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Sclerotinia sclerotiorum is a fungal plant pathogen notorious for its ability to infect hundreds of plant species by triggering the death of plant cells rapidly. It is responsible for “Sclerotinia stem mold” and “white mold” diseases that regularly devastate rapeseed, sunflower and tomato fields notably, and it can also infect wild plant species such as thale cress (Arabidopsis thaliana).  

The quest for quantitative disease resistance genes in plants

Disease resistance in plants relies on diverse molecular bases according to pathogen lifestyles. Within a single plant species, some varieties are able to slow down infection by Sclerotinia. This type of immune mechanisms is designated as quantitative disease resistance. Scientists at LIPM are trying to understand why quantitative disease resistance is more efficient in some plant varieties than others. To do so, they confronted genome sequence and quantitative resistance in about a hundred varieties of thale cress collected across Europe. The approach consist in mapping association at the whole genome scale. This analysis revealed a gene, called POQR, that exists in two forms in Nature, among which one is consistently present in resistant thale cress varieties. By inactivating this gene through mutagenesis, plants become susceptible, indicating that POQR is a gene for resistance to Sclerotinia.

Evolution stuttering: convergence between thale cress and tomato

The team at LIPM was surprised to notice that two forms of the POQR gene naturally occur in multiple plant species. In tomato (Solanaceae family) for instance, one POQR copy exhibits several features of the “resistant” variant from thale cress (Brassicacea family). The scientists used a virus-mediated gene silencing approach to show that one the POQR variants is indeed a resistance gene in tomato. The common ancestor of thale cress and tomato, that existed about 120 million years ago, likely had a single copy of POQR. This gene duplicated and evolved similarly in parallel in Brassicacea and Solanacea to become a quantitative disease resistance genes in several modern plant species.

These findings broaden the range of molecular functions associated with plant quantitative disease resistance and suggest that evolution of this trait can be repeated, to some extent, across lineages of the plant kingdom.