Quantitative Immunity in Plants: Research overview
The importance and complexity of pathogen perception and signaling pathways in the regulation and execution of plant immune responses have become apparent during the last years. Notably, R gene-mediated immunity has been shown to provide complete resistance in plant populations. However R-gene mediated resistance is usually rapidly overcome by pathogens in the field, and it is never observed in response to certain pathogens. Additional forms of resistance have gained increasing attention for breeding purposes, such as quantitative resistance, but they are still poorly understood.
In the absence of qualitative resistance, an incomplete resistance designated as quantitative resistance is often observed, leading to the reduction rather than absence of disease. A number of loci conferring quantitative resistance were found durable, but very little is known on the molecular mechanisms underlying quantitative immune responses in plants.
Figure legend: LEFT: A typical R-gene mediated resistance response in the wild, observed on oaks infected by the sudden oak death pathogen Phytophthora ramorum (Photo credits: D. Rizzo, National Science Foundation); the scheme below illustrates in a simplified manner the underlying molecular bases (brown = pathogen cell, green= plant cell). RIGHT: Typical quantitative resistance response observed in the wild (Photo credits: G. Gilbert, National Science Foundation), with a simplified model of the hypothetical molecular bases shown below. Recommended reading:Roux F, Voisin D, Badet T, Balagué C, Barlet X, Huard-Chauveau C, Roby D, Raffaele S. Resistance to phytopathogens e tutti quanti: placing plant quantitative disease resistance on the map. Mol Plant Pathol. 2014 Jun;15(5):427-32.
To decipher the molecular mechanisms associated with quantitative immunity, we are developing a multidisciplinary research programme focused on the use of the model plant Arabidopsis thaliana challenged with the bacterial pathogen Xanthomonas campestris or the fungal pathogen Sclerotinia sclerotiorum. These are two major pathogens of Brassica plants (rapeseed, cabbage, turnip, mustard...) with distinct lifestyles allowing to explore the diversity of quantitative immunity in plants.
Xanthomonas species are a major constraint for seed and plant production in vegetables crops, especially in Brassica and Solanaceae, causing black rot and bacterial spot, respectively. Plant pathogenic Xanthomonas species have a wide geographical distribution, causing reduction in yield and quality and are expected to increase in incidence and range under climate change. Beside resistance gene deployment, there is no other efficient method to control Xanthomonas species. Therefore, we adopted an interdisciplinary strategy using molecular genetics and evolutionary biology in collaboration with Fabrice Roux (LIPM, Toulouse) for the identification of key components of QDR to Xanthomonas. Our aims are the following: (i) decipher the resistance pathways operating in QDR and their potential placement in known disease resistance pathways; (ii) elucidate the ecological and evolutionary forces shaping the natural genetic diversity observed at these QDR genes.
The white and stem mold pathogen Sclerotinia sclerotiorum is a generalist fungal pathogen, infecting a broad range of host species (>400) in nature. It is among the most devastating plant pathogens worldwide and causes disease on many crops including soybean, rapeseed, sunflower and most vegetables, but very limited solutions for the genetic control of the disease exist to date. S. Sclerotiorum naturally infects wild and cultivated Brassica species, including the model plant Arabidopsis thaliana. We are developing a multidisciplinary research programme centered on the A. thaliana – S. sclerotiorum to address the following questions: (i) Which plant genes are involved in quantitative immunity to Sclerotinia? How do they contribute to disease resistance? (ii) What are the mechanisms used by Sclerotinia to colonize its hosts? How did they evolve?
Figure legend: Illustration of the two main pathosystems used in the group. Left picture: A Xanthomonas campestris (Xcc) bacterium culture in vitro; Right picture: Sclerotinia sclerotiorum (Ss) fungus cultures in vitro. Middle: A composite image of an Arabidopsis thaliana plant showing symptoms caused by Xcc on the left side and by Ss on the right side.