plant surface microbiology.pdf

10 Development Interactions Between Clavicipitaleans and Their Host Plants 165
potent mycoparasite of many plant pathogenic fungi (Papavizas 1985; Chet
1987). Because of this property, it is being investigated as a potential biocontrol
agent in crop production.
The physiological roles of the endophytic proteinase, chitinase, and endob-1,6-glucanase
are not yet known. The endophytic chitinase transcript is
very abundant as determined from a blot of total RNA isolated from the
infected plants. This is similar to the situation with the endophytic proteinase
(Reddy et al. 1996). The endo-b-1,6-glucanase appears to be expressed at
lower levels. Several roles have been proposed for chitinases and endoglucanases
from filamentous fungi. Roles in hyphal growth, branching, and
autolysis have been proposed (Bartnicki-Garcia 1973; Gooday and Gow 1990;
Peberdy 1990) as well as roles in mycoparasitism. Functions in fungal growth
and/or in mycoparasitism would be relevant to endophytic infection. Interestingly,
the homologous proteinase, chitinase, and endo-b-1,6-glucanase from
T. harzianum are believed to be synergistic components of its mycoparasitic
activity (Geremia et al. 1993; Garcia et al. 1994; Lora et al. 1995). These
hydrolytic enzymes function together to break down the cell walls of the fungal
hosts allowing entry of the T. harzianum hyphae.
Expression of these hydrolytic enzymes in endophyte-infected plants
raises the possibility that they may also function as a mycolytic system for the
endophyte. Such a system could provide the endophyte with a source of nutrients
in addition to plant derived nutrients found in the apoplast. With a
mycolytic system, the endopytic hyphae located on the surface of the plant
(Moy et al. 2000) would have access to additional sources of nutrients from
other surface-located fungi. By attacking invading fungi, an endophytic
mycolytic system could also protect the plants from pathogenic fungi, perhaps
resulting in enhanced disease resistance. Current research is aimed at
determining the roles of these enzymes in endophyte infection.
6 Modifications of Plant Tissues for Nutrient Acquisition
6.1 Development of the Stroma in Epichloë
The development of sexual reproductive structures in plants poses some special
problems for Clavicipitaceae. Larger quantities of nutrients are needed to
provide the fuel for construction of the external mycelial stroma on which are
produced first spermatia, then perithecia and ascospores (White and Bultman
1987; Bultman et al. 1995). To obtain large quantities of nutrients from hosts,
many other groups of biotrophic fungi,e.g.,powdery mildews,downy mildews,
and rusts may produce haustoria to suck nutrients from individual host cells
(Alexopoulos et al. 1996). However, clavicipitalean plant biotrophs have
another strategy.They grow on meristematic tissues before the cuticle has been
formed and by some unknown mechanism prevent development of the waxy