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Underground "peace talks" Plants are able to establish symbiotic associations with several microorganisms. The most agriculturally and ecologically important symbioses are the interactions with mycorrhizal fungi and nitrogen-fixing bacteria. The vast majority of higher plants are able to form arbuscular mycorrhizae, an association with Glomeromycota fungi which highly improves their phosphorous, nitrogen and water uptake from the soil. Infected plants also obtain an increased resistance against biotic and abiotic stresses. In contrast, the endo-symbiosis with nitrogen-fixing bacteria is limited to only a few plant families. The best-studied nitrogen-fixing symbiosis is formed between legumes and bacteria named rhizobia. This symbiosis serves as one of the major sources of biologically available nitrogen in the biosphere, and the use of legumes in crop rotation with non-legumes has been fundamental to productivity from antiquity to modern agricultural systems. The visible manifestation of symbiosis is the development of a novel organ, the "nodule", on the roots of legume plants, wherein the plant provides energy in the form of carbon in exchange for ammonia. In these two types of plant - microbe symbiotic interactions, a molecular dialogue serves as an identity check.
During nodulation, this dialog determines the range of legumes with which a particular bacterial genotype can associate. Flavonoid compounds from the legume roots trigger gene expression and synthesis of a second signal in the bacterium that is perceived by the plant root. The bacterially-derived signals, so-called "Nod factors", consist of modified chitin oligomers, where specific chemical modifications determine the host range of the bacterium. Nod factors allow bacterial entry and initiate programs for bacterial infection and root nodule development. Extremely low concentrations (10-9 to 10-12 M) of purified Nod factors induce many of the same responses in the roots of legume hosts that are induced by symbiotic bacteria, including specific ion fluxes, root hairs deformations, “calcium spiking”, gene expression, and cortical cell divisions.
During
mycorrhization, strigolactones produced by the host roots act as a
'branching factor' that increase the probability of contacts between
fungi and plants. In turn, the fungus releases a diffusible signal that
is recognized by the plant and that leads to symbiosis-related gene
activation. Understanding
how Myc factors and Nod factors are perceived and transduced is an
issue of long standing interest to plant biologists, and represents one
of the
highlights of plant biology research over the past years.
Myc and Nod factor signaling
The
high affinity and specificity with which Nod and Myc factors elicit
plant
responses
suggests that they are perceived by plant receptors. However, the
precise
nature and number of receptors utilized in the symbiotic interaction
has yet to
be elucidated. Physiological and genetic studies suggest the
involvement of
multiple Nod receptors or a single Nod receptor with multiple, distinct
activities. A
previous study by Ardourel et al.
observed that diverse Nod factors structures can initiate nodule
development
but bacterial infection requires specific decorations on the cognate
Nod
factors. They proposed a high stringency “entry receptor” that mediates
bacterial entry into the root hair, and a low stringency “signaling
receptor”.
Recent molecular genetic studies in the model
legumes Medicago truncatula and Lotus japonicus indicate that a family
of
LysM receptor kinases initiates the developmental and infection Nod
factors
responses.
Genetic
screens for Nod- mutants identified several other loci
downstream of
the putative Nod and Myc factors receptors. The M. truncatula dmi1, dmi2, and
dmi3 (does
not make infections, dmi1/2/3)
mutants do not form root nodules upon infection with S.
meliloti. Intriguingly, these mutants
do not
associate with arbuscular mycorrhizal (AM) fungi, indicating that the
signaling
pathways leading to nodulation and mycorrhization share common
components ("common symbiotic pathway").
Phenotypic analyses allowed the ordering of dmi
mutants relative to each other and to Nod factors responses. Positional cloning
efforts have molecularly characterized the DMI
genes and the corresponding proteins, providing insights into the mechanism of symbiotic signal
transduction.
Systems
used in our laboratory to dissect symbiotic interactions
Industrial and Economical Relevance Nod factors and Myc factors stimulate plant development. Nod factors are now commercialized by the EMD Crop BioScience Company (former Nitragin company) as a plant growth promoter for legume (soybean, alfalfa and peanuts) and non-legume crops (cotton) under the name of OptimizeTM. Our research aims at understanding how Nod factors are perceived by the plants and their effects on symbiotic interactions, plant development and plant-pathogen interactions. As a summary, we are interested in understanding the relationships between: Arbuscular mycorrhization and legume nodulation Symbiotic signaling and plant development Symbiotic responses and plant defense reactions Feel free to
contact us if you have any question!
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