ABSTRACT
An endophyte is literally dened as one plant living within another organism. Applying the generally accepted vekingdom system of Whittaker (1969), the term “endomycete” would, thus, seem more appropriate for a fungus living internally in a plant. is term is, however, ambiguous because it may lead to confusion with members of the Endomycetes, a class introduced by von Arx (1967) to accommodate ascomycetous yeasts and fungi with an yeast like growth phase. De Bary (1866) coined the term “endophyte” to distinguish organisms
that invade and reside within host tissues or cells from “epiphytes,” those fungi living on the outer surfaces of plants. Various plant/fungus symbioses constitute a continuum from antagonism to mutualism, and the type of symbiosis may change over time and space. Diseases caused by highly virulent pathogens or well-developed ectomycorrhizae are immediately obvious also to the nonspecialist, whereas the result of a plant/fungus association in between these two extremes may escape observation even by a specialist. e impossibility to unequivocally dene the behavior of an endophyte as antagonistic or mutualistic becomes obvious by the ndings of Freeman and Rodriguez (1993) who observed a fungal plant pathogen to convert to a nonpathogenic, endophytic mutualist by mutation at a single locus. us, a pragmatic denition of endophytism should be applied, which includes all organisms located within apparently healthy, functional root tissues at the moment of sample collection. e currently available methods to detect endophytes are destructive and, thus, the addendum “at the moment of sample collection” was necessary to account for the dynamic nature of plant-fungus interactions. Only fungi will be considered in this chapter although bacteria can live endophytically too (Chanway 1996; Schulz et al. 2006). Endophytic bacteria have been shown to be of vital importance in some symbioses, for example, actinorhizal symbiosis between tree roots (Alnus spp.) and Frankia spp., symbiosis between sugarcane (Saccharum officinarum L.) and Acetobacter diazotrophicus, a nitrogen-xing bacterial endophyte (Dong et al. 1994), or plant growth-promoting rhizobacteria (Wei et al. 1994). e fungal partners in mycorrhizal associations are clearly also endophytes because part or all of their thallus is localized within the roots. Comprehensive articles, reviews, and books have been provided for many of
I. Introduction ...........................................................................................................................38-1 II. Species and Hosts of Root Endophytes ..............................................................................38-2
Dark Septate Endophytes III. Methods of Detection........................................................................................................... 38-4 IV. Root Endophytes of Herbaceous Plants .............................................................................38-5
VI. Conclusions and Outlook ...................................................................................................38-38 References .........................................................................................................................................38-38
the groups of classical mycorrhizal associations, namely, ecto-(ECM), ectendo-(EEM), or arbuscular mycorrhizae (AM) and the ericoid, arbutoid, orchid, or monotropoid mycorrhizae (Harvais and Hadley 1967; Read 1983; Mikola 1988; Allen et al. 1991; Egger et al. 1991; Allen and Allen 1992; Currah and Zelmer 1992; Read 1992, 1996; Brundrett 2004; Smith and Read 2008; Brundrett 2009). Emphasis in this chapter will be laid on root-fungus associations that are not regarded as mycorrhizae in the classical sense although literally they also form “fungus-root” entities of tightly interwoven fungusplant tissues. Fungal root endophytes are ubiquitous and probably more abundant than classical mycorrhizae because they are not conned to the root tips and can occur everywhere in the root system. ey are regularly isolated during studies about classical mycorrhizae but are then rather regarded as nuisance than interesting research objects. Since fungal root endophytes have been largely neglected, information about their functions is rare. Aer all, the number of publications about fungal root endophytes started to increase exponentially since about a decade ago, and there is hope that this group of organisms will eventually obtain the attention it deserves. In addition to mycorrhizae, root-colonizing obligate biotrophs with a prolonged latent phase such as certain smuts (GarciaGuzman et al. 1996) or rust fungi, that is, Tranzschelia fusca (G. Winter) Dietel in the rhizomes of Anemone nemorosa L., will not be discussed in this chapter.
