plant surface microbiology.pdf

518
Frank B. Dazzo
In contrast, quantitative microscopy revealed that similar treatment of
white clover roots with LPS from heterologous wild-type rhizobia (e.g., R.
leguminosarum bv. viciae or S. meliloti) resulted in very incompatible root
hair responses (Dazzo et al. 1991). These included a reduction in frequency of
successful infections made by wild-type R. leguminosarum bv. trifolii, a corresponding
increase in proportion of aborted infections accompanied by accumulation
of intensely autofluorescent material at the arrested infection thread
within the root hairs, and the suppression in levels of some of the newly synthesized
root hair proteins plus elevation in levels of other specific root hair
proteins (Dazzo et al. 1991). These results indicate that Rhizobium LPS is a
potent signal molecule that rapidly communicates with host root hairs before
bacterial penetration, triggering signal transduction of various molecular
and physiological changes in these host cells that modulate infection thread
development and compatibility/incompatibility events during primary host
infection (Dazzo et al. 1991).
2.9 Chitolipooligosaccharide Nod Factors
Microscopy has played a major role in showing that chitolipooligosaccharides
(CLOS), first described by Lerouge et al. in S. meliloti (Lerouge et al.
1990), are one group of several different types of Nod factor molecules made
by R. leguminosarum bv. trifolii capable of inducing Had and Ccd/Noi on
white clover roots (Hollingsworth et al. 1989; Philip-Hollingsworth et al.
1991, 1997; Orgambide et al. 1994, 1995, 1996; Dazzo et al. 1996a; Dazzo et al.
1996b; ). Consistent with their amphiphilic physicochemistry, CLOSs of true
wild type (i.e., not genetically manipulated) R. leguminosarum bv. trifolii
accumulate three log cycles higher in their cellular membranes rather than
in the extracellular milieu, and comprise a diverse family of at least 23 different
types of CLOS that vary in O-acetyl and N-fattyacyl substitution, and
in degree of oligomerization (Orgambide et al. 1995; Philip-Hollingsworth et
al. 1995).
Because these wild-type Nod factors are primarily associated with membranes
rather than secreted extracellularly (contrary to dogma), it was important
to establish if they represent the symbiotically relevant forms. Quantitative
microscopy bioassays on axenic seedlings showed that this was definitely
the case. The family of wild-type membrane CLOSs from R. leguminosarum
bv. trifolii was fully active in its ability to induce Had, Ccd and Noi in white
clover roots at subnanomolar concentrations (Orgambide et al. 1996). Furthermore,
these symbiotic activities of R. leguminosarum bv. trifolii membrane
CLOSs were host-specific in that they elicited no mitogenic Ccd or Noi
activity in hairy vetch or alfalfa roots (heterologous legumes of different
cross-inoculation groups), no Had in alfalfa at any concentration tested, and
only elicited a weak Had response in hairy vetch requiring a 10 4 -fold higher