plant surface microbiology.pdf

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James F. White Jr. et al.
endophyte has not been found to possess insect deterrent properties, instead
growth enhancements have been proposed (Faeth et al. 2000). Finally several
species of fine fescues infected by Epichloë festucae were found to have an
increased resistance to the dollar spot disease caused by Sclerotinia homeocarpa.
It seems evident that plants benefit tremendously from the colonization of
symbiotic microbes. The benefits to hosting mutualistic microbes likely outweigh
losses in terms of nutrient use by the microbes. The widespread level of
infection of grasses by Epichloë and asexual forms in Neotyphodium are evidence
that these associations also have evolutionary value. In one study of
endophytes in grasses, infection levels in some hosts (e.g., Achnatherum
robustum and Festuca versuta) were estimated to be greater than 90 % in populations
throughout the ranges of the grasses (White 1987).
3 Coevolution of Clavicipitalean Fungi with Grass Hosts
It is evident that Epichloë (Clavicipitaceae; Ascomycetes) and related asexual
endophytes have co-evolved with the cool-season (C-3) grasses in which they
perennate (White 1988). Species of these endophytes are unknown from
warm-season (C-4) grasses (White 1987). On the other hand, endophytic
species in genus Balansia, also in family Clavicipitaceae, appear to have coevolved
with warm-season grasses and are rarely or never found on cool-season
hosts (White and Owens 1992). In co-evolving with grasses, it is logical to
expect that their interactions with hosts became more sophisticated.
4 The Jump from Insects to Plants
4.1 Trans-Kingdom Jump
Analysis of rDNA 26S sequence data indicates that the predominantly insectinfecting
subfamily Cordycipitoideae (Clavicipitaceae) is the most deeply
rooted group and is, therefore likely ancestral to grass-infecting species (Sullivan
et al. 2000).A trans-kingdom host jump is postulated to have occurred to
plants. Such a jump could have occurred gradually through intermediate
forms that were parasitic on both insects and plants.
4.2 Intermediate Stages in the Transition to Plants
Several Cordycipitoideae exhibit stages of such a transition. Most of the
Cordycipitoideae (e.g., Cordyceps militaris and C. sinensis) infect insect hosts
and mummify them, using their necrotrophied bodies as energy to fuel fun-