plant surface microbiology.pdf

10 Development Interactions Between Clavicipitaleans and Their Host Plants 159
gal development (Tzean et al. 1997). In these associations, there is no association
with plants and no opportunity for the fungi to adapt to plants as hosts.
However, some species of Cordycipitoideae infect scale insects that are sedentary
and parasitic on plant hosts by use of stylets with which they penetrate
and suck sugars from host vascular tissues. In these species some simple
adaptations for parasitism of plants are evident. In Hypocrella africana, H.
gaertneriana,andH. schizostachyi, infection of the scale insect is biotrophic
with the fungus obtaining nutrients from the plant through the living body of
the insect (Hywel-Jones and Samuels 1998). Here, the parasitized scale insect
is a bridge to obtain plant nutrients; however, the fungus does not interface
directly with the plant in any way. This is an indirect adaptation to parasitism
on plants. The quantity of nutrients available to Hypocrella in this type of
association far exceeds that available in the body of the insect. Hywel-Jones
and Samuels (1998) estimated that the stroma attained some 1000 times or
more the mass of the body of the insect. Hyperdermium bertonii exhibits
another step toward direct parasitism of plants. This species infects scale
insects, necrophytizes them, then develops epibiotically on the surface of the
plant, nourishing itself on sugars that continue to flow from the stylet wound
left by the scale insect (Sullivan et al. 2000). Although H. bertonii relies on
scale insects to prepare its parasitism site on plants, it directly absorbs and
utilizes plant sugars. It is also possible that H. bertonii produces compounds
that interfere with scar tissue development to prevent the stylet wound from
sealing. This possibility should be further evaluated. However, at present we
have no evidence that wound retardant compounds or growth regulator compounds
are being produced by H. bertonii. Regardless, it is evident that H.
bertonii has taken physiological steps in adapting to growth on plant sugars.
In experiments, conducted in vitro where H. bertonii is grown on a minimal
medium containing minerals and combinations of simple sugars glucose and
fructose, we have demonstrated that mycelium and conidial production are
stimulated by equal ratios of glucose to fructose; while higher levels of fructose
in media induce the fungus to differentiate pigmentation and its mature
stromal morphology. Hyperdermium bertonii has adapted to utilize changes
in host sugar content on which it nourishes itself to guide its development.
Sucrose, leaking directly from the stylet wound, is cleaved to its component
monomers glucose and fructose. The glucose is likely preferentially absorbed.
As a result, fructose is left behind to accumulate in the liquid film of sugars on
which the fungus grows. Increasing concentrations of fructose, or the fructan
polymers of it, are the probable cues employed by the epiphyte to shift its
growth from early stroma development to differentiation and maturation.
The possession of invertases by H. bertonii may also be evidence of adaptation
to plants. Sucrose is only available in plant tissues. It is a short step from
the condition of Hyperdermium to infection of plants without the use of
insects.