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BOLOGNA/ZURICH - Past efforts to classically breed apple cultivars with
resistance to apple scab disease have proved insufficient. Direct gene
transfer experiments involving genes from scab-resistant wild apples,
however, have lead to promising results, July 2004 by Mark Finlayson,
Checkbiotech.

Apple scab is a fungal disease of major importance to apple growers and is
caused by Venturia inaequalis. This fungus, of the family ascomycete, spends
winter on dead leaves beneath trees. After spring time rainfall, it then
sporulates and infects the leaves and fruit of nearby trees, causing
undersized, gnarled and thus worthless apples.

Commercial orchards have retreated to removing fallen leaves in autumn,
combined with rigorous and costly fungicide spray schedules throughout most
of the season, with costs exceeding $100 per acre a year.

An obvious alternative solution is the use of resistant cultivars. Several
small-fruited, scab-resistant wild species have been investigated, in
particular Malus floribunda 821. In M. Floribunda, scientists have
identified the genes of the so-called Vf region as an important factor in
resistance to V. inaequalis.

Conventional breeding methods have been used to develop several
scab-resistant cultivars from M. floribunda, but none of them has yet met
the fruit-quality standards of other commercially successful cultivars such
as Golden Delicious or Gala.

A group led by Enrico Belfanti at the University of Bologna and Eve
Silfverberg-Dilworth at the Swiss Federal Institute of Technology in Zurich
has now conducted a gene transfer experiment in which one of the Vf genes,
HcrVf2, was introduced into the susceptible cultivar Gala by means of
A.tumefaciens-mediated transformation.

The five resulting transgenic lines were tested in greenhouse conditions for
scab resistance. The leaves of three lines showed no signs of infection. In
rare cases, some pinpoint pits, which are signs of resistance reactions,
were observed. Another line showed some restricted fungal development. In
comparison, untreated Gala and one (only partially transformed) line were
severely damaged.

The experiment successfully demonstrated the capability of HcrVf2 to confer
resistance to the scab disease. Field trials such as this one could lead the
way to elaborate gene therapies and change the future of pest control in
commercial apple orchards, not only cutting costs for growers, but also
reducing the usage of fungicide.

Further studies will seek to provide insight into the exact mechanisms of
scab resistance. The precise function of the other genes of the Vf cluster,
for example, still remains to be examined.

Mark Finlayson is a Biology student at the University of Basel and a Science
Writer for Checkbiotech. Contact him at info@finsta.ch about this article.

[www.checkbiotech.org];
subtopic_id=7&doc_id=8293

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