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Film d'animation du projet Road Movie. © INRA

Plants take up their arms

At INRA Angers-Nantes, IRHS researchers are studying plant mechanisms of resistance against disease, and in particular against apple scab. Within the framework of a regional project, an animated film entitled “Road Movie” was made to explain how scientists are identifying genes that have a durable resistance. Don’t miss it!

By Patricia Léveillé, translated by Inge Laino
Updated on 02/11/2016
Published on 01/29/2016

One of life’s small pleasures is sinking one’s teeth into a crispy apple! Except if the apple is infected with apple scab… This disease, caused by the Ventura ineaqualis fungus, really takes a toll on an apple’s appeal with brownish spots and bruises. It can reduce, or even wipe out, the production of an apple tree without actually killing the tree. Generally speaking, where there are apple trees, there is apple scab. More than 90% of European apple orchards are planted with varieties that are susceptible to the disease. Apples are the world’s third fruit crop, and Europe’s first, and the industry now seeks nothing short of total eradication of the disease. In retail, bruised fruit is not considered shelf-worthy, sounding the death knell for growers.

To fight against the disease, growers have recourse to several methods, including the use of resistant varieties. Unfortunately, the fungus responsible for apple scab often finds a way to adapt to natural resistance. “In the race to find a solution, it’s often the pathogen that wins, because it has an incredible capacity to adapt. That is why we have to protect resistance with complimentary methods, once the varieties are introduced into orchards” explains Bruno Le Cam, INRA director of research at the Research Institute of Horticulture and Seeds (IRHS) in Angers and coordinator of the Road Movie project.

Gene for gene: searching for durable resistance

Faced with this “resistance failure” vis-à-vis disease, researchers are testing a new strategy based on knowledge of the mechanisms that come into play during an infection. “When a plant gets infected, the pathogen secretes hundreds of little proteins”, explains Bruno Le Cam. Meanwhile, the plant is able to recognize the presence of the pathogen thanks to receptor proteins encoded by resistance genes. All it takes is for one single fungal protein to enter into contact with one of the plant’s receptors for the plant to trigger defence mechanisms and stave off the attack. That is what is referred to as gene for gene interaction: for every resistance gene there is a corresponding avirulence gene. This system works like a charm as long as the pathogen is recognised. However, it is a known fact of nature that random mutations take place within a pathogen’s genome. If the gene that encodes the avirulent protein mutates, the plant’s receptor protein won’t recognise it, and the disease can wreak havoc. Based on this knowledge, scientists are trying to predict the durability of resistance genes. They must identify avirulent proteins in pathogens that are indispensable to the survival of the fungus, and for which a mutation would be fatal. “If an avirulent protein never mutates, the corresponding resistance gene - which still has to be identified in our genetic databases - should be durable. That’s what we’re betting on!” explains Bruno Le Cam.

A huge asset for IRHS is the access it has to a large range of strains that allows scientists to look for fungal proteins that never underwent a mutation. “We have already sequenced 90 genomes of the Ventura ineaqualis strainfrom wild and cultivated apple trees in five continents, or from other plants of the Rosaceae family such as firethorn and medlar. If we don’t observe any mutations among these ‘candidate genes’ despite the genetic and geographic diversity of the samples, that means that the configuration or modus operandi of these proteins is truly indispensable to the survival of the fungus”. Since it has substantial genetic resources for apple trees, IRHS can bombard apple trees with “candidate” fungal proteins to discover the famous durable genes of resistance. In that plant material lies, quite possibly, the source of durable resistance that will bless the varietals of tomorrow.

Scientific contact(s):

Associated Division(s):
Plant Health and Environment, Plant Biology and Breeding
Associated Centre(s):
Pays de la Loire

Apple scab, or the success story of a fungus from Asia

Map of the trajectory of apple scab. © INRA
Map of the trajectory of apple scab © INRA

The Road Movie project

The Road Movie project is about “Apples’ resistance to disease: virulence mechanisms and the identification of effectors”. Several teams - EcoFun, ResPom, Bioinfo and FungiSem - from the Research Institute of Horticulture and Seeds took part in the project, with the support of the Loire region.