plant surface microbiology.pdf

17 Fungal Endophytes 283
increase directly as a result of host survival. The association where only one
fungus is associated within the host plant is more likely to be mutualistic
(Hammon and Faeth 1992).
The endophytes associated with grasses have received much attention, and
many of these have been found to produce physiologically active alkaloids
that cause their hosts to be toxic to mammals and increase their resistance to
insect herbivores (Funk et al. 1983; Clay 1988; Cheplick and Clay 1988;
Prestidge and Gallagher 1988). In the grasses and other plant hosts, endophytes
have also been shown to enhance plant growth, reduce infection by
nematodes, increase stress tolerance and increase nitrogen uptake in nitrogen
deficit-soils (Latch et al. 1985; Clay 1987, 1990; Kimmons 1990; Bacon 1993;
Gasoni and Stegman De Gurfinkel 1997; Rommert et al. 1998; Verma et al.
1999; Bultman and Murphy 2000;). Several reviews are available on secondary
metabolite production by endophytes (Miller 1986; Clay 1991; Petrini et al.
1992). Endophytes in culture can produce biologically active compounds
(Brunner and Petrini 1992) including several alkaloids, paxilline, lolitrems
and tertraenone steroids (Dahlman et al. 1991), antibiotics (Fisher et al. 1984a,
b) and plant growth promoting factors (Petrini et al. 1992). Endophytes are
increasingly being identified as a group of organisms capable of providing a
source of secondary metabolites for use in biotechnology and agriculture
(Bills and Polishook 1992).
4 Modes of Endophytic Infection and Colonization
The colonization of plant tissues by endophytes, plant pathogens and mycorrhizae
involves several steps involving host recognition, spore germination,
penetration of the epidermis and tissue colonization (Petrini 1991, 1996). The
inoculum source of fungal endophytes is widely considered to be the airborne
spores, and also seed transmission and transmission of propagules by insect
vectors (Petrini 1991). A high level of genetic diversity of endophyte isolates
suggests that infection foci arise from different strains of fungi derived from
constant new inoculum (Hammerli et al. 1992; Rodrigues et al. 1993). In terms
of mechanical and enzymatic elements of penetration by endophytic fungi, it
can be assumed that endophytes adopt the same strategy for penetration of
host tissue as pathogens (Petrini et al. 1992). Fungi can invade plant tissues by
direct cuticular penetration, via appressoria formed on the cuticle, after
which penetration occurs through the cuticle and epidermal cell wall or via
natural openings like stomata (O’Donnell and Dickinson 1980; Muirhead and
Deverall 1981; Kulik 1988; Cabral et al. 1993; Viret et al. 1993; Viret and Petrini,
1994). Following penetration the infection may be inter-cellular or intra-cellular
and may be limited to one cell or in a limited area around the penetration
site. Limited cytological work on nonclavicipitaceous endophytes have
shown that the infection of these endophytes in host plants may be inter- or