Population genetics to help in the fight against Ralstonia solanacearum

We interview Jérémy Guinard, who is beginning the second year of his PhD * in the Center for Plant Protection (3P). He is researching the route by which bacterium Ralstonia solanacearum finds a way around the aubergine’s natural resistance.

Tell us briefly about the bacteria you study ?

- Ralstonia solanacearum is a soil bacterium, a plant pathogen. It is present on every continent, particularly tropical and subtropical regions. Depending on the region it is in, the bacteria may attack more than 250 different plant species in more than 50 botanical families, including commercial crops such as tomato, potato, banana or tobacco.

But you are studying its effect on another plant of the nightshade family : the aubergine?

- I actually try to understand the biology of this bacterium to study the way it circumvents the aubergine’s natural bacterial resistance. Aside from antibiotic treatments, which are banned in France and several European countries, no effective solution exists at the moment to deal with Ralstonia solanacearum. The idea is to find an alternative solution to give these plants a natural resistance to this bacterium via genetic crossing and new selection techniques.

Why study the aubergine?

- Certain plants have been identified with a tolerance to Ralstonia solanacearum, but no real resistance. The tomato remains, despite its tolerance, very sensitive to this bacterium. However, we have discovered a major resistance gene in aubergines, ERS1, which allows the plant to grow and reproduce even in the presence of bacteria or a high inoculum level (the amount of pathogen needed to infect a host plant) in the soil. Aubergine resistance is unique, and is currently the only known source of resistance in the nightshade family.

Your thesis advocates a holistic approach to Ralstonia solanacearum resistance in aubergines. What does this mean?

- A holistic approach is based on the study of genes involved in the pathogenicity of the bacterium and its ability to multiply and reproduce within the plant : end effectors. After a rigourous bibliographic study, we selected a number of end effectors in order to study their distribution and allelic variability in a comprehensive collection of strains. This collection includes bacteria with different patterns of infection, i.e. capable of infecting the resistant / vulnerable aubergine or causing latent infections. The aim is to better understand the circumvention of resistance at the genomic level.

Then you plan to combine this holistic approach with a second, more targeted approach?

- The second approach targets population genetics on a small scale (field). We will thus be able to study different parameters of bacteria population genetics, including : the distribution of the bacteria within the same field, whether the resistant aubergine has an impact on bacteria genetics and whether the successive cropping cycles of one variety of aubergine will affect the genetics of the bacteria. I will try to answer these questions by setting up two crop cycles per year of aubergine (each comprising two varieties, one vulnerable and one resistant) on the same plot.

Do you think these two approaches will work well together?

- Knowledge of both the genotypic and phenotypic diversity of Ralstonia solanacearum and of genetic determinism of resistance are, in my opinion, necessary to establish an effective strategy against this bacterium. This dual approach will help identify and implement appropriate molecular markers to predict any circumvention of resistance, oversee bacterial control methods and help set policy for the sustainable management of plant resistance at the local and regional scale.

* Title of thesis : « Dynamique évolutive du contournement de la résistance de l’aubergine par Ralstonia solanacearum - Approches globale et locale de la durabilité de résistance »
Thesis supervisor : Emmanuel Wicker
Thesis advisor : Stéphane Poussier