NENA, a Lotus japonicus homolog of Sec13, is required for rhizodermal infection by arbuscular mycorrhiza fungi and rhizobia but dispensable for cortical endosymbiotic development

Cell and developmental biology of arbuscular mycorrhiza symbiosis

The default mineral nutrient acquisition strategy of land plants is the symbiosis with arbuscular mycorrhiza (AM) fungi. Research into the cell and developmental biology of AM revealed fascinating insights into the plasticity of plant cell development and of interorganismic communication. It is driven by the prospect of increased exploitation of AM benefits for sustainable agriculture. The plant cell developmental program for intracellular accommodation of AM fungi is activated by a genetically defined signaling pathway involving calcium spiking in the nucleus as second messenger. Calcium spiking is triggered by chitooligosaccharides released by AM fungi that are probably perceived via LysM domain receptor kinases. Fungal infection and calcium spiking are spatiotemporally coordinated, and only cells committed to accommodating the fungus undergo high-frequency spiking. Delivery of mineral nutrients by AM fungi occurs at tree-shaped hyphal structures, the arbuscules, in plant cortical cells. Nutrients are taken up at a plant-derived periarbuscular membrane, which surrounds fungal hyphae and carries a specific transporter composition that is of direct importance for symbiotic efficiency. An elegant study has unveiled a new and unexpected mechanism for specific protein localization to the periarbuscular membrane, which relies on the timing of gene expression to synchronize protein biosynthesis with a redirection of secretion. The control of AM development by phytohormones is currently subject to active investigation and has led to the rediscovery of strigolactones. Nearly all tested phytohormones regulate AM development, and major insights into the mechanisms of this regulation are expected in the near future.

Host-specific Nod-factors associated with Medicago truncatula nodule infection differentially induce calcium influx and calcium spiking in root hairs

Rhizobial nodulation (Nod) factors activate both nodule morphogenesis and infection thread development during legume nodulation. Nod factors induce two different calcium responses: intra-nuclear calcium oscillations and a calcium influx at the root hair tip. Calcium oscillations activate nodule development; we wanted to test if the calcium influx is associated with infection. Sinorhizobium meliloti nodL and nodF mutations additively reduce infection of Medicago truncatula. Nod-factors made by the nodL mutant lack an acetyl group; mutation of nodF causes the nitrogen (N)-linked C16:2 acyl chain to be replaced by C18:1. We tested whether these Nod-factors differentially induced calcium influx and calcium spiking. The absence of the NodL-determined acetyl group greatly reduced the induction of calcium influx without affecting calcium spiking. The calcium influx was even further reduced if the N-linked C16:2 acyl group was replaced by C18:1. These additive effects on calcium influx correlate with the additive effects of mutations in nodF and nodL on legume infection. Infection thread development is inhibited by ethylene, which also inhibited Nod-factor-induced calcium influx. We conclude that Nod-factor perception differentially activates the two developmental pathways required for nodulation and that activation of the pathway involving the calcium influx is important for efficient infection.

Nuclear calcium signaling in plants

Plant cell nuclei can generate calcium responses to a variety of inputs, tantamount among them the response to signaling molecules from symbiotic microorganisms .

CERBERUS and NSP1 of Lotus japonicus are common symbiosis genes that modulate arbuscular mycorrhiza development

Arbuscular mycorrhizal symbiosis (AMS) and root nodule symbiosis (RNS) are mutualistic plant-microbe interactions that confer nutritional benefits to both partners. Leguminous plants possess a common genetic system for intracellular symbiosis with AM fungi and with rhizobia. Here we show that CERBERUS and NSP1, which respectively encode an E3 ubiquitin ligase and a GRAS transcriptional regulator and which have previously only been implicated in RNS, are involved in AM fungal infection in Lotus japonicus. Hyphal elongation along the longitudinal axis of the root was reduced in the cerberus mutant, giving rise to a lower colonization level. Knockout of NSP1 decreased the frequency of plants colonized by AM fungi or rhizobia. CERBERUS and NSP1 showed different patterns of expression in response to infection with symbiotic microbes. A low constitutive level of CERBERUS expression was observed in the root and an increased level of NSP1 expression was detected in arbuscule-containing cells. Induction of AM marker gene was triggered in both cerberus and nsp1 mutants by infection with symbiotic microbes; however, the mutants showed a weaker induction of marker gene expression than the wild type, mirroring their lower level of colonization. The common symbiosis genes are believed to act in an early signaling pathway for recognition of symbionts and for triggering early symbiotic responses. Our quantitative analysis of symbiotic phenotypes revealed developmental defects of the novel common symbiosis mutants in both symbioses, which demonstrates that common symbiosis mechanisms also contribute to a range of functions at later or different stages of symbiont infection.

Two Lotus japonicus symbiosis mutants impaired at distinct steps of arbuscule development

Arbuscular mycorrhiza (AM) fungi form nutrient-acquiring symbioses with the majority of higher plants. Nutrient exchange occurs via arbuscules, highly branched hyphal structures that are formed within root cortical cells. With a view to identifying host genes involved in AM development, we isolated Lotus japonicus AM-defective mutants via a microscopic screen of an ethyl methanesulfonate-mutagenized population. A standardized mapping procedure was developed that facilitated positioning of the defective loci on the genetic map of L. japonicus, and, in five cases, allowed identification of mutants of known symbiotic genes. Two additional mutants representing independent loci did not form mature arbuscules during symbiosis with two divergent AM fungal species, but exhibited signs of premature arbuscule arrest or senescence. Marker gene expression patterns indicated that the two mutants are affected in distinct steps of arbuscule development. Both mutants formed wild-type-like root nodules upon inoculation with Mesorhizobium loti, indicating that the mutated loci are essential during AM but not during root nodule symbiosis.

Evolution of the plant-microbe symbiotic 'toolkit'

Beneficial associations between plants and arbuscular mycorrhizal fungi play a major role in terrestrial environments and in the sustainability of agroecosystems. Proteins, microRNAs, and small molecules have been identified in model angiosperms as required for the establishment of arbuscular mycorrhizal associations and define a symbiotic 'toolkit' used for other interactions such as the rhizobia-legume symbiosis. Based on recent studies, we propose an evolutionary framework for this toolkit. Some components appeared recently in angiosperms, whereas others are highly conserved even in land plants unable to form arbuscular mycorrhizal associations. The exciting finding that some components pre-date the appearance of arbuscular mycorrhizal fungi suggests the existence of unknown roles for this toolkit and even the possibility of symbiotic associations in charophyte green algae.

Splice variants of the SIP1 transcripts play a role in nodule organogenesis in Lotus japonicus

SymRK-interacting protein 1 (SIP1) has previously been shown to interact with the symbiosis receptor kinase, SymRK, in Lotus japonicus. A longer variant of the SIP1 transcript, SIP1L, was isolated and characterized. SIP1L contains an additional 17 amino acids that make its C-terminus a complete heat shock protein 20 (Hsp20)-like domain. In contrast to SIP1S, the longer splicing variant SIP1L could not interact with SymRK. Both SIP1L and SIP1S transcripts could be detected in developing nodules and other plant tissues, although the former was always more abundant than the latter. SIP1L and SIP1S formed heteromeric protein complexes, which were co-localized in the plasma membrane, cytoplasm and nuclei. Expression of SIP1-RNAi in transgenic hairy roots resulted in impairment in the nodule and arbuscular mycorrhizal development, suggesting an important role of SIP1 in the common symbiosis pathway. Overexpression of either SIP1L or SIP1S increased the number of nodules formed on transgenic hairy roots, indicating a positive role of SIP1 in nodulation. The SIP1S-like transcript was not detected in other higher plants tested, and the SIP1L-like proteins of these plants were capable of interacting with the SymRK orthologs. It is proposed that the loss of the ability of SIP1L to interact with SymRK in Lotus is compensated by the expression of a shorter splicing variant, SIP1S, which binds SymRK and may play a role in relaying the symbiosis signals to downstream cellular events.

Speak, friend, and enter: signalling systems that promote beneficial symbiotic associations in plants

Plants associate with a wide range of microorganisms, with both detrimental and beneficial outcomes. Central to plant survival is the ability to recognize invading microorganisms and either limit their intrusion, in the case of pathogens, or promote the association, in the case of symbionts. To aid in this recognition process, elaborate communication and counter-communication systems have been established that determine the degree of ingress of the microorganism into the host plant. In this Review, I describe the common signalling processes used by plants during mutualistic interactions with microorganisms as diverse as arbuscular mycorrhizal fungi and rhizobial bacteria.

Speak, friend, and enter: signalling systems that promote beneficial symbiotic associations in plants

Plants associate with a wide range of microorganisms, with both detrimental and beneficial outcomes. Central to plant survival is the ability to recognize invading microorganisms and either limit their intrusion, in the case of pathogens, or promote the association, in the case of symbionts. To aid in this recognition process, elaborate communication and counter-communication systems have been established that determine the degree of ingress of the microorganism into the host plant. In this Review, I describe the common signalling processes used by plants during mutualistic interactions with microorganisms as diverse as arbuscular mycorrhizal fungi and rhizobial bacteria.

Symbiosis and the social network of higher plants

In the Internet era, communicating with friends and colleagues via social networks constitutes a significant proportion of our daily activities. Similarly animals and plants also interact with many organisms, some of which are pathogens and do no good for the plant, while others are beneficial symbionts. Almost all plants indulge in developing social networks with microbes, in particular with arbuscular mycorrhizal fungi, and emerging evidence indicates that most employ an ancient and widespread central 'social media' pathway made of signaling molecules within what is called the SYM pathway. Some plants, like legumes, are particularly active recruiters of friends, as they have established very sophisticated and beneficial interactions with nitrogen-fixing bacteria, also via the SYM pathway. Interestingly, many members of the Brassicaceae, including the model plant Arabidopsis thaliana, seem to have removed themselves from this ancestral social network and lost the ability to engage in mutually favorable interactions with arbuscular mycorrhizal fungi. Despite these generalizations, recent studies exploring the root microbiota of A. thaliana have found that in natural conditions, A. thaliana roots are colonized by many different bacterial species and therefore may be using different and probably more recent 'social media' for these interactions. In general, recent advances in the understanding of such molecular machinery required for plant-symbiont associations are being obtained using high throughput genomic profiling strategies including transcriptomics, proteomics and metabolomics. The crucial mechanistic understanding that such data reveal may provide the infrastructure for future efforts to genetically manipulate crop social networks for our own food and fiber needs.

Insights into post-transcriptional regulation during legume-rhizobia symbiosis

During the past ten years, changes in the transcriptome have been assessed at different stages of the legume-rhizobia association by the use of DNA microarrays and, more recently, by RNA sequencing technologies. These studies allowed the identification of hundred or thousand of genes whose steady-state mRNA levels increase or decrease upon bacterial infection or in nodules as compared with uninfected roots. However, transcriptome based-approaches do not distinguish between mRNAs that are being actively translated, stored as messenger ribonucleoproteins (mRNPs) or targeted for degradation. Despite that the increase in steady-state levels of an mRNA does not necessarily correlate with an increase in abundance or activity of the encoded protein, this information has been commonly used to select genes that are candidates to play a role during nodule organogenesis or bacterial infection. Such criterion does not take into account the post-transcriptional mechanisms that contribute to the regulation of gene expression. One of such mechanisms, which has significant impact on gene expression, is the selective recruitment of mRNAs to the translational machinery.  Here, we review the post-transcriptional mechanisms that contribute to the regulation of gene expression in the context of the ecological and agronomical important symbiotic interaction established between roots of legumes and the nitrogen fixing bacteria collectively known as rhizobia. In addition, we discuss how the development of new technologies that allow the assessment of these regulatory layers would help to understand the genetic network governing legume rhizobia symbiosis.

Assessing the function of the plant nuclear pore complex and the search for specificity

Plant cells encounter a wide variety of molecules that influence their gene expression and development. A key component of most signal transduction pathways involves the regulated movement of molecules into and out of the nucleus. The plant nuclear pore complex (NPC) is a critical controlling element in this nucleocytoplasmic movement of protein and RNA. The NPC is comprised of approximately 30 nucleoporin proteins arranged in radial symmetry around the central pore. Over recent years our understanding of how the NPC impacts different signalling pathways has increased following the identification of a range of nucleoporin mutant plants. These mutants allow us to gain insight into how the response to hormonal, abiotic, and biotic stresses are effected by changes in nuclear transport. Importantly we have little information regarding the specific molecules whose nuclear transport is altered in these processes and the identification of these proteins is a significant challenge. Here is presented an overview as to how the members of the plant NPC affect signalling pathways, highlighting the progress and difficulties within this research area.

Nuclear pore complex composition: a new regulator of tissue-specific and developmental functions

Nuclear pore complexes (NPCs) are multiprotein aqueous channels that penetrate the nuclear envelope connecting the nucleus and the cytoplasm. NPCs consist of multiple copies of roughly 30 different proteins known as nucleoporins (NUPs). Due to their essential role in controlling nucleocytoplasmic transport, NPCs have traditionally been considered as structures of ubiquitous composition. The overall structure of the NPC is indeed conserved in all cells, but new evidence suggests that the protein composition of NPCs varies among cell types and tissues. Moreover, mutations in various nucleoporins result in tissue-specific diseases. These findings point towards a heterogeneity in NPC composition and function. This unexpected heterogeneity suggests that cells use a combination of different nucleoporins to assemble NPCs with distinct properties and specialized functions.

Analysis of the Lotus japonicus nuclear pore NUP107-160 subcomplex reveals pronounced structural plasticity and functional redundancy

Mutations in the Lotus japonicus nucleoporin genes, NUP85, NUP133, and NENA (SEH1), lead to defects in plant-microbe symbiotic signaling. The homologous proteins in yeast and vertebrates are part of the conserved NUP84/NUP107-160 subcomplex, which is an essential component of the nuclear pore scaffold and has a pivotal role in nuclear pore complex (NPC) assembly. Loss and down-regulation of NUP84/NUP107-160 members has previously been correlated with a variety of growth and molecular defects, however, in L. japonicus only surprisingly specific phenotypes have been reported. We investigated whether Lotus nup85, nup133, and nena mutants exhibit general defects in NPC composition and distribution. Whole mount immunolocalization confirmed a typical nucleoporin-like localization for NUP133, which was unchanged in the nup85-1 mutant. Severe NPC clustering and aberrations in the nuclear envelope have been reported for Saccharomyces cerevisiae nup85 and nup133 mutants. However, upon transmission electron microscopy analysis of L. japonicus nup85, nup133 and nena, we detected only a slight reduction in the average distances between neighboring NPCs in nup133. Using quantitative immunodetection on protein-blots we observed that loss of individual nucleoporins affected the protein levels of other NUP107-160 complex members. Unlike the single mutants, nup85/nup133 double mutants exhibited severe temperature dependent growth and developmental defects, suggesting that the loss of more than one NUP107-160 member affects basal functions of the NPC.

Selective recruitment of mRNAs and miRNAs to polyribosomes in response to rhizobia infection in Medicago truncatula

Translation of mRNAs is a key regulatory step that contributes to the coordination and modulation of eukaryotic gene expression during development or adaptation to the environment. mRNA stability or translatability can be regulated by the action of small regulatory RNAs (sRNAs), which control diverse biological processes. Under low nitrogen conditions, leguminous plants associate with soil bacteria and develop a new organ specialized in nitrogen fixation: the nodule. To gain insight into the translational regulation of mRNAs during nodule formation, the association of mRNAs and sRNAs to polysomes was characterized in roots of the model legume Medicago truncatula during the symbiotic interaction with Sinorhizobium meliloti. Quantitative comparison of steady-state and polysomal mRNAs for 15 genes involved in nodulation identified a group of transcripts with slight or no change in total cellular abundance that were significantly upregulated at the level of association with polysomes in response to rhizobia. This group included mRNAs encoding receptors like kinases required either for nodule organogenesis, bacterial infection or both, and transcripts encoding GRAS and NF-Y transcription factors (TFs). Quantitative analysis of sRNAs in total and polysomal RNA samples revealed that mature microRNAs (miRNAs) were associated with the translational machinery, notably, miR169 and miR172, which target the NF-YA/HAP2 and AP2 TFs, respectively. Upon inoculation, levels of miR169 pronouncedly decreased in polysomal complexes, concomitant with the increased accumulation of the NF-YA/HAP2 protein. These results indicate that both mRNAs and miRNAs are subject to differential recruitment to polysomes, and expose the importance of selective mRNA translation during root nodule symbiosis.

Rhizobial and mycorrhizal symbioses in Lotus japonicus require lectin nucleotide phosphohydrolase, which acts upstream of calcium signaling

Nodulation in legumes requires the recognition of rhizobially made Nod factors. Genetic studies have revealed that the perception of Nod factors involves LysM domain receptor-like kinases, while biochemical approaches have identified LECTIN NUCLEOTIDE PHOSPHOHYDROLASE (LNP) as a Nod factor-binding protein. Here, we show that antisense inhibition of LNP blocks nodulation in Lotus japonicus. This absence of nodulation was due to a defect in Nod factor signaling based on the observations that the early nodulation gene NODULE INCEPTION was not induced and that both Nod factor-induced perinuclear calcium spiking and calcium influx at the root hair tip were blocked. However, Nod factor did induce root hair deformation in the LNP antisense lines. LNP is also required for infection by the mycorrhizal fungus Glomus intraradices, suggesting that LNP plays a role in the common signaling pathway shared by the rhizobial and mycorrhizal symbioses. Taken together, these observations indicate that LNP acts at a novel position in the early stages of symbiosis signaling. We propose that LNP functions at the earliest stage of the common nodulation and mycorrhization symbiosis signaling pathway downstream of the Nod factor receptors; it may act either by influencing signaling via changes in external nucleotides or in conjunction with the LysM receptor-like kinases for recognition of Nod factor.

Buffering capacity explains signal variation in symbiotic calcium oscillations

Legumes form symbioses with rhizobial bacteria and arbuscular mycorrhizal fungi that aid plant nutrition. A critical component in the establishment of these symbioses is nuclear-localized calcium (Ca(2+)) oscillations. Different components on the nuclear envelope have been identified as being required for the generation of the Ca(2+) oscillations. Among these an ion channel, Doesn't Make Infections1, is preferentially localized on the inner nuclear envelope and a Ca(2+) ATPase is localized on both the inner and outer nuclear envelopes. Doesn't Make Infections1 is conserved across plants and has a weak but broad similarity to bacterial potassium channels. A possible role for this cation channel could be hyperpolarization of the nuclear envelope to counterbalance the charge caused by the influx of Ca(2+) into the nucleus. Ca(2+) channels and Ca(2+) pumps are needed for the release and reuptake of Ca(2+) from the internal store, which is hypothesized to be the nuclear envelope lumen and endoplasmic reticulum, but the release mechanism of Ca(2+) remains to be identified and characterized. Here, we develop a mathematical model based on these components to describe the observed symbiotic Ca(2+) oscillations. This model can recapitulate Ca(2+) oscillations, and with the inclusion of Ca(2+)-binding proteins it offers a simple explanation for several previously unexplained phenomena. These include long periods of frequency variation, changes in spike shape, and the initiation and termination of oscillations. The model also predicts that an increase in buffering capacity in the nucleoplasm would cause a period of rapid oscillations. This phenomenon was observed experimentally by adding more of the inducing signal.

Medicago truncatula ERN transcription factors: regulatory interplay with NSP1/NSP2 GRAS factors and expression dynamics throughout rhizobial infection

Rhizobial nodulation factors (NFs) activate a specific signaling pathway in Medicago truncatula root hairs that involves the complex interplay of Nodulation Signaling Pathway1 (NSP1)/NSP2 GRAS and Ethylene Response Factor Required for Nodulation1 (ERN1) transcription factors (TFs) to achieve full ENOD11 transcription. ERN1 acts as a direct transcriptional regulator of ENOD11 through the activation of the NF-responsive "NF box." Here, we show that NSP1, when combined with NSP2, can act as a strong positive regulator of ERN1 and ENOD11 transcription. Although ERN1 and NSP1/NSP2 both activate ENOD11, two separate promoter regions are involved that regulate expression during consecutive symbiotic stages. Our findings indicate that ERN1 is required to activate NF-elicited ENOD11 expression exclusively during early preinfection, while NSP1/NSP2 mediates ENOD11 expression during subsequent rhizobial infection. The relative contributions of ERN1 and the closely related ERN2 to the rhizobial symbiosis were then evaluated by comparing their regulation and in vivo dynamics. ERN1 and ERN2 exhibit expression profiles compatible with roles during NF signaling and subsequent infection. However, differences in expression levels and spatiotemporal profiles suggest specialized functions for these two TFs, ERN1 being involved in stages preceding and accompanying infection thread progression while ERN2 is only involved in certain stages of infection. By cross complementation, we show that ERN2, when expressed under the control of the ERN1 promoter, can restore both NF-elicited ENOD11 expression and nodule formation in an ern1 mutant background. This indicates that ERN1 and ERN2 possess similar biological activities and that functional diversification of these closely related TFs relies primarily on changes in tissue-specific expression patterns.

Negative regulation of CCaMK is essential for symbiotic infection

One of the earliest responses of legumes to symbiotic signalling is oscillation of the calcium concentration in the nucleoplasm of root epidermal cells. Integration and decoding of the calcium-spiking signal involve a calcium- and calmodulin-dependent protein kinase (CCaMK) and its phosphorylation substrates, such as CYCLOPS. Here we describe the Lotus japonicus ccamk-14 mutant that originated from a har1-1 suppressor screen. The ccamk-14 mutation causes a serine to asparagine substitution at position 337 located within the calmodulin binding site, which we determined to be an in vitro phosphorylation site in CCaMK. We show that ccamk-14 exerts cell-specific effects on symbiosis. The mutant is characterized by an increased frequency of epidermal infections and significantly compromised cortical infections by Mesorhizobium loti and also the arbuscular mycorrhiza fungus Rhizophagus irregularis. The S337 residue is conserved across angiosperm CCaMKs, and testing discrete substitutions at this site showed that it participates in a negative regulation of CCaMK activity, which is required for the cell-type-specific integration of symbiotic signalling.

How membranes shape plant symbioses: signaling and transport in nodulation and arbuscular mycorrhiza

As sessile organisms that cannot evade adverse environmental conditions, plants have evolved various adaptive strategies to cope with environmental stresses. One of the most successful adaptations is the formation of symbiotic associations with beneficial microbes. In these mutualistic interactions the partners exchange essential nutrients and improve their resistance to biotic and abiotic stresses. In arbuscular mycorrhiza (AM) and in root nodule symbiosis (RNS), AM fungi and rhizobia, respectively, penetrate roots and accommodate within the cells of the plant host. In these endosymbiotic associations, both partners keep their plasma membranes intact and use them to control the bidirectional exchange of signaling molecules and nutrients. Intracellular accommodation requires the exchange of symbiotic signals and the reprogramming of both interacting partners. This involves fundamental changes at the level of gene expression and of the cytoskeleton, as well as of organelles such as plastids, endoplasmic reticulum (ER), and the central vacuole. Symbiotic cells are highly compartmentalized and have a complex membrane system specialized for the diverse functions in molecular communication and nutrient exchange. Here, we discuss the roles of the different cellular membrane systems and their symbiosis-related proteins in AM and RNS, and we review recent progress in the analysis of membrane proteins involved in endosymbiosis.

Lotus japonicus ARPC1 is required for rhizobial infection

Remodeling of the plant cell cytoskeleton precedes symbiotic entry of nitrogen-fixing bacteria within the host plant roots. Here we identify a Lotus japonicus gene encoding a predicted ACTIN-RELATED PROTEIN COMPONENT1 (ARPC1) as essential for rhizobial infection but not for arbuscular mycorrhiza symbiosis. In other organisms ARPC1 constitutes a subunit of the ARP2/3 complex, the major nucleator of Y-branched actin filaments. The L. japonicus arpc1 mutant showed a distorted trichome phenotype and was defective in epidermal infection thread formation, producing mostly empty nodules. A few partially colonized nodules that did form in arpc1 contained abnormal infections. Together with previously described L. japonicus Nck-associated protein1 and 121F-specific p53 inducible RNA mutants, which are also impaired in the accommodation of rhizobia, our data indicate that ARPC1 and, by inference a suppressor of cAMP receptor/WASP-family verpolin homologous protein-ARP2/3 pathway, must have been coopted during evolution of nitrogen-fixing symbiosis to specifically mediate bacterial entry.

Epidermal and cortical roles of NFP and DMI3 in coordinating early steps of nodulation in Medicago truncatula

Legumes have evolved the capacity to form a root nodule symbiosis with soil bacteria called rhizobia. The establishment of this symbiosis involves specific developmental events occurring both in the root epidermis (notably bacterial entry) and at a distance in the underlying root cortical cells (notably cell divisions leading to nodule organogenesis). The processes of bacterial entry and nodule organogenesis are tightly linked and both depend on rhizobial production of lipo-chitooligosaccharide molecules called Nod factors. However, how these events are coordinated remains poorly understood. Here, we have addressed the roles of two key symbiotic genes of Medicago truncatula, the lysin motif (LysM) domain-receptor like kinase gene NFP and the calcium- and calmodulin-dependent protein kinase gene DMI3, in the control of both nodule organogenesis and bacterial entry. By complementing mutant plants with corresponding genes expressed either in the epidermis or in the cortex, we have shown that epidermal DMI3, but not NFP, is sufficient for infection thread formation in root hairs. Epidermal NFP is sufficient to induce cortical cell divisions leading to nodule primordia formation, whereas DMI3 is required in both cell layers for these processes. Our results therefore suggest that a signal, produced in the epidermis under the control of NFP and DMI3, is responsible for activating DMI3 in the cortex to trigger nodule organogenesis. We integrate these data to propose a new model for epidermal/cortical crosstalk during early steps of nodulation.

Activation of calcium- and calmodulin-dependent protein kinase (CCaMK), the central regulator of plant root endosymbiosis

The key molecular event during the development of arbuscular mycorrhiza and the root nodule symbiosis is the activation of calcium- and calmodulin-dependent protein kinase (CCaMK). Its regulation is complex and involves positive as well as negative regulation facilitated by autophosphorylation of two conserved sites. Deregulated versions of CCaMK are sufficient for mediating both organogenesis and infection processes. Epistasis tests demonstrated that a main function of signaling components upstream of calcium spiking is the activation of CCaMK. Despite CCaMK being a central signaling hub, specificity for both symbioses exists, resulting in differential transcriptional gene expression patterns. While the specificity upstream of CCaMK can be conceptualized by the specific perception of rhizobial and fungal lipo-chitooligosaccharides via cognate LysM receptors, the mechanisms conferring transcriptional specificity downstream of CCaMK are likely conferred by a variety of transcriptional regulators, mediating symbiosis appropriate gene regulation.

Putative members of the Arabidopsis Nup107-160 nuclear pore sub-complex contribute to pathogen defense

In eukaryotic cells, transduction of external stimuli into the nucleus to induce transcription and export of mRNAs for translation in the cytoplasm is mediated by nuclear pore complexes (NPCs) composed of nucleoporin proteins (Nups). We previously reported that Arabidopsis MOS3, encoding the homolog of vertebrate Nup96, is required for plant immunity and constitutive resistance mediated by the de-regulated Toll interleukin 1 receptor/nucleotide-binding/leucine-rich repeat (TNL)-type R gene snc1. In vertebrates, Nup96 is a component of the conserved Nup107-160 nuclear pore sub-complex, and implicated in immunity-related mRNA export. Here, we used a reverse genetics approach to examine the requirement for additional subunits of the predicted Arabidopsis Nup107-160 complex in plant immunity. We show that, among eight putative complex members, beside MOS3, only plants with defects in Nup160 or Seh1 are impaired in basal resistance. Constitutive resistance in the snc1 mutant and immunity mediated by TNL-type R genes also depend on functional Nup160 and have a partial requirement for Seh1. Conversely, resistance conferred by coiled coil-type immune receptors operates largely independently of both genes, demonstrating specific contributions to plant defense signaling. Our functional analysis further revealed that defects in nup160 and seh1 result in nuclear accumulation of poly(A) mRNA, and, in the case of nup160, considerable depletion of EDS1, a key positive regulator of basal and TNL-triggered resistance. These findings suggest that Nup160 is required for nuclear mRNA export and full expression of EDS1-conditioned resistance pathways in Arabidopsis.

The recent evolution of a symbiotic ion channel in the legume family altered ion conductance and improved functionality in calcium signaling

Arbuscular mycorrhiza and the rhizobia-legume symbiosis are two major root endosymbioses that facilitate plant nutrition. In Lotus japonicus, two symbiotic cation channels, CASTOR and POLLUX, are indispensable for the induction of nuclear calcium spiking, one of the earliest plant responses to symbiotic partner recognition. During recent evolution, a single amino acid substitution in DOES NOT MAKE INFECTIONS1 (DMI1), the POLLUX putative ortholog in the closely related Medicago truncatula, rendered the channel solo sufficient for symbiosis; castor, pollux, and castor pollux double mutants of L. japonicus were rescued by DMI1 alone, while both Lj-CASTOR and Lj-POLLUX were required for rescuing a dmi1 mutant of M. truncatula. Experimental replacement of the critical serine by an alanine in the selectivity filter of Lj-POLLUX conferred a symbiotic performance indistinguishable from DMI1. Electrophysiological characterization of DMI1 and Lj-CASTOR (wild-type and mutants) by planar lipid bilayer experiments combined with calcium imaging in Human Embryonic Kidney-293 cells expressing DMI1 (the wild type and mutants) suggest that the serine-to-alanine substitution conferred reduced conductance with a long open state to DMI1 and improved its efficiency in mediating calcium oscillations. We propose that this single amino acid replacement in the selectivity filter made DMI1 solo sufficient for symbiosis, thus explaining the selective advantage of this allele at the mechanistic level.

Lotus japonicus E3 ligase SEVEN IN ABSENTIA4 destabilizes the symbiosis receptor-like kinase SYMRK and negatively regulates rhizobial infection

The Lotus japonicus SYMBIOSIS RECEPTOR-LIKE KINASE (SYMRK) is required for symbiotic signal transduction upon stimulation of root cells by microbial signaling molecules. Here, we identified members of the SEVEN IN ABSENTIA (SINA) E3 ubiquitin-ligase family as SYMRK interactors and confirmed their predicted ubiquitin-ligase activity. In Nicotiana benthamiana leaves, SYMRK-yellow fluorescent protein was localized at the plasma membrane, and interaction with SINAs, as determined by bimolecular fluorescence complementation, was observed in small punctae at the cytosolic interface of the plasma membrane. Moreover, fluorescence-tagged SINA4 partially colocalized with SYMRK and caused SYMRK relocalization as well as disappearance of SYMRK from the plasma membrane. Neither the localization nor the abundance of Nod-factor receptor1 was altered by the presence of SINA4. SINA4 was transcriptionally upregulated during root symbiosis, and rhizobia inoculated roots ectopically expressing SINA4 showed reduced SYMRK protein levels. In accordance with a negative regulatory role in symbiosis, infection thread development was impaired upon ectopic expression of SINA4. Our results implicate SINA4 E3 ubiquitin ligase in the turnover of SYMRK and provide a conceptual mechanism for its symbiosis-appropriate spatio-temporal containment.

The half-size ABC transporters STR1 and STR2 are indispensable for mycorrhizal arbuscule formation in rice

The central structure of the symbiotic association between plants and arbuscular mycorrhizal (AM) fungi is the fungal arbuscule that delivers minerals to the plant. Our earlier transcriptome analyses identified two half-size ABCG transporters that displayed enhanced mRNA levels in mycorrhizal roots. We now show specific transcript accumulation in arbusculated cells of both genes during symbiosis. Presently, arbuscule-relevant factors from monocotyledons have not been reported. Mutation of either of the Oryza sativa (rice) ABCG transporters blocked arbuscule growth of different AM fungi at a small and stunted stage, recapitulating the phenotype of Medicago truncatula stunted arbuscule 1 and 2 (str1 and str2) mutants that are deficient in homologous ABCG genes. This phenotypic resemblance and phylogenetic analysis suggest functional conservation of STR1 and STR2 across the angiosperms. Malnutrition of the fungus underlying limited arbuscular growth was excluded by the absence of complementation of the str1 phenotype by wild-type nurse plants. Furthermore, plant AM signaling was found to be intact, as arbuscule-induced marker transcript accumulation was not affected in str1 mutants. Strigolactones have previously been hypothesized to operate as intracellular hyphal branching signals and possible substrates of STR1 and STR2. However, full arbuscule development in the strigolactone biosynthesis mutants d10 and d17 suggested strigolactones to be unlikely substrates of STR1/STR2. Interestingly, rice STR1 is associated with a cis-natural antisense transcript (antiSTR1). Analogous to STR1 and STR2, at the root cortex level, the antiSTR1 transcript is specifically detected in arbusculated cells, suggesting unexpected modes of STR1 regulation in rice.

Nuclear-localized and deregulated calcium- and calmodulin-dependent protein kinase activates rhizobial and mycorrhizal responses in Lotus japonicus

The common symbiosis pathway is at the core of symbiosis signaling between plants and soil microbes. In this pathway, calcium- and calmodulin-dependent protein kinase (CCaMK) plays a crucial role in integrating the signals both in arbuscular mycorrhizal symbiosis (AMS) and in root nodule symbiosis (RNS). However, the molecular mechanism by which CCaMK coordinates AMS and RNS is largely unknown. Here, we report that the gain-of-function (GOF) variants of CCaMK without the regulatory domains activate both AMS and RNS signaling pathways in the absence of symbiotic partners. This activation requires nuclear localization of CCaMK. Enforced nuclear localization of the GOF-CCaMK variants by fusion with a canonical nuclear localization signal enhances signaling activity of AMS and RNS. The GOF-CCaMK variant triggers formation of a structure similar to the prepenetration apparatus, which guides infection of arbuscular mycorrhizal fungi to host root cells. In addition, the GOF-CCaMK variants without the regulatory domains partly restore AMS but fail to support rhizobial infection in ccamk mutants. These data indicate that AMS, the more ancient type of symbiosis, can be mainly regulated by the kinase activity of CCaMK, whereas RNS, which evolved more recently, requires complex regulation performed by the regulatory domains of CCaMK.

Receptor kinase signaling pathways in plant-microbe interactions

Plant receptor-like kinases (RLKs) function in diverse signaling pathways, including the responses to microbial signals in symbiosis and defense. This versatility is achieved with a common overall structure: an extracytoplasmic domain (ectodomain) and an intracellular protein kinase domain involved in downstream signal transduction. Various surfaces of the leucine-rich repeat (LRR) ectodomain superstructure are utilized for interaction with the cognate ligand in both plant and animal receptors. RLKs with lysin-motif (LysM) ectodomains confer recognitional specificity toward N-acetylglucosamine-containing signaling molecules, such as chitin, peptidoglycan (PGN), and rhizobial nodulation factor (NF), that induce immune or symbiotic responses. Signaling downstream of RLKs does not follow a single pattern; instead, the detailed analysis of brassinosteroid (BR) signaling, innate immunity, and symbiosis revealed at least three largely nonoverlapping pathways. In this review, we focus on RLKs involved in plant-microbe interactions and contrast the signaling pathways leading to symbiosis and defense.

Rhizobial and fungal symbioses show different requirements for calmodulin binding to calcium calmodulin-dependent protein kinase in Lotus japonicus

Ca(2+)/calmodulin (CaM)-dependent protein kinase (CCaMK) is a key regulator of root nodule and arbuscular mycorrhizal symbioses and is believed to be a decoder for Ca(2+) signals induced by microbial symbionts. However, it is unclear how CCaMK is activated by these microbes. Here, we investigated in vivo activation of CCaMK in symbiotic signaling, focusing mainly on the significance of and epistatic relationships among functional domains of CCaMK. Loss-of-function mutations in EF-hand motifs revealed the critical importance of the third EF hand for CCaMK activation to promote infection of endosymbionts. However, a gain-of-function mutation (T265D) in the kinase domain compensated for these loss-of-function mutations in the EF hands. Mutation of the CaM binding domain abolished CaM binding and suppressed CCaMK(T265D) activity in rhizobial infection, but not in mycorrhization, indicating that the requirement for CaM binding to CCaMK differs between root nodule and arbuscular mycorrhizal symbioses. Homology modeling and mutagenesis studies showed that the hydrogen bond network including Thr265 has an important role in the regulation of CCaMK. Based on these genetic, biochemical, and structural studies, we propose an activation mechanism of CCaMK in which root nodule and arbuscular mycorrhizal symbioses are distinguished by differential regulation of CCaMK by CaM binding.

Ecological and agronomic importance of the plant genus Lotus. Its application in grassland sustainability and the amelioration of constrained and contaminated soils

The genus Lotus comprises around 100 annual and perennial species with worldwide distribution. The relevance of Lotus japonicus as a model plant has been recently demonstrated in numerous studies. In addition, some of the Lotus species show a great potential for adaptation to a number of abiotic stresses. Therefore, they are relevant components of grassland ecosystems in environmentally constrained areas of several South American countries and Australia, where they are used for livestock production. Also, the fact that the roots of these species form rhizobial and mycorrhizal associations makes the annual L. japonicus a suitable model plant for legumes, particularly in studies directed to recognize the mechanisms intervening in the tolerance to abiotic factors in the field, where these interactions occur. These properties justify the increased utilization of some Lotus species as a strategy for dunes revegetation and reclamation of heavy metal-contaminated or burned soils in Europe.

Cytokinin induction of root nodule primordia in Lotus japonicus is regulated by a mechanism operating in the root cortex

Cytokinin plays a central role in the formation of nitrogen-fixing root nodules following inoculation with rhizobia. We show that exogenous cytokinin induces formation of discrete and easily visible nodule primordia in Lotus japonicus roots. The expression of nodulin genes was up-regulated upon cytokinin treatment, suggesting that the genuine nodulation program was indeed activated. This offers a simple approach for dissecting the underlying mechanism. Cytokinin-induced nodule primordia formation was unperturbed in several loss-of-function mutants impaired in epidermal responses to either rhizobial infection, Nod factor application, or both. However, absence of primordia in nsp1, nsp2, and nin mutants showed the requirement for these transcriptional regulators in the cytokinin-mediated activation of the root cortex. Distinguishing the epidermal and cortical responses further, we found that external cytokinin application induced expression of the Nin::GUS reporter gene within the root cortex but not in the root epidermis. Using L. japonicus lhk1-1 and har1 mutants, we demonstrate that discrete activation of root cortical cells by cytokinin depends on the LHK1 cytokinin receptor and is subjected to HAR1-mediated autoregulation.

Medicago truncatula IPD3 is a member of the common symbiotic signaling pathway required for rhizobial and mycorrhizal symbioses

Legumes form endosymbiotic associations with nitrogen-fixing bacteria and arbuscular mycorrhizal (AM) fungi which facilitate nutrient uptake. Both symbiotic interactions require a molecular signal exchange between the plant and the symbiont, and this involves a conserved symbiosis (Sym) signaling pathway. In order to identify plant genes required for intracellular accommodation of nitrogen-fixing bacteria and AM fungi, we characterized Medicago truncatula symbiotic mutants defective for rhizobial infection of nodule cells and colonization of root cells by AM hyphae. Here, we describe mutants impaired in the interacting protein of DMI3 (IPD3) gene, which has been identified earlier as an interacting partner of the calcium/calmodulin-dependent protein, a member of the Sym pathway. The ipd3 mutants are impaired in both rhizobial and mycorrhizal colonization and we show that IPD3 is necessary for appropriate Nod-factor-induced gene expression. This indicates that IPD3 is a member of the common Sym pathway. We observed differences in the severity of ipd3 mutants that appear to be the result of the genetic background. This supports the hypothesis that IPD3 function is partially redundant and, thus, additional genetic components must exist that have analogous functions to IPD3. This explains why mutations in an essential component of the Sym pathway have defects at late stages of the symbiotic interactions.

Phylogenetic perspectives on the origins of nodulation

Recent refinements to the phylogeny of rosid angiosperms support the conclusion that nodulation has evolved several times in the so-called N(2)-fixing clade (NFC), and provide dates for these origins. The hypothesized predisposition that enabled the evolution of nodulation occurred approximately 100 million years ago (MYA), was retained in the various lineages that radiated rapidly shortly thereafter, and was functional in its non-nodulation role for at least an additional 30 million years in each nodulating lineage. Legumes radiated rapidly shortly after their origin approximately 60 MYA, and nodulation most likely evolved several times during this radiation. The major lineages of papilionoid legumes diverged close to the time of origin of nodulation, accounting for the diversity of nodule biology in the group. Nodulation symbioses exemplify the concept of "deep homology," sharing various homologous components across nonhomologous origins of nodulation, largely due to recruitment from existing functions, notably the older arbuscular mycorrhizal symbiosis. Although polyploidy may have played a role in the origin of papilionoid legume nodules, it did not do so in other legumes, nor did the prerosid whole-genome triplication lead directly to the predisposition of nodulation.

How does phosphate status influence the development of the arbuscular mycorrhizal symbiosis?

Most terrestrial plant roots form mutualistic symbiosis with soil-borne arbuscular mycorrhizal fungi (AMF), a characteristic feature of which is nutrient exchange between the two symbiotic partners. Phosphate (Pi) is the main benefit the host plants acquired from the AMF. It has long been a common realization that high Pi supply could suppress the AMF development. However, the direct molecular regulatory mechanisms underlying this plant directed suppression are lacking. Here, we reviewed the recent work providing the evidences that high Pi supply induces transcriptional alteration, leading to the inhibition of AMF development at different stages of AM symbiosis, and gave our view on potential cross-talk among Pi starvation, AM as well as phytohormone signaling.

Lotus japonicus symRK-14 uncouples the cortical and epidermal symbiotic program

SYMRK is a leucine-rich-repeat (LRR)-receptor kinase that mediates intracellular symbioses of legumes with rhizobia and arbuscular mycorrhizal fungi. It participates in signalling events that lead to epidermal calcium spiking, an early cellular response that is typically considered as central for intracellular accommodation and nodule organogenesis. Here, we describe the Lotus japonicus symRK-14 mutation that alters a conserved GDPC amino-acid sequence in the SYMRK extracellular domain. Normal infection of the epidermis by fungal or bacterial symbionts was aborted in symRK-14. Likewise, epidermal responses of symRK-14 to bacterial signalling, including calcium spiking, NIN gene expression and infection thread formation, were significantly reduced. In contrast, no major negative effects on the formation of nodule primordia and cortical infection were detected. Cumulatively, our data show that the symRK-14 mutation uncouples the epidermal and cortical symbiotic program, while indicating that the SYMRK extracellular domain participates in transduction of non-equivalent signalling events. The GDPC sequence was found to be highly conserved in LRR-receptor kinases in legumes and non-legumes, including the evolutionarily distant bryophytes. Conservation of the GDPC sequence in nearly one-fourth of LRR-receptor-like kinases in the genome of Arabidopsis thaliana suggests, however, that this sequence might also play an important non-symbiotic function in this plant.

AM fungal exudates activate MAP kinases in plant cells in dependence from cytosolic Ca(2+) increase

The molecular dialogue occurring prior to direct contact between the fungal and plant partners of arbuscular-mycorrhizal (AM) symbioses begins with the release of fungal elicitors, so far only partially identified chemically, which can activate specific signaling pathways in the host plant. We show here that the activation of MAPK is also induced by exudates of germinating spores of Gigaspora margarita in cultured cells of the non-leguminous species tobacco (Nicotiana tabacum), as well as in those of the model legume Lotus japonicus. MAPK activity peaked about 15 min after the exposure of the host cells to the fungal exudates (FE). FE were also responsible for a rapid and transient increase in free cytosolic Ca(2+) in Nicotiana plumbaginifolia and tobacco cells, and pre-treatment with a Ca(2+)-channel blocker (La(3+)) showed that in these cells, MAPK activation was dependent on the cytosolic Ca(2+) increase. A partial dependence of MAPK activity on the common Sym pathway could be demonstrated for a cell line of L. japonicus defective for LjSym4 and hence unable to establish an AM symbiosis. Our results show that MAPK activation is triggered by an FE-induced cytosolic Ca(2+) transient, and that a Sym genetic determinant acts to modulate the intensity and duration of this activity.

Nuclear membranes control symbiotic calcium signaling of legumes

Nuclear-associated oscillations in calcium act as a secondary messenger in the symbiotic signaling pathway of legumes. These are decoded by a nuclear-localized calcium and calmodulin-dependent protein kinase, the activation of which is sufficient to drive downstream responses. This implies that the calcium oscillations within the nucleus are the predominant signals for legume symbiosis. However, the mechanisms that allow targeted release of calcium in the nuclear region have not been defined. Here we show that symbiosis-induced calcium changes occur in both the nucleoplasm and the perinuclear cytoplasm and seem to originate from the nuclear membranes. Reaction diffusion simulations suggest that spike generation within the nucleoplasm is not possible through transmission of a calcium wave from the cytoplasm alone and that calcium is likely to be released across the inner nuclear membrane to allow nuclear calcium changes. In agreement with this, we found that the cation channel DMI1, which is essential for symbiotic calcium oscillations, is preferentially located on the inner nuclear membrane, implying an essential function for the inner nuclear membrane in symbiotic calcium signaling. Furthermore, a sarco/endoplasmic reticulum calcium ATPase (SERCA) essential for symbiotic calcium oscillations is targeted to the inner nuclear membrane, as well as the outer nuclear membrane and endoplasmic reticulum (ER). We propose that release of calcium across the inner nuclear membrane allows targeted release of the ER calcium store, and efficient reloading of this calcium store necessitates the capture of calcium from the nucleoplasm and nuclear-associated cytoplasm.

The rules of engagement in the legume-rhizobial symbiosis

Rhizobial bacteria enter a symbiotic association with leguminous plants, resulting in differentiated bacteria enclosed in intracellular compartments called symbiosomes within nodules on the root. The nodules and associated symbiosomes are structured for efficient nitrogen fixation. Although the interaction is beneficial to both partners, it comes with rigid rules that are strictly enforced by the plant. Entry into root cells requires appropriate recognition of the rhizobial Nod factor signaling molecule, and this recognition activates a series of events, including polarized root-hair tip growth, invagination associated with bacterial infection, and the promotion of cell division in the cortex leading to the nodule meristem. The plant's command of the infection process has been highlighted by its enforcement of terminal differentiation upon the bacteria within nodules of some legumes, and this can result in a loss of bacterial viability while permitting effective nitrogen fixation. Here, we review the mechanisms by which the plant allows bacterial infection and promotes the formation of the nodule, as well as the details of how this intimate association plays out inside the cells of the nodule where a complex interchange of metabolites and regulatory peptides force the bacteria into a nitrogen-fixing organelle-like state.

Regulation of signal transduction and bacterial infection during root nodule symbiosis

Among plant-microbe interactions, root nodule symbiosis is one of the most important beneficial interactions providing legume plants with nitrogenous compounds. Over the past years a number of genes required for root nodule symbiosis has been identified but most recently great advances have been made to dissect signalling pathways and molecular interactions triggered by a set of receptor-like kinases. Genetic and biochemical approaches have not only provided evidence for the cross talk between bacterial infection of the host plant and organogenesis of a root nodule but also gained insights into dynamic regulation processes underlying successful infection events. Here, we summarise recent progress in the understanding of molecular mechanisms that regulate and trigger cellular signalling cascades during this mutualistic interaction.

Lipo-chitooligosaccharide signaling in endosymbiotic plant-microbe interactions

The arbuscular mycorrhizal (AM) and the rhizobia-legume (RL) root endosymbioses are established as a result of signal exchange in which there is mutual recognition of diffusible signals produced by plant and microbial partners. It was discovered 20 years ago that the key symbiotic signals produced by rhizobial bacteria are lipo-chitooligosaccharides (LCO), called Nod factors. These LCO are perceived via lysin-motif (LysM) receptors and activate a signaling pathway called the common symbiotic pathway (CSP), which controls both the RL and the AM symbioses. Recent work has established that an AM fungus, Glomus intraradices, also produces LCO that activate the CSP, leading to induction of gene expression and root branching in Medicago truncatula. These Myc-LCO also stimulate mycorrhization in diverse plants. In addition, work on the nonlegume Parasponia andersonii has shown that a LysM receptor is required for both successful mycorrhization and nodulation. Together these studies show that structurally related signals and the LysM receptor family are key components of both nodulation and mycorrhization. LysM receptors are also involved in the perception of chitooligosaccharides (CO), which are derived from fungal cell walls and elicit defense responses and resistance to pathogens in diverse plants. The discovery of Myc-LCO and a LysM receptor required for the AM symbiosis, therefore, not only raises questions of how legume plants discriminate fungal and bacterial endosymbionts but also, more generally, of how plants discriminate endosymbionts from pathogenic microorganisms using structurally related LCO and CO signals and of how these perception mechanisms have evolved.

Successful joint ventures of plants: arbuscular mycorrhiza and beyond

Among the oldest symbiotic associations of plants are arbuscular mycorrhiza (AM) with fungi of the phylum Glomeromycota. Although many of the symbiotic signaling components have been identified on the side of the plant, AM fungi have long evaded genetic analysis owing to their strict biotrophy and their exceptional genetics. Recently, the identification of the fungal symbiosis signal (Myc factor) and of a corresponding Myc factor receptor, and new insights into AM fungal genetics, have opened new avenues to address early communication and functional aspects of AM symbiosis. These advances will pave the way for breeding programs towards adapted AM fungi for crop production, and will shed light on the ecology and evolution of this remarkably successful symbiosis.

Activation of a Lotus japonicus subtilase gene during arbuscular mycorrhiza is dependent on the common symbiosis genes and two cis-active promoter regions

The subtilisin-like serine protease SbtM1 is strongly and specifically induced during arbuscular mycorrhiza (AM) symbiosis in Lotus japonicus. Another subtilase gene, SbtS, is induced during early stages of nodulation and AM. Transcript profiling in plant symbiosis mutants revealed that the AM-induced expression of SbtM1 and the gene family members SbtM3 and SbtM4 is dependent on the common symbiosis pathway, whereas an independent pathway contributes to the activation of SbtS. We used the specific spatial expression patterns of SbtM1 promoter β-d-glucuronidase (GUS) fusions to isolate cis elements that confer AM responsiveness. A promoter deletion and substitution analysis defined two cis regions (region I and II) in the SbtM1 promoter necessary for AM-induced GUS activity. 35S minimal promoter fusions revealed that either of the two regions is sufficient for AM responsiveness when tested in tandem repeat arrangement. Sequence-related regions were found in the promoters of AM-induced subtilase genes in Medicago truncatula and rice, consistent with an ancient origin of these elements predating the divergence of the angiosperms.

Invasion by invitation: rhizobial infection in legumes

Nodulation of legume roots typically begins with rhizobia attaching to the tip of a growing root-hair cell. The attached rhizobia secrete Nod factors (NF), which are perceived by the plant. This initiates a series of preinfection events that include cytoskeletal rearrangements, curling at the root-hair tip, and formation of radially aligned cytoplasmic bridges called preinfection threads (PIT) in outer cortical cells. Within the root-hair curl, an infection pocket filled with bacteria forms, from which originates a tubular invagination of cell wall and membrane called an infection thread (IT). IT formation is coordinated with nodule development in the underlying root cortex tissues. The IT extends from the infection pocket down through the root hair and into the root cortex, where it passes through PIT and eventually reaches the nascent nodule. As the IT grows, it is colonized by rhizobia that are eventually released into cells within the nodule, where they fix nitrogen. NF can also induce cortical root hairs that appear to originate from PIT and can become infected like normal root hairs. Several genes involved in NF signaling and some of the downstream transcription factors required for infection have been characterized. More recently, several genes with direct roles in infection have been identified, some with roles in actin rearrangement and others with possible roles in protein turnover and secretion. This article provides an overview of the infection process, including the roles of NF signaling, actin, and calcium and the influence of the hormones ethylene and cytokinin.

Function and evolution of nodulation genes in legumes

Root nodule (RN) symbiosis has a unique feature in which symbiotic bacteria fix atmospheric nitrogen. The symbiosis is established with a limited species of land plants, including legumes. How RN symbiosis evolved is still a mystery, but recent findings on legumes genes that are necessary for RN symbiosis may give us a clue.

Germinating spore exudates from arbuscular mycorrhizal fungi: molecular and developmental responses in plants and their regulation by ethylene

Arbuscular mycorrhizal (AM) fungi stimulate root development and induce expression of mycorrhization-specific genes in both eudicots and monocots. Diffusible factors released by AM fungi have been shown to elicit similar responses in Medicago truncatula. Colonization of roots by AM fungi is inhibited by ethylene. We compared the effects of germinating spore exudates (GSE) from Glomus intraradices in monocots and in eudicots, their genetic control, and their regulation by ethylene. GSE modify root architecture and induce symbiotic gene expression in both monocots and eudicots. The genetic regulation of root architecture and gene expression was analyzed using M. truncatula and rice symbiotic mutants. These responses are dependent on the common symbiotic pathway as well as another uncharacterized pathway. Significant differences between monocots and eudicots were observed in the genetic control of plant responses to GSE. However, ethylene inhibits GSE-induced symbiotic gene expression and root development in both groups. Our results indicate that GSE signaling shares similarities and differences in monocots versus eudicots, that only a subset of AM signaling pathways has been co-opted in legumes for the establishment of root nodulation with rhizobia, and that regulation of these pathways by ethylene is a feature conserved across higher land plants.

Common and not so common symbiotic entry

Great advances have been made in our understanding of the host plant's common symbiosis functions, which in legumes mediate intracellular accommodation of both nitrogen-fixing bacteria and arbuscular mycorrhiza (AM) fungi. However, it has become apparent that additional plant genes are required specifically for bacterial entry inside the host root. In this opinion article, we consider Lotus japonicus nap1 and pir1 symbiotic mutants within the context of other deleterious mutations that impair an intracellular accommodation of bacteria but have no impact on the colonization of roots by AM fungi. We highlight a clear delineation of early signaling events during bacterial versus AM symbioses while suggesting a more intricate origin of the plant's ability for intracellular accommodation of bacteria.