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Max Planck Researchers in Cologne, Germany, unravel mechanism of resistance
to fungal infection in barley, August 2004.

Powdery mildew is a typical fungal infection in crop plants and only the
regular application of fungicides prevent huge yield losses in agriculture.
Some crops, however, hold a natural resistance against powdery mildew like
cultivars of the European barley with a mutation in the Mlo gene. Scientists
from the Max Planck Institute for Plant Breeding Research (MPIZ) in Cologne
have collaborated with colleagues from Great Britain, France and Denmark to
solve the mystery of the resistance mechanism and to highlight the cultural
history of plant breeding (Nature, 19.08.2004, cover story).

Plants have - similar to animals and humans - a sophisticated multi-level
immune system which enables them to identify parasites and destroy them. The
detection of parasites is based on an armada of plant receptors - a plant
radar system which signalizes pathogen invasion. To circumvent the immune
system, a parasite has to either slip through the plant radar system or
affect the cellular immune response following its detection. The mildew
chose the latter strategy and therefore manipulates the so called MLO
protein in the cell membrane of Barley cultivars that is encoded by the
corresponding Mlo-gene in the genome.

Although laboratory experiments with mutated mlo-genes confirmed a
correlation to the resistance, the detailed genetic analysis revealed no
causal differences between the DNA sequence of the mlo gene from resistant
and susceptible plants. Thus, the precise mechanism behind this resistance
remained unknown.

A specific mlo-resistance-gene recovered from a natural habitat was
originally retrieved from Ethiopian landraces, primitive forms of Barley
cultivars. They were collected during an expedition in 1937. Nowadays this
mutation plays a crucial role in mildew resistance; it was introduced by
traditional plant breeding methods into approximately 70 percent of the
cultivated European spring barley elite varieties since the 1970?s. Barley
is the raw material for beer and whiskey production. The mlo-resistant
cultivars have proved valuable in agriculture for over thirty years,
reducing the need for agro-chemical fungicide treatment.

The mystery underlying this mildew resistance strategy was disclosed when
the research groups of Ralph Panstruga and Paul Schulze-Lefert discovered a
mlo gene fragment which occurs in several repeats in the genome of the
mutant. About ten adjacent repeats could be detected during analysis. They
are located "upstream" of the wild-type gene on the DNA and are directly
linked to mildew resistance. "The repeats are read along with the normal
gene," explains Schulze-Lefert. "The original reading frame cannot be
recognized anymore and the MLO protein is therefore no longer produced in
the cells." Even in the rare cases where the reading frame is detected by
enzymes, the MLO protein can be produced in minimal amounts only and mildew
will grow just marginally on the leaves.

But another question also interested the scientists: When did the mutation
of the mlo gene first occur in nature? A genetic fingerprint of the
Ethiopian landrace disclosed that this mutation occurred very recently -
less than 10,000 years ago. "We assume that mlo resistance arose only once,
presumably in Ethiopia, some time after the crop had been domesticated by
native Ethiopians," says Ralph Panstruga.

Today?s agriculturally used cultivars of Barley are genetically closely
related: there are not more than three basic kinds in contrast to the almost
unlimited natural diversity of wild type barley. These new results emphasize
the need to maintain and characterize the natural biodiversity of crop
plants as a source of agriculturally important traits and underline the
increasing power of molecular studies for understanding the mechanisms
underlying functional biodiversity.

Contact:
Claudia Lorenz
Max Planck Institute for Plant Breeding Research, Carl-von-Linné-Weg 10,
50829 Cologne
Tel.: +49 221 5062-672
Fax: +49 221 5062-674
E-mail: lorenz@mpiz-koeln.mpg.de

[www.mpg.de]
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