Agriculture Reference
In-Depth Information
suggested by Muresu et al. (2008), needs further investigation, as does the possibility
that a high level of non-nodulating bacteria may be associated with nonculturable
rhizobia. Judging by the work of Li et al. (2008) this is not always the case.
4.4 Specificity
Specificity is another area where things are changing rapidly and, again, this is largely
due to the fact that more work is now being done on legumes outside the narrow
confines of temperate agriculture. However, the most detailed work on the recognition
between host and symbiont is still being carried out on temperate species, in particular
the model legumes
Medicago truncatula
and
Lotus japonicus
(see Chapters 3 and 5).
The most stringent conditions are when there is a gene for gene 'pairing' between the
host and bacterium. So far such a high level of specificity has only been demonstrated
for temperate papilionoid legumes. The early work of Nutman (1969) on species of
clover was followed by the discovery that a variety of pea from markets in Iran,
subsequently referred to as
Pisum sativum
cv Afghanistan, could not nodulate with the
standard rhizobia used in European agriculture (Lie et al., 1976). More recent studies
are unravelling the molecular bases for these differences (e.g. Firmin et al., 1993, for
peas; and Roddam et al., 2002, for subterranean clover).
In order to form nodules, a series of molecular exchanges between the host plant
and bacteria is involved. Of the various types of compound produced by the host
plant, phenolic compounds, especially flavonoids, have been the most widely studied.
These are secreted by roots of many plants, apparently being an ancient characteristic.
However, legumes use this general class of compounds to communicate with rhizobia
and stimulate the production of Nod factors. Nodulation genes and Nod factors have
been extensively studied and described. Unfortunately not everyone uses the same
terminology, which makes it difficult for the non-expert to follow the trail from soil to
functional nodules. In this section the early stages of interaction involving specificity
will be summarised. Chapter 5 will deal with later interactions that lead to various
nodule types and levels of effectiveness.
All nodulating bacteria that have been studied have the common nodulation gene
nodD
that, in the presence of suitable flavonoids, leads to the production of NodD
proteins. These in turn interact with one or more
nod
boxes, leading to the production
of Nod factors that are implicated in specificity (Kobayashi & Broughton, 2008). One
nod
box consists of the common nodulation genes,
nodABCIJ
. The first three of these are
concerned with the production of Nod factors and the last two their secretion. It was
noted earlier that some photosynthetic bradyrhizobia lack
nodABC
, whose function has
only been proven for the root hair infection pathway. The common feature of all
nodABC
products is the synthesis of a backbone of
1-4 linked
N
-acteylglucosamine residues,
similar to the chitin molecules found in insect exoskeletons and fungal cell walls
(Fig. 4.1). This backbone is decorated at various pointswith fatty acid and other residues
that are involved in host specificity and in the type of nodule formed (determinate or
indeterminate); see the reviewbyD'Haeze andHolsters (2002). In general, themore dif-
ferent decorations a strain can produce, themore host species it can nodulate, the record
