Melampsora laricis-populina v1.0
Urediniospores (in yellow-orange pustules) of Melampsora laricis-populina are produced on Populus leaves, serving as inoculum for rust epidemics on Populus throughout the summer (Photo: F Martin). Spores of Melampsora laricis-populina (Photo: © D. Le Thiec, INRA Nancy).

The poplar leaf rust fungus Melampsora is the most devastating and widespread pathogen of poplars, and has limited the use of poplars for environmental and wood production goals in many parts of the world. Almost all known poplar cultivars are susceptible to Melampsora laricis-populina, and new virulent strains are continuously developing. This disease therefore has a strong potential impact on current and future poplar plantations used for production of forest products (principally pulp and consolidated wood products), carbon sequestration, biofuels production, and bioremediation. There is a pressing need to develop a thorough understanding of the Melampsora species that are poplar pathogens so that new control approaches can be established.

Melampsora laricis-populina belongs to the Basidiomycota (Pucciniomycotina ; Pucciniomycetes ; Pucciniales ; Melampsoraceae). It requires a Populus and a Larix host to complete its life cycle. The rust overwinters as teliospores on dead Populus leaves on the ground. These spores germinate in the spring, producing windborne basidiospores, which results in infection of larch needles. A few days later, masses of yellow orange aeciospores are produced on needles of the coniferous host. They serve as inoculum for infection of live Populus leaves during the spring. Urediniospores (in yellow-orange pustules) are then produced on Populus leaves, serving as inoculum for rust epidemics on Populus throughout the summer. In late summer, teliospores (the overwintering spores) are again produced on Populus leaves, completing the rust's life cycle. Melampsora laricis-populina is a close relative of other economically important rusts (Pucciniales), including Puccinia and other cereal rusts.

Besides its commercial importance, Melampsora shares a long coevolutionary history with Populus, with a constant interplay of resistance and pathogenicity. An improved understanding of the defense mechanism in poplar leaves may help to reduce the damage in plantations of the economically important poplar species and provide basic insights on the evolutionary biology of host-pathogen interactions.

To elucidate the genetic basis of the Melampsora-Populus interaction, we have sequenced the hundred million base-pair genome of the dicaryotic strain 98AG31 to high draft using a whole genome shotgun method. This is the first genome of a tree pathogen to be sequenced.

Finally, the comparison of the genomes of mutualistic (Laccaria bicolor) and pathogenic (Melampsora laricis-populina) basidiomycetes interacting with Populus will provide insights into pathogenicity/symbiosis mechanisms and into differences in evolutionary processes developed by the different types of biotrophic fungi.