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Calendar Name: seminars
Scheduled for: Monday, May 9 2011, 11:00 - 12:30
Event text:    Dr Pascal Ratet
	       
	       Institut des Sciences du Végétal
	       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.

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