Agriculture Reference
In-Depth Information
Chapter 5
Development and Functioning of Nodules
In order for nitrogen fixation by legume nodules to be fully effective, i.e. allowing plants
to grow at least as well as they can on combined nitrogen, host and bacterial processes
need to be closely integrated. How this may be achieved in some advanced papilionoid
species will be considered first. Studies on these have centred on the model legumes,
Medicago truncatula
(Jones et al., 2007) and
Lotus japonicus
(Udvardi et al., 2005) and
a few other species such as
Pisum sativum
and
Glycine max
. These are often thought
to illustrate typical indeterminate (
Medicago, Pisum
) and determinate (
Lotus, Glycine
)
nodules, respectively. Table 5.1 lists the major differences between these types of nod-
ules. Initial infection in both types is similar and occurs through root hairs. As briefly
mentioned earlier (Chapter 3), hair infection is preceded by the secretion of flavonoids
and related substances by the host root. Not all of these will act as
nod
inducers in
any or all legumes, and they may serve other purposes such as substrates for rhizo-
bial growth (Cooper, 2004). The latter paper lists those flavonoids that are known to
induce
nod
genes in various legumes. Dakora (2000) found considerable commonality
among
nod
gene-inducing flavonoids from five members of legume tribe Phaseoleae.
This and other evidence suggests that production of flavonoids may also have host
taxonomic significance. In order for sufficient bacteria to reach the root surface a num-
ber of other processes are involved. These include motility and chemotaxis (Poole
et al., 2008). When sufficient bacteria are present they may affect gene expression by a
density-dependent phenomenon known as quorum sensing (reviewed by Poole et al.,
2008, and recently shown by Suarez-Moreno et al., 2008, to be present also in
Burkholde-
ria
spp.). Quorum sensing may be associated with biofilm formation, thought to be an
ancestral process found in both
-rhizobia and controlled by the common
nod
genes,
nodABCD1
(Fujishige et al., 2008) As mentioned in Chapter 4,
nodABC
genes
code for the biosynthesis of the lipochitin oligosaccharide backbone of the Nod factor
(Fig. 4.1).
NodEH
code for the decorations on the LPS backbone that lead to host
specificity: wide host range strains have several of these genes. In
Azorhizobium caulin-
odans
some decorations are essential for root hair, but not crack infections (Oldroyd &
Downie, 2008). Further components of the pre-infection stage are lectins on the root
hairs, which aid rhizobial adhesion to the hairs and surface polysaccharides on the
rhizobia. Whether the latter act as signal molecules or as a means of evading plant
-and
