plant surface microbiology.pdf

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Ajit Varma et al.
parasite on other fungi. Basidiomycetes, e.g., include saprobes, mycorrhizaforming
fungi, plant parasites, but also fungi which are parasites of other
fungi. Hosts are both Basidiomycetes and Ascomycetes. Ultrastructural investigations
of this kind of organismic interaction (Chap. 16) revealed two main
types, the formation of colacosomes and the fusion between pathogen and
host fungus cells. Colacosomes are unique organelles, which appear at the
interface between parasite and host while fusion is based on specialized interactive
cells (haustoria), which establish a direct cytoplasmic connection.
Many microorganisms coexist with plants in ways that do not lead to plant
disease, symbiosis, or other specific interactions. Some fungi or bacteria can
be latent pathogens. Some have little or no influence on the plant, but may
form toxic compounds that are damaging to grazing animals. Microorganisms
that form more or less benign associations with plants are generally
termed ‘endophytes’ and are genetically diverse. A large number of fungal
endophytes can be difficult to identify because they include a high proportion
with sterile mycelia (Chap. 17). Overall, the roles of many of these organisms
are poorly understood.
The mechanisms for entry of endophytic organisms into plants can be
investigated using methodologies such as those applied to elucidate the
cytoskeletal rearrangements of plant cells and fungal hyphae at the plant–
microbe interface during colonization of roots by mycorrhizal fungi (Chap.
18). Invading organisms have been shown to influence the expression of plant
genes for some filamentous structures within the cell cytoskeleton. Indirect
immunofluorescence microscopy has been used to investigate the cytoskeleton
of some mycorrhizal associations demonstrating the separation and
invagination of the plasma membrane from the plant cell wall in response to
growth of fungi inside the cell wall.
Colonization of plants by related and unrelated groups of microorganisms
may occur simultaneously. For example, saprophytes, pathogens and mycorrhizal
fungi may be associated with the same root systems and colonize roots
to different degrees.Several species of arbuscular mycorrhizal fungi can simultaneously
colonize the same sections of root,although they are generally separated
in different cells or parts of the root cortex. Prior colonization by one
organism can influence sequential colonization by other organisms. This
occurs to varying degrees for different groups of plant endophytes, symbionts
and pathogens. The relative extent to which roots become colonized by several
species of arbuscular mycorrhizal fungi present in the same soil depends on
the relative abundance of propagules of the fungi in the soil,the developmental
stage of the hyphae associated with fungal propagules, the susceptibility of the
roots to invasion and the physiological responses of the root to different
species of fungi (Chap. 19). Investigations of the molecular communication
between these fungi and their host plants during root colonization and nutrient
acquisition are now beginning to be understood in terms of gene expression
in plants and fungi. This provides a basis for predicting physiological