Thermotolerance and post-fire growth in Rhizina undulata is associated with the expansion of heat stress-related protein families.

BACKGROUND: Rhizina undulata is an important tree pathogen, infecting a wide variety of conifer species, including those in the genus Pinus. The fungus relies on heat shock-mediated activation of its ascospores, which require high temperatures to initiate germination. Consequently, disease due to R. undulata often occurs after fire events in both natural and managed forests. The genetic mechanisms contributing to the pyrophilous nature of R. undulata have not been investigated. We sought to identify key genes that may be responsible for thermotolerance in the fungus. A comparative genomics approach was used, by sequencing the genome of R. undulata for the first time and comparing its predicted proteome to those from other Pezizomycetes, both with and without known associations with fire. RESULTS: Three protein families were shown to be expanded in R. undulata; heat shock protein 20 (HSP20), glutathione-S transferases (GST), and aromatic compound dioxygenases (ACD). While HSP20 was uniquely over-represented in R. undulata, the expansions of the GST and ACD families were also identified in other fire-associated species. CONCLUSION: HSP20s are known to protect cells against heat stress, GSTs are involved in the detoxification of reactive oxygen species, and ACDs play a role in the metabolism of recalcitrant compounds present in post-fire environments. The expansion of these families thus suggests that they may play an important role in protecting and stimulating R. undulata ascospores during and after fire-induced heat shock, activating the fungus, and enabling it to colonise the root systems of conifers.