Endophytic Fungi in Forest Trees: Are They Mutualists?
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fungal biology reviews 21 (2007) 75–89 journal homepage: www.elsevier.com/locate/fbr Review Endophytic fungi in forest trees: are they mutualists? Thomas N. SIEBER* Forest Pathology and Dendrology, Institute of Integrative Biology (IBZ), ETH Zurich, CH-8092 Zurich, Switzerland article info abstract Article history: Forest trees form symbiotic associations with endophytic fungi which live inside healthy Received 26 February 2007 tissues as quiescent microthalli. All forest trees in temperate zones host endophytic fungi. Received in revised form The species diversity of endophyte communities can be high. Some tree species host more 24 April 2007 than 100 species in one tissue type, but communities are usually dominated by a few host- Accepted 15 May 2007 specific species. The endophyte communities in angiosperms are frequently dominated by Published online 14 June 2007 species of Diaporthales and those in gymnosperms by species of Helotiales. Divergence of angiosperms and gymnosperms coincides with the divergence of the Diaporthales and the Keywords: Helotiales in the late Carboniferous about 300 million years (Ma) ago, indicating that the Antagonism Diaporthalean and Helotialean ancestors of tree endophytes had been associated, respec- Apiognomonia errabunda tively, with angiosperms and gymnosperms since 300 Ma. Consequently, dominant tree Biodiversity endophytes have been evolving with their hosts for millions of years. High virulence of Commensalism such endophytes can be excluded. Some are, however, opportunists and can cause disease Evolution after the host has been weakened by some other factor. Mutualism of tree endophytes is Fomes fomentarius often assumed, but evidence is mostly circumstantial. The sheer impossibility of producing Mutualism endophyte-free control trees impedes proof of mutualism. Some tree endophytes exhibit Nectria coccinea either a pathogenic or a putatively mutualistic behaviour depending on the situation. Pathogenicity The lifestyle (mutualism, commensalism, parasitism) of most tree endophytes is, however, Quiescence not known. They are just there in the tissue and resume growth at the onset of natural se- Taxonomy nescence of the host tissue on which they eventually sporulate. Density of colonization of conifer needles by endophytes increases with needle age. It is postulated that the needles die as soon as colonization density reaches a threshold value. Normally, the threshold is not reached before the onset of natural senescence. The threshold value may, however, be reached earlier under some adverse conditions, e.g. lack of light in dense stands. As a consequence, endophytes kill the needles prematurely. Needle endophytes could, thus, be useful to eliminate ‘‘parasitic’’ needle mass, i.e. needles which consume more than they produce. ª 2007 The British Mycological Society. Published by Elsevier Ltd. All rights reserved. 1. Introduction however, are specialized to attack and infect living organisms. Some of these are pathogens, disease symptoms becoming Fungi are omnipresent on organic compounds. The majority manifest after a comparatively short period of incubation. are saprobes and decompose dead organic matter. Many, Others infect living organisms but symptoms do not develop, * Tel.: þ41 44 632-5521; fax: þ41 44 632-1380. E-mail address: [email protected] 1749-4613/$ – see front matter ª 2007 The British Mycological Society. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.fbr.2007.05.004 76 T. N. Sieber because, once inside the tissue, they assume a quiescent (la- shoots of chestnut (Bissegger & Sieber 1994). Carroll (1995) tent) state either for the whole lifetime of the infected plant microdissected Douglas fir needles and was able to isolate tissue or for an extended period of time, i.e. until environmen- up to four different species per needle. Not only species diver- tal conditions are favourable for the fungus or the phase dis- sity but also within-species diversity, i.e. genotype diversity, position of the host changes to the advantage of the fungus. within small volumes of plant tissue can be high. Isolates These fungi are considered ‘endophytes’. If the symbiosis oc- from Norway spruce needles revealed single needles colo- curs in leaves or needles the plant-fungus entity is sometimes nized by several different genotypes of Lophodermium piceae termed ‘mycophylla’ corresponding with ‘mycorrhiza’ - the (Mu¨ ller et al. 2001). symbiosis of primary roots and fungi. Endophyte isolates often remain sterile making morphol- Two groups of endophytic fungi are recognized, clavicipita- ogy-based identification impossible. If sporulation occurs lean and non-clavicipitalean endophytes (Carroll 1988; Petrini identification is frequently possible to the genus only because 1991; Schulz & Boyle 2005; Stone & Petrini 1997; Stone et al. the species is either not described or the morphology of the 2004). Species of the Clavicipitaceae form symbioses almost fructification in culture deviates significantly from the one exclusively with grass hosts. Grass endophytes colonize their produced on the host. Most species descriptions are based hosts systemically (except the roots) and several species are on fungal morphology formed on the host, making identifica- transmitted vertically by seeds to the next host generation. tion based on cultures difficult. Comparison of the culture Grass endophytes enhance host fitness by the production of morphology of endophyte isolates with that of pure cultures both alkaloids, that inhibit insect herbivory, and metabolites originating from fructifications formed on host tissues some- that stimulate plant growth (Clay 1991). In contrast, host colo- times helps. Alternatively, DNA sequences of both endophyte nization by non-clavicipitalean endophytes is non-systemic isolates and cultures originating from fructifications on the and is restricted to disjunctive, endophytic microthalli which host can be compared. However, many fungi occur as endo- may consist of only a few cells (Stone 1987). Non-clavicipita- phytes on a broad range of hosts but sporulate only on one lean endophytes represent a broad range of species from sev- or a few of them (Baayen et al. 2002; Petrini & Petrini 1985), eral families of ascomycetes and probably occur in all plant making such ‘comparison’ approaches a Herculean effort. In species of the temperate zones including grasses (Sieber addition, the high diversity of fungal endophyte-DNA se- et al. 1988). quences, which do not match any of the sequences currently Many excellent reviews about endophytic fungi in woody available from DNA databases (Higgins et al. 2007), and my plants have already been published (Carroll 1988; Petrini own experience with sporulating but unidentifiable cultures 1991; Schulz & Boyle 2005; Stone & Petrini 1997; Stone et al. of endophytic fungi indicate that many endophytes are unde- 2004). In this review, I will explore the different lifestyles scribed species. Probabilistic considerations lead to the same (mutualism, commensalism, parasitism) of endophytic fungi. conclusion. If there were two host-specific fungal endophytes A list of hosts, number of species isolated, names of dominant per plant species, a minimum of between 500’000 and 600’000 endophytes and their taxonomic affiliation are presented as endophyte species would exist, assuming that there are be- a basis to discuss evolution of endophytes. Confirmation of tween 250’000 and 300’000 plant species worldwide (Schmit co-evolution of host and endophytes will lead me to explore & Mueller 2007; Wilson 1988). About 79’000 species of fungi mutualism and parasitism of endophytes. Emphasis is on en- have been described with only 35’000 of them being plant- dophytes in aerial plant tissues of forest trees in the temperate associated microfungi (Schmit & Mueller 2007). If we assume zones. Those interested in root endophytes are referred to that all of them were occurring as endophytes, at least other reviews (Addy et al. 2005; Jumpponen & Trappe 1998; 465’000 endophyte species would be undescribed. Sieber 2002; Sieber and Gru¨ nig 2006). Referencing is not com- Since host tissues can be sampled methodically, studies on prehensive, but I have tried to include key references which endophytic fungi are useful to discover and estimate fungal may provide access to additional literature on fungal diversity and to monitor changes of this diversity. The con- endophytes. comitant use of several selective isolation methods is, how- ever, important to get a complete picture of the hidden diversity (Bills 1996). Extraction and amplification of fungal 2. Diversity, taxonomy and evolution DNA directly from plant tissues for detection, quantification and identification of endophytes is an alternative. Identifica- Surveys of many tree species during the past 30 y have shown tion is performed by comparisons of DNA sequences with that colonization by endophytic fungi is ubiquitous (Tables 1 those available from databases (Ganley & Newcombe 2006). and 2). The number of species detected depends on biotic, abi- This approach speeds-up diversity surveys and will allow otic and experimental factors, e.g. the host species, type and identification of fungi that do not grow or do not sporulate phase disposition of the plant organ, edaphic and climatic in culture. Elimination of epiphytic DNA might, however, con- conditions, the isolation procedure and the number and size stitute a problem since classical surface-sterilization, which is of samples. Species diversity of internal mycobiota is