France
August 18, 2011
Led by INRA, an international consortium including researchers from INRA, CEA-Genoscope and CNRS has completed the whole genome sequencing and analysis of the fungus responsible for stem canker of oilseed rape, Leptosphaeria maculans. The fungus is the main pathogen of oilseed rape and causes significant yield losses. Its genome has an unusual bipartite structure, each compartment of which evolves differently. This ‘two-speed’genome underpins the evolutionary potential of the phytopathogen and its ability to adapt rapidly to the disease resistance of its host. L. maculans is thus a constantly renewed threat to oilseed rape growers. In-depth analysis of the genome is expected to improve the selection of durably resistant varieties of oilseed rape. Full results of the study are available in the Advance Online Publication of the 15 February 2011 edition of NATURE Communications.
Stem canker (also termed ‘Blackleg’) is the main disease affecting oilseed rape (canola) world-wide. It causes 5 to 20% losses at the national scale and can destroy entire fields at the local level. Because chemical treatments are largely ineffective and difficult to implement, efforts to control the disease focus on the selection and use of plant varieties which are naturally resistant to the fungus. Unfortunately the causal agent shows an extremely high evolutionary potential and can overcome varietal resistance in just three cropping seasons.
To identify genes interacting with oilseed rape and decipher the adaptation processes of Leptosphaeria maculans, the genome of the fungus was sequenced. Sequence analysis revealed that the genome of this species has recently been invaded by a bulk of transposable elements (TE) (1) . These elements, which comprise one third of the genome, are repeated multiple times and grouped together in specific genome compartments. They play an important role in the organisation, reshaping and dynamics of this genome. This characteristic may explain the fungus’ ability to rapidly adapt to the host plant. In-depth analysis of the sequence showed that genes which play a significant role in infection processes are grouped in these TE-rich regions. In particular, 120 of the 650 genes encoding infection-enabling proteins – known as effectors – are found in these regions. These effectors (2) are unique in that they have no known role and are specific to Leptosphaeria maculans. These data suggest that the recent insertion of transposable elements which ‘parasite’ the genome is conducive to the multiplication of genes encoding effectors.
This sequencing sheds light on the way in which Leptosphaeria maculans has adapted and continues to adapt to oilseed rape and new types of resistance. Current genome analysis of other pathogenic Leptosphaeria species with various levels of pathogenicity towards oilseed rape and related crucifers will make it possible to identify specific factors involved in the emergence of pathogenicity and adaptation to oilseed rape. In the long term, in-depth study of the content, evolution and role of effector proteins is expected to improve oilseed rape resistance management.
(1) A transposable element is a sequence of DNA that parasitizes a genome by autonomously transposing itself into the genome and multiplying. Transposable elements do not usually have a function within the invaded genome.
(2) An effector is a protein which enables infection, e.g. by suppressing a plant’s defences. Effectors can be “recognised” by resistant plants and activate their defence mechanisms. These are known as ‘avirulence proteins’.
Scientific leaders:
Thierry Rouxel
Unité Bioger CPP (Biologie et gestion des risques en agriculture - Champignons pathogènes des plantes)
Inra-Bioger, UR1290, Avenue Lucien Brétignières, BP 01, F-78850 Thiverval-Grignon
Joëlle Amselem
Unité de recherche Génomique-Info
Inra-URGI, Route de Saint Cyr, 78026 Versailles Cedex
For further information:
Effector diversification within compartments of the Leptosphaeria maculans genome affected by Repeat Induced Point mutations. Thierry Rouxel et al. NATURE Communications, 15 february 2011. http://dx.doi.org/ : 10.1038/ncomms1189.
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