Mycorrhizal fungi are a heterogeneous group of diverse fungal taxa, associated with the roots of over 90% of all plant species. Recently, state-of-the-art molecular and genetic tools, coupled to ...high-throughput sequencing and advanced microscopy, have led to the genome and transcriptome analysis of several symbionts. Signalling pathways between plants and fungi have now been described and the identification of several novel nutrient transporters has revealed some of the cellular processes that underlie symbiosis. Thus, the contributions of each partner in a mycorrhizal association are starting to be unravelled. This new knowledge is now available for use in agricultural practices.
Arbuscular mycorrhizas (AM) are one of the most widespread symbiosis on earth. This plant-fungus interaction involves around 72% of plant species, including most crops. AM symbiosis improves plant ...nutrition and tolerance to biotic and abiotic stresses. The fungus, in turn, receives carbon compounds derived from the plant photosynthetic process, such as sugars and lipids. Most studies investigating AM and their applications in agriculture requires a precise quantification of the intensity of plant colonization. At present, the majority of researchers in the field base AM quantification analyses on manual visual methods, prone to operator errors and limited reproducibility. Here we propose a novel semi-automated approach to quantify AM fungal root colonization based on digital image analysis comparing three methods: (i) manual quantification (ii) image thresholding, (iii) machine learning. We recognize machine learning as a very promising tool for accelerating, simplifying and standardizing critical steps in analysing AM quantification, answering to an urgent need by the scientific community studying this symbiosis.
Recent years have witnessed major advances in our knowledge of plant mutualistic symbioses such as the rhizobium-legume symbiosis (RLS) and arbuscular mycorrhizas (AM). Some of these findings caused ...the revision of longstanding hypotheses, but one of the most solid theories is that a conserved set of plant proteins rules the transduction of symbiotic signals from beneficial glomeromycetes and rhizobia in a so-called common symbiotic pathway (CSP). Nevertheless, the picture still misses several elements, and a few crucial points remain unclear. How does one common pathway discriminate between - at least - two symbionts? Can we exclude that microbes other than AM fungi and rhizobia also use this pathway to communicate with their host plants? We here discuss the possibility that our current view is biased by a long-lasting focus on legumes, whose ability to develop both AM and RLS is an exception among plants and a recent innovation in their evolution; investigations in non-legumes are starting to place legume symbiotic signaling in a broader perspective. Furthermore, recent studies suggest that CSP proteins act in a wider scenario of symbiotic and non-symbiotic signaling. Overall, evidence is accumulating in favor of distinct activities for CSP proteins in AM and RLS, depending on the molecular and cellular context where they act.
Since the Green Revolution, intensive application of agrochemicals has increased productivity in agriculture, at a great cost in terms of water pollution, loss of soil fertility and biodiversity, and ...negative effects on human health. Scientific advance and increasing public awareness are driving a change toward sustainable practices. In such a context, the symbiosis between plants and arbuscular mycorrhizal (AM) fungi is extremely promising: AM interaction improves plant mineral nutrition and stress tolerance. In turn, AM fungi receive plant photosynthesis-derived carbon. A complex chemical dialogue mediates plant-fungus recognition and symbiosis establishment: AM fungi perceive root-secreted strigolactones, which promote spore germination, hyphal growth, branching and metabolism. Host roots recognize their symbionts through chitin-derived molecules. Such Myc-factors activate a range of symbiotic responses, preparing the plant to a successful association. Here we review the most recent advances in knowledge of AM signaling molecules, with a focus on their possible application.
Arbuscular mycorrhizas (AMs) are widespread symbiotic associations that are commonly described as the result of co-evolution events between fungi and plants where both partners benefit from the ...reciprocal nutrient exchange. Here, we review data from fossil records, characterizations of AM fungi in basal plants and live cell imaging of angiosperm colonization processes from an evolutionary-developmental perspective. The uniformity of plant cell responses to AM colonization in haploid gametophytes and diploid sporophytes, in non-root organs, and throughout many seed plant clades highlights the ancient origin of the interaction and suggests the existence of common molecular and cellular processes. The possibility that pre-existing mechanisms involved in plant cell division were recruited by plants to accommodate AM fungi is discussed.
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.
Endosymbiotic interactions are characterized by the formation of specialized membrane compartments, by the host in which the microbes are hosted, in an intracellular manner. Two well-studied ...examples, which are of major agricultural and ecological importance, are the widespread arbuscular mycorrhizal symbiosis and the Rhizobium–legume symbiosis. In both symbioses, the specialized host membrane that surrounds the microbes forms a symbiotic interface, which facilitates the exchange of, for example, nutrients in a controlled manner and, therefore, forms the heart of endosymbiosis. Despite their key importance, the molecular and cellular mechanisms underlying the formation of these membrane interfaces are largely unknown. Recent studies strongly suggest that the Rhizobium–legume symbiosis coopted a signaling pathway, including receptor, from the more ancient arbuscular mycorrhizal symbiosis to form a symbiotic interface. Here, we show that two highly homologous exocytotic vesicle-associated membrane proteins (VAMPs) are required for formation of the symbiotic membrane interface in both interactions. Silencing of these Medicago VAMP72 genes has a minor effect on nonsymbiotic plant development and nodule formation. However, it blocks symbiosome as well as arbuscule formation, whereas root colonization by the microbes is not affected. Identification of these VAMP72s as common symbiotic regulators in exocytotic vesicle trafficking suggests that the ancient exocytotic pathway forming the periarbuscular membrane compartment has also been coopted in the Rhizobium–legume symbiosis.
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.
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