plant surface microbiology.pdf

160
James F. White Jr. et al.
4.3 Parasitism of Grass Meristematic Tissues
On the meristematic tissues of grasses, wounds created by insects are unnecessary
since the tissues of meristems, such as the inflorescence primordia, are
bathed in sucrose. Atkinsonella hypoxylon illustrates this point. Atkinsonella
hypoxylon grows superficially on young leaves of grasses as an epiphyte, perhaps
degrading wax in the cuticle to obtain nutrients for epiphytic growth
(White et al. 1991). When the grass begins to produce an inflorescence primordium,
sucrose is mobilized into the primordium to provide energy for its
development. The primordium is surrounded by nutrients in liquid, which in
turn is surrounded by layers of developing leaves. It is believed that the sudden
increase in sucrose availability and concentration triggers rapid mycelial
growth that eventually results in formation of the stroma (White and Chambless
1991). It is interesting that leaves and inflorescence primordia within the
stroma never develop a cuticular layer that would impede flow of nutrients
and moisture to the fungus. Prevention of cuticular development and prevention
of maturation of the inflorescence primordial tissues may be a growth
regulator effect, although they have not yet been identified for this species.
5 Developmental Differentiation of Endophytic and
Epiphyllous Mycelium
5.1 Plant Cell Wall Alteration
Epichloë sp. illustrate many of the physiological capacities needed by
Clavicipitaceae to colonize grasses (White et al. 1991). Epichloë inhabits leaf
sheaths and growing tillers of grass plants. Endophytic mycelium is largely
nonbranched and exclusively intercellular (Fig. 1) and often seen to adhere
closely to parenchyma cell walls as if attached by glue. This may be due to the
partial degradation of cell wall components by the endophytic mycelium.
While it is possible that cell walls are modified by endophytes, they remain
largely intact as evidenced by electron microscopic studies. It is notable that
during stroma development, profound changes have been observed in cell
walls of the grass epidermis.Walls of epidermal cells appear to lose structural
integrity with mycelium of the endophyte frequently penetrating the wall
(Fig. 2).
5.2 Endophytic Mycelial Growth
Endophytic mycelium in young leaves or elongating tillers is frequently narrow
(1 mm across), straight, oriented parallel to the axis of expansion of the
cells and plant organ (Fig. 3). Sometimes in very young tissues, the hyphae