Dark septate endophytes (DSE) of the Phialocephala fortinii s.l. – Acephala applanata species complex in tree roots: classification, population biology, and ecology

Abstract

Dark septate endophytes (DSE), a diverse group of ascomycetes, are dominant root colonizers in many ecosystems. The most frequent DSE in natural forest ecosystems in the Northern hemisphere belong to the Phialocephala fortinii s.l. – Acephala applanata species complex (PAC). Recently, species rank was assigned to seven cryptic species (CSP) of P. fortinii s.l.: Phialocephala fortinii s. str. C.J.K. Wang & H.E. Wilcox, Phialocephala europaea C.R. Grünig et T.N. Sieber, Phialocephala helvetica C.R. Grünig et T.N. Sieber, Phialocephala letzii C.R. Grünig et T.N. Sieber, Phialocephala subalpina C.R. Grünig et T.N. Sieber, Phialocephala turiciensis C.R. Grünig et T.N. Sieber, and Phialocephala uotolensis C.R. Grünig et T.N. Sieber. PAC species occur on all parts of the root system of trees, from mycorrhizal root tips to the stem base. Up to 80% of fine roots in forest stands can be colonized by them, and up to eight species occur sympatrically. The present work is a mixture of review and reconsideration of published work in the light of the subdivision of P. fortinii s.l. into several species. We review the current knowledge related to taxonomy, geographical distribution, population biology, and ecology of PAC species. We identified strains of P. fortinii s.l. from previously published studies to species level. The reassessment of earlier studies indicates that PAC species behave in a versatile manner along the mutualism-parasitism continuum and lifestyle designation is complicated by the use of different experimental systems. Finally we define the most promising research areas, which will contribute to elucidate the ecological role of root endophytes in general and PAC species in particular.