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personal observations). These features are under the control of the host plant. In the
vast majority of legumes studied, in all three sub-families, bacteroids are rod-shaped.
Pleiomorphy (X, Y and T shapes) seems to be restricted to some, but not all, members
of tribes Trifolieae and Fabeae (Vicieae). However, in a survey of bacteroids frommany
tribes and of all types of nodule morphology, Sprent (2001) found great variation in
bacteroid length. Only tribes Phaseoleae, Desmodieae and determinate nodules within
Loteae had consistently short rods. There is no information about the DNA content
or viability of bacteroids from any of the genera outside the few studied by Mergaert
et al. (2006). First reported in
Arachis hypogaea
are large spherical non-viable bacteroids,
although recent evidence suggests that these may revert to rod-shaped forms when
nodules senesce (Khetmala & Bal, 2005). Similar large coccoid bacteroids, one per sym-
biosome, have been reported from several, but not all species of
Aeschynomene
(James
et al., 2001), but whether or not these are viable is unknown
5.4.2 The role of poly-
-hydroxybutyrate (PHB)
One of the differences between the determinate and indeterminate nodules listed
in Table 5.1 concerns the production by bacteria of the reserve polymer, poly-
β
-
hydroxybutyrate (PHB). Trainer and Charles (2006) concluded that PHB is a char-
acteristic of determinate, but not indeterminate nodules. Soybean bacteroids are the
classic case, where over 50% of the dry weight of bacteroids can be stored as PHB. It is
also often found, but usually to a lesser extent, in nodules of
Phaseolus
beans. However,
it is not seen in nodules of several species of the closely related legume genus
Vigna
(Sprent & Gibson, unpublished data). Since PHB is a polymer that is widely produced
by bacteria under oxygen-deficient conditions, it could be argued that its accumulation
in soybean bacteroids is a sign of inefficiency. This is consistent with the data of Peralta
et al. (2004), who found that engineering
Rhizobium etli
so that PHB accumulation was
abolished, led to enhanced nitrogen fixation in
Phaseolus vulgaris
. Bacteria in indeter-
minate nodules can have PHB in infection threads, but not usually in later stages, and
it was suggested by Lodwig et al. (2005) that it is used to fuel bacteroid differentiation,
and that bacteroids in nodules of plants such as pea use glycogen rather than PHB as
a storage product. It is clear that bacteria in indeterminate nodules have the ability to
make PHB, and the biochemical pathways for this are set out in Trainer and Charles
(2006). Ratcliff et al. (2008) discuss the trade-offs between using reductant to make
PHB and to reduce nitrogen and conclude that (at least in
Sinorhizobium meliloti
)PHB
is produced to support growth and survival under starvation conditions. In work with
Cyclopia
nodules Elliott et al. (2007b) showed electron micrographs in which PHB was
present in nodules formed with a less effective strain of
Burkholderia tuberum
,butnot
in an effective strain. Gross et al. (2002) studied infection of the mimosoid legume
Anadenanthera peregrina
by rhizobia and found major accumulation of PHB in bac-
teroids in a symbiosis that was poorly effective. PHB is also often found in nodules of
species of
Mimosa
formed with
Burkholderia phymatum
(Elliott et al., 2007a). These ref-
erences show that PHB accumulation is not confined to determinate nodules, is found
in nodules from widely different legumes, can be found in
-rhizobia
and may be associated with nodules that are not fully effective. Further, there is no
- as well as
