Current Organic Chemistry, 2007, 11, 707-720 707 1385-2728/07 $50.00+.00 © 2007 Bentham Science Publishers Ltd. Fungal Endophytes and their Role in Plant Protection C. Giménez 1 , R. Cabrera 1 , M. Reina 2 and A. González-Coloma 3,* 1 Dpt. Biología Vegetal-Fitopatología, Facultad de Biología, Universidad de La Laguna, Avda. Astrofísico F. Sánchez 3, 38206 La Laguna, Tenerife, Spain 2 Instituto de Productos Naturales y Agrobiología, CSIC, Avda. Astrofísico F. Sánchez 3, 38206 La Laguna, Tenerife, Spain 3 Instituto de Ciencias Agrarias-CCMA, CSIC, Serrano 115-dpdo, 28006 Madrid, Spain Abstract: Endophytes (fungal and bacterial organisms) have attracted great interest over the past few years because their presence benefits the host plant (development and defence) and they are a source of secondary metabolites of potential in- terest and thus play an important role in the regulation of plant communities and their herbivores. There are an increasing number of reports on their identification and on the production of secondary metabolites. Their role in plant-pathogen and plant-insect interactions is receiving increasing attention because of their potential use in pest control, however, little is known about their physiology and the regulation processes of the plant-endophyte interaction. INTRODUCTION The presence of epiphytic organisms in plants is well known. However, endophytes (fungal and bacterial organ- isms) live in the intercellular space (apoplasts) of the host plant tissues as well as inside the cells (symplasts) [1] caus- ing no apparent damage. Therefore, an association is estab- lished between these two organisms previously considered a classical symbiosis. Plants with associated endophytes have advantages over those without the association including de- fences against herbivores resulting in improved biomass and distribution. Moreover, the endophyte gains access to im- proved resource availability [2]. The term endophyte means "inside the plant" (endon= in- side; phytón=plant) and therefore this term can refer to fun- gus and bacteria or even other organisms (virus, algae, nema- todes). This term has been used since the 19 th century to refer to endophyte-host interactions when Leveille and de Bary first using it to refer to fungus living inside plants. There are endophyte algae [3] and mutualistic bacterial endophytes [4], [5] that contribute to improved plant productivity through enhanced nitrogen fixation [6-9] and defence [10-12]. Fur- thermore, there are algae-infecting fungal endophytes that produce secondary metabolites of pharmacological and agro- chemical interest. There are mutualistic fungal mycorrizae that have been considered endophytes, although it is possible to distinguish between them due to the former’s lack of ex- ternal hyphae [1]. Some authors consider as endophytes fun- gal species with long asymptomatic periods, including pathogens and latent saprophytes [1]. The term endophyte has been considered similar to mutualism, although there are clear examples of antagonism between the fungus and the plant [13]. The most accepted concept refers to fungi which colonize plants without producing any symptoms during their development [14]. However some dark-septate endo- phytes act as mutualistic endophytes in plant roots while others become pathogens. A fungal endophyte can cause disease symptoms to the host plant under stress conditions [14]. Furthermore, an *Address correspondence to this author at the Instituto de Ciencias Agrarias-CCMA, CSIC, Serrano 115-dpdo, 28006 Madrid, Spain; E-mail: azu@ccma.csic.es endophyte can act as a pathogen to plant species other that its host [15]. Additionally, it is possible to isolate a latent or weak pathogen from an asymptomatic host plant suggesting that some pathogens may have evolved from endophyte an- cestors [16]. On the other hand, endophytes could be single- gene mutant of wild pathogenic types [17]. Therefore, it is not clear whether some of the plant-isolated fungi thought to be endophytes are actually latent pathogens. For example, Mycosphaerella spp. have been isolated as endophytes from many host plant species [18], Glomerella and its anamorph, the genus Cladosporium, well known plant pathogens with a wide distribution, have been isolated as endophytes from several host plant species. Diaphorte phaseolorum (Cooke & Ellis) Sacc (anamorph: Phomopsis phaseoli (Desmaz.) Sacc) is a known pathogen causing pod and stem blight in legumes, but it can also live as an endophyte and produce spores once the host plant is dead [19]. The term endophyte refers to a complex set of interac- tions including the interactions between the different fungal species that comprise the endophyte community and the host-plant defence mechanisms to prevent these fungal spe- cies from becoming pathogens. At the cellular level, the presence of active defence symptoms in the host plant sug- gests the presence of a pathogen, while their absence indi- cates the presence of endophytes [20,21]. This plant-fungus interaction is an evolutionary driving force [1,14] where equilibrium between these organisms must be established which will, in turn, also depend on the abiotic conditions. This interaction could, therefore, be considered a true sym- biosis [15]. True endophytes will exist once equilibrium is achieved between fungal activity and the plant reaction and is maintained over time. Traditionally endophytes have been considered an impor- tant component of plant defence mechanisms against herbi- vores and stress tolerance but there are more considerations at the ecosystem level, i.e. the role those endophytes may play in plant litter degradation. Pine endophytes have been described as the first microorganisms involved in leaf de- composition; the materials formed are the substrate that al- lows the formation of reproductive organs and fungal disper- sion [22]. Not For Distribution