plant surface microbiology.pdf

27 Applications of Quantitative Microscopy in Plant Surface Microbiology 511
2.5 In Situ Molecular Interactions Between Legumes Roots and Surface-
Colonizing Rhizobia
Microscopy has played a central role in elucidating molecular events important
to the development of the Rhizobium-legume root-nodule symbiosis. The
use of various molecular probes combined with the awesome resolving power
of the microscope has made it possible to dissect and locate key molecules
that participate in primary host infection, including the cell surface interfaces
during symbiotic recognition, attachment, deformation, and root hair penetration,
and also in root nodule development. Various types of microscopy
that can view intact living cells noninvasively have added new dimensions to
unraveling the symbiotic interactions of potent rhizobial signal molecules
with host cells, including the precise localization of specific binding receptor
sites on the host root surface, the rapid internalization of certain rhizobial signal
communication molecules within root hairs and their transfer to underlying
cortical cells, and various other infection-related host cell responses. The
significance of all of these studies is improved when the various microscopical
techniques are accompanied by quantitative methods of data acquisition.
Some examples of in situ “molecular microscopy” in studies of plant surface
microbiology are illustrated here.
2.6 Cross-Reactive Surface Antigens and Trifoliin A Host Lectin
Rhizobium leguminosarum bv. trifolii and white clover roots share related
surface components that are antigenically cross-reactive (Dazzo and Hubbell
1975; Dazzo and Brill 1979). Quantitative immunofluorescence microscopy
indicates that these cell-surface antigens are transient, symbiont-specific,
infection-related, and participate in the host lectin-mediated stage of symbiont
recognition on the clover root hair surface (Dazzo and Hubbell 1975;
Dazzo and Brill 1979; Dazzo et al. 1979). Transformation of Azotobacter
vinelandii with DNA from R. leguminosarum bv. trifolii resulted in hybrid
recombinants that expressed these symbiotic cross-reactive antigens (Bishop
et al. 1977), and these recombinants gained the ability to carry out the phase
1A pattern of bacterial cell attachment to white clover root hair tips (Dazzo
and Brill 1979). The cell surface location of these epitopes plus their infection-related
symbiont-specificity, interaction with the multivalent white
clover root lectin, and role in cell attachment formed the basis for proposing
their involvement as cell-surface receptors in a lectin cross-bridging model
of symbiont recognition during early stages of primary host infection
(Dazzo and Hubbell 1975; Dazzo and Brill 1979). Recent studies using plant
molecular biology techniques have provided substantial evidence supporting
the validity of this cross-bridging model (van Rhijn et al. 1998; Hirsch
1999).