plant surface microbiology.pdf

10 Development Interactions Between Clavicipitaleans and Their Host Plants 169
ate impacts of clavicipitalean-produced secondary metabolites on plant tissues
themselves. The current scientific wisdom holds that clavicipitalean secondary
metabolites have impacts on animal tissues as feeding deterrents and
other defensive compounds. Whether ergot alkaloids, auxin-like compounds
or other secondary metabolites of these fungi are involved in effecting
changes in plant tissues embedded within or adjacent to stromal mycelium
must be further evaluated. It seems likely that this will be a fruitful area for
future investigation.
6.4 Evaporative-Flow Mechanism for Nutrient Acquisition
The stroma of Epichloë maintains a constant flow of water and nutrients into its
mycelium through an evaporation-driven process (White and Camp 1996;
White et al. 1997). Water evaporates rapidly from the surface of stromata. As
water evaporates from the stroma it is replaced by water from the plant. This
process establishes a flow of water and dissolved nutrients into the stroma from
mycelium interfacing with the vascular bundles and other tissues embedded
within the stroma. Evaporative flow mimics the enhanced transpiration that
occurs in developing inflorescences during elongation of the flowering tillers
of uninfected grasses, but here the stroma is the recipient of the nutrients.
6.5 The Cytokinin Induction Hypothesis
In Atkinsonella hypoxylon stromata form on the inflorescence primordium
and include parts of several leaves as well (Fig. 7). The stromata are gray
(sometimes with areas of a yellow pigment) and produce several different
spore states, including cup-shaped sporodochia that produce moist masses of
ephelidial conidia, and a layer of neotyphodial conidia borne on tips of elongate
conidiogenous cells. It is reasonable to expect that the fungus would
coordinate its development with that of its host grass. For example, the fungus
mycelium must be able to detect when it is growing on an inflorescence primordium
rather than on the tiller meristems. On the tiller meristems it will
produce a low biomass of nonpigmented mycelium and ephelidial conidia,
but no neotyphodial conidia or other structures; while on the inflorescence
primordium the entire suite of morphological structures is produced. One
way for the fungus to coordinate its development to that of the host plant
would be to use compounds present in the host during different stages of
development as ‘cues’ to initiate developmental stages in the fungus. Our
approach to the search for host compounds that may serve as cues for fungal
development has been a trial and error approach. Over several years, we have
screened hundreds of compounds that might be present in grass tissues to
determine how they affect differentiation of A. hypoxylon.