Urediniospores (in yellow-orange pustules) of Melampsora larici-populina are produced on Populus leaves, serving as inoculum for rust epidemics on Populus throughout the summer (Photo: F Martin).

Spores of Melampsora larici-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 larici-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 larici-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 larici-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 dikaryotic 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 larici-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.

Melampsora larici-populina version 2.0 is a reassembly of original data guided by a genetic map provided by Uffe Helsten which was based on 85 offspring, and has map position information at 29,168 unique positions in the genome.  Scaffold breaks were identified using an abrupt change in the linkage group.  A total of 62 breaks were made. Scaffolds were then oriented, ordered, and joined together using the map and assembled into 18 chromosomes.  A total of 186 joins were made during this process.