Unusual evolutionary mechanisms to escape effector-triggered immunity in the fungal phytopathogen Leptosphaeria maculans.
Leptosphaeria maculans is the fungus responsible for the stem canker disease of oilseed rape (Brassica napus). AvrLm3 and AvrLm4-7, two avirulence effector genes of L. maculans, are involved in an unusual relationship: AvrLm4-7 suppresses the Rlm3-mediated resistance. Here, we assessed AvrLm3 polymorphism in a collection of 235 L. maculans isolates. No field isolates exhibited deletion or inactivating mutations in AvrLm3, as observed for other L. maculans avirulence genes. Eleven isoforms of the AvrLm3 protein were found. In isolates virulent towards both Rlm3 and Rlm7 (a3a7), the loss of the Rlm3-mediated resistance response was due to two distinct mechanisms. First, when AvrLm4-7 was inactivated (deletion or inactivating mutations), amino acid substitutions in AvrLm3 generated virulent isoforms of the protein. Second, when only point mutations were observed in AvrLm4-7, a3a7 isolates still contained an avirulent allele of AvrLm3. Directed mutagenesis confirmed that some point mutations in AvrLm4-7 were sufficient for the fungus to escape Rlm7-mediated resistance while maintaining the suppression of the AvrLm3 phenotype. Signatures of positive selection were also identified in AvrLm3. The complex evolutionary mechanisms enabling L. maculans to escape Rlm3-mediated resistance while preserving AvrLm3 integrity, along with observed reduced aggressiveness of isolates silenced for AvrLm3, serves to emphasize the importance of this effector in pathogenicity towards B. napus. While the common response to resistance gene pressure is local selection of isolates depleted in the cognate avirulence gene, this example contributes to complexify the gene-for-gene concept of plant-pathogen evolution with a 'camouflaged' model allowing retention of nondispensable avirulence effectors.
UMR BIOGER, INRA, AgroParisTech, Université Paris-Saclay, Avenue Lucien Brétignières, BP 01, F-78850, Thiverval-Grignon, France.|Plant Pathology, Institute of Integrative Biology, ETH Zurich, Universitätstrasse 16, 8092, Zürich, Switzerland.
C
C
Plissonneau
UMR BIOGER, INRA, AgroParisTech, Université Paris-Saclay, Avenue Lucien Brétignières, BP 01, F-78850, Thiverval-Grignon, France.
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F
Blaise
UMR BIOGER, INRA, AgroParisTech, Université Paris-Saclay, Avenue Lucien Brétignières, BP 01, F-78850, Thiverval-Grignon, France.
B
B
Ollivier
Terres Inovia, Avenue Lucien Brétignières, F-78850, Thiverval-Grignon, France.
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M
Leflon
Terres Inovia, Avenue Lucien Brétignières, F-78850, Thiverval-Grignon, France.
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J
Carpezat
UMR BIOGER, INRA, AgroParisTech, Université Paris-Saclay, Avenue Lucien Brétignières, BP 01, F-78850, Thiverval-Grignon, France.
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T
Rouxel
UMR BIOGER, INRA, AgroParisTech, Université Paris-Saclay, Avenue Lucien Brétignières, BP 01, F-78850, Thiverval-Grignon, France.
M-H
http://orcid.org/0000-0001-5523-9180
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Balesdent
D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D001203:Ascomycota / Q000235:genetics* / Q000472:pathogenicity; D005075:Biological Evolution; D029688:Brassica napus / Q000382:microbiology*; D060467:Disease Resistance; D005787:Gene Frequency; D005800:Genes, Fungal; D005828:Genetics, Population; D009154:Mutation; D010641:Phenotype; D010935:Plant Diseases / Q000382:microbiology*; D057865:Plant Immunity; D011110:Polymorphism, Genetic; D017433:Protein Structure, Secondary; D012641:Selection, Genetic; D014774:Virulence / Q000235:genetics
effector; evolution; fitness; gene-for-gene interaction; oilseed rape; stem canker