From Arabidopsis to Prunus: towards the development of fruit trees resistant to plum pox virus

Collaborative work by INRA in Bordeaux and Avignon has focused on creating resistant varieties using methods other than transgenesis.  The aim is to search for resistance genes that exist naturally in the Prunus by exploring the genetic diversity of the species.  However, it is difficult to carry out such research in host trees, because the identification and isolation of candidate genes requires large populations covering several generations of crosses.  Furthermore, tests of viral susceptibility in a fruit tree require at least four years of follow-up.  For this reason, the researchers have chosen to use Arabidopsis thaliana, a model plant with a short cycle (4 to 5 weeks), on which experiments are facilitated and whose genome is already known.  Once they have identified resistance or susceptibility genes in Arabidopsis, the researchers may be able to search for their equivalents in Prunus, which will thus accelerate the entire process.

Recent studies have confirmed the validity of the approach via Arabidopsis.  Although this small plant belongs to the family of crucifers (cabbage, rapeseed, mustard), it appears to constitute a good model for the study of plum pox virus.  Indeed, the inoculation of five different isolates of the virus in the leaves of 12 different ecotypes of Arabidopsis² caused a broad range of responses in the plant, a phenomenon that can also be observed in Prunus.

The researchers observed that most combinations of viral isolates and Arabidopsis ecotypes (52 out of 60) did not trigger the appearance of symptoms.  They thus focused on these asymptomatic situations in the hope of finding clues to the development of trees which did not react with the virus.
Of these asymptomatic combinations, some (30 out of 60) were said to be "tolerant", i.e. the virus developed and spread to neighbouring tissues from the infected leaf but did not cause any symptoms.
In the others (22 out of 60), true resistance was seen, i.e. spread of the virus was halted.  What was interesting was that at least two types of resistance were demonstrated, thus indicating the existence of several genes controlling distinctive mechanisms of resistance to plum pox virus.  In the first type of resistance, the virus was eliminated; it did not develop, not even at the sites of primary infection.  In the second type, invasion of the plant was blocked: the virus developed in infected cells but did not spread to other tissues.  The researchers also showed that these two types of resistance corresponded to different genes.

The perspectives consist in firstly, the transfer of knowledge already acquired in Arabidopsis to cultivated fruit species, and secondly, the identification in Arabidopsis of other, as yet unknown resistance genes so that their equivalents can be determined in Prunus.
For the moment, the resistances identified in Arabidopsis concern cellular protein genes that the virus circumvents to its benefit in order to multiply and/or spread.  In resistant plants, these genes code for forms of these factors that the viruses cannot use to their benefit.  This type of resistance also seems to exist in Prunus.  In the short term, and backed by a research contract funded by Viniflhor, the aim is thus to assess their representativeness in Prunus.

By cumulating different complementary mechanisms for resistance to plum pox virus, it may be possible to create sustainably resistant varieties of Prunus.

¹ The plants are grafted on a root-stock of a different variety in order to cumulate the advantages of both varieties.
² Ecotype: characterises organisms sampled from the same ecosystem and sharing a certain number of phenotypic and genotypic traits.

Véronique DECROOCQ coordinates the European Sharco programme (2007-2011) which aims to combine prophylactic solutions (detection, crop protection) and genetic solutions (development of resistance) in order to prevent or limit the prevalence of plum pox virus.  This programme involves thirteen research institutions from eleven countries in Europe and North America.