Towards evolutionary and functional characterization of an avirulence gene in the poplar rust fungus
Abstract
Poplar rust, caused by the pathogenic fungus Melampsora larici-populina (Basidiomycota,
Pucciniales), is the main phytosanitary constraint for commercial poplar cultivation in Europe
and other parts of the world. It affects also wild poplars (Populus nigra) but to a lesser
extent. In the last 50 years many rust-resistant cultivars were bred and released, but all the
qualitative resistance genes (i.e. major resistance genes) released were overcome by
pathogen evolution within a short period. Qualitative resistance is particularly subject to
breakdown by pathogen evolution for perennial hosts, such as poplar trees, because of the
wide inequality between the pathogen's rapid generation time and the time needed to
deploy new host varieties. For instance, resistance R7 was released in poplar plantations in
the 1980's and some R7 cultivars were widely planted in northwestern Europe. In 1994,
breakdown of the R7 resistance was detected in Belgium and northern France, and virulent 7
M. larici-populina individuals spread all over Western Europe in less than five years, causing
very destructive epidemics, and leading to a complete replacement of the pathogen's
populations, both on cultivated and wild poplars. Through a genome-wide association study
(GWAS), we recently identified a locus in the genome of M. larici-populina, which likely
corresponds to the Avr7 avirulence locus, whose mutation is responsible for R7 resistance
breakdown in poplar. The candidate avirulence gene possesses some characteristics of
canonical effectors (coding a small cysteine rich protein, specific to M. larici-populina, early
expressed and of unknown function), but lacks a secretion signal. Our aim is functionally
validate this avirulence locus and to study the evolution of this locus over time in natural
populations of the pathogen. This would be the first avirulence gene identified in this
biotrophic fungal species. A better understanding of the evolution of the pathogen's genome
and the genetic architecture of virulence would help defining breeding strategies for durable
resistance, especially for perennial crops, such as poplars.