Botrytis cinerea

Introduction

Botrytis cinerea (the asexual stage of the teleomorph Botryotinia fuckeliana) is one of the principal pathogenic fungi of grapevines, and is responsible for major viticulture losses in France and the rest of Europe.
B. cinerea is a haploid Euascomycete fungus. It belongs to the class Leotiomycetes, with a genome size of 30 Mb. It is a model species for the infectious process in phytopathogenic fungi. This fungus causes infections which are characterized by the rapid destruction of the tissues of its host plant as it proceeds to colonize it (necrotrophy).

Why sequence the genome of Botrytis cinerea?

The sequencing project for the genome of the fungus, B. cinerea, is being done within the framework of the molecular analysis of the “vineyards and its pathogens” ecosystem including the Stolbur phytoplasma. The characterization of the genomes of the grape and its principal pathogens (B. cinerea, Stolbur) provides the opportunity to study the mechanisms implicated in the interactions between the grapvine and its pathogens as well as the dynamics of these pathogens in the “vineyard” ecosystem.
The production of genomic sequences will make it possible to construct DNA chips for the host plant and its pathogens which will be used to identify the genes induced or repressed during an infection. The validation of these transcriptional analyses will be facilitated by the development of functional genomics tools for each organism. The molecular analysis of both partners in a plant-pathogen interaction is necessary to better understand these diseases and to discover new methods of defense against them.

Economic importance of Botrytis cinerea

gray mold in vineyards thanks to B. cinerea (Photo Anne-Sophie Walker © INRA)

Filamentous fungi are the principal pathogenic agents of plants and the damage provoked by these microbes is responsible for about 20% of the crop losses in the world. B. cinerea is the causative agent of gray mold in vineyards and of diseases in several hundred other host plants. Losses due to this fungus account for 20% of harvests of the affected crops in the world, and their cost is estimated at 10-100 billion euros per year.
Estimated losses for vineyards in France amount to 15-40% of harvests depending on climatic conditions. Other losses are estimated at 20-25% of the strawberry crops in Spain, and 20% of cut flowers in Holland.
The fight against fungal diseases provoked by B. cinerea requires ths use of fungicides, the modification of cultivation techniques and the development of resistant or tolerant cultivars. Fungicidal treatments against B. cinerea cost about 540 million euros in 2001, which represents 10% of the world fungicide market (Annual Report, UIPP, 2002). Furthermore, the magnitude of the fungicidal treatments against this fungus has provoked the appearance of resistant strains, necessitating the development of new molecules (Leroux et al., 2002 Pest Manag. Sci. 58:876).
The damage caused by gray mold have a disastrous economic impact on numerous crops, notably vineyards in France. There are, however, specific climatic conditions under which B. cinerea causes symptoms of noble rot which is essential for the production of syrupy wines with high profitability such as Sauternes.

Importance of Botrytis cinerea in phytopathology

Because of its polyphagic and necrotrophic characteristics, B. cinerea is a good model for the study of fungal infectious processes. This fungus preferentially attacks fruits (grapes, strawberries, tomatoes) and flowers, upon which it produces a gray rot. It is also capable of attacking stems, leaves and seeds. Furthermore, it has been shown in the laboratory that the same isolate is capable of attacking very different plant hosts (bean, grapevine, tomato, Arabidopsis, rose) and different organs of the same plant (fruits, leaves, petals).
B. cinerea causes infections characterized by rapid destruction (maceration, necrosis) of the tissues of its plant host as it colonies it (necrotrophy). B. cinerea is therefore an excellent model for the study of this type of infectious process which is characteristic of numerous fungal diseases of plants. This fungus utilizes a large range of pathogenic factors (lytic enzymes, activated oxygen forms, toxins, plant hormones) to attack its host plants, including grapevines. Access to its genome should lead to the identification of genes and functions implicated in this type of infectious process.

Phylogenetic interest of Botrytis cinerea

The majority of fungal species which are pathogenic for plants are Ascomycetes from three classes: Sordariomycetes, Loculoascomycetes and Leotiomycetes.
The Sordariomycete group includes non-pathogenic model species such as Neurospora crassa and Posdospora anserina which are used for basic research on essential biological processes and pathogenic species of plants such as Magnaporthe grisea and Fusarium graminearum.

The genomes of N. crassa, M. grisea and F. graminearum have been sequenced (NSF-Broad Institute, USA), as well as that of P. anserina (Genoscope). There are no sequences for the Leotiomycetes or the

Loculoascomycetes which are accessible to the scientific community. A sequencing project for the phytopathogenic fungus, Sclerotinia sclerotiorum, which belongs to the class Leotiomycetes, has been initiated in the framework of a NSF-Broad Institute grant program in 2004. This fungus, although closely related taxonomically to B. cinerea, differs in many biological and molecular characteristics (see paragraph below). The sequencing of B. cinerea should therefore provide a new genomic resource as a phytopathogenic representative of the Leotiomycetes.

Botrytis cinerea as a molecular and genetic model

The complete sequencing of Botrytis cinerea will be facilitated because of the haploid nature of its genome and its relatively small size (30 Mb). By comparison with the genomes of Ascomycetes which have already been sequenced, it is estimated that this fungus has 10,000 to 12,000 genes spread over about 10 chromosomes. Furthermore, although the molecular biology of B. cinerea has only been studied for about a decade, all the tools of classical and molecular genetics (crosses, transformation, homologous recombination and replacement of genes, insertional mutagenesis and collections of mutants, ESTs, macroarrays, genomic and expression libraries) are available for performing studies of functional genomics.