Peace talks in the underground
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.
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.
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!