Within the plant microbiota, mutualistic fungal and bacterial symbionts are striking examples of microorganisms playing crucial roles in nutrient acquisition. They have coevolved with their hosts ...since initial plant adaptation to land. Despite the evolutionary distances that separate mycorrhizal and nitrogen-fixing symbioses, these associations share a number of highly conserved features, including specific plant symbiotic signaling pathways, root colonization strategies that circumvent plant immune responses, functional host-microbe interface formation, and the central role of phytohormones in symbiosis-associated root developmental pathways. We highlight recent and emerging areas of investigation relating to these evolutionarily conserved mechanisms, with an emphasis on the more ancestral mycorrhizal associations, and consider to what extent this knowledge can contribute to an understanding of plant-microbiota associations as a whole.
The primary objective of this study was to identify the molecular signals present in arbuscular mycorrhizal (AM) germinated spore exudates (GSEs) responsible for activating nuclear Ca2+ spiking in ...the Medicago truncatula root epidermis.
Medicago truncatula root organ cultures (ROCs) expressing a nuclear-localized cameleon reporter were used as a bioassay to detect AM-associated Ca2+ spiking responses and LC-MS to characterize targeted molecules in GSEs.
This approach has revealed that short-chain chitin oligomers (COs) can mimic AM GSE-elicited Ca2+ spiking, with maximum activity observed for CO4 and CO5. This spiking response is dependent on genes of the common SYM signalling pathway (DMI1/DMI2) but not on NFP, the putative Sinorhizobium meliloti Nod factor receptor. A major increase in the CO4/5 concentration in fungal exudates is observed when Rhizophagus irregularis spores are germinated in the presence of the synthetic strigolactone analogue GR24. By comparison with COs, both sulphated and nonsulphated Myc lipochito-oligosaccharides (LCOs) are less efficient elicitors of Ca2+ spiking in M. truncatula ROCs.
We propose that short-chain COs secreted by AM fungi are part of a molecular exchange with the host plant and that their perception in the epidermis leads to the activation of a SYM-dependent signalling pathway involved in the initial stages of fungal root colonization.
The rice lysin-motif (LysM) receptor-like kinase OsCERK1 is now known to have a dual role in both pathogenic and symbiotic interactions. Following the recent discovery that the Oscerk1 mutant is ...unable to host arbuscular mycorrhizal (AM) fungi, we have examined whether OsCERK1 is directly involved in the perception of the short-chain chitin oligomers (Myc-COs) identified inAMfungal exudates and shown to activate nuclear calcium (Ca2+) spiking in the rice root epidermis.
An Oscerk1 knockout mutant expressing the cameleon NLS-YC2.60 was used to monitor nuclear Ca2+ signaling following root treatment with either crude fungal exudates or purified Myc-COs.
Compared with wild-type rice, Ca2+ spiking responses toAMfungal elicitation were absent in root atrichoblasts of the Oscerk1 mutant. By contrast, rice lines mutated in OsCEBiP, encoding the LysM receptor-like protein which associates with OsCERK1 to perceive chitin elicitors of the host immune defense pathway, responded positively to Myc-COs.
These findings provide direct evidence that the bi-functional OsCERK1 plays a central role in perceiving short-chain Myc-CO signals and activating the downstream conserved symbiotic signal transduction pathway.
Arbuscular mycorrhizas (AM) are widespread, ancient endosymbiotic associations that contribute significantly to soil nutrient uptake in plants. We have previously shown that initial fungal ...penetration of the host root is mediated via a specialized cytoplasmic assembly called the prepenetration apparatus (PPA), which directs AM hyphae through the epidermis (Genre et al., 2005). In vivo confocal microscopy studies performed on Medicago truncatula and Daucus carota, host plants with different patterns of AM colonization, now reveal that subsequent intracellular growth across the root outer cortex is also PPA dependent. On the other hand, inner root cortical colonization leading to arbuscule development involves more varied and complex PPA-related mechanisms. In particular, a striking alignment of polarized PPAs can be observed in adjacent inner cortical cells of D. carota, correlating with the intracellular root colonization strategy of this plant. Ultrastructural analysis of these PPA-containing cells reveals intense membrane trafficking coupled with nuclear enlargement and remodeling, typical features of arbusculated cells. Taken together, these findings imply that prepenetration responses are both conserved and modulated throughout the AM symbiosis as a function of the different stages of fungal accommodation and the host-specific pattern of root colonization. We propose a model for intracellular AM fungal accommodation integrating peri-arbuscular interface formation and the regulation of functional arbuscule development.
Root endosymbioses are beneficial associations formed between terrestrial plants and either bacterial or fungal micro-organisms. A common feature of these intracellular symbioses is the requirement ...for mutual recognition between the two partners before host-regulated microbial entry. As part of this molecular dialogue, symbiosis-specific microbial factors set in motion a highly conserved plant signal transduction pathway, of which a central component is the activation of sustained nuclear Ca2+ oscillations in target cells of the host epidermis. Here, we focus on recent findings concerning this crucial Ca2+-dependent signalling step for endosymbiotic associations involving either arbuscular mycorrhizal fungi or nitrogen-fixing Frankia actinomycetes, and in particular how this knowledge is contributing to the identification of the respective microbial factors.
The objectives of this study were to investigate an association between the magnitude of flow-mediated aortic wall shear stress (WSS) and medial wall histopathology in patients with bicuspid aortic ...valve (BAV) with aortopathy.
Patients with BAV (n = 27; 52 ± 15 years; 3 women; proximal thoracic aorta diameter = 4.4 ± 0.7 and 4.6 ± 0.5 cm) who underwent prophylactic aortic resection received preoperative 3-dimensional time-resolved phase-contrast magnetic resonance imaging with 3-dimensional velocity encoding to quantify WSS relative to a population of healthy age- and sex-matched tricuspid aortic valve control participants (n = 20). Quantitative histopathology was conducted on BAV aorta tissue samples resected at surgery (n = 93), and correlation was performed between elastic fiber thickness and in vivo aortic WSS as continuous variables. Validation of elastic fiber thickness was achieved by correlation relative to tissue stiffness determined using biaxial biomechanical testing (n = 22 samples).
Elastic fibers were thinner and WSS was higher along the greater curvature compared with other circumferential regions (vs anterior wall: P = .003 and P = .0001, respectively; lesser curvature: both P = .001). Increased regional WSS was associated with decreased elastic fiber thickness (r = −0.25; P = .02). Patient stratification with subanalysis showed an increase in the correlation between WSS and histopathology with aortic valve stenosis (r = −0.36; P = .002) and smaller aortic diameters (<4.5 cm: r = −0.39; P = .03). Elastic fiber thinning was associated with circumferential stiffness (r = −0.41; P = .06).
For patients with BAV, increased aortic valve-mediated WSS is significantly associated with elastic fiber thinning, particularly with aortic valve stenosis and in earlier stages of aortopathy. Elastic fiber thinning correlates with impaired tissue biomechanics. These novel findings further implicate valve-mediated hemodynamics in the progression of BAV aortopathy.
In many legumes, root entry of symbiotic nitrogen-fixing rhizobia occurs via host-constructed tubular tip-growing structures known as infection threads (ITs). Here, we have used a confocal microscopy ...live-tissue imaging approach to investigate early stages of IT formation inMedicago truncatularoot hairs (RHs) expressing fluorescent protein fusion reporters. This has revealed that ITs only initiate 10 to 20 h after the completion of RH curling, by which time major modifications have occurred within the so-called infection chamber, the site of bacterial entrapment. These include the accumulation of exocytosis (M. truncatulaVesicle-Associated Membrane Protein721e)- and cell wall (M. truncatulaEARLY NODULIN11)-associated markers, concomitant with radial expansion of the chamber. Significantly, the infection-defectiveM. truncatula nodule inception-1mutant is unable to create a functional infection chamber. This underlines the importance of theNIN-dependent phase of host cell wall remodeling that accompanies bacterial proliferation and precedes IT formation, and leads us to propose a two-step model for rhizobial infection initiation in legume RHs.
The arbuscular mycorrhizal symbiosis associates soil fungi with the roots of the majority of plants species and represents a major source of soil phosphorus acquisition. Mycorrhizal interactions ...begin with an exchange of molecular signals between the two partners. A root signaling pathway is recruited, for which the perception of fungal signals triggers oscillations of intracellular calcium concentration. High phosphate availability is known to inhibit the establishment and/or persistence of this symbiosis, thereby favoring the direct, non-symbiotic uptake of phosphorus by the root system. In this study, Medicago truncatula plants were used to investigate the effects of phosphate supply on the early stages of the interaction. When plants were supplied with high phosphate fungal attachment to the roots was drastically reduced. An experimental system was designed to individually study the effects of phosphate supply on the fungus, on the roots, and on root exudates. These experiments revealed that the most important effects of high phosphate supply were on the roots themselves, which became unable to host mycorrhizal fungi even when these had been appropriately stimulated. The ability of the roots to perceive their fungal partner was then investigated by monitoring nuclear calcium spiking in response to fungal signals. This response did not appear to be affected by high phosphate supply. In conclusion, high levels of phosphate predominantly impact the plant host, but apparently not in its ability to perceive the fungal partner.
Rhizobia and arbuscular mycorrhizal fungi produce signals that are perceived by host legume receptors at the plasma membrane and trigger sustained oscillations of the nuclear and perinuclear ...Ca2+concentration (Ca2+spiking), which in turn leads to gene expression and downstream symbiotic responses. The activation of Ca2+spiking requires the plasma membrane-localized receptor-like kinase Does not Make Infections 2 (DMI2) as well as the nuclear cation channel DMI1. A key enzyme regulating the mevalonate (MVA) pathway, 3-Hydroxy-3-Methylglutaryl CoA Reductase 1 (HMGR1), interacts with DMI2 and is required for the legume–rhizobium symbiosis. Here, we show that HMGR1 is required to initiate Ca2+spiking and symbiotic gene expression inMedicago truncatularoots in response to rhizobial and arbuscular mycorrhizal fungal signals. Furthermore, MVA, the direct product of HMGR1 activity, is sufficient to induce nuclear-associated Ca2+spiking and symbiotic gene expression in both wild-type plants anddmi2mutants, but interestingly not indmi1mutants. Finally, MVA induced Ca2+spiking in Human Embryonic Kidney 293 cells expressing DMI1. This demonstrates that the nuclear cation channel DMI1 is sufficient to support MVA-induced Ca2+spiking in this heterologous system.
• The aim of this study was to investigate Ca²⁺ responses to endosymbiotic arbuscular mycorrhizal (AM) fungi in the host root epidermis following pre-infection hyphopodium formation in both legumes ...and nonlegumes, and to determine to what extent these responses could be mimicked by germinated fungal spore exudate. • Root organ cultures of both Medicago truncatula and Daucus carota, expressing the nuclear-localized cameleon reporter NupYC2.1, were used to monitor AM-elicited Ca²⁺ responses in host root tissues. • Ca²⁺ spiking was observed in cells contacted by AM hyphopodia for both hosts, with highest frequencies correlating with the epidermal nucleus positioned facing the fungal contact site. Treatment with AM spore exudate also elicited Ca²⁺ spiking within the AM-responsive zone of the root and, in both cases, spiking was dependent on the M. truncatula common SYM genes DMI1/2, but not on the rhizobial Nod factor perception gene NFP. • These findings support the conclusion that AM fungal root penetration is preceded by a SYM pathway-dependent oscillatory Ca²⁺ response, whose evolutionary origin predates the divergence between asterid and rosid clades. Our results further show that fungal symbiotic signals are already generated during spore germination, and that cameleon-expressing root organ cultures represent a novel AM-specific bio-assay for such signals.
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