Agriculture Reference
In-Depth Information
leaves and tend to use carbon rather than nitrogen compounds for protection against
herbivory (Orians & Milewski, 2007). In such conditions, it has been argued (Sprent,
2007) that acacias (sl) can afford to have nodules with a range of effectiveness, since
water, rather than nitrogen is often the limiting factor for growth. Nodules on such
plants may be of more benefit to the bacteria than to the host. By living inside a
nodule, bacteria are protected from adverse soils conditions and also supplied, via
roots, with essential nutrients from a larger volume of soil than they could access for
themselves.
Sprent (2007) pointed out that the time at which nodulated legumes evolved was
one of great environmental change. At about 55 Ma, there was an abrupt increase in
temperature of 5 to 10
◦
C over a wide range of latitudes together with a major release
of methane and carbon dioxide from sea floor sediments (Bowen et al., 2004; Sluijs
et al., 2007). These conditions could have led to a degree of nitrogen limitation for plant
growth, favouring the evolution of nitrogen-fixing symbioses. At about the same time,
at least in parts of the neotropics, the fossil record shows a transient increase in plant
biodiversity (Jaramillo et al., 2006). Since the major groups of legumes also had their
origin at this time, it would be interesting to know if some of the diversity that did not
survive included less efficient forms of legume nodulation.
The possibility that primitive rhizobia evolved from parasitic forms was discussed
above. An alternative scenario is that the first nodulating bacteria were photosyn-
thetic bradyrhizobia, capable of infecting plants via wounds, such as those forming
stem nodules on
Aeschynomene
(Sprent & Raven, 1992). Photosynthetic ability was not
needed when nodules formed underground, but some bradyrhizobia still retain the
key enzyme, rubisco. Since these various arguments were put forward, the two genera
Rhizobium
(close to
Agrobacterium
)and
Bradyrhizobium
have been shown to belong to
different families (Chapter 4). However, when these families separated is not known,
so the bacterial side of nodule evolution, as well as the host plant side, currently has
many unanswered questions. The whole position in the bacteria is complicated by
the ability of many to exchange genetic material, a point that will be raised again in
Chapter 4. For example, the wide host range strain NGR 234 contains genes with a
high level of similarity to those of
Agrobacterium tumefaciens, Bradyrhizobium japonicum,
Mesorhizobium loti
and the plant pathogen
Erwinia carotovora
(Streit et al., 2004).
3.5 Model legumes
In order to analyse the various stages of nodulation and nitrogen fixation in legumes,
two species have been chosen as models,
Lotus japonicus
and
Medicago truncatula
(com-
monly known as barrel medic). These species have small genomes, small seed size
(enabling them to be grown in bulk in test tubes) and endosymbionts whose genomes
have been sequenced. In addition, they have determinate (
Lotus
) or indeterminate
(
Medicago
) nodules. Using these species, a great deal of information about nodule
development has been acquired, some of which will be dealt with in more detail in
Chapter 5. The endosymbiont in use for
M. truncatula
is
Sinorhizobium meliloti
1021,
widely used as an inoculant for
M. sativa
(lucerne, alfalfa). Unfortunately the effective-
ness of this host/rhizobial combination was not checked and it has now been shown
