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Towards more sustainable resistances to downy mildew in sunflower

Downy mildew is a sunflower disease that can lead to an almost total loss of yield in the case of serious attacks. Thanks to long work on valorization of natural genomic resources and to the development of new genotyping technologies, scientists and technicians at the plant-microorganisms interactions laboratory (LIPM) have improved understanding of infection mechanisms which should help to reinforce disease resistance of sunflower in the field.

Sunflower with downy mildew
Updated on 12/17/2018
Published on 12/10/2018

Sunflower downy mildew, a parasite with strong pathogenicity

Sunflower downy mildew is caused by the pathogenic oomycete Plasmopara halstedii, a fungal-like parasite. Sunflower resistance to downy mildew is conferred by resistance genes called Pl, 22 of which had been identified and used in cultivated varieties to protect them naturally from this devastating disease. However, over the last 40 years, the large-scale use of one or a few Pl genes in sunflower varieties led to the emergence of new, more virulent downy mildew strains (pathotypes or races) overcoming existing resistances. It was therefore important to identify in sunflower as wide a range of resistance genes as possible that are effective against many downy mildew pathotypes. These resistances can be reasonably used in cultivated varieties, either by combination or alternation, to ensure their effectiveness and improve their durability in the field.

Identify and understand virulence factors *

Between downy mildew (pathogen) and the sunflower (host plant), a molecular dialogue is established, often called an "arms race". The pathogen has virulence factors, called "effectors", that the plant "learns" to recognize and against which it develops targeted resistance, through a mechanism of adaptive evolution. When an effector is thus "unmasked", it triggers in the plant a resistance reaction against the pathogen.

The first approach of the work carried out was based on the research for downy mildew virulence factors whose genomic sequences are the most conserved between the different strains, and which are therefore potentially essential to development in sunflower. Thirty of these conserved virulence factors have been characterized: more than half play a role in virulence since they suppress a plant resistance mechanism, the role of the other half is still unclear. The localization of expression of these effectors has been performed in sunflower cells, in which they target different cell compartments.

Their ability to be recognized in sunflower lines has been applied to identify new resistances. Effector recognition in sunflower leaves is visualized by reactions of localized cell death called hypersensitive response (HR) characteristic of the plant's resistance to infection. Five effectors, among the 30 studied, trigger HR in sunflower lines carrying broad-spectrum resistance, some of which had not yet been published.

For one of these conserved effectors of downy mildew, we were able to identify the precise region of the sunflower genome that carries the gene giving resistance to this effector. We thus propose a pair of avirulence (virulence factor unmasked by the plant) / resistance genes never described in sunflower before. The specific response to such effectors makes it possible to differentiate between sunflower resistances, in particular when they are very close on the sunflower genome. This approach simplifies and speeds up the genomic localization of new resistance to downy mildew.

Ten new mapped resistance genes **

The second approach has been the characterization in sunflower of new broad-spectrum sources of resistance to downy mildew, effective against at least 16 pathotypes of downy mildew with various modes of virulence, and therefore against any attack of downy mildew in Europe.
These resistances were found in wild sunflower (Helianthus annuus), or in related Helianthus species, and were crossed with cultivated sunflower. The genes responsible for these resistances were mapped on the sunflower reference genome, using high density Axiom® DNA chips to determine genetic variations between the different sunflowers. Ten new resistance genes have been identified and differentiated according to their position on the genome, their origin, and the hypersensitive response to a downy mildew effector.
This first large-scale physical mapping of downy mildew resistance genes on the sunflower genome made it possible to locate the 10 new genes and also 10 already published Pl genes. This knowledge should help to increase the range of resistance genes used in sunflower varieties and so improve durability of effective downy mildew resistances.

Optimize genetic resistance in sunflower

The use of effectors conserved in all the pathotypes of downy mildew of varied virulence and thus playing probably an important role for the pathogen, makes it possible to discover sunflower resistance genes which should be more durable.

The use of these resistances within the framework of a reasoned agriculture, by combining them or alternating them in place or time should limit the impact of the pathogen. The key is the construction of cropping systems and technical itineraries that are less risky for growers.

This work was carried out by the research teams of the Plant-Microorganisms Interaction Laboratory (LIPM) in Toulouse, including the Sunflower Genetics and Genomics and Bioinformatics groups, the Genome and Transcriptome platform (GET-Plage) at Toulouse, the Genotyping and Sequencing platform Gentyane and the Genetics Diversity and Ecophysiology of Cereals (GDEC) laboratory at Clermont-Ferrand.

Scientific contact(s):


*Sunflower resistance to multiple downy mildew pathotypes revealed by recognition of conserved effectors of the oomycete Plasmopara halstedii. Yann Pecrix, Luis Buendia, Charlotte Penouilh-Suzette, Maude Maréchaux, Ludovic Legrand, Olivier Bouchez, David Rengel, Jérôme Gouzy, Ludovic Cottret, Felicity Vear, Laurence Godiard. Plant J. 2018 Nov 13. doi: 10.1111/tpj.14157

**Ten broad spectrum resistances to downy mildew physically mapped on the sunflower genome. Yann Pecrix, Charlotte Penouilh-Suzette, Stéphane Muños, Felicity Vear and Laurence Godiard. Front. Plant Sci. doi: 10.3389/fpls.2018.01780

Genetic resources of wild and cultivated sunflowers

Biological Resources Centre (BRC) for sunflower. © INRA
Biological Resources Centre (BRC) for sunflower © INRA
As a public research institute, INRA explores and preserves biodiversity and genetic resources of several crops. These resources are available to public and private collaborators according to national policies and international agreements. In order to maintain a wide genetic diversity for research and breeding, the Biological Resources Centre (BRC) for sunflower has been developing its collection since the 1960s and is now operating at INRA Occitanie-Toulouse.

  • 2313 cultivated lines
  • 403 cultivated populations (open-pollinated varieties, landraces, breeding pools)
  • 537 related wild-type ecotypes from the Helianthus genus

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