Calendar Name: seminars
Scheduled for: Monday, May 9 2011, 11:00 - 12:30
Event text: Dr Pascal Ratet
Institut des Sciences du V�g�tal
CNRS
Gif-sur-Yvette
FRANCE
Details: �Insertion mutants in Medicago truncatula as tools to
study symbiotic nodule identity and bacterial
recognition.�
ABSTRACT
The legume species Medicago truncatula establishes a
symbiotic interaction with the nitrogen-fixing bacterium
Sinorhizobium meliloti. A complex molecular dialogue
between the two partners is necessary for microbial
colonization and plant organogenesis leading to a
functional nitrogen-fixing nodule. During formation of
this organ, specific cellular differentiation is seen
both for host cells and for the endosymbiotic bacteria,
called bacteroids. This process is accompanied by novel
gene expression patterns for both plant and bacteria.
Our laboratory has demonstrated that insertion mutant
collections can be developed in this model legume plant
using the tobacco retrotransposons Tnt1 or the
endogenous retroptransposon MERE1. In collaboration with
the Noble Foundation (OK-USA), we characterized tagged
symbiotic plant mutants that form no nodules,
nonfunctional nodules or nodules with altered
development during the interaction with S. meliloti.
Among these mutants, we identified three alleles of a
new Fix- symbiotic mutant locus. The Fix- nodules
produce fluorescent compounds reminiscent of phenolic
molecules normally produced during defense reactions.
This observation suggests a shift from symbiosis to
pathogenesis during the interaction between the two
symbiotic partners. A detailed analysis indicates that
bacteroid differentiation is altered in this plant
mutant and that the mutated gene may be important for
the plant to recognize bacteria. The tagged locus was
cloned and we are studying the biological function of
this gene in order to understand its role in the
recognition between the two partners.
We also characterized 4 alleles of an original mutant
that we called noot (nodule root). This noot mutant
develops nodules that appear functional for nitrogen
fixation in the presence of bacteria but are altered in
the pattern of development and organization. During
development, a root appears in apical position of the
nodules. The noot mutation is sufficient to convert a
nodule into a root suggesting that root and nodule
morphogenetic pathways share more common elements that
previously admitted. We have cloned the gene responsible
for this phenotype. This mutant unravels one element of
the molecular determinants controlling the symbiotic
nodule organ identity but also how during evolution
nodules evolved from legume roots.
