Xerotolerance of Penicillium and Phialocephala fungi, dominant taxa of fine lateral roots of woody plants in the intermountain Pacific Northwest, USA

In the intermountain Pacific Northwest (PNW), USA, soil water potentials can fall below − 10 MPa in dry summers, leading to high fine lateral root turnover in woody plants across the region with implications for root endophyte communities. In a pilot study of three western conifers, we identified xerotolerant endophytic fungi dominant in the roots. We hypothesized that xerotolerant fungi dominate the root endophyte communities of woody plants in summer across the arid intermountain PNW without regard to host. We isolated fungi in summer from fine lateral roots of ten gymnosperm species and nine angiosperm species, spanning both ecto- and arbuscular mycorrhizal symbioses, using both a xerophile-selective medium with a water potential at − 20 MPa and a non-selective medium with adequate water availability. Penicillium and Phialocephala together constituted 69% (4742-6872) of all culturable isolates across all host species in fine lateral roots of woody plants across the region, regardless of mycorrhizal strategy or whether the hosts were gymnosperm or angiosperm. In paired t-tests, Penicillium spp. exhibited greater xerotolerance (53% of all isolates on glycerol-amended PDA belonged to Penicillium, but only 16% of PDA isolates did (P < 0.0001)) across all plants sampled. Phialocephala exhibited the opposite pattern: 21% and 42% of all isolates on glycerol-amended and standard PDA, respectively, with a significant difference among paired samples across plant species (P < 0.0001). Whereas 95.5% of representative isolates of sampled regional phylotypes of Penicillium were xerotolerant, only 22.2% of Phialocephala were. Xerotolerance was much more common in roots than in foliage: 54.2% of all root isolates were xerotolerant versus only 10.2% of needle isolates (P < 0.0001) in Pseudotsuga menziesii. Given these findings, reciprocal shifts in relative abundance between Penicillium and Phialocephala might be expected to be mediated by drought.