plant surface microbiology.pdf

which are perhaps intermediates in the synthesis of C23 (w-1) hydroxy fatty
acyl groups in lipopolysaccharides, can be present in Nod factors of Sinorhizobium
meliloti (Demont et al. 1994). Figure 4 and Table 2 summarize the
large variations of Nod factors identified so far. A novel lipochitin oligosaccharide
has recently been found in Rhizobium etli KIM5S (Pacios-Bras et al.
2002). This is the first case where the major LCO contains six oligosaccharide
residues and differs by this point from all other rhizobia analyzed so far. An
additional specificity was that the chitin backbone was deacetylated in one or
two of the GlcNAc moieties, although these were only minor compounds. The
fatty acids of these Nod factors were C16:0, C16:1, C18:0, C18:1 and C17:1. In
this respect, the fatty acids are much more variable than those of Rhizobium
etli strain CE3. Moreover, the host range of strain KIM5S of Rhizobium etli
was different from the Rhizobium type strain CE3, since it could not nodulate
Lotus japonicus, although it did nodulate Siratro.In Sinorhizobium meliloti it
has been shown that an enzymatic N-deacetylation of the Nod factors
decreases their biological activity, but increases the stability in the rhizosphere
(Staehelin et al. 2000).
In all rhizobia the nodABC genes are essential for the synthesis of the core
LCO: NodC synthesizes the chito-oligosaccharide backbone and nodB
removes N-acetyl groups from the sugar at its nonreducing end.All other nod,
nol and noe genes are responsible for the modifications of this general structure,
as indicated in Table 2. NodD is a positive transcription regulator from
the Lysr family and present in all rhizobia. In some rhizobial species such as
Sinorhizobium meliloti, nodD genes are present in multiple forms and their
proteins respond to different groups of flavonoids. NodG has the enzymatic
activity of an 3-oxoacyl-acyl carrier protein reductase and is thereby homologous
to FabG involved generally in fatty acid elongation (López-Lara and
Geiger 2001).
3.2 Cyclic Glucans
6 Signalling in the Rhizobia–Legumes Symbioses 109
Cyclic glucans in rhizobia are small molecules linked either by b-(1,2) glycosidic
bonds with 17–40 units in Rhizobium and Sinorhizobium or by b-(1,3)
and b-(1,6) glycosidic bonds in Bradyrhizobium japonicum. Dominant substituents
can be either sn-1-phosphoglycerol (Breedveld and Miller 1998) or
phosphocholine (Rolin et al. 1992). The function of the cyclic glucans in Rhizobium,
Sinorhizobium and Bradyrhizobium is to protect against hypoosmotic
conditions. Rhizobia also produce, however, large quantities of cyclic
glucans in the endosymbiotic stage. A specific function during this stage is
assumed to be an increase in the solubility of flavonoids and Nod factors
(Morris et al. 1991; Schlaman et al. 1997). Another hypothesis is, that b-glucans
play a decisive role in the suppression of the host plant defense response
with rhizobia, compared to phytopathogenic bacteria.