Cell death by incompatibility in the fungus Podospora

A novel gene, phcA from Pseudomonas syringae induces programmed cell death in the filamentous fungus Neurospora crassa

The phytopathogen Pseudomonas syringae competes with other epiphytic organisms, such as filamentous fungi, for resources. Here we characterize a gene in P. syringae pv. syringae B728a and P. syringae pv. tomato DC3000, termed phcA, that has homology to a filamentous fungal gene called het-c. phcA is conserved in many P. syringae strains, but is absent in one of the major clades, which includes the P. syringae pathovar phaseolicola. In the filamentous fungus Neurospora crassa, HET-C regulates a conserved programmed cell death pathway called heterokaryon incompatibility (HI). Ectopic expression of phcA in N. crassa induced HI and cell death that was dependent on the presence of a functional het-c pin-c haplotype. Further, by co-immunoprecipitation experiments, a heterocomplex between N. crassa HET-C1 and PhcA was associated with phcA-induced HI. P. syringae was able to attach and extensively colonize N. crassa hyphae, while an Escherichia coli control showed no association with the fungus. We further show that the P. syringae is able to use N. crassa as a sole nutrient source. Our results suggest that P. syringae has the potential to utilize phcA to acquire nutrients from fungi in nutrient-limited environments like the phyllosphere by the novel mechanism of HI induction.

The fungus-specific HET domain mediates programmed cell death in Podospora anserina

Vegetative incompatibility is a programmed cell death reaction that occurs when fungal cells of unlike genotypes fuse. Genes defining vegetative incompatibility (het genes) are highly polymorphic, and most if not all incompatibility systems include a protein partner bearing the fungus-specific domain termed the HET domain. The nonallelic het-C/het-E incompatibility system is the best-characterized incompatibility system in Podospora anserina. Cell death is triggered by interaction of specific alleles of het-C, encoding a glycolipid transfer protein, and het-E, encoding a HET domain and a WD repeat domain involved in recognition. We show here that overexpression of the isolated HET domain from het-E results in cell death. This cell death is characterized by induction of autophagy, increased vacuolization, septation, and production of lipid droplets, which are hallmarks of cell death by incompatibility. In addition, the HET domain lethality is suppressed by the same mutations as vegetative incompatibility, but not by the inactivation of het-C. These results establish the HET domain as the mediator of cell death by incompatibility and lead to a modular conception of incompatibility systems whereby recognition is ensured by the variable regions of incompatibility proteins and cell death is triggered by the HET domain.