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Querina (R) florina, apple variety resistant to apple scab. © INRA, LE LEZEC Marcel

Novel mechanisms for more sustainable resistance to disease

Drifting to avoid drowning (pepper)

A unique mechanism of quantitative resistance was brought to light by the pepper/potyvirus duo. This quantitative resistance works in synergy with a monogenetic resistance: it boosts sustainability by hindering the adaptation of the virus, thereby delaying circumvention.

By Pascale Mollier, translated by Inge Laino
Updated on 03/30/2015
Published on 01/22/2015

When a gene that is resistant to a certain pathogen is introduced into a plant, sooner or later the pathogen begins to circumvent the resistance.  By a mechanism of adaptation, the pathogen develops variants (mutants) that are virulent, that is, capable of sidestepping the plant’s resistance.

Scientists at INRA have brought to light a particular type of QTL (1) in the pepper/potyvirus combination that slows down this process. When these QTLs are associated with a major resistance gene (pvr2), the viral population in the plant decreases and the frequency and speed of circumvention is curtailed: the QTLs delay or prevent the onset of variants that have adapted to pvr2, that is, variants that can circumvent resistance and become aggressive again.

Genetic drift vs. resistant gene shipwreck

Raft of the Medusa, Géricault. Louvre museum, Paris.
Raft of the Medusa, Géricault. Louvre museum, Paris.

“The action of these QTLs can be explained by a mechanism known as ‘genetic drift’, by which the frequency of variants within a population changes randomly from one generation to the next. It is therefore a counterbalance to selection, by which the best-adapted variants are selected”, explains Alain Palloix, head of the programme (2).

Using a mathematical model, researchers have found that when one or more QTLs that reduce the size of a viral population by 105 to 50 particles is combined with a major pvr2 gene, the likelihood of circumvention is reduced eight-fold.

Searching for genetic drift QTLs

Scientists use findings from experiments on genetic evolution to identify new genetic drift QTLs. These experiments have brought to light:

  • several pepper genotypes that carry the major resistance gene pvr2 and different QTL combinations;
  • a viral population consisting of a mixture of variants more or less adapted to pvr2.

Several weeks after inoculation of the viral variant mixture in plants, the virus is extracted and the variants that still managed to infect the plant identified. Thanks to a mathematical model developed by researchers and capable of simulating average evolutionary trajectories of each viral variant, the genetic drift effect caused by QTLs can be deduced. The best QTL combinations can therefore be identified in varietal collections and genetic resources to “protect” monogenetic resistance. This research will allow scientists to select new varieties based not only on their resistance, but also on how well they control the evolution of bio-aggressors in the long term.

(1) QTL (quantitative trait loci): region of the genome whose polymorphic nature explains in significant part the variability of a measured trait.

(2) Key action TAKE CONTROL of the metaprogramme SMACH.

Scientific contact(s):

Associated Division(s):
Plant Biology and Breeding, Plant Health and Environment, Applied Mathematics and Informatics
Associated Centre(s):
Provence-Alpes-Côte d'Azur


The potyvirus group is the largest of all plant virus groups, its members numbering at least 180. All of them are capable of infecting more than 30 known plant families worldwide, including Solanaceae, Cucurbitaceae, Brassicaceae and Asteraceae. A potyviral infection causes discoloration and deformation of leaves, vein necrosis that can spread to the entire plant, a significant reduction in plant size, and severe productivity loss.

Of all the methods available for controlling viral infections in plants, the use of genetically resistant varieties is one of the best. It provides efficient protection, does not incur extra costs or labour for the grower, is environmentally friendly, and safe for consumers.

As the seed market becomes increasingly international, genes that control resistance to different potyviruses must be selected and used. Because potyviral infections can take a devastating economic toll, research on multi-resistant plant varieties is key.