plant surface microbiology.pdf

10 Development Interactions Between Clavicipitaleans and Their Host Plants 171
key signal for the fungus to begin a sequence of developmental events that
end in production of the mature stroma. Some preliminary differential display
studies were also conducted to examine genes that may be upregulated
and downregulated when A. hypoxylon was exposed to cytokinin. The results
of these differential display studies showed that several genes were turned on
while several others were turned off, however, none of the genes were identified.
One preliminary study on another clavicipitalean-producing stromata
on inflorescence primordia, Epichloë festucae, employed a monoclonal antibody-based
cytokinin detection kit (Phytodetek-t-ZR, Sigma, St. Louis, Missouri)
to compare levels of cytokinin in stromata and other tissues of the
grass. The result of this test suggested that cytokinin was present in high concentrations
within the stromata. However, this test must be considered preliminary
because of the possibility for cross-reactivity of the antibody with
other compounds. More precise tests for the presence of cytokinins must be
employed to evaluate levels in the stromata. Presently, the hypothesis that
cytokinins are a key cue for development of stromata on the grass inflorescence
primordium for the grass inflorescence-colonizing clavicipitaleans is
an interesting hypothesis. However, additional work must be done to evaluate
this hypothesis.
7 Evolution of Asexual Derivatives of Epichloë
7.1 Reproduction and Loss of Sexual Reproduction
One notable feature of genus Epichloë is the abundance of asexual species,
often classified in form genus Neotyphodium. Formation of asexual derivatives
is apparently a relatively frequent phenomenon based on how common
these asexual forms are in grasses (White 1987). In the sexual cycle of
Epichloë stromata are produced on grasses, and on stromata spermatia
develop. In a heterothallic mating process symbiotic flies in genus
Botanophila (Anthomyidae) vector spermatia between stromata of the opposite
mating type (Bultman et al. 1995). Following deposition of spermatia on
a compatible stroma, an ascogenous (dikaryotic) mycelium develops in
which perithecia and ascospores form. Meiosis takes place within the asci to
result in the haploid ascospores that are ejected from asci onto surrounding
vegetation, where they may germinate to form wind-disseminated conidia
(White and Bultman 1987). Precisely how primary infections of grasses
occur is still unknown, but may involve a period of epiphyllous growth prior
to penetration of plant tissues (Moy et al. 2000). Other investigators (Diehl
1950; Chung and Schardl 1997) have suggested that ovules may be the site of
entry into plants. However, definitive data that will answer this question are
still lacking. The asexual forms of Epichloë are seed-transmitted and stromata
do not form on grass inflorescences. Since these asexual forms do not