Agriculture Reference
In-Depth Information
It has been suggested that root nodule bacteria evolved from parasitic forms, largely
because
Rhizobium
and the gall-forming
Agrobacterium
are so closely related (possibly
con-generic; see Chapter 4) (Sprent & Raven, 1985). This is also consistent with the
primitive mode of infection not being through root hairs, as agrobacteria are wound
pathogens. However, there are also problems with this suggestion, including the fact
that agrobacteria transfer phytohormone-coding DNA into host cells (Sprent & Raven,
1992). Also on the bacterial side (see Chapter 4), it has generally been assumed that,
in order to infect plants, rhizobia must have the so-called common nodulation genes,
nodABC
. However, it has recently been shown that some photosynthetic bacteria nodu-
lating stems of two species of
Aeschynomene
lack these genes, but are not defective in
nodulation (Giraud et al., 2007). This again is consistent with the non-root-hair infec-
tion pathway being ancestral to the hair infection pathway.
Early in plant evolution, at least when meristems were formed, signalling molecules
(hormones) were needed. All currently known plant hormones appear to have a role in
nodule formation: these are summarised in Table 5.2 (p. 82) and are discussed in more
detail in Manoury et al. (2008) and in Chapter 5. Plant hormones are also produced by
many soil bacteria, including rhizobia, and these toomay affect nodulation processes. A
recent noteworthy result suggests that bacterial cytokinins may be a key to nodulation
in Nod factor-independent nodulation, such as that described for
Aeschynomene
above
(Frugier et al., 2008).
Flavonoids are secreted by roots of many vascular plants, and in the case of legumes
have been recruited to act as signal molecules and also in nodule development. Reddy
et al. (2007) have recently reviewed the role of flavonoids in nodule initiation and
development; this topic will be explored in further in Chapter 5.
3.4 Why was nodulation necessary?
A true mutualism, such as effective legume nodulation, brings benefits to both part-
ners. Here the benefits to plants and bacteria will be explored in turn. In a thoughtful
and much-quoted paper, McKey (1994) argued that the key to legume evolution in-
volved a high nitrogen life style. Where this could not be maintained by taking up
soil nitrogen, legumes developed symbioses with fungi and bacteria (mycorrhizas and
nodules). The high nitrogen life style enabled them to produce leaves rich in nitrogen,
which they could afford to recycle, and to develop nitrogen-rich seeds. Supporting ev-
idence came from various sources, including analysis of the nitrogen content of leaves
from all three sub-families, where it was found to be similar in non-nodulating and
nodulating species, as also found by Sprent et al. (1996) for Brazilian legumes from
the Cerrado and nearby areas. At the time McKey's paper was written, legumes were
still believed to have originated in the humid tropics. He argued that by producing
leaves high in nitrogen, photosynthetic capacity was increased, allowing plants to
grow rapidly in favourable conditions and survive unfavourable conditions such as
drought and fire. These are very persuasive arguments, but they do not always hold
up when plants from some environments are studied. In particular, many of the sclero-
phyllous nodulated legumes that are found in Australia have long-lived, nutrient-poor
