plant surface microbiology.pdf

27 Applications of Quantitative Microscopy in Plant Surface Microbiology 515
Fig. 6. Role of Rhizobium
acidic heteropolysaccharide
in symbiotic development
with legumes. A
Direct detection of symbiont-specific
receptor
sites for R. leguminosarum
bv. trifolii acidic heteropolysaccharide
on root
hairs of white clover
seedlings. B Quantitative
microscopy of symbiotic
phenotypes of an R. leguminosarum
bv. trifolii
ANU437 Exo – mutant relative
to its Exo + wild-type
ANU794 parent scored on
the white clover host. The
significant requirement of
the bacterial acidic heteropolysaccharide
in
expression of its important
Roa-3, Hac, and Inf symbiotic
phenotypes is clearly
indicated. Reprinted with
permission from the
American Society for
Microbiology
match the cellular distribution of trifoliin A on the root surface, and are
specifically hapten-inhibitable, thus implicating an involvement of this root
hair lectin in recognition of the rhizobial acidic heteropolysaccharide (Dazzo
and Brill 1977; Dazzo et al. 1978).
Further symbiotic roles of the acidic heteropolysaccharide from R. leguminosarum
bv. trifolii in clover root nodulation were shown by detailed
microscopy of the phenotypes exhibited by mutants blocked in its synthesis.
A common symbiotic phenotype of “exo-minus” mutants of many fast-growing
rhizobia is their defective ability to invade nodules on their respective
host plant (Leigh et al. 1987; Lopez-Lara et al. 1993, 1995; Rolfe et al. 1996;
Sanchez et al. 1997). Figure 6B summarizes the results of detailed, quantitative
microscopical analysis of symbiotic phenotypes in exo-minus mutants of R.
leguminosarum bv. trifolii scored on white clover seedling roots prior to nodule
invasion (Rolfe et al. 1996). These quantitative microscopy results clearly
indicate that the acidic heteropolysaccharide of R. leguminosarum bv. trifolii
plays a crucial role in several early events of the infection process, including