Unusual evolutionary mechanisms to escape effector‐triggered immunity in the fungal phytopathogen Leptosphaeria maculans

Abstract

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.

A Six-Year Investigation of the Dynamics of Avirulence Allele Profiles, Blackleg Incidence, and Mating Type Alleles ofLeptosphaeria maculansPopulations Associated with Canola Crops in Manitoba, Canada

“…A hide and seek relationship between AvrLm3 and AvrLm4-7 was reported by Plissonneau et al (2016), where L. maculans isolates carrying AvrLm4-7 also carried AvrLm3 (but the function of AvrLm3 was blocked by AvrLm4-7). A very recent paper by Plissonneau et al (2017) demonstrated the "camouflaged model," where the loss of Rlm3-mediated resistance response in isolates virulent toward Rlm3 occurred by two methods. The first method was through deletion or inactivation mutations of AvrLm4-7, which resulted in virulent isoforms of the AvrLm3 protein.…”

A two genes – for – one gene interaction between Leptosphaeria maculans and Brassica napus

“…Another unusual particularity of the AvrLm10A-AvrLm10B pair lies in its effect on lesion size. Until now, all three tested L. maculans avirulence genes have been shown to promote lesion development (Huang et al, 2010;Nov akov a et al, 2016;Plissonneau et al, 2017). Isogenic lines or isolates complemented with the avirulence gene induced larger lesions than those in which the gene was absent.…”

Rapid evolution in plant–microbe interactions – a molecular genomics perspective

“…1). Recently reported examples include SNPs (Lu et al, 2016;Plissonneau et al, 2017b;Zhong et al, 2017;Meile et al, 2018), deletions , transposable element (TE) insertions (Wu et al, 2015;Zhang et al, 2015), and epigenetic gene silencing (Qutob et al, 2013) of avirulence genes, all of which can result in a gain of virulence. An instance of great agronomical relevance is the emergence of the rice blast fungus as a novel pathogen of wheat that was promoted by the loss of the critical PWT3 Avr gene (Inoue et al, 2017).…”