plant surface microbiology.pdf

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James F. White Jr. et al.
cuticle and alter the epidermis itself, effectively removing a key barrier to the
flow of nutrients to the stromal mycelium. The following two examples will
illustrate this method of nutrient acquisition by these clavicipitaleans. In
Epichloë the abundance of sucrose in the developing inflorescence primordium
triggers the fungus to proliferate rapidly and permeate the young
inflorescence and the leaf sheath of a leaf that surrounds it.This process is comparable
to that already suggested for stroma development in Atkinsonella
hypoxylon, except that in Epichloë the mycelium is endophytic and frequently
permeates vascular tissues as well as nonvascular tissues (White et al. 1991).
The stroma is composed of a mix of plant tissues and fungal mycelium. These
stromata are much like those of the scale insect parasites Hypocrella africana,
H. gaertneriana,andH. schizostachyi,in that the host tissues embedded within
the stromata remain alive, but are modified so that nutrients will flow freely
into the developing stromata. Plant tissues embedded within the stroma are
not only permeated by mycelium, but also possess epidermal cells that are
hypertrophied, often collapsed, and lack waxy cuticles (White et al. 1997).
Through these modifications of the host tissues, the endophyte removes all
barriers to nutrient flow into the stromal mycelium. The development of
mycelium within the vascular bundle enhances the transfer of nutrients to the
fungal stroma. By mummifying the living inflorescence primordium and the
sheath of the leaf that surrounds it, the fungus can intercept all nutrients that
are transported into the flowering tiller. Mature stromata of Epichloë always
possess the stromal leaf blade emergent from the top of the stroma (Fig.4).The
reason for this emergent leaf blade is unknown, but may be a source of plant
hormones that are needed as a signal to the plant to continue to send nutrients
into the culm. Experimental work is needed to evaluate this hypothesis.
6.2 Stroma Development in Myriogenospora
A second clavicipitalean biotroph that modifies host tissues for nutrient
acquisition during stroma development is the epiphytic fungus Myriogenospora
atramentosa. Myriogenospora atramentosa grows superficially on
the epidermis of young leaves at the crown of many warm-season grasses and
sedges. As the leaves develop, conidia of M. atramentosa proliferate on the
folded leaves of the grass. The leaves continue to expand and the conidial
stroma develops into a linear black perithecial stroma, composed of a single
line of perithecia (Figs. 5, 6). The plant leaf tissues beneath the stroma are
modified with hypertrophied epidermal cells that lack a cuticular layer
(Rykard et al. 1985; White and Glenn 1994). The absence of a cuticle layer on
the leaf epidermis and modification of the epidermal cells by the fungus permits
M. atramentosa to absorb nutrients directly through the epidermis of the
leaf blades to provide energy for stroma development.