plant surface microbiology.pdf

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James F. White Jr. et al.
itive correlation between the size of stromata and the growth rate on a selection
of sugars. Apparently, the larger the stroma formed on a particular host,
the faster an endophyte must grow to develop that stroma. It was further
found that endophytes that failed to reach the critical growth rate on any of
the sugars, tended to produce fewer stromata per plant than endophytes that
grew rapidly on all of the sugars. From an evolutionary perspective, selection
against stroma development may be selection for endophytes that grow more
slowly on nutrients available in host tissues. This hypothesis is consistent with
at least one important observation. Many asexual endophytes (e.g., Neotyphodium
coenophialum and N. lolii) are slow growing in culture while stromaforming
endophytes grow comparably faster.
7.4 The Shift from Pathogen to Mutualist
Much is known of the biochemistry and genetics of the interactions between
plants and pathogenic organisms and how these interactions result in disease
or in plant resistance (Oliver and Osbourne 1995; Hammond-Kosack and
Jones 1996). Mutualistic associations, such as those between the fungal endophytes
and their grass hosts, are believed to have evolved from pathogenic
associations (Clay 1988). Little is known regarding the genetic changes that
result in a change from a pathogenic to a mutualistic lifestyle.
Plant fungal pathogens typically secrete a number of plant cell wall degrading
enzymes such as cellulases, glucanases, xylanases, and polygalacturonases.
It is likely that expression of these cell wall degrading enzymes plays
some role in pathogenicity (Oliver and Osbourne 1995; Mendgen et al. 1996),
although disruption of individual genes has not resulted is reduced virulence
(Scott-Craig et al. 1990; Apel et al. 1993; Schaeffer et al. 1994; Bowen et al. 1995;
Sposato et al. 1995). The presence of other genes encoding the same enzyme
activity and synergistic activity of different cell wall degrading enzymes in
pathogenicity may explain these results.
Claviceps purpurea, a plant pathogen closely related to the Epichloë and
Neotyphodium endophytes, secretes a polygalacturonase during infection of
rye ovaries (Tenberge et al. 1996). Polygalacturonase activity is believed to be
important in splitting the host middle lamellae allowing intercellular growth
of the fungus (Tenberge et al. 1996). Since the fungal endophytes also have an
intercellular mode of growth, we have investigated the possibility of endophytic
polygalacturonase expression in the Neotyphodium sp. endophyte that
infects the grass Poa ampla. No hybridization was detected in a DNA blot
using the cloned C. purpurea gene as a probe. Also, nothing was detected in
PCR reactions using degenerate primers based on conserved amino acid
regions of polygalacturonase genes from diverse organisms. It appears that
this endophytic fungus may have lost the gene(s) for polygalacturonase. Perhaps
loss of this cell wall degrading activity is a factor in the evolution of