plant surface microbiology.pdf

510
Frank B. Dazzo
Fig. 4. Phase contrast micrograph of primary
host infection in the Rhizobium–legume symbiosis.
Note the prominent infection thread
(arrow) within the deformed root hair cell.
Scale bar 10 mm
elongate during the inward growth of the infection thread, which proceeds at
approximately the same elongation rate. This inward growth of the infection
thread is led by a mobile nucleus and a flurry of cytoplasmic streaming within
the root hair (Nutman et al. 1973). Successful infections are best quantitated
by visual counting while viewed by phase contrast microscopy; light staining
of the infection thread with methylene blue can enhance contrast to detect
them. Distinctions of successful vs. unsuccessful infections can be made by
detailed microscopical examination to assess whether the infection thread
has grown to the root hair base and penetrated into the underlying subepidermal
cortical cell. Infective rhizobia engineered with Gus or green fluorescent
protein reporter genes can facilitate the detection of infected root hairs,
but this is overkill for skilled microscopists.
An alternate primitive route of primary host infection of legumes leading
to effective nodule formation is the crack entry of rhizobia into natural
wounds of the host plant epidermis. This commonly occurs in many tropical
legumes (Napoli et al. 1975b) and some temperate legumes, but can also occur
infrequently in anomalous ineffective nodulations by rhizobia outside their
normal cross-inoculation group (Hrabak et al. 1985). In the aquatic legume
Neptunia natans where root hairs do not normally develop, the natural splitting
of the epidermis during development of the spongy aerenchyma and
emergence of adventitious and lateral roots create openings that allow “crack
entry” of the rhizobial symbiont, Allorhizobium undicola, Rhizobium undicola,
or Devosia neptuniae, as the normal mode of primary host infection
(Subba-Rao et al. 1995).
Recently, an interesting novel combination of infection events has been
found to occur in development of the root-nodule symbiosis of rhizobia with
tagasaste (Chamaecytisus proliferus L.), a legume indigenous to the Canary
Islands near the west coast of Africa. In this symbiosis, primary host infection
initially involves rhizobial deformation and penetration of host root hairs, but
all these primary host infections abort and the rhizobia then revert to a crack
entry mode of invasion directly into the emerging root nodules without
development of infection threads (Vega-Hernandez et al. 2001). Quite a
remarkable, unique mode of plant infection by surface rhizobia!