Genopole Evry


Accueil > Research > Biotic Interactions > P. Ratet Team

Genetic Control of the symbiosis

Grain (such as peas, faba beans, soybean, chickpea) and forage (alfalfa, clover, lupinus) legume plants are important crops worldwide for both animal and human consumption as they are major protein sources. Legumes are also rich in a range of secondary plant compounds and other high added value compounds that open new fields of legume use for non-food purposes (bio-active molecules, bioenergy, biopolymers…). Legumes are grown in rotation with other crops for their capacity to restore soil fertility in organic farming practices. Their use in agriculture can reduce the application of nitrogen fertilizers, as they can grow in nitrogen deficient soils. Establishment of a symbiotic interaction between legume plants and beneficial soil bacteria, collectively named Rhizobia, occurs when the soil nitrogen source is limiting and leads to the de-novo formation of symbiotic nodules, generally formed on roots of the plants. Nodules host bacteria that are able to fix atmospheric nitrogen and made it available for the plant. This symbiotic association thus allows the plant to overcome nitrogen limitation. In return, the plant furnishes carbon derivatives to its invaders.

The first steps of this symbiotic association and the formation of the symbiotic organ have been described in detail (Ferguson et al., 2010, J. Int Plant Biol., 52, 61-76 ; Oldroyd et al., 2011, Annu. Rev. Genet., 45, 119–144) but organ identity and later steps of the interaction related to bacterial accommodation and immunity are less understood. Rhizobia often invade the plant root using specialized symbiotic structures called infection threads. In the mature nitrogen fixing nodule, the rhizobia reside in symbiotic nodule cells within organelle-like structures, called symbiosomes. In contrast to what is generally observed during microbial invasion, legumes do not elicit apparent defense reactions during symbiosis despite that the bacterial population reaches massive densities in nodules.

The team has pioneered the development of Tnt1 mutagenesis in the model legume plant Medicago truncatula (d’Erfurth et al., 2003, Plant J. 34(1):95-106 ; Tadege et al., 2005, Trends Plant Sci. 10(5):229-35 ; Tadege et al., 2008, Plant J. 54(2):335-47 ; Pislariu et al., 2012, Plant Physiol. 159(4):1686-99) together with the isolation of Medicago mutants and transformation of Medicago (Cosson et al., 2006, Methods Mol Biol. 343:115-27). The team is using reverse and forward genetic strategies to study legume-rhizobium interactions.
Two main axes of research are developed by the team :

  1. The nodule organ identity : how nodules are formed and how their identity (nodule versus root identity) is genetically controlled ?
  2. The immune status of the nodules : how the plant can deal with a massive colonization of rhizobia ? Emphasis is made on signaling events that are involved in bacterial perception and control of the immune response of the root.