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.

Fig.: The Barley spike without awns from an Ethiopian landrace has a specific resistance gene, called Mlo, and has protected the plants from powdery mildew. The resistance gene was introduced into European cultivars (spike with awns) by traditional breeding methods in the 1970's.
Image: Ralph Panstruga, Max Planck Institute for Plant Breeding Research

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.