Chapter 9. Emerging Perspectives on the Ecological Roles of Endophytic Fungi in Tropical Plants

DK3133_C009.fm Page 182 Monday, April 18, 2005 4:21 PM



182



Van Bael et al.



2002). Infected plants often harbor a single fungal genotype, and asexual endophytes are

typically transmitted vertically from maternal plants to their offspring via seeds. Endophytes associated with some domesticated grasses are generally thought to act as mutualistic symbionts (see Clay, 1991; Clay and Schardl, 2002; see also Faeth, 2002). These

endophytes, which are intimately associated with their hosts, can confer an array of benefits

upon their hosts, including tolerance to heavy metals, increased drought resistance, reduced

herbivory, defense against pathogens, and enhanced growth and competitive ability

(reviewed by Saikkonen et al., 1998). However, vertical transmission, high specificity, and

low within-host fungal diversity appear to represent a special case that does not provide

a general model for the majority of host–endophyte associations (Saikkonen et al., 1998;

Stone et al., 2000; Faeth and Fagan, 2002).

Whether endophytes of woody angiosperms also confer benefits to their hosts is a

subject of current debate. While studies with temperate-zone trees show that in some cases

endophyte densities are negatively correlated with herbivores and galling insects (Wilson

and Carroll, 1994, 1997; Wilson, 1995b; Gange, 1996; Preszler et al., 1996; Wilson and

Faeth, 2001), some authors have argued that defensive mutualisms between endophytes

and woody plants are likely to be rare (see Carroll, 1986, 1991; Faeth, 2002). In particular,

it has been suggested that herbivorous insects may actually promote endophyte infection

via folivory, especially in the case of leaf-mining insects (Faeth and Hammon, 1997; Faeth,

2002). However, considering that endophytes are symbionts that obtain resources from

and grow within their hosts, it is highly plausible that endophytes of woody plants have

evolved ways to defend their hosts, and thus themselves, from being eaten by herbivores

or damaged by pathogens (see Frank, 1996; Herre et al., 1999; Arnold, 2002).

Despite this intriguing possibility for mutualistic interactions between endophytes

and their hosts, endophyte research in tropical areas has generally been limited to describing the endophyte species found on particular host plants (e.g., Lodge et al., 1996, Bayman

et al., 1998; Rajagopal and Suryanarayanan, 2000). Recent studies in tropical areas have

demonstrated that endophytes can be extremely diverse within host plants, even within a

single leaf. For example, tropical endophytes represent at least five classes of Ascomycota,

with 3 to 20 species often coexisting as highly localized infections within individual leaves

(Lodge et al., 1996; Arnold et al., 2000). However, compared with endophyte–grass

systems, the ecological roles of endophytic fungi associated with leaves of tropical woody

plants are poorly known. Only a few recent studies have focused on the basic ecology of

these fungi and their interactions with hosts (Fröhlich and Hyde, 1999; Arnold et al., 2001,

2003; Arnold and Herre, 2003; Suryanarayanan et al., 2003).

In contrast to vertical transmission of endophytes in grasses, endophytes associated

with foliage of tropical woody plants appear to be predominantly transmitted horizontally

via sporefall (Bayman et al., 1998; Lebrón et al., 2001; Arnold and Herre, 2003; Mejia et

al., 2003). Leaves are flushed endophyte-free, and then shortly after emergence, they

become densely infected with endophytes. There is some evidence to suggest that insect

folivory may influence the abundance and diversity of endophytes (A.E. Arnold, unpublished), but the majority of endophyte infections occur without leaf damage as a precursor.

Recent studies indicate that young leaves accumulate endophytes shortly after emergence

via epiphytic germination of fungal propagules, which then infect leaves via cuticular

penetration or growth through stomates (Arnold and Herre, 2003, Mejia et al., 2003).

We have conducted multiyear surveys of endophytes that are associated with Theobroma cacao (Malvaceae) and several other plant hosts in Panama. We outline our major

findings on diversity, host affinity, transmission, interactions, and pathogen resistance in

Table 9.1. Additionally, we discuss the following unanswered questions: (1) Which fungal

species occupy which hosts? (2) What is the mechanism for differential host affinity? (3)



DK3133_C009.fm Page 183 Monday, April 18, 2005 4:21 PM



Emerging Perspectives on the Ecological Roles of Endophytic Fungi



183



Table 9.1 Summary of Selected Findings from Field Surveys and Experimental Work on

Endophytic Fungi

Major Findings

1. Endophytic fungal (EF) diversity is extremely high within a single host species. In a sample

of 126 T. cacao leaves (32 mm2 of tissue sampled per leaf), 1172 isolates representing 344

morphotaxa were recovered. Within that sample, 20 morphotaxa accounted for roughly 60%

of all isolates, with most morphotaxa found only rarely (Arnold et al., 2003). This result is

consistent with surveys of endophyte diversity in other families of tropical woody plants

(Fröhlich and Hyde, 1999; Arnold et al., 2000; Arnold, 2002), and the prevalence of rare

morphotaxa reflects a general pattern among tropical plant-associated fungi (see Gilbert,

2002, Gilbert and Sousa, 2002, Gilbert et al., 2002).

2. EF communities exhibit considerable heterogeneity at small and large spatial scales (Bayman

et al., 1998; Arnold et al., 2000, 2003). Although the aggregate fungal communities found

on conspecific trees growing within 50 km of each other show relatively high Morsita–Horn

similarity (>0.65), that similarity drops off sharply with larger distances (see also Fröhlich

and Hyde, 1999).

3. EF transmission is horizontal (among hosts) rather than vertical. Leaves are flushed

endophyte-free, and EF are acquired from the habitat over time (see Arnold and Herre,

2003). Leaves appear to saturate in EF density after roughly 2 to 4 weeks.

4. The species diversity of EF communities within leaves increases up to the point of saturation

of EF density, generally at 4 to 8 weeks after leaf flush (Rojas et al., unpublished data).

5. EF exhibit differential host affinity. EF communities associated with different host species

show striking differences, even when those species are growing in close proximity (Arnold

et al., 2000). Specifically, the EF species that tend to dominate the communities in a given

host tend to be rare, if they are found at all, in other hosts (Arnold et al., 2003; Herre et

al., unpublished).

6. EF growth in vitro is strongly affected by the medium. Generally, EF that are commonly

found in a given host usually grow best in media that contain extracts of that host species

(Arnold and Herre, 2003; Arnold et al., 2003).

7. EF species show a range of dominance interactions in vitro, ranging from indifference to

active inhibition (Herre et al., unpublished data). The outcome of interactions between any

two EF species depends on the medium (Arnold et al., 2003). EF species that commonly

occur on a given host generally tend to dominate interactions with more rarely occurring

species when tested on medium containing extracts of that host.

8. Hosts with EF-free leaves can be produced by preventing freshly flushed leaves from surface

wetting, which is conducive to spore germination and subsequent hyphal infection (Arnold

and Herre, 2003). Selected EF can be introduced into leaves in order to conduct experimental

tests of the effects of the EF (Arnold et al., 2003, Mejia et al., 2003).

9. Greenhouse trials demonstrate that EF-inoculated leaves resist Phytophthora sp. (pathogen)

damage, compared with EF-free leaves (Arnold et al., 2003). EF can enhance host

antipathogen defenses.

10. Field trials show that EF inoculations can help protect T. cacao fruits from loss to pathogen

damage (Phytophthora sp.) (Mejia et al., 2003, Mejia et al., unpublished).



DK3133_C009.fm Page 184 Monday, April 18, 2005 4:21 PM



184



Van Bael et al.



What is the complete life cycle of the fungi? (4) What is the mechanism of endophytemediated host defense?



9.2



QUESTIONS



9.2.1

Which Fungal Species Are in Which Hosts?

Given the diversity of tropical fungi and their hosts, we have not yet begun to scratch

the surface of describing how fungi are distributed across hosts. To date, we have isolated

endophytic fungi from leaves of eight plant hosts (three vines and five woody plants)

in Panama using standard methods (outlined in Arnold et al., 2003). Fungi were grouped

to morphotaxa using vegetative features that appeared to conservatively uphold species

boundaries as defined by molecular markers (Arnold et al., 2000; Arnold, 2002; Lacap

et al., 2003). For the most common and several rare endophytic morphotaxa associated

with each host plant species, we used analyses of nrDNA sequence divergence and

conducted interaction trials among different isolates to confirm the species boundaries

suggested by morphology (see Arnold et al., 2003; Herre et al., unpublished). Further,

we used a basic local alignment search tool (BLAST) in order to assign tentative

names to the morphospecies (Table 9.2). We emphasize that caution must be used in

interpreting the species names given by sequence matches from the BLAST search,

primarily due to the incomplete and uneven sampling of taxa in the GenBank database.

Therefore, we include the names of our top matches to provide a general idea of the

genera and possible species that are commonly found as endophytes in these plants. We

note that there is often genetic divergence between isolates that yield the same name as

top matches. Given that even small genetic differences can translate to large functional

differences (Freeman and Rodriguez, 1993), these observations are consistent with the

inference that functional diversity of endophytes is likely to be much greater than the

diversity reflected in species names.

To compare differences in host affinity among endophytes, we surveyed and compared the endophytic fungi within two host plant groups. One group consisted of three

woody trees on Barro Colorado Island, while the second group consisted of three vines

and one woody shrub, all growing in nearby Parque Soberania. Among the endophyte

morphotaxa recovered from the trees in the first group (T. cacao [Malvaceae], n = 9 leaves;

Heisteria cocinna [Olacaceae], Ouratea lucens [Ochnaceae], n = 3 leaves; Table 9.2),

65.5% were recovered from only one host species (Arnold et al., 2003). Moreover, the

most common morphotaxa from one woody host species was usually absent or rare in the

other host species. Among the morphotaxa recovered from the second group (Ipomoea

phillomega, Ipomoea squamata, Merremia umbellata [Convolvulaceae], n = 16 leaves/host

species; Witheringia solanacea [Solanaceae], n = 8 leaves; Table 9.2), 75.6% were recovered from only one host species (Van Bael et al., unpublished data). In contrast to the first

group, however, several of the most common endophyte–host species were very closely

related to the common endophytes in the other host plant species (Table 9.2). This observation of high overlap or similarity among common endophytes in the second group may

reflect the relatively higher phylogenetic affinities of these hosts (three Convolvulaceae

and one Solanaceae). This raises the question: Do closely related hosts share similar

endophytes? A further possibility is that the most common endophytes are more likely to

be host generalists, as has been demonstrated for polypores (Gilbert et al., 2002). Further

work, in which structured sampling of hosts with different degrees of phylogenetic affinity

is done, is needed.



DK3133_C009.fm Page 185 Monday, April 18, 2005 4:21 PM



Emerging Perspectives on the Ecological Roles of Endophytic Fungi

Table 9.2 Species of Endophytic Fungi That Were Frequently

Isolated from Leaves of Several Host Plants in Panama

Host Plant Family, Species

Olacaceae

Heisteria cocinna



Malvaceae

Theobroma cacao



Ochnaceae

Ouratea lucens

Convolvulaceaef

Ipomoea phillomega

Ipomoea squamata



Merremia umbellata



Solanaceaef

Witheringia solanacea

Rubiaceae

Faramea occidentalis



Top GenBank Matchesa



Guignardia magniferae

Xylaria hypoxylon

Xylaria arbuscula A

Botryosphaeria luteab

Colletotrichum gloeosporoidesc A

Botryosphaeria dothidead A

Botryosphaeria dothideae B

Colletotrichum gloeosporoidesf B

Phomopsis sp.

Colletotrichum gloeosporoides C

Xylaria longipes A

Guignardia endophyllicola

Phyllosticta sp.

Glomerella cingulatag A

Xylaria arbuscula B

Glomerella cingulata B

Curvularia affinis

Colletotrichum truncatum A

Xylaria longipes B

Colletotrichum gloeosporoides D

Colletotrichum truncatum B

Glomerella cingulata C

Colletotrichum truncatum C

Xylaria sp.

Glomerella cingulata D



Note: Identities are based on BLAST searches of the National Center for

Biotechnology Information GenBank database using internal transcribed

spacer (ITS) sequences (Altschul et al., 1990).

a



Listed are the fungal species present in GenBank with which endophytes

showed the highest affinity. Letters signify samples that were genetically

distinct, despite receiving the same name.

b–e Ranking for the most frequently encountered endophyte species in one T.

cacao collection of 10 leaves (Rojas et al., in preparation).

f Identifications represent the two or three most common fungi per plant

species in these families.

g Note that C. gloeosporoides is an anamorph of G. cingulata.



185



DK3133_C009.fm Page 186 Monday, April 18, 2005 4:21 PM



186



Van Bael et al.



9.2.2

What Is the Mechanism for Differential Host Affinity?

In addition to carefully designed surveys of leaves from different species, experiments are

important for distinguishing true host affinity from spatial artifacts (i.e., localized dispersal

within host crowns) and for examining the mechanisms behind host affinity when it is

observed (Arnold et al., 2003). Recent experimental work has demonstrated growth differences among endophyte morphotaxa frequently collected from T. cacao, H. cocinna,

and O. lucens when they were plated on separate media containing leaf extracts from each

host species. In >75% of the cases, growth rates were higher on media containing extracts

of the host species from which they were most frequently isolated in the field surveys

(Arnold et al., 2003). Moreover, the growth rates of endophytes in vitro (with host plant

extracts) corresponded to their relative abundance in planta, with common taxa from a

given host growing better than rare taxa. In sum, host-specific leaf chemistry appears to

favor the growth of some endophytes over others, and highest growth rates are observed

when endophytes were cultivated on extracts of the host species for which they displayed

highest affinity in the field. By mediating the growth of particular endophyte species, hostspecific leaf chemistry may also influence the outcomes of competitive interactions among

endophytes or among endophytes, herbivores, and pathogens.

9.2.3

What Is the Life Cycle for Tropical Endophytic Fungi?

Very little work has been done to establish the complete life cycles of the fungal endophytes

identified from woody angiosperms. Reproductive structures of some of the fungal associates are readily observed in nature. Fungi typically identified as the most prevalent

dicotyledonous taxa (e.g., Xylaria spp. and Colletotrichum spp.) are also often encountered

on the tropical forest floor developing from leaf and wood litter (Bischoff, personal

observations). The current dogma is that the fungi contained within the plant reproduce

after the plant tissue (e.g., leaves and stems) senesces or abscises (Wilson, 2000). These

fruiting structures then provide inocula that lead to new infections of developing leaf and

branch tissue (Malloch and Blackwell, 1992).

Although horizontal transmission via spores after leaf senescence is a likely method

of dispersal, it is doubtful that it is the only form in which horizontal transmission occurs

among the endophytes of woody angiosperms. Species of the grass endophytic genera

Epichloë and Balansia are known to vertically transmit by systemic infection of the host

embryo (Freeman, 1902; Clay, 1986). In contrast to these clavicipitaceous endophytes,

there has been little evidence of vertical transmission among endophytes of woody dicots.

As in previous studies (Bayman et al., 1998; Lebrón et al., 2001), we have observed that

seedlings at germination and leaves at emergence lack cultivable endophytes. However,

endophytic species have been found associated with host seeds while attached to the parent

plant (Petrini et al., 1992; Wilson and Carroll, 1994). These fungi may then disperse with

the angiosperm seed, sporulate, and thus provide the inoculum for the newly established

seedling. This would help maintain a host–symbiont relationship even in founder events

of dispersal. Grass endophytes living asymptomatically in plant tissue were discovered

over 100 years ago (Vogl, 1898). Despite extensive work focused on this plant–host

interaction over the ensuing years, it was not until 1996 that Neotyphodium sp. (the

anamorph of Epichloë) was found to develop a mycelial net and conidiogenous cells along

the leaf surface of Agrostis hiemalis and Poa rigidifolia (White et al., 1996). The authors

determined that the epiphyllous conidia are likely responsible for some of the horizontal

transmission occurring in the grass–Epichloë interaction. It is possible that this inconspicuous mode of dispersal is also occurring in some of the woody endophytic species.

When discussing the spore dispersal and life cycles of endophytes associated with

woody plants, we find that there are more questions than answers. This is especially true



DK3133_C009.fm Page 187 Monday, April 18, 2005 4:21 PM



Emerging Perspectives on the Ecological Roles of Endophytic Fungi



187



Figure 9.1 On the left is an image of a T. cacao leaf with endophytes (E+) that have been

introduced experimentally and appear as black lines. On the right is an image of an endophyte-free

(E–) T. cacao leaf. (Photos by L. Mejia.)



of the tropical woody angiosperms. For example, why fungi wait until senescence to

reproduce, what cues their reproduction, and how within-leaf competition influences

endophyte fitness require further research. Further, due to the high diversity of these

endophytes (Arnold et al., 2000), it is likely that many different types of life cycles will

be found among these fungi. For example, many of these endophytes are also regarded as

pathogens of particular hosts. It may be that these organisms are able to live in an

asymptomatic manner in one host but cause disease in another. Detailed studies of these

organisms and their dispersal methods may provide clues to host shifting and the origins

of symptomatic pathogens in susceptible hosts.

9.2.4

What Is the Mechanism of Host Defense?

Two recent studies have demonstrated that in at least some cases, endophytes can enhance

host defenses against pathogens (Arnold et al., 2003; Mejia et al., 2003).Two key methodological discoveries allowed this work to occur. First, we found that by keeping leaves

dry as they grew, the leaves remained endophyte-free (E–) (Arnold, 2002; Arnold and

Herre, 2003; Mejia et al., 2003; see also Wilson and Carroll, 1994; Wilson et al., 1997).

Second, we were able to introduce endophytes into E– leaves, in combinations and

concentrations of our choosing, and thereby create endophyte positive (E+) leaves (Mejia

et al., 2003; Arnold et al., 2003). Leaves that were E– and E+ could be generated within

individual seedlings of T. cacao (Figure 9.1).

In a greenhouse experiment (Arnold et al., 2003), we generated seedlings (n = 70)

in which half of the focal leaves were inoculated with a group of seven endophyte species

(from the genera Colletotrichum, Xylaria, and Nectria/Fusarium) that had shown previous

in vitro activity against a foliar pathogen, Phytophthora sp. Thus, each seedling contained

endophyte treated (E+) and untreated (E–) leaves. Eighteen days after endophyte treatments, we applied a strain of Phytophthora sp., isolated previously from symptomatic T.

cacao in Panama, to a subset of E+ and E– leaves. The final experiment included all

factorial combinations of endophyte (E) and pathogen (P) presence and absence. After 15

additional days, we assessed pathogen damage by determining leaf mortality and the area

of damage on surviving leaves.

Leaves without endophytes and with Phytophthora (E–P+) experienced leaf death

and abscission 2.8 times more frequently than did leaves inoculated with endophytes

(E+P+). Moreover, on P+ leaves that did survive, necrotic lesions were significantly larger

on leaves without endophytes (E–P+) than on leaves with endophytes (E+P+). Although

the protection by endophytes was apparently localized to individual leaves, entire host

plants were affected by the presence or absence of endophytes. For example, when we



DK3133_C009.fm Page 188 Monday, April 18, 2005 4:21 PM



188



Van Bael et al.



considered both leaf loss and leaf damage on retained leaves, surface area available for

photosynthesis decreased by 32.3% for E–P+ treatments relative to E–P–, but only by

14.1% for E+P+ treatments relative to E+P– (Arnold et al., 2003).

While this experiment demonstrated that endophytes limit pathogen damage in T.

cacao, the mechanism for this defense remains unclear. One clue, however, was the

apparent localization of defense to endophyte-infected tissues. This observation, combined

with observations of interactions among endophytes in vitro (Herre et al., unpublished

data), suggested that interspecific interactions among endophytes and pathogens may play

an important role in mediating host defense. To explore this hypothesis, we assessed in

vitro interactions between 50 endophyte morphotaxa isolated from T. cacao and three

major cacao pathogens (Phytophthora sp., Moniliophthora roreri, and Crinipellis perniciosa; Mejia et al., 2003, unpublished data). In interactions on standard media (2% malt

extract agar), 40% of the endophyte morphotaxa appeared to antagonize at least one of

the pathogen species, while the remaining endophytes had no effect or were themselves

antagonized. Interestingly, when we repeated the interaction trials on media containing

leaf extracts of T. cacao, the outcomes differed qualitatively and quantitatively. Together,

these observations suggest that direct interactions among endophytes and pathogens are

complex, diverse, and sensitive to host-specific leaf chemistry. The diversity of endophytes

and their interactions may contribute to effective antipathogen defense in woody plants.

Because host plants must deal with ever-changing and diverse pathogens in tropical forests,

this form of defense is likely to be enhanced when endophytes are highly diverse within

and among leaves, plants, and host species.



9.3



CONCLUSIONS



We are only beginning to understand the ecological role of endophytes in natural tropical

communities and to realize their applied potential. It is clear that horizontally transmitted

endophytes can enhance and supplement host defense against pathogens. The mechanism

of defense appears to be in part affected by the outcome of interspecific competition among

endophytes and pathogenic fungi, which in turn appears to be influenced by plant chemistry. There are still many outstanding questions about mechanisms of defense and about

the potential mutualism between endophytes and their hosts. For example, what are the

costs of harboring endophytes to hosts? What is the relative importance of abundance,

diversity, and species composition of endophytes in determining whether antipathogen

defense occurs? Do endophytes in woody plants provide other types of defense to their

hosts, such as against herbivores? An additional obvious need is to expand the work into

other host species, in order to assess the generality and frequency of such endophytemediated effects.

In addition, the extent to which the interactions among endophytes and their hosts

represent true mutualisms deserves further study. In general, mutualistic interactions

between hosts and vertically transmitted symbionts can be easily reconciled with existing

theory (reviews by Herre et al., 1999; Leigh, 1999). In contrast, horizontally transmitted

symbionts are expected to behave less mutualistically and may tend toward antagonism.

Nonetheless, several recent examples of horizontally transmitted mutualists, such as pollinators (Herre, 1999), mycorrhizal fungi (Husband et al., 2002), and endophytic fungi,

may challenge the existing theory.



DK3133_C009.fm Page 189 Monday, April 18, 2005 4:21 PM



Emerging Perspectives on the Ecological Roles of Endophytic Fungi



189



ACKNOWLEDGMENTS

The authors thank Greg Gilbert, Tom Gianfagna, and Prakash Hebbar for essential technical

advice and training. They also thank the Smithsonian Institution, the Smithsonian Migratory Bird Center, the Andrew W. Mellon Foundation, the National Science Foundation

(DEB 9902346 to Lucinda McDade and A.E.A.), the American Cacao Research Institute,

and the World Cacao Foundation for financial support.



REFERENCES

Arnold, A.E. (2002). Neotropical Fungal Endophytes: Diversity and Ecology. Ph.D. thesis, University

of Arizona, Tucson.

Arnold, A.E., Herre, E.A. (2003). Canopy cover and leaf age affect colonization by tropical fungal

endophytes: ecological pattern and process in Theobroma cacao (Malvaceae). Mycologia

95:388–398.

Arnold, A.E., Maynard, Z., Gilbert, G.S. (2001). Fungal endophytes in dicotyledonous neotropical

trees: patterns of abundance and diversity. Mycol. Res. 105:1502–1507.

Arnold, A.E., Maynard, Z., Gilbert, G.S., Coley, P.D., Kursar, T.A. (2000). Are tropical fungal

endophytes hyperdiverse? Ecol. Lett. 3:267–274.

Arnold, A.E., Mejia, L.C., Kyllo, D., Rojas, E., Maynard, Z., Robbins, N., Herre, E.A. (2003).

Fungal endophytes limit pathogen damage in a tropical tree. Proc. Natl. Acad. Sci. U.S.A.

100:15649–15654.

Altschul, S.F., Gish, W., Miller, W., Myers, E.W., Lipman, D.J. (1990). Basic local alignment search

tool. J. Mol. Biol. 215:403–410.

Bayman, P., Angulo-Sandoval, P., Baez-Ortiz, Z., Lodge, D.J. (1998). Distribution and dispersal of

Xylaria endophytes in two tree species in Puerto Rico. Mycol. Res. 102:944–948.

Carroll, G. (1986). The biology of endophytism in plants with particular reference to woody

perennials. In Microbiology of the Phyllosphere, Fokkema, N.J., Van den Huevel, J., Eds.

Cambridge, U.K., Cambridge University Press, pp. 205–222.

Carroll, G. (1988). Fungal endophytes in stems and leaves: from latent pathogen to mutualistic

symbionts. Ecology 69:2–9.

Carroll, G.C. (1991). Beyond pest deterrence: alternative strategies and hidden costs of endophytic

mutualisms in vascular plants. In Microbial Ecology of Leaves, Andrews, J.H., Hirano, S.S.,

Eds. New York, Springer-Verlag, pp. 358–375.

Clay, K. (1986). Induced vivipary in the sedge Cyperus virens and the transmission of the fungus

Balansia cyperi (Clavicipitaceae). Can. J. Bot. 64:2984–2988.

Clay, K. (1988). Fungal endophytes of grasses: a defensive mutualism between plants and fungi.

Ecology 69:10–16.

Clay, K. (1991). Endophytes as antagonists of plant pests. In Microbial Ecology of Leaves, Andrews,

J.H., Hirano, S.S., Eds. New York, Springer-Verlag, pp. 331–357.

Clay, K., Schardl, C. (2002). Evolutionary origins and ecological consequences of endophyte symbiosis with grasses. Am. Nat. 160:S99–S127.

Davis, E.C., Franklin, J.B., Shaw, A.J., Vilgalys, R. (2003). Endophytic Xylaria (Xylariaceae) among

liverworts and angiosperms: phylogenetics, distribution, and symbiosis. Am. J. Bot.

90:1661–1667.

Evans, H.C., Holmes, K.A., Thomas, S.E. (2003). Endophytes and mycoparasites associated with

an indigenous forest tree, Theobroma gileri, in Ecuador and a preliminary assessment of

their potential as biocontrol agents of cacao diseases. Mycol. Prog. 2:149–160.

Faeth, S.H. (2002). Are endophytic fungi defensive plant mutualists? Oikos 98:25–36.

Faeth, S.H., Fagan, W.F. (2002). Fungal endophytes: common host plant symbionts but uncommon

mutualists. Integrative Comp. Biol. 42:360–368.



DK3133_C009.fm Page 190 Monday, April 18, 2005 4:21 PM



190



Van Bael et al.



Faeth, S.H., Hammon, K.E. (1997). Fungal endophytes in oak trees: experimental analyses of

interactions with leafminers. Ecology 78:820–827.

Frank, S.A. (1996). Host-symbiont conflict over the mixing of symbiotic lineages. Proc. R. Soc.

Lond. Ser. B Biol. Sci. 263:339–344.

Freeman, E.M. (1902). The seed-fungus of Lolium temulentum L., the darnel. Philos. Trans.

196:1–29.

Freeman, S., Rodriguez, R.J. (1993). Genetic conversion of a fungal plant pathogen to a nonpathogenic, endophytic mutualist. Science 260:75–78.

Fröhlich, J., Hyde, K.D. (1999). Biodiversity of palm fungi in the tropics: are global fungal diversity

estimates realistic? Biodiversity Conserv. 8:977–1004.

Gange, A.C. (1996). Positive effects of endophyte infection on sycamore aphids. Oikos 75:500–510.

Gilbert, G.S. (2002). Interacciones entre microorganismos y plantas. In Conservación de Bosques

Tropicales, Guariguata, M., Kattan, G., Eds. Cartago, Costa Rica, Libro Universitario

Regional, pp. 435–465.

Gilbert, G.S., Sousa, W.P. (2002). Host specialization among wood-decay polypore fungi in a

Caribbean mangrove forest. Biotropica 34:396–404.

Gilbert, G.S., Ferrer, A., Carranza, J. (2002). Polypore fungal diversity and host density in a moist

tropical forest. Biodiversity Conserv. 11:947–957.

Herre, E.A. (1999). Laws governing species interactions? Encouragement and caution from figs and

their associates. In Levels of Selection, Keller, L., Ed. Princeton, NJ, Princeton Press, pp.

209–237.

Herre, E.A., Knowlton, N., Mueller, U.G., Rehner, S.A. (1999). The evolution of mutualisms:

exploring the paths between conflict and cooperation. Trends Ecol. Evol. 14:49–53.

Husband, R., Herre, E.A., Turner, S.L., Gallery, R., Young, J.P.W. (2002). Molecular diversity of

arbuscular mycorrhizal fungi and patterns of host association over time and space in a

tropical forest. Molecular Ecology 11:2669–2678.

Lacap, D.C., Hyde, K.D., Liew, E.C.Y. (2003). An evaluation of the fungal “morphotype” concept

based on ribosomal DNA sequences. Fungal Diversity 12:53–66.

Lebrón, L., Lodge, D.J., Laureano, S., Bayman, P. (2001). Where is the gate to the party? Phytopathology 91:116.

Leigh, E.G., Jr. (1999). Tropical Forest Ecology: A View from Barro Colorado Island. Oxford, Oxford

University Press.

Lodge, D.J., Fisher, P.J., Sutton, B.C. (1996). Endophytic fungi of Manilkara bidentata leaves in

Puerto Rico. Mycologia 88:733–738.

Malloch, D., Blackwell, M. (1992). Dispersal of fungal diasporas. In The Fungal Community: Its

Organization and Role in the Ecosystem, Carroll, G.C., Wicklow, D.T., Eds. New York,

Marcel Dekker, pp. 147–171.

Mejia, L.C., Rojas, E., Maynard, Z., Arnold, A.E., Kyllo, D., Robbins, N., Herre, E.A. (2003).

Inoculation of beneficial endophytic fungi into Theobroma cacao tissues. In Proceedings

of the 14th International Cocoa Research Conference, Accra, Ghana.

Petrini, O. (1991). Fungal endophytes of tree leaves. In Microbial Ecology of Leaves, Andrews, J.H.,

Hirano, S.S., Eds. New York, Springer-Verlag, pp. 179–197.

Petrini, O., Sieber, T.N., Toti, L., Viret, O. (1992). Ecology, metabolite production and substrate

utilization in endophytic fungi. Nat. Toxins 1:185–196.

Preszler, R.W., Gaylord, E.S., Boecklen, W.J. (1996). Reduced parasitism of a leaf-mining moth on

trees with high infection frequencies of an endophytic fungus. Oecologia 108:159–166.

Rajagopal, K., Suryanarayanan, T.S. (2000). Isolation of endophytic fungi from leaves of neem

(Azadirachta indica A. Juss.). Curr. Sci. 78:1375–1378.

Saikkonen, K., Faeth, S.H., Helander, M., Sullivan, T.J. (1998). Fungal endophytes: a continuum of

interactions with host plants. Ann. Rev. Ecol. Syst. 29:319–343.

Stone, J.K., Bacon, C.W., White, J.R. (2000). An overview of endophytic microbes: endophytism

defined. In Microbial Endophytes, Bacon, C.W., White, J.F., Eds. New York, Marcel Dekker,

pp. 3–29.



DK3133_C009.fm Page 191 Monday, April 18, 2005 4:21 PM



Emerging Perspectives on the Ecological Roles of Endophytic Fungi



191



Suryanarayanan, T.S., Venkatesan, G., Murali, T.S. (2003). Endophytic fungal communities in leaves

of tropical forest trees: diversity and distribution patterns. Curr. Sci. 85:489–493.

Vogl, A. (1898). Mehl und die anderen Mehlprodukte der Cerealien und Leguminosen. Zeitschrift

Nahrungsmittle Untersuchung, Hyg. Warenkunde 12:25–29.

White, J.F., Jr., Martin, T.I., Cabral, D. (1996). Endophyte-host associations in grasses. XXII. Conidia

formations by Acremonium endophytes on the phylloplanes of Agrostis Hiemalis and Poa

rididifolia. Mycologia 88:174–178.

Wilson, D. (1995a). Endophyte: the evolution of a term, and clarification of its use and definition.

Oikos 73:274–276.

Wilson, D. (1995b). Fungal endophytes which invade insect galls: insect pathogens, benign

saprophytes, or fungal inquilines. Oecologia 103:255–260.

Wilson, D. (2000). Ecology of woody plant endophytes. In Microbial Endophytes, Bacon, C.W.,

White, J.F., Jr., Eds. New York, Mercel Dekker, pp. 389–420.

Wilson, D., Carroll, G.C. (1994). Infection studies of Discula-quercina, an endophyte of Quercusgarryana. Mycologia 86:635–647.

Wilson, D., Carroll, G.C. (1997). Avoidance of high-endophyte space by gall-forming insects.

Ecology 78:2153–2163.

Wilson, D., Faeth, S.H. (2001). Do fungal endophytes result in selection for leafminer ovipositional

preference? Ecology 82:1097–1111.

Wilson, D., Barr, M.E., Faeth, S.H. (1997). Ecology and description of a new species of Ophiognomonia endophytic in the leaves of Quercus emoryi. Mycologia 89:537–546.



DK3133_C009.fm Page 192 Monday, April 18, 2005 4:21 PM