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Rapeseed: phoma stem canker on a leaf. © INRA, Thierry Rouxel

Phoma stem canker of rapeseed: infective mechanisms under epigenetic control  

The control of Leptosphaeria maculans, which causes phoma stem canker in rapeseed, requires a clear understanding of the biology of this pathogenic fungus, and particularly how it operates its infective process.  INRA researchers in Versailles-Grignon and their American colleagues have evidenced a process that adjusts production of the proteins necessary to infect the plant.

Updated on 09/19/2014
Published on 03/07/2014

At a global scale, phoma (or phoma stem canker) is one of the most harmful diseases that affect rapeseed; it can reduce yield by up to 50%.  While chemical control has proved little effective, the selection and use of rapeseed varieties that are naturally resistant to the fungus which causes the infection, Leptosphaeria maculans, is a widely employed approach.  In order to better understand the infective processes of this fungus, INRA research scientists in Versailles-Grignon, and their American colleagues, focused on the proteins secreted by the pathogenic agent that allow infection of the plant and are referred to as "effectors".

In 2011, an international research consortium managed by this same INRA team in Versailles-Grignon managed to sequence the genome of L. maculans. At that time, the scientists revealed that the genes involved in the infective process are grouped in regions with characteristics reminiscent of those of heterochromatin(1), and notably their wealth of transposable elements (mobile DNA sequences that are able to propagate autonomously in the genome where generally they do not have an identified function).  This localisation may favour the diversification of these genes and hence the very rapid adaptation of the fungus to its host plant.

In light of these previous findings, the researchers focused their explorations on the role played by the genomic environment in the expression of effector-coding genes.  To achieve this, they modified the spatial environment of these genes by inactivating two proteins known for their important involvement in assembling and maintaining hematochromatin structure.  The results revealed the importance of the genomic localisation of effector-coding genes to the control of their expression.  They showed that this control caused the repression of effector expression during the vegetative growth of L. maculans and only activated it during the infection of plants.  This expression adjusted to the infectious phases of the fungus means that it is able to produce its different effectors in a concerted manner and adapt rapidly to a new environment through the action of a simple mechanism.  Except for the critical period when they enter the plant, the effectors are little or not expressed and thus hidden, which probably limits the possibility that the plant may recognise these signals.

This global mechanism is original for its epigenetic nature(2) linked to the structure of the regions harbouring the effector-coding genes without affecting the DNA sequence.  It also allows L. maculans to economise on the production of proteins under conditions where they are not necessary, and to optimise this production when it does become necessary, notably during the very early phases of plant colonisation.  Overall, these findings offer a promising scientific picture of infective mechanisms which it may be possible to modulate in order to better control this fungus.

(1) In the cell nucleus, DNA is associated with proteins to form the chromatin.  As the base element of chromosomes, it presents two forms: the heterochromatin, a compact structure generally associated with the repression of gene expression, and the euchromatin, a loose structure which by contrast facilitates their expression.

(2) Epigenetic: a heritable change to gene expression that occurs in the absence of DNA mutations (this change is dependent on a process that is not purely genetic but which varies as a function of the environment).

Contact(s)
Scientific contact(s):

  • Isabelle Fudal (+33 1 30 81 45 90 ) Biology and Risk Management in Agriculture - Plant Pathogenic Fungi
Press Relations:
INRA News Office (+33 1 42 75 91 86)

Reference

Jessica Louise Soyer, Mennat El Ghalid, Nicolas Glaser, Bénédicte Ollivier, Juliette Linglin, Jonathan Grandaubert, Marie-Hélène Balesdent, Lanelle R. Connolly, Michael Freitag, Thierry Rouxel, and Isabelle Fudal. Epigenetic control of effector gene expression in the plant pathogenic fungus Leptosphaeria maculans. PLOS Genetics, 6 March 2014. DOI:  10.1371/journal.pgen.1004227