobial infection, not only in root epidermal cells but also during nodule development.

Nodulation phenotypes of gibberellin and brassinosteroid mutants of pea

The initiation and development of legume nodules induced by compatible Rhizobium species requires a complex signal exchange involving both plant and bacterial compounds. Phytohormones have been implicated in this process, although in many cases direct evidence is lacking. Here, we characterize the root and nodulation phenotypes of various mutant lines of pea (Pisum sativum) that display alterations in their phytohormone levels and/or perception. Mutants possessing root systems deficient in gibberellins (GAs) or brassinosteroids (BRs) exhibited a reduction in nodule organogenesis. The question of whether these reductions represent direct or indirect effects of the hormone deficiency is addressed. For example, the application of GA to the roots of a GA-deficient mutant completely restored its number of nodules to that of the wild type. Grafting studies revealed that a wild-type shoot or root also restored the nodule number of a GA-deficient mutant. These findings suggest that GAs are required for nodulation. In contrast, the shoot controlled the number of nodules that formed in graft combinations of a BR-deficient mutant and its wild type. The root levels of auxin and GA were similar among these latter graft combinations. These results suggest that BRs influence a shoot mechanism that controls nodulation and that the root levels of auxin and GA are not part of this process. Interestingly, a strong correlation between nodule and lateral root numbers was observed in all lines assessed, consistent with a possible overlap in the early developmental pathways of the two organs.

Seven Lotus japonicus genes required for transcriptional reprogramming of the root during fungal and bacterial symbiosis

A combined genetic and transcriptome analysis was performed to study the molecular basis of the arbuscular mycorrhiza (AM) symbiosis. By testing the AM phenotype of nodulation-impaired mutants and complementation analysis, we defined seven Lotus japonicus common symbiosis genes (SYMRK, CASTOR, POLLUX, SYM3, SYM6, SYM15, and SYM24) that are required for both fungal and bacterial entry into root epidermal or cortical cells. To describe the phenotype of these mutants at the molecular level, we screened for differentiating transcriptional responses of mutant and wild-type roots by large-scale gene expression profiling using cDNA-amplified fragment length polymorphism. Two percent of root transcripts was found to increase in abundance during AM development, from which a set of AM-regulated marker genes was established. A Ser-protease (SbtS) and a Cys-protease (CysS) were also activated during root nodule development. AM-induced transcriptional activation was abolished in roots carrying mutations in common symbiosis genes, suggesting a central position of these genes in a pathway leading to the transcriptional activation of downstream genes. By contrast, AM fungus-induced gene repression appeared to be unaffected in mutant backgrounds, which indicates the presence of additional independent signaling pathways.

Formation of organelle-like N2-fixing symbiosomes in legume root nodules is controlled by DMI2

In most legume nodules, the N2-fixing rhizobia are present as organelle-like structures inside their host cells. These structures, named symbiosomes, contain one or a few rhizobia surrounded by a plant membrane. Symbiosome formation requires the release of bacteria from cell-wall-bound infection threads. In primitive legumes, rhizobia are hosted in intracellular infection threads that, in contrast to symbiosomes, are bound by a cell wall. The formation of symbiosomes is presumed to represent a major step in the evolution of legume-nodule symbiosis, because symbiosomes facilitate the exchange of metabolites between the two symbionts. Here, we show that the genes, which are essential for initiating nodule formation, are also actively transcribed in mature Medicago truncatula nodules in the region where symbiosome formation occurs. At least one of these genes, encoding the receptor kinase DOES NOT MAKE INFECTIONS 2 (DMI2) is essential for symbiosome formation. The protein locates to the host cell plasma membrane and to the membrane surrounding the infection threads. A partial reduction of DMI2 expression causes a phenotype that resembles the infection structures found in primitive legume nodules, because infected cells are occupied by large intracellular infection threads instead of by organelle-like symbiosomes.

SrSymRK, a plant receptor essential for symbiosome formation

The symbiosis between legumes and rhizobia is essential for the nitrogen input into the life cycle on our planet. New root organs, the nodules, are established, which house N2-fixing bacteria internalized into the host cell cytoplasm as horizontally acquired organelles, the symbiosomes. The interaction is initiated by bacterial invasion via epidermal root hair curling and cell division in the cortex, both triggered by bacterial nodulation factors. Of the several genes involved in nodule initiation that have been identified, one encodes the leucine-rich repeat-type receptor kinase SymRK. In SymRK mutants of Lotus japonicus or its orthologs in Medicago sp. and Pisum sativum, nodule initiation is arrested at the level of the root hair interaction. Because of the epidermal block, the role of SymRK at later stages of nodule development remained enigmatic. To analyze the role of SymRK downstream of the epidermis, the water-tolerant legume Sesbania rostrata was used that has developed a nodulation strategy to circumvent root hair responses for bacterial invasion. Evidence is provided that SymRK plays an essential role during endosymbiotic uptake in plant cells.

Nodulation signaling in legumes requires NSP2, a member of the GRAS family of transcriptional regulators

Rhizobial bacteria enter a symbiotic interaction with legumes, activating diverse responses in roots through the lipochito oligosaccharide signaling molecule Nod factor. Here, we show that NSP2 from Medicago truncatula encodes a GRAS protein essential for Nod-factor signaling. NSP2 functions downstream of Nod-factor-induced calcium spiking and a calcium/calmodulin-dependent protein kinase. We show that NSP2-GFP expressed from a constitutive promoter is localized to the endoplasmic reticulum/nuclear envelope and relocalizes to the nucleus after Nod-factor elicitation. This work provides evidence that a GRAS protein transduces calcium signals in plants and provides a possible regulator of Nod-factor-inducible gene expression.

NSP1 of the GRAS Protein Family Is Essential for Rhizobial Nod Factor-Induced Transcription

Rhizobial Nod factors induce in their legume hosts the expression of many genes and set in motion developmental processes leading to root nodule formation. Here we report the identification of the Medicago GRAS-type protein Nodulation signaling pathway 1 (NSP1), which is essential for all known Nod factor-induced changes in gene expression. NSP1 is constitutively expressed, and so it acts as a primary transcriptional regulator mediating all known Nod factor-induced transcriptional responses, and therefore, we named it a Nod factor response factor.

Analysis of promoter activity of the early nodulin Enod40 in Lotus japonicus

Our comparative studies on the promoter (pr) activity of Enod40 in the model legume Lotus japonicus in stably transformed GusA reporter lines and in hairy roots of L. japonicus demonstrate a stringent regulation of the Enod40 promoter in the root cortex and root hairs in response to Nod factors. Interestingly, the L. japonicus Enod40-2 promoter fragment also shows symbiotic activity in the reverse orientation. Deletion analyses of the Glycine max (Gm) Enod40 promoter revealed the presence of a minimal region -185 bp upstream of the transcription start. Stable transgenic L. japonicus reporter lines were used in bioassays to test the effect of different compounds on early symbiotic signaling. The responses of prGmEnod40 reporter lines were compared with the responses of L. japonicus (Lj) reporter lines based on the LjNin promoter. Both reporter lines show very early activity postinoculation in root hairs of the responsive zone of the root and later in the dividing cells of nodule primordia. The LjNin promoter was found to be more responsive than the GmEnod40 promoter to Nod factors and related compounds. The use of prGmEnod40 reporter lines to analyze the effect of nodulin genes on the GmEnod40 promoter activity indicates that LJNIN has a positive effect on the regulation of the Enod40 promoter, whereas the latter is not influenced by ectopic overexpression of its own gene product. In addition to pointing to a difference in the regulation of the two nodulin genes Enod40 and Nin during early time points of symbiosis, the bioassays revealed a difference in the response to the synthetic cytokinin 6-benzylaminopurine (BAP) between alfalfa and clover and L. japonicus. In alfalfa and clover, Enod40 expression was induced upon BAP treatment, whereas this seems not to be the case in L. japonicus; these results correlate with effects at the cellular level because BAP can induce pseudonodules in alfalfa and clover but not in L. japonicus. In conclusion, we demonstrate the applicability of the described L. japonicus reporter lines in analyses of the specificity of compounds related to nodulation as well as for the dissection of the interplay between different nodulin genes.

Characterization of the Lotus japonicus symbiotic mutant lot1 that shows a reduced nodule number and distorted trichomes

We isolated a recessive symbiotic mutant of Lotus japonicus that defines a genetic locus, LOT1 (for low nodulation and trichome distortion). The nodule number per plant of the mutant was about one-fifth of that of the wild type. The lot1 mutant showed a moderate dwarf phenotype and distorted trichomes, but its root hairs showed no apparent differences to those of the wild type. Infection thread formation after inoculation of Mesorhizobium loti was repressed in lot1 compared to that in the wild type. The nodule primordia of lot1 did not result in any aborted nodule-like structure, all nodules becoming mature and exhibiting high nitrogen fixation activity. The mutant was normally colonized by mycorrhizal fungi. lot1 also showed higher sensitivity to nitrate than the wild type. The grown-up seedlings of lot1 were insensitive to any ethylene treatments with regard to nodulation, although the mutant showed normal triple response on germination. It is conceivable that a nodulation-specific ethylene signaling pathway is constitutively activated in the mutant. Grafting experiments with lot1 and wild-type seedlings suggested that the root genotype mainly determines the low nodulation phenotype of the mutant, while the trichome distortion is regulated by the shoot genotype. Grafting of har1-4 shoots to lot1 roots resulted in an intermediate nodule number, i.e. more than that of lot1 and less than that of har1-4. Putative double mutants of lot1 and har1 also showed intermediate nodulation. Thus, it was indicated that LOT1 is involved in a distinct signal transduction pathway independent of HAR1.

Legume Haemoglobins: Symbiotic Nitrogen Fixation Needs Bloody Nodules

How do plants create an environment in which symbiotic bacteria can reduce enough N2 to provide the plant with sufficient ammonium for growth? Gene silencing has now been used to show that legume haemoglobins are crucial.

The evolution of chronic infection strategies in the alpha-proteobacteria

Many of the alpha-proteobacteria establish long-term, often chronic, interactions with higher eukaryotes. These interactions range from pericellular colonization through facultative intracellular multiplication to obligate intracellular lifestyles. A common feature in this wide range of interactions is modulation of host-cell proliferation, which sometimes leads to the formation of tumour-like structures in which the bacteria can grow. Comparative genome analyses reveal genome reduction by gene loss in the intracellular alpha-proteobacterial lineages, and genome expansion by gene duplication and horizontal gene transfer in the free-living species. In this review, we discuss alpha-proteobacterial genome evolution and highlight strategies and mechanisms used by these bacteria to infect and multiply in eukaryotic cells.

Pharmacological evidence that multiple phospholipid signaling pathways link Rhizobium nodulation factor perception in Medicago truncatula root hairs to intracellular responses, including Ca2+ spiking and specific ENOD gene expression

Rhizobium nodulation (Nod) factors are specific lipochito-oligosaccharide signals essential for initiating in root hairs of the host legume developmental responses that are required for controlled entry of the microsymbiont. In this article, we focus on the Nod factor signal transduction pathway leading to specific and cell autonomous gene activation in Medicago truncatula cv Jemalong in a study making use of the Nod factor-inducible MtENOD11 gene. First, we show that pharmacological antagonists that interfere with intracellular ion channel and Ca2+ pump activities are efficient blockers of Nod factor-elicited pMtENOD11-beta-glucuronidase (GUS) expression in root hairs of transgenic M. truncatula. These results indicate that intracellular Ca2+ release and recycling activities, essential for Ca2+ spiking, are also required for specific gene activation. Second, pharmacological effectors that inhibit phospholipase D and phosphoinositide-dependent phospholipase C activities are also able to block pMtENOD11-GUS activation, thus underlining a central role for multiple phospholipid signaling pathways in Nod factor signal transduction. Finally, pMtENOD11-GUS was introduced into all three Nod-/Myc- dmi M. truncatula mutant backgrounds, and gene expression was evaluated in response to the mastoparan peptide agonist Mas7. We found that Mas7 elicits root hair MtENOD11 expression in dmi1 and dmi2 mutants, but not in the dmi3 mutant, suggesting that the agonist acts downstream of DMI1/DMI2 and upstream of DMI3. In light of these results and the recently discovered identities of the DMI gene products, we propose an integrated cellular model for Nod factor signaling in legume root hairs in which phospholipids play a key role in linking the Nod factor perception apparatus to downstream components such as Ca2+ spiking and ENOD gene expression.

Molecular genetics of the arbuscular mycorrhizal symbiosis

During arbuscular mycorrhiza (AM) development, fungal hyphae grow throughout root epidermal, exodermal and cortical cell layers to reach the inner cortex where the symbiosis' functional units, the arbuscles, develop. Three essential components of a plant signalling network, a receptor-like kinase, a predicted ion-channel and a calmodulin-dependent protein kinase have been identified. A detailed morphological study of symbiotic plant mutants revealed that different subsets of plant genes support the progress of fungal infection in successive root cell layers. Moreover, evidence of a diffusible fungal signalling factor that triggers gene activation in the root has recently been obtained.