The Bcvic1 and Bcvic2 vegetative incompatibility genes in Botrytis cinerea encode proteins with domain architectures involved in allorecognition in other filamentous fungi

Abstract

Vegetative incompatibility is a fungal allorecognition system characterised by the inability of genetically distinct conspecific fungal strains to form a viable heterokaryon, and is controlled by multiple polymorphic loci termed vic (vegetative incompatibility) or het (heterokaryon incompatibility). We have genetically identified and characterised the first vic locus in the economically important, plant-pathogenic, necrotrophic fungus Botrytis cinerea. A bulked segregant approach coupled with whole genome Illumina sequencing in near-isogenic lines of cinerea was used to map a 60-kb genomic region for a vic locus. Within that locus, we identified two adjacent, highly polymorphic open reading frames, Bcvic1 and Bcvic2, which encode predicted proteins that contain domain architectures implicated in vegetative incompatibility in other filamentous fungi. Bcvic1 encodes a predicted protein containing a putative serine esterase domain, a NACHT family of NTPases domain, and several Ankyrin repeats. Bcvic2 encodes a putative syntaxin protein containing a SNARE domain; such proteins typically function in vesicular transport. Deletion of Bcvic1 and Bcvic2 individually had no effect on vegetative incompatibility. However, deletion of the region containing both Bcvic1 and Bcvic2 resulted in mutant lines that were severely restricted in growth and showed loss of vegetative incompatibility. Complementation of these mutants by ectopic expression restored the growth and vegetative incompatibility phenotype, indicating that Bcvic1 and Bcvic2 are controlling vegetative incompatibility at this vic locus. Author SummaryFungal colonies are characterised by radiating filaments, termed hyphae, which often fuse to form a highly interconnected individual. This is advantageous since it enables efficient water and nutrient utilisation across a colony network. However, hyphal fusion is not necessarily restricted to within an individual colony, with potential for hyphal fusion between individuals belonging to the same species. There are, however, drawbacks to this. For instance, viruses that detrimentally affect a colony may be transmitted, with their infection leading to a reduction in the virulence of a pathogenic species. Fungi have therefore developed complex systems to prevent fusion between genetically distinct individuals of the same species. This phenomenon is termed vegetative incompatibility and results in the death of fused cells and cessation of transfer of cellular contents from one individual to another. We have identified the first genes in the fungal plant pathogen Botrytis cinerea that control this phenomenon. They resemble genes that control vegetative incompatibility in other fungi, and genes involved in immunity in plants and animals. Uncovering further genes involved in vegetative incompatibility in B. cinerea may pave the way for the development of a super donor strain capable of overriding vegetative incompatibility to transmit viruses, thus enabling their exploitation as potent control agents against this damaging plant pathogen.