Agriculture Reference
In-Depth Information
6
5
OH
OH
O
O
H
2
C
H
2
C
H
2
C
H
2
C
n
O
O
O
O
4
O
O
O
O
O
7
O
O
HO
O
NH
3
N
2
10
NH
9
NH
O
C
C
O
1
O
C
CH
3
CH
3
8
Figure 4.1
Basic features of lipochitin-oligosaccharides, or Nod-factors. They all have a backbone of
0-3. The
backbone is decorated with up to 10 groups of varying complexity. The list gives the main ones included
in the review of D'Haeze & Holsters (2002). For a recent account of the synthesis of Nod factors, see
Kobayashi & Broughton (2008). Positions 1 and 6 are especially critical for nodule type (determinate
or indeterminate) and specificity, respectively. (1, fatty acyl chains of C14-C20, with varying degrees
of unsaturation; 2, Me or H; 3, 4, 5, carbamoyl, acteyl or H; 6, fucosyl, arabinosyl. sulphate or H; 7,
usually H, rarely mannosyl or glycerol; 8, usually Me, sometimes H; 9, usually H, sometimes arabinosyl
or fucosyl; 10, usually H, rarely acetyl or fucosyl.)
1.4 linked N-acetyl glucosamine, with a varying number of residues. In the figure, n
=
being held by NGR234, an organism first isolated by Trinick in Papua NewGuinea and
since widely studied by Broughton's group in Geneva. In their 1999 paper, Pueppke
and Broughton give a nice historical account of studies on specificity in general and on
NGR234 in particular. They list hundreds of host legumes, from all three sub-families,
that can be nodulated, not always effectively by this strain. However, most of these
hosts are tropical or subtropical and there are some interesting anomalies; for example,
Australian acacias are generally nodulated by NGR234, but African ones are not.
Variation in specificity within species has been noted above for
P. sativum
.This
situation is not uncommon in legume species that have been domesticated for many
years, as is well illustrated by
P. vulgaris
(Graham, 2008). Grahamnotes that this species
is nodulated by more than a dozen different rhizobia, from three genera (
Bradyrhizo-
bium, Rhizobium, Sinorhizobium
) and yet shows some degree of specificity in different
geographical regions. In its centre of origin in South America,
R. etli
is the major en-
dophyte, whereas in parts of Europe, North Africa and the United States,
R. gallicum
is
often used. Perhaps surprisingly,
R. leguminosarum
bv
phaseoli
is found less often, but is
common in France and Tunisia. Further details of this and other examples can be found
in Graham (2008). Cultivated
P. vulgaris
is less promiscuous in the number of rhizobia
with which it nodulates than ancestral forms (Souza et al., 1994). This may be a general
phenomenon in unimproved legumes from tropical areas, since many of the species
studied have been shown to nodulate with a wide variety of rhizobia, for example, both
African and Australian acacias, with widely different levels of effectiveness (Burdon
et al., 1999; Nick, 1998; Odee et al., 2002; Thrall et al., 2000, 2007). However, this may
also be true for temperate regions, as Mutch and Young (2004) found that wild species
of legumes in eastern England were more promiscuous than crop species grown in the
same soil in their ability to nodulate with strains of
R. leguminosarum
bv
viciae
.
Variations in species of bacteria nodulating different species within a host genus are
common, as is shown in the examples given in Tables 4.2 to 4.6. This may extend to
