Agriculture Reference
In-Depth Information
did not, and in the latter host nitrate reduced the proportion of plant nitrogen de-
rived from fixation whereas ammonium did not. These and other data are reviewed
in Sprent (1999). More recently Ruffel et al. (2008) have shown in
Medicago truncat-
ula
that different nitrogen sources may affect different gene regulatory systems in
different ways. In particular under conditions of low nitrogen supply, root systems
up-regulated genes for nitrate uptake, but not those for ammonium uptake or nitro-
gen fixation. In some species of
Mimosa
that normally nodulate with
-rhizobia in
the field in Brazil, but that can also accept
-rhizobia, the balance of competition be-
tween these two types of rhizobia is affected by nitrate and/or ammonium (Elliott
et al., 2008).
4.6 Stability and genetic exchange
Most of the early work on rhizobial genetics was carried out on species of
Rhizobium
because the DNA for nodulation and nitrogen-fixing characteristics is located on plas-
mids, rather than on the chromosomes. Plasmids can be readily transferred between
strains and even species (see the review by Farrand, 1998). Slow-growing rhizobia of
the Bradyrhizobiaceae have both their 'housekeeping' genes and their nitrogen-fixing
genes on chromosomes, making experimentation more difficult. However, Sullivan
et al. (1995) found that
Mesorhizobium loti
(then called
Rhizobium loti
) had symbiotic
islands in its chromosome, which could be transferred to other rhizobia in soil. Sim-
ilar results have been found for
Mesorhizobium cicerae
biovar
biserrulae
(Nandasene et
al., 2006). These differences are important for a number of reasons. In practical terms,
lateral transfer of genes between rhizobia in soil causes problems for persistence of
inoculant strains (Nandasene et al., 2006). It could also be of benefit to native and
introduced legumes. For example, Chen et al. (2008) found that there appeared to be
considerable lateral transfer amongst species of
Mesorhizobium
, enabling those that
normally nodulate other genera (such as
M. tianshanense
for
Glycyrrhiza
) to nodulate
species of
Caragana
. In the very inhospitable Tibet plateau, Hou et al. (2008) found
extensive lateral transfer between rhizobial genera and concluded that this may be
associated with the need for the bacteria to adapt to local extreme conditions. In a
wide range of areas of temperate China, the large genus
Astragalus
wasfoundtobe
nodulated with a wide variety of rhizobia, again with extensive lateral transfer among
them (Zhou et al., 2008). On the other hand, Zhang et al. (2008) found that symbiotic
genes in bradyrhizobia nodulating
Vigna
species in China were chiefly maintained by
vertical, rather than horizontal (lateral) transfer. No good evidence from field studies
has been found that shows lateral transfer in bradyrhizobia. So far, none of these ex-
tensive studies from China, nor others from temperate regions outside parts of South
Africa, have reported any
-rhizobia nodulating native legumes.
In common with many bacteria, rhizobia function at a population level, for example
in the process of quorum sensing, where a population can react metabolically to its
environment (see Chapter 5). This is true of both
-rhizobia (Oldroyd&Downie,
2008; Suarez-Moreno et al., 2008), and may have been an early evolutionary event
(Diggle et al., 2007).
-and
