Comparative analysis of the tubulin cytoskeleton organization in nodules of Medicago truncatula and Pisum sativum: bacterial release and bacteroid positioning correlate with characteristic microtubule rearrangements

Cellular basis of legume-rhizobium symbiosis

The legume-rhizobium symbiosis represents the most important system for terrestrial biological nitrogen fixation on land. Efficient nitrogen fixation during this symbiosis depends on successful rhizobial infection and complete endosymbiosis, which are achieved by complex cellular events including cell-wall remodeling, cytoskeletal reorganizations, and extensive membrane expansion and trafficking. In this review, we explore the dynamic remodeling of the plant-specific cell wall-membrane system-cytoskeleton (WMC) continuum during symbiotic nitrogen fixation. We focus on key processes linked to efficient nitrogen fixation, including rhizobial uptake, infection thread formation and elongation, rhizobial droplet release, cytoplasmic bridge formation, and rhizobial endosymbiosis. Additionally, we discuss the advanced techniques for investigating the cellular basis of root-nodule symbiosis and provide insights into the unsolved mysteries of robust symbiotic nitrogen fixation.

Effects of Elevated Temperature on Pisum sativum Nodule Development: I-Detailed Characteristic of Unusual Apical Senescence

Despite global warming, the influence of heat on symbiotic nodules is scarcely studied. In this study, the effects of heat stress on the functioning of nodules formed by Rhizobium leguminosarum bv. viciae strain 3841 on pea (Pisum sativum) line SGE were analyzed. The influence of elevated temperature was analyzed at histological, ultrastructural, and transcriptional levels. As a result, an unusual apical pattern of nodule senescence was revealed. After five days of exposure, a senescence zone with degraded symbiotic structures was formed in place of the distal nitrogen fixation zone. There was downregulation of various genes, including those associated with the assimilation of fixed nitrogen and leghemoglobin. After nine days, the complete destruction of the nodules was demonstrated. It was shown that nodule recovery was possible after exposure to elevated temperature for 3 days but not after 5 days (which coincides with heat wave duration). At the same time, the exposure of plants to optimal temperature during the night leveled the negative effects. Thus, the study of the effects of elevated temperature on symbiotic nodules using a well-studied pea genotype and Rhizobium strain led to the discovery of a novel positional response of the nodule to heat stress.

Cytoskeleton as a roadmap navigating rhizobia to establish symbiotic root nodulation in legumes

Legumes enter into symbiotic associations with soil nitrogen-fixing rhizobia, culminating in the creation of new organs, root nodules. This complex process relies on chemical and physical interaction between legumes and rhizobia, including early signalling events informing the host legume plant of a potentially beneficial microbe and triggering the nodulation program. The great significance of this plant-microbe interaction rests upon conversion of atmospheric dinitrogen not accessible to plants into a biologically active form of ammonia available to plants. The plant cytoskeleton consists in a highly dynamic network and undergoes rapid remodelling upon sensing various developmental and environmental cues, including response to attachment, internalization, and accommodation of rhizobia in plant root and nodule cells. This dynamic nature is governed by cytoskeleton-associated proteins that modulate cytoskeletal behaviour depending on signal perception and transduction. Precisely localized cytoskeletal rearrangements are therefore essential for the uptake of rhizobia, their targeted delivery, and establishing beneficial root nodule symbiosis. This review summarizes current knowledge about rhizobia-dependent rearrangements and functions of the cytoskeleton in legume roots and nodules. General patterns and nodule type-, nodule stage-, and species-specific aspects of actin filaments and microtubules remodelling are discussed. Moreover, emerging evidence is provided about fine-tuning the root nodulation process through cytoskeleton-associated proteins. We also consider future perspectives on dynamic localization studies of the cytoskeleton during early symbiosis utilizing state of the art molecular and advanced microscopy approaches. Based on acquired detailed knowledge of the mutualistic interactions with microbes, these approaches could contribute to broader biotechnological crop improvement.

Comparison of the Formation of Plant-Microbial Interface in Pisum sativum L. and Medicago truncatula Gaertn. Nitrogen-Fixing Nodules

Different components of the symbiotic interface play an important role in providing positional information during rhizobial infection and nodule development: successive changes in cell morphology correspond to subsequent changes in the molecular architecture of the apoplast and the associated surface structures. The localisation and distribution of pectins, xyloglucans, and cell wall proteins in symbiotic nodules of Pisum sativum and Medicago truncatula were studied using immunofluorescence and immunogold analysis in wild-type and ineffective mutant nodules. As a result, the ontogenetic changes in the symbiotic interface in the nodules of both species were described. Some differences in the patterns of distribution of cell wall polysaccharides and proteins between wild-type and mutant nodules can be explained by the activation of defence reaction or premature senescence in mutants. The absence of fucosylated xyloglucan in the cell walls in the P. sativum nodules, as well as its predominant accumulation in the cell walls of uninfected cells in the M. truncatula nodules, and the presence of the rhamnogalacturonan I (unbranched) backbone in meristematic cells in P. sativum can be attributed to the most striking species-specific features of the symbiotic interface.

Plant-Environment Response Pathway Regulation Uncovered by Investigating Non-Typical Legume Symbiosis and Nodulation

Nitrogen is an essential element needed for plants to survive, and legumes are well known to recruit rhizobia to fix atmospheric nitrogen. In this widely studied symbiosis, legumes develop specific structures on the roots to host specific symbionts. This review explores alternate nodule structures and their functions outside of the more widely studied legume-rhizobial symbiosis, as well as discussing other unusual aspects of nodulation. This includes actinorhizal-Frankia, cycad-cyanobacteria, and the non-legume Parasponia andersonii-rhizobia symbioses. Nodules are also not restricted to the roots, either, with examples found within stems and leaves. Recent research has shown that legume-rhizobia nodulation brings a great many other benefits, some direct and some indirect. Rhizobial symbiosis can lead to modifications in other pathways, including the priming of defence responses, and to modulated or enhanced resistance to biotic and abiotic stress. With so many avenues to explore, this review discusses recent discoveries and highlights future directions in the study of nodulation.

Microtubule Regulation in Plants: From Morphological Development to Stress Adaptation

Microtubules (MTs) are essential elements of the eukaryotic cytoskeleton and are critical for various cell functions. During cell division, plant MTs form highly ordered structures, and cortical MTs guide the cell wall cellulose patterns and thus control cell size and shape. Both are important for morphological development and for adjusting plant growth and plasticity under environmental challenges for stress adaptation. Various MT regulators control the dynamics and organization of MTs in diverse cellular processes and response to developmental and environmental cues. This article summarizes the recent progress in plant MT studies from morphological development to stress responses, discusses the latest techniques applied, and encourages more research into plant MT regulation.

Effects of Phytotoxic Nonenolides, Stagonolide A and Herbarumin I, on Physiological and Biochemical Processes in Leaves and Roots of Sensitive Plants

Phytotoxic macrolides attract attention as prototypes of new herbicides. However, their mechanisms of action (MOA) on plants have not yet been elucidated. This study addresses the effects of two ten-membered lactones, stagonolide A (STA) and herbarumin I (HBI) produced by the fungus Stagonospora cirsii, on Cirsium arvense, Arabidopsis thaliana and Allium cepa. Bioassay of STA and HBI on punctured leaf discs of C. arvense and A. thaliana was conducted at a concentration of 2 mg/mL to evaluate phenotypic responses, the content of pigments, electrolyte leakage from leaf discs, the level of reactive oxygen species, Hill reaction rate, and the relative rise in chlorophyll a fluorescence. The toxin treatments resulted in necrotic and bleached leaf lesions in the dark and in the light, respectively. In the light, HBI treatment caused the drop of carotenoids content in leaves on both plants. The electrolyte leakage caused by HBI was light-dependent, in contrast with that caused by STA. Both compounds induced light-independent peroxide generation in leaf cells but did not affect photosynthesis 6 h after treatment. STA (10 µg/mL) caused strong disorders in root cells of A. thaliana leading to the complete dissipation of the mitochondrial membrane potential one hour post treatment, as well as DNA fragmentation and disappearance of acidic vesicles in the division zone after 8 h; the effects of HBI (50 µg/mL) were much milder. Furthermore, STA was found to inhibit mitosis but did not affect the cytoskeleton in cells of root tips of A. cepa and C. arvense, respectively. Finally, STA was supposed to inhibit the intracellular vesicular traffic from the endoplasmic reticulum to the Golgi apparatus, thus interfering with mitosis. HBI is likely to have another main MOA, probably inhibiting the biosynthesis of carotenoids.

A legume kinesin controls vacuole morphogenesis for rhizobia endosymbiosis

Symbioses between legumes and rhizobia require establishment of the plant-derived symbiosome membrane, which surrounds the rhizobia and accommodates the symbionts by providing an interface for nutrient and signal exchange. The host cytoskeleton and endomembrane trafficking systems play central roles in the formation of a functional symbiotic interface for rhizobia endosymbiosis; however, the underlying mechanisms remain largely unknown. Here we demonstrate that the nodulation-specific kinesin-like calmodulin-binding protein (nKCBP), a plant-specific microtubule-based kinesin motor, controls central vacuole morphogenesis in symbiotic cells in Medicago truncatula. Phylogenetic analysis further indicated that nKCBP duplication occurs solely in legumes of the clade that form symbiosomes. Knockout of nKCBP results in central vacuole deficiency, defective symbiosomes and abolished nitrogen fixation. nKCBP decorates linear particles along microtubules, and crosslinks microtubules with the actin cytoskeleton, to control central vacuole formation by modulating vacuolar vesicle fusion in symbiotic cells. Together, our findings reveal that rhizobia co-opted nKCBP to achieve symbiotic interface formation by regulating cytoskeletal assembly and central vacuole morphogenesis during nodule development.

Host nuclear repositioning and actin polarization towards the site of penetration precedes fungal ingress during compatible pea-powdery mildew interactions

MAIN CONCLUSION

Actin polarization and actin-driven host nuclear movement towards the fungal penetration site facilitates successful host colonization during compatible pea-Erysiphe pisi interactions. Proper nuclear positioning in plant cells is crucial for developmental processes and response to (a)biotic stimuli. During plant-fungal interactions, the host nucleus moves toward the infection site, a process regulated by the plant cytoskeleton. Notably, rearrangement of the plant cytoskeleton is one of the earliest cellular responses to pathogen invasion and is known to impact penetration efficiency. Yet, the connection between host nuclear movement and fungal ingress is still elusive, particularly in legumes. Here, we investigated the host nuclear dynamics during compatible interactions between Pisum sativum (pea) and the adapted powdery mildew (PM) fungus Erysiphe pisi to gain insights into the functional relevance of PM-induced nuclear movement in legumes. We show that the host nucleus moves towards the fungal appressorium before penetration and becomes associated with the primary haustorium. However, the nucleus migrates away from the primary infection site as the infection progresses toward colony expansion and sporulation. Treatment of pea leaves with the actin-polymerization inhibitor, cytochalasin D, abolished host nuclear movement towards the fungal penetration site and restricted PM growth. In contrast, treatment with oryzalin, a microtubule-polymerization inhibitor, had no effect. In addition to nuclear movement, strong polarization of host actin filaments towards the site of appressorial contact was evident at early infection stages. Our results suggest that actin focusing mediates host nuclear movement to the fungal penetration site and facilitates successful colonization during compatible pea-PM interactions.

Tubulin Cytoskeleton Organization in Cells of Determinate Nodules

Plant cell differentiation is based on rearrangements of the tubulin cytoskeleton; this is also true for symbiotic nodules. Nevertheless, although for indeterminate nodules (with a long-lasting meristem) the organization of microtubules during nodule development has been studied for various species, for determinate ones (with limited meristem activity) such studies are rare. Here, we investigated bacteroid morphology and dynamics of the tubulin cytoskeleton in determinate nodules of four legume species: Glycine max, Glycine soja, Phaseolus vulgaris, and Lotus japonicus. The most pronounced differentiation of bacteroids was observed in G. soja nodules. In meristematic cells in incipient nodules of all analyzed species, the organization of both cortical and endoplasmic microtubules was similar to that described for meristematic cells of indeterminate nodules. In young infected cells in developing nodules of all four species, cortical microtubules formed irregular patterns (microtubules were criss-crossed) and endoplasmic ones were associated with infection threads and infection droplets. Surprisingly, in uninfected cells the patterns of cortical microtubules differed in nodules of G. max and G. soja on the one hand, and P. vulgaris and L. japonicus on the other. The first two species exhibited irregular patterns, while the remaining two exhibited regular ones (microtubules were oriented transversely to the longitudinal axis of cell) that are typical for uninfected cells of indeterminate nodules. In contrast to indeterminate nodules, in mature determinate nodules of all four studied species, cortical microtubules formed a regular pattern in infected cells. Thus, our analysis revealed common patterns of tubulin cytoskeleton in the determinate nodules of four legume species, and species-specific differences were associated with the organization of cortical microtubules in uninfected cells. When compared with indeterminate nodules, the most pronounced differences were associated with the organization of cortical microtubules in nitrogen-fixing infected cells. The revealed differences indicated a possible transition during evolution of infected cells from anisotropic growth in determinate nodules to isodiametric growth in indeterminate nodules. It can be assumed that this transition provided an evolutionary advantage to those legume species with indeterminate nodules, enabling them to host symbiosomes in their infected cells more efficiently.

The Regulation of Pea (Pisum sativum L.) Symbiotic Nodule Infection and Defense Responses by Glutathione, Homoglutathione, and Their Ratio

In this study, the roles of glutathione (GSH), homoglutathione (hGSH), and their ratio in symbiotic nodule development and functioning, as well as in defense responses accompanying ineffective nodulation in pea (Pisum sativum) were investigated. The expression of genes involved in (h)GSH biosynthesis, thiol content, and localization of the reduced form of GSH were analyzed in nodules of wild-type pea plants and mutants sym33-3 (weak allele, "locked" infection threads, occasional bacterial release, and defense reactions) and sym33-2 (strong allele, "locked" infection threads, defense reactions), and sym40-1 (abnormal bacteroids, oxidative stress, early senescence, and defense reactions). The effects of (h)GSH depletion and GSH treatment on nodule number and development were also examined. The GSH:hGSH ratio was found to be higher in nodules than in uninoculated roots in all genotypes analyzed, with the highest value being detected in wild-type nodules. Moreover, it was demonstrated, that a hGSHS-to-GSHS switch in gene expression in nodule tissue occurs only after bacterial release and leads to an increase in the GSH:hGSH ratio. Ineffective nodules showed variable GSH:hGSH ratios that correlated with the stage of nodule development. Changes in the levels of both thiols led to the activation of defense responses in nodules. The application of a (h)GSH biosynthesis inhibitor disrupted the nitrogen fixation zone in wild-type nodules, affected symbiosome formation in sym40-1 mutant nodules, and meristem functioning and infection thread growth in sym33-3 mutant nodules. An increase in the levels of both thiols following GSH treatment promoted both infection and extension of defense responses in sym33-3 nodules, whereas a similar increase in sym40-1 nodules led to the formation of infected cells resembling wild-type nitrogen-fixing cells and the disappearance of an early senescence zone in the base of the nodule. Meanwhile, an increase in hGSH levels in sym40-1 nodules resulting from GSH treatment manifested as a restriction of infection similar to that seen in untreated sym33-3 nodules. These findings indicated that a certain level of thiols is required for proper symbiotic nitrogen fixation and that changes in thiol content or the GSH:hGSH ratio are associated with different abnormalities and defense responses.

Structure and Development of the Legume-Rhizobial Symbiotic Interface in Infection Threads

The intracellular infection thread initiated in a root hair cell is a unique structure associated with Rhizobium-legume symbiosis. It is characterized by inverted tip growth of the plant cell wall, resulting in a tunnel that allows invasion of host cells by bacteria during the formation of the nitrogen-fixing root nodule. Regulation of the plant-microbial interface is essential for infection thread growth. This involves targeted deposition of the cell wall and extracellular matrix and tight control of cell wall remodeling. This review describes the potential role of different actors such as transcription factors, receptors, and enzymes in the rearrangement of the plant-microbial interface and control of polar infection thread growth. It also focuses on the composition of the main polymers of the infection thread wall and matrix and the participation of reactive oxygen species (ROS) in the development of the infection thread. Mutant analysis has helped to gain insight into the development of host defense reactions. The available data raise many new questions about the structure, function, and development of infection threads.

General Patterns and Species-Specific Differences in the Organization of the Tubulin Cytoskeleton in Indeterminate Nodules of Three Legumes

The tubulin cytoskeleton plays an important role in establishing legume–rhizobial symbiosis at all stages of its development. Previously, tubulin cytoskeleton organization was studied in detail in the indeterminate nodules of two legume species, Pisum sativum and Medicago truncatula. General as well as species-specific patterns were revealed. To further the understanding of the formation of general and species-specific microtubule patterns in indeterminate nodules, the tubulin cytoskeleton organization was studied in three legume species (Vicia sativa, Galega orientalis, and Cicer arietinum). It is shown that these species differ in the shape and size of rhizobial cells (bacteroids). Immunolocalization of microtubules revealed the universality of cortical and endoplasmic microtubule organization in the meristematic cells, infected cells of the infection zone, and uninfected cells in nodules of the three species. However, there are differences in the endoplasmic microtubule organization in nitrogen-fixing cells among the species, as confirmed by quantitative analysis. It appears that the differences are linked to bacteroid morphology (both shape and size).

Overexpression of alfalfa SIMK promotes root hair growth, nodule clustering and shoot biomass production

Nitrogen-fixing rhizobia and legumes have developed complex mutualistic mechanism that allows to convert atmospheric nitrogen into ammonia. Signalling by mitogen-activated protein kinases (MAPKs) seems to be involved in this symbiotic interaction. Previously, we reported that stress-induced MAPK (SIMK) shows predominantly nuclear localization in alfalfa root epidermal cells. Nevertheless, SIMK is activated and relocalized to the tips of growing root hairs during their development. SIMK kinase (SIMKK) is a well-known upstream activator of SIMK. Here, we characterized production parameters of transgenic alfalfa plants with genetically manipulated SIMK after infection with Sinorhizobium meliloti. SIMKK RNAi lines, causing strong downregulation of both SIMKK and SIMK, showed reduced root hair growth and lower capacity to form infection threads and nodules. In contrast, constitutive overexpression of GFP-tagged SIMK promoted root hair growth as well as infection thread and nodule clustering. Moreover, SIMKK and SIMK downregulation led to decrease, while overexpression of GFP-tagged SIMK led to increase of biomass in above-ground part of plants. These data suggest that genetic manipulations causing downregulation or overexpression of SIMK affect root hair, nodule and shoot formation patterns in alfalfa, and point to the new biotechnological potential of this MAPK.

New methods for confocal imaging of infection threads in crop and model legumes

BACKGROUND

The formation of infection threads in the symbiotic infection of rhizobacteria in legumes is a unique, fascinating, and poorly understood process. Infection threads are tubes of cell wall material that transport rhizobacteria from root hair cells to developing nodules in host roots. They form in a type of reverse tip-growth from an inversion of the root hair cell wall, but the mechanism driving this growth is unknown, and the composition of the thread wall remains unclear. High resolution, 3-dimensional imaging of infection threads, and cell wall component specific labelling, would greatly aid in our understanding of the nature and development of these structures. To date, such imaging has not been done, with infection threads typically imaged by GFP-tagged rhizobia within them, or histochemically in thin sections.

RESULTS

We have developed new methods of imaging infection threads using novel and traditional cell wall fluorescent labels, and laser confocal scanning microscopy. We applied a new Periodic Acid Schiff (PAS) stain using rhodamine-123 to the labelling of whole cleared infected roots of Medicago truncatula; which allowed for imaging of infection threads in greater 3D detail than had previously been achieved. By the combination of the above method and a calcofluor-white counter-stain, we also succeeded in labelling infection threads and plant cell walls separately, and have potentially discovered a way in which the infection thread matrix can be visualized.

CONCLUSIONS

Our methods have made the imaging and study of infection threads more effective and informative, and present exciting new opportunities for future research in the area.

Visualization of cortical microtubule networks in plant cells by live imaging and immunostaining

Cortical microtubules (CMTs) play pivotal roles during plant cell growth and division. The organization of CMTs undergoes important changes during different cellular and developmental processes. Here, we describe two methods for the visualization of CMT organization in plant cells using confocal laser scanning microscopy. CMT networks in the outer tissue layers can be directly visualized by live imaging of a fluorescent reporter line, and a protocol combining sectioning and immunostaining is applied for visualization of CMTs throughout tissues.

For complete details on the use and execution of this protocol, please refer to Zhao et al. (2020).

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Methods for live imaging of cortical microtubules in outer cell layers of plant tissues

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Agarose-gel embedding and sectioning method for exposing inner plant cells

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Methods for immunostaining of cortical microtubules in inner cells of plant tissues

Symbiotic Regulatory Genes Controlling Nodule Development in Pisum sativum L

Analyses of natural variation and the use of mutagenesis and molecular-biological approaches have revealed 50 symbiotic regulatory genes in pea (Pisum sativum L.). Studies of genomic synteny using model legumes, such as Medicago truncatula Gaertn. and Lotus japonicus (Regel) K. Larsen, have identified the sequences of 15 symbiotic regulatory genes in pea. These genes encode receptor kinases, an ion channel, a calcium/calmodulin-dependent protein kinase, transcription factors, a metal transporter, and an enzyme. This review summarizes and describes mutant alleles, their phenotypic manifestations, and the functions of all identified symbiotic regulatory genes in pea. Some examples of gene interactions are also given. In the review, all mutant alleles in genes with identified sequences are designated and still-unidentified symbiotic regulatory genes of great interest are considered. The identification of these genes will help elucidate additional components involved in infection thread growth, nodule primordium development, bacteroid differentiation and maintenance, and the autoregulation of nodulation. The significance of symbiotic mutants of pea as extremely fruitful genetic models for studying nodule development and for comparative cell biology studies of legume nodules is clearly demonstrated. Finally, it is noted that many more sequences of symbiotic regulatory genes remain to be identified. Transcriptomics approaches and genome-wide sequencing could help address this challenge.

Rhizobial infection triggers systemic transport of endogenous RNAs between shoots and roots in soybean

Legumes have evolved a symbiotic relationship with rhizobial bacteria and their roots form unique nitrogen-fixing organs called nodules. Studies have shown that abiotic and biotic stresses alter the profile of gene expression and transcript mobility in plants. However, little is known about the systemic transport of RNA between roots and shoots in response to rhizobial infection on a genome-wide scale during the formation of legume-rhizobia symbiosis. In our study, we found that two soybean (Glycine max) cultivars, Peking and Williams, show a high frequency of single nucleotide polymorphisms; this allowed us to characterize the origin and mobility of transcripts in hetero-grafts of these two cultivars. We identified 4,552 genes that produce mobile RNAs in soybean, and found that rhizobial infection triggers mass transport of mRNAs between shoots and roots at the early stage of nodulation. The majority of these mRNAs are of relatively low abundance and their transport occurs in a selective manner in soybean plants. Notably, the mRNAs that moved from shoots to roots at the early stage of nodulation were enriched in many nodule-related responsive processes. Moreover, the transcripts of many known symbiosis-related genes that are induced by rhizobial infection can move between shoots and roots. Our findings provide a deeper understanding of endogenous RNA transport in legume-rhizobia symbiotic processes.

Mutational analysis indicates that abnormalities in rhizobial infection and subsequent plant cell and bacteroid differentiation in pea (Pisum sativum) nodules coincide with abnormal cytokinin responses and localization

BACKGROUND AND AIMS

Recent findings indicate that Nod factor signalling is tightly interconnected with phytohormonal regulation that affects the development of nodules. Since the mechanisms of this interaction are still far from understood, here the distribution of cytokinin and auxin in pea (Pisum sativum) nodules was investigated. In addition, the effect of certain mutations blocking rhizobial infection and subsequent plant cell and bacteroid differentiation on cytokinin distribution in nodules was analysed.

METHODS

Patterns of cytokinin and auxin in pea nodules were profiled using both responsive genetic constructs and antibodies.

KEY RESULTS

In wild-type nodules, cytokinins were found in the meristem, infection zone and apical part of the nitrogen fixation zone, whereas auxin localization was restricted to the meristem and peripheral tissues. We found significantly altered cytokinin distribution in sym33 and sym40 pea mutants defective in IPD3/CYCLOPS and EFD transcription factors, respectively. In the sym33 mutants impaired in bacterial accommodation and subsequent nodule differentiation, cytokinin localization was mostly limited to the meristem. In addition, we found significantly decreased expression of LOG1 and A-type RR11 as well as KNOX3 and NIN genes in the sym33 mutants, which correlated with low cellular cytokinin levels. In the sym40 mutant, cytokinins were detected in the nodule infection zone but, in contrast to the wild type, they were absent in infection droplets.

CONCLUSIONS

In conclusion, our findings suggest that enhanced cytokinin accumulation during the late stages of symbiosis development may be associated with bacterial penetration into the plant cells and subsequent plant cell and bacteroid differentiation.

Bacterial release is accompanied by ectopic accumulation of cell wall material around the vacuole in nodules of Pisum sativum sym33-3 allele encoding transcription factor PsCYCLOPS/PsIPD3

Pisum sativum symbiotic mutant SGEFix--2 carries the sym33-3 allele of the gene Sym33, encoding transcription factor PsCYCLOPS/PsIPD3. Previously, strong host cell defence reactions were identified in nodules of this mutant. In the present study, new manifestations of defence reactions were revealed in 28-day-old white nodules in which bacterial release had occurred. These nodules were investigated using histochemical staining of pectin and suberin and by immunogold localisation of three components of pectin: highly methyl-esterified homogalacturonan (HG) recognised by monoclonal antibody JIM7, low methyl-esterified HG recognised by JIM5 and linear (1-4)-β-D-galactan side-chain of rhamnogalacturonan I (RG I) recognised by LM5. In the mutant, but not in the wild-type, cell wall material was deposited around the vacuole in the uninfected cells, in cells containing infection threads and in the infected cells. The deposits around the vacuole were marked with JIM7 and LM5 antibodies but not with JIM5, suggesting that they contain newly formed cell wall material. Deposition was accompanied by suberin accumulation. This is the first report that deposition of cell wall material around the vacuole may be associated with the defence reaction in ineffective nodules. In addition, hypertrophic infection droplets labelled with JIM7 were identified. In the matrix of some infection threads, RG I recognised a pectic gel component. Callose deposits in the cell walls and in the walls of infection threads were occasionally observed. The observations suggest that an important function of transcriptional factor CYCLOPS/IPD3 is the suppression of defence reactions during establishment of the legume-rhizobial symbiosis.

Comparative analysis of remodelling of the plant-microbe interface in Pisum sativum and Medicago truncatula symbiotic nodules

Infection of host cells by nitrogen-fixing soil bacteria, known as rhizobia, involves the progressive remodelling of the plant-microbe interface. This process was examined by using monoclonal antibodies to study the subcellular localisation of pectins and arabinogalactan proteins (AGPs) in wild-type and ineffective nodules of Pisum sativum and Medicago truncatula. The highly methylesterified homogalacturonan (HG), detected by monoclonal antibody JIM7, showed a uniform localisation in the cell wall, regardless of the cell type in nodules of P. sativum and M. truncatula. Low methylesterified HG, recognised by JIM5, was detected mainly in the walls of infection threads in nodules of both species. The galactan side chain of rhamnogalacturonan I (RG-I), recognised by LM5, was present in the nodule meristem in both species and in the infection thread walls in P. sativum, but not in M. truncatula. The membrane-anchored AGP recognised by JIM1 was observed on the plasma membrane in nodules of P. sativum and M. truncatula. In P. sativum, the AGP epitope recognised by JIM1 was present on mature symbiosome membranes of wild-type nodules, but JIM1 labelling was absent from symbiosome membranes in the mutant Sprint-2Fix- (sym31) with undifferentiated bacteroids, suggesting a possible involvement of AGP in the maturation of symbiosomes. Thus, the common and species-specific traits of cell wall remodelling during nodule differentiation were demonstrated.

Symbiotic Performance of Sinorhizobium meliloti Lacking ppGpp Depends on the Medicago Host Species

Host specificity in the root-nodule symbiosis between legumes and rhizobia is crucial for the establishment of a successful interaction and ammonia provision to the plant. The specificity is mediated by plant-bacterial signal exchange during early stages of interaction. We observed that a Sinorhizobium meliloti mutant ∆relA, which is deficient in initiating the bacterial stringent response, fails to nodulate Medicago sativa (alfalfa) but successfully infects Medicago truncatula. We used biochemical, histological, transcriptomic, and imaging approaches to compare the behavior of the S. meliloti ∆relA mutant and wild type (WT) on the two plant hosts. ∆relA performed almost WT-like on M. truncatula, except for reduced nitrogen-fixation capacity and a disorganized positioning of bacteroids within nodule cells. In contrast, ∆relA showed impaired root colonization on alfalfa and failed to infect nodule primordia. Global transcriptome analyses of ∆relA cells treated with the alfalfa flavonoid luteolin and of mature nodules induced by the mutant on M. truncatula revealed normal nod gene expression but overexpression of exopolysaccharide biosynthesis genes and a slight suppression of plant defense-like reactions. Many RelA-dependent transcripts overlap with the hypo-osmolarity-related FeuP regulon or are characteristic of stress responses. Based on our findings, we suggest that RelA is not essential until the late stages of symbiosis with M. truncatula, in which it may be involved in processes that optimize nitrogen fixation.

Lateral Root Initiation in the Parental Root Meristem of Cucurbits: Old Players in a New Position

While in most higher plants, including the model system Arabidopsis thaliana, the formation of lateral root primordia is induced in the elongation zone of the parental root, in seven plant families, including Cucurbitaceae, an alternative root branching mechanism is established such that lateral roots are initiated directly in the apical meristem of the parental root. In Arabidopsis, the transcription factor GATA23 and MEMBRANE-ASSOCIATED KINASE REGULATOR4 (MAKR4) are involved in the gene regulatory network of lateral root initiation. Among all marker genes examined, these are the earliest known marker genes up-regulated by auxin during lateral root initiation. In this study, putative functional orthologs of Arabidopsis GATA23 and MAKR4 were identified in cucumber (Cucumis sativus) and squash (Cucurbita pepo). Both cucurbits contained 26 genes encoding GATA family transcription factors and only one MAKR4 gene. Phylogenetic and transcriptional analysis of up-regulation by auxin led to the identification of GATA23 putative functional orthologs in Cucurbitaceae - CpGATA24 and CsGATA24. In squash, CpMAKR4 was up-regulated by naphthylacetic acid (NAA) and, similar to MAKR4 in Arabidopsis, indole-3-butyric acid (IBA). A detailed analysis of the expression pattern of CpGATA24 and CpMAKR4 in squash roots from founder cell specification until emergence of lateral root primordia was carried out using promoter-fluorescent reporter gene fusions and confocal microscopy. Their expression was induced in the protoxylem, and then expanded to founder cells in the pericycle. Thus, while the overall expression pattern of these genes was significantly different from that in Arabidopsis, in founder cells their expression was induced in the same order as in Arabidopsis. Altogether, these findings suggest that in Cucurbitaceae the putative functional orthologs of GATA23 and MAKR4 might play a role in founder cell specification and primordium positioning during lateral root initiation. The role of the protoxylem in auxin transport as a trigger of founder cells specification and lateral root initiation is discussed.

The host actin cytoskeleton channels rhizobia release and facilitates symbiosome accommodation during nodulation in Medicago truncatula

In plants, the actin cytoskeleton plays a central role in regulating intracellular transport and trafficking in the endomembrane system. Work in legumes suggested that during nodulation, the actin cytoskeleton coordinates numerous cellular processes in the development of nitrogen-fixing nodules. However, we lacked live-cell visualizations demonstrating dynamic remodeling of the actin cytoskeleton during infection droplet release and symbiosome development. Here, we generated transgenic Medicago truncatula lines stably expressing the fluorescent actin marker ABD2-GFP, and utilized live-cell imaging to reveal the architecture and dynamics of the actin cytoskeleton during nodule development. Live-cell observations showed that different zones in nitrogen-fixing nodules exhibit distinct actin architectures and infected cells display five characteristic actin architectures during nodule development. Live-cell imaging combined with three-dimensional reconstruction demonstrated that dense filamentous-actin (F-actin) arrays channel the elongation of infection threads and the release of infection droplets, an F-actin network encircles freshly-released rhizobia, and short F-actin fragments and actin dots around radially distributed symbiosomes. Our findings suggest an important role of the actin cytoskeleton in infection droplet release, symbiosome development and maturation, and provide significant insight into the cellular mechanisms underlying nodule development and nitrogen fixation during legume-rhizobia interactions.

Role of a receptor-like kinase K1 in pea Rhizobium symbiosis development

MAIN CONCLUSION

The LysM receptor-like kinase K1 is involved in regulation of pea-rhizobial symbiosis development. The ability of the crop legume Pisum sativum L. to perceive the Nod factor rhizobial signals may depend on several receptors that differ in ligand structure specificity. Identification of pea mutants defective in two types of LysM receptor-like kinases (LysM-RLKs), SYM10 and SYM37, featuring different phenotypic manifestations and impaired at various stages of symbiosis development, corresponds well to this assumption. There is evidence that one of the receptor proteins involved in symbiosis initiation, SYM10, has an inactive kinase domain. This implies the presence of an additional component in the receptor complex, together with SYM10, that remains unknown. Here, we describe a new LysM-RLK, K1, which may serve as an additional component of the receptor complex in pea. To verify the function of K1 in symbiosis, several P. sativum non-nodulating mutants in the k1 gene were identified using the TILLING approach. Phenotyping revealed the blocking of symbiosis development at an appropriately early stage, strongly suggesting the importance of LysM-RLK K1 for symbiosis initiation. Moreover, the analysis of pea mutants with weaker phenotypes provides evidence for the additional role of K1 in infection thread distribution in the cortex and rhizobia penetration. The interaction between K1 and SYM10 was detected using transient leaf expression in Nicotiana benthamiana and in the yeast two-hybrid system. Since the possibility of SYM10/SYM37 complex formation was also shown, we tested whether the SYM37 and K1 receptors are functionally interchangeable using a complementation test. The interaction between K1 and other receptors is discussed.

The Symbiosome: Legume and Rhizobia Co-evolution toward a Nitrogen-Fixing Organelle?

In legume nodules, symbiosomes containing endosymbiotic rhizobial bacteria act as temporary plant organelles that are responsible for nitrogen fixation, these bacteria develop mutual metabolic dependence with the host legume. In most legumes, the rhizobia infect post-mitotic cells that have lost their ability to divide, although in some nodules cells do maintain their mitotic capacity after infection. Here, we review what is currently known about legume symbiosomes from an evolutionary and developmental perspective, and in the context of the different interactions between diazotroph bacteria and eukaryotes. As a result, it can be concluded that the symbiosome possesses organelle-like characteristics due to its metabolic behavior, the composite origin and differentiation of its membrane, the retargeting of host cell proteins, the control of microsymbiont proliferation and differentiation by the host legume, and the cytoskeletal dynamics and symbiosome segregation during the division of rhizobia-infected cells. Different degrees of symbiosome evolution can be defined, specifically in relation to rhizobial infection and to the different types of nodule. Thus, our current understanding of the symbiosome suggests that it might be considered a nitrogen-fixing link in organelle evolution and that the distinct types of legume symbiosomes could represent different evolutionary stages toward the generation of a nitrogen-fixing organelle.

Cell differentiation in nitrogen-fixing nodules hosting symbiosomes

The nitrogen-fixing nodule is a unique ecological niche for rhizobia, where microaerobic conditions support functioning of the main enzyme of nitrogen fixation, nitrogenase, which is highly sensitive to oxygen. To accommodate bacteria in a symbiotic nodule, the specialised infected cells increase in size owing to endoreduplication and are able to shelter thousands of bacteria. Bacteria are isolated from the cytoplasm of the plant cell by a membrane-bound organelle-like structure termed the symbiosome. It is enclosed by a symbiosome membrane, mainly of plant origin but with some inclusion of bacterial proteins. Within the symbiosome, bacterial cells differentiate into bacteroids a form that is specialised for nitrogen fixation. In this review, we briefly summarise recent advances in studies of differentiation both of symbiosomes and of the infected cells that accommodate them. We will consider the role of CCS52A, DNA topoisomerase VI, tubulin cytoskeleton rearrangements in differentiation of infected cells, the fate of the vacuole, and the distribution of symbiosomes in the infected cells. We will also consider differentiation of symbiosomes, paying attention to the role of NCR peptides, vesicular transport to symbiosomes, and mutant analysis of symbiosome development in model and crop legumes. Finally, we conclude that mechanisms involved in redistribution organelles, including the symbiosomes, clearly merit much more attention.

Regulation of cellulose synthesis in response to stress

The cell wall is a complex polysaccharide network that provides stability and protection to the plant and is one of the first layers of biotic and abiotic stimuli perception. A controlled remodeling of the primary cell wall is essential for the plant to adapt its growth to environmental stresses. Cellulose, the main component of plant cell walls is synthesized by plasma membrane-localized cellulose synthases moving along cortical microtubule tracks. Recent advancements demonstrate a tight regulation of cellulose synthesis at the primary cell wall by phytohormone networks. Stress-induced perturbations at the cell wall that modify cellulose synthesis and microtubule arrangement activate similar phytohormone-based stress response pathways. The integration of stress perception at the primary cell wall and downstream responses are likely to be tightly regulated by phytohormone signaling pathways in the context of cellulose synthesis and microtubule arrangement.