Hemileia vastatrix

Introduction

Basidiomycetes responsible for rust diseases regularly cause dramatic damages on plants of economical importance such as Coffee-tree, Poplars, Wheat or Flax. There is a strong lack of knowledge concerning the mechanisms that control rust disease development in host-plants. Rust fungi are obligate biotrophs (i.e. they develop in live host plant) that cannot be cultivated, which is a serious limitation to their study in the lab. The recent release of the genome sequences of the wheat rust Puccinia (Broad Institute) and the poplar rust Melampsora (Joint Genome Institute) opens unparallel perspectives for a better understanding of plant-pathogen interactions and to define new control approaches of rust disease.

This collaborative project between INRA, IRD and the Genoscope will focus on the leaf rusts Melampsora larici-populina and Hemileia vastatrix attacking Poplars and Coffee-trees respectively. The aim of this project is to use the 454-pyrosequencing technique to identify transcripts coding for rust virulence factors in the two pathosystems. Comparison of expressed sequence tags with genome sequences and catalogs of predicted genes available for plant and fungal species in international genomic databases will allow to derive specific sequences belonging to the plant host or the pathogen. A particular attention will be given to transcripts encoding small secreted fungal proteins that likely contain putative effectors.

Hemileia vastatrix

Hemileia vastatrix (Berk. & Broome) is a basidiomycete that belongs to the Pucciniales order. H. vastatrix is an obligatory parasite specific of coffee plants (Coffea sp.) and some Psilanthus. Coffee leaf rust is one of the most destructive disease of the cash crop Coffea arabica that produces the high cup-quality beverage called arabica coffee.

The fungus is widely distributed in all regions of the world where coffee is grown and can represent serious limitation to economically sustainable coffee production (Bettencourt and Rodrigues, 1988). Little information is available on the fungal genetic factors that condition the coffee-rust interaction. Orange rust is regarded as the most harmful cryptogamic disease of the coffee-tree and it is responsible for heavy losses which amount millions of dollars for coffee culture in the world. The production losses due to rust are estimated at 30% if no treatment is employed.

H. vastatrix is a macrocyclic rust with no alternate host identified so far. The infection process of H. vastatrix on coffee leaves involves specific events including appressorium formation over stomata, penetration into the leaf and colonization of living host cells by intracellular specialized fungal structures (haustoria). The fungus colonizes all the leaf mesophyll by intercellular hyphae, which will give rise to new spores 15 to 24 days after infection (a lesion may produce between 300,000 and 400,000 spores). To date, almost no genetic or molecular data is available for H. vastatrix.

See more : Melampsora larici-populina