plant surface microbiology.pdf

508
Frank B. Dazzo
degrade the bacterial polysaccharides; it exhibits host-selectivity and is found
on approximately 95 % of successfully infected root hairs in the Rhizobiumwhite
clover symbiosis (Dazzo and Hubbell 1975; Dazzo et al. 1976, 1982, 1984;
Dazzo and Brill 1979; Sherwood et al. 1984; Rolfe et al. 1996; Smit et al. 1992).
The Phase 1A pattern of randomly oriented attachment occurs within 15 min
of inoculation, and involves an initial nonhost-specific interaction of a rhizobial
surface protein “rhicadhesin” on individual bacteria with the root hair tip
(Smit et al. 1992), followed within the first hour by a more host-specific aggregation
of bacterial cells immobilized at the root hair tip and mediated by an
excreted, multivalent host lectin. Cells that have not yet attached to the host
root become polarly encapsulated in the external root environment during
the next 4–8 h (Phase 1B), due to the combined action of “polarase” enzymes
in root exudate and de novo synthesis of a new capsule at one cell pole (Dazzo
et al. 1982; Sherwood et al. 1984). Beginning approximately 4 h after inoculation,
these polarly encapsulated cells attach “end-on”, i.e., perpendicular to
the surface along the sides of the same root hair (phase 1C). Phase 1 attachment
is distinguished from phase 2 adhesion by the significantly increased
strength of adhesion of attached cells detected approximately 12 h after inoculation,
concurrent with the elaboration of extracellular microfibrils that
increase the degree of contact of the attached bacteria to the root hair surface
(Dazzo et al. 1984). Indeed, this strength of Phase 2 rhizobial adhesion to
legume host root hairs is immense, exceeding that which anchors some root
hairs onto the root itself! Figure 2A–F is a series of phase contrast light micrographs
and scanning electron micrographs that illustrate each of these distinct
patterns of rhizobial attachment to white clover root hairs (Dazzo and
Brill 1979; Dazzo et al. 1984).
2.3 Rhizobium-Induced Root Hair Deformations
Root hairs on axenic seedlings are straight, but become deformed (Had [Hair
deformation] phenotype) during growth in response to various bioactive
metabolites made by rhizobia. Four different morphotypes of white clover
Had are induced under axenic conditions by minute quantities of purified
bioactive Nod metabolites made by R. leguminosarum bv. trifolii. These are
root hair distortions, tip swellings, branches, and corkscrews induced by rhizobial
membrane chitolipooligosaccharides, N-acetylglutamic acid, and
diglycosyl diacylglycerol glycolipids (Philip-Hollingsworth et al. 1991;
Orgambide et al. 1994, 1996; Dazzo et al. 1996a, b;). Collectively called moderate
Had, these various types of root hair deformations are less symbiont-specific
than marked curling of the root hair tip (commonly referred to as the
“Shepherd’s crook” Hac [Hair curling] phenotype). This Hac morphotype is
illustrated in Fig. 3 and requires close proximity of viable cells of the homologous
symbiont (Li and Hubbell 1969; Yao and Vincent 1976). This figure is a