Characterization of symbiotic mutants affected in organ identity
The NOOT1 gene is necessary for nodule identity (Couzigou et al., 2012) :
The Medicago noot mutant isolated in our laboratory develops roots in apical position of the symbiotic organ. The coch mutant from pea has a symbiotic phenotype similar to noot but also shows stipule to leaf conversion and flower developmental modifications. We have isolated the NOOT and COCH genes and shown that they are necessary for the robust persistence of the nodule. NOOT and COCH are Arabidopsis BOP orthologs and their functions in the identity of the lateral aerial organs are conserved in M. truncatula and P. sativum (Couzigou et al., 2012 ; Norberg et al., 2005, Development, 132, 2203-13 ; Hepworth et al., 2005, Plant Cell, 17, 1434-48). In the noot and coch mutants the ectopic roots do not derive from the nodule meristem but originate from the vascular initials. This suggests that the nodule vascular strands are ontologically related to roots.
Characterization of symbiotic mutants affected in symbiotic immunity
DNF2 is necessary for bacteroid persistence (Bourcy et al, 2013 ; Berrabah et al., 2014a) :
We have identified and characterized the DNF2 gene (Starker et al., 2006, Plant Physiology, 140, 671–80) that encodes a Phosphoinositol-Phospholipase C-XD-containing protein. The dnf2 mutant plants develop nodules which are correctly invaded during the first stages of the symbiosis but in which the symbiotic process aborts once bacteria are released into the plant cell. This arrest of the symbiotic process is accompanied by defense-like reactions. The phenotype of this dnf2 mutant unravels a new aspect of the symbiotic interaction suggesting an active mechanism responsible for the repression of plant defense within the symbiotic nodule cells.
The SYMCRK gene is necessary for repression of immunity (Berrabah et al., 2014b) :
In order to identify new genes required for symbiotic inhibition of defence-like reactions, we hypothesized that some DNF2 co-regulated genes might be involved in the mechanism. Four candidate genes were chosen and the symbiotic capacity of the corresponding mutant lines was tested. Amongst the tested lines, one mutant lines altered in a genes encoding for a receptor-like kinase was found to have a phenotype similar to DNF2 (i.e. necrotic Fix- nodules). Our work shows that this gene is also necessary to suppress defense reaction during symbiosis.
Role of the Salycilic acid pathway in Medicago :
In order to understand the role of the salycilic acid pathway during symbiosis and other plant-microbe interactions we are studying the role of the NPR gene family in Medicago. For this, we are characterizing mutants in the NPR1, NPR2 and NPR3 genes and will study their responses to known pathogens and to the symbiotic bacteria.