plant surface microbiology.pdf

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Frank B. Dazzo
cated that the spatial density of these rhizobia-associated eroded pits was significantly
higher on the root epidermis of host rather than nonhost legume
combinations, was inhibited by high nitrate supply, and was not induced by
immobilized wild-type R. leguminosarum bv. trifolii chitolipooligosaccharide
Nod factors reversibly adsorbed to latex beads. Transmission electron microscope
(TEM) examination of these highly localized epidermal pits indicated
that they were only partially eroded, i.e., only the outer amorphous region of
the plant wall in direct contact with the bacterial cell was disrupted, whereas
the underlying highly ordered portion(s) of the wall remained ultrastructurally
intact (Fig. 9B). Further studies using phase contrast and polarized
light microscopy indicated that (1) the structural integrity of clover root hair
walls is dependent on wall polymers that are valid substrates for the purified
cell-bound polysaccharide-degrading enzymes (e.g., C2 cellulase isozyme)
from rhizobia (Fig. 9C–G); (2) the major site where these rhizobial cell-bound
enzymes can completely erode through the root hair wall is highly localized at
the isotropic, noncrystalline apex of the root hair tip (Fig. 9C–G), and (3) the
degradability of clover root hair walls by these rhizobial polysaccharidedegrading
enzymes is enhanced by modifications induced during growth in
the presence of CLOS Nod factors from wild-type clover rhizobia. These
results suggest that these eroded plant structures represent incomplete
attempts of bacterial penetration that had only progressed through isotropic,
noncrystalline layers of the plant cell wall, and that the rhizobial cell-bound
glycanases and chitolipooligosaccharides participate in complementary roles
that ultimately create the localized transmuro portal of entry for successful
primary host infection (Munoz et al. 1998; Mateos et al. 2001).
2.11 Elicitation of Root Hair Wall Peroxidase by Rhizobia
Many investigators have proposed that successful infection of legumes by rhizobia
may depend on the microsymbiont’s ability to escape, suppress, or avoid
host defense responses that normally protect plants against invasive microorganisms
(Vance 1983; Djordjevic et al. 1987; Parniske et al. 1990, 1991). To test
this hypothesis, we performed in situ enzyme cytochemistry at subcellular
resolution using brightfield microscopy followed by in vitro enzyme assays to
detect changes in activity of plant wall-bound peroxidase as an indication of
a localized host defense response following inoculation of white clover and
pea roots with compatible and incompatible combinations of rhizobial symbionts
(R. leguminosarum biovars trifolii and viciae; Salzwedel and Dazzo
1993). For compatible combinations, elevated peroxidase activity was initially
delayed, but subsequently located precisely at infection-related sites: the center
of markedly deformed shepherd’s crooks and at penetration sites of incipient
infection thread formation, but not elsewhere on the infected root hairs
including the intracellular infection thread itself. In contrast, the incompati-