Agriculture Reference
In-Depth Information
Thermopsideae are only known to nodulate with
-rhizobia, but within that group, a
range of fast- and slow-growing species. In Podalyrieae, five species of
Cyclopia
(Elliott
et al., 2007b), three from
Podalyria
plus
Virgilia oroboides
(James & Sprent, unpublished
data) can be nodulated by the
-rhizobium
Burkholderia tuberum
. In terms of nodule
structure and morphology, both groups have indeterminate nodules, often branched
and the infected tissue contains both infected and uninfected cells. Infection has not
been widely studied, but in
Cyclopia
occurs through root hairs (Elliott et al., 2007b).
Where information is available this is not true of tribes Crotalarieae and Genisteae
(Sprent, 2007).
Crotalarieae and Genisteae
Genera in tribes Crotalarieae and Genisteae are given in Table 1.9. Both are likely to be
revised. Whilst there appear to be similarities in nodule structure and possible infection
mechanism, these two tribes have very different geographical ranges. Crotalarieae is
largely African and this tribe will be considered first. Most species (about 510) of
the largest genus,
Crotalaria
, are found in Africa, but there are significant numbers
found in Asia and Australia (van Wyk, 2005). It was the first genus of legume where
nodulation by non-classical rhizobia was reported. Sy et al. (2001) showed that a
group of species of
Crotalaria
from Senegal could be split into two, depending on
whether or not they could be nodulated by a methanol metabolising bacterium from
the
-Proteobacteria,
Methylobacterium nodulans
. This division of species according to
endophyte does not coincide with intraspecific groupings based on other characteristics
(personal observations). Since then, similar results have been reported for species
of
Lotononis
(Jafthe et al., 2002), a mainly South African genus, but extending into
Mediterranean regions. Yates et al. (2007) showed that some, but not all species could
be nodulated by a species of
Methylobacterium
, which in this case could not metabolise
methanol. More details of these bacteria will be given in Chapter 4.
Aspalathus
is
endemic to South Africa, with most species belonging to the Cape Floristic Region
(CFR). Because of the importance of
A. linearis
for production of rooibos (red bush)
tea, this species has been widely studied. However, the exact nature of its endophyte
is still not clear. The genus as a whole nodulates with a variety of
-rhizobia (Deschodt
& Strijdom, 1976), but a report of nodulation by
Burkholderia
(Moulin et al., 2001) has
not been substantiated and Elliott et al. (2007b) found that several species could not
be nodulated by
B. tuberum
, a species that nodulates several species of another CFR
endemic,
Cyclopia
(tribe Podalyrieae). The genus
Lebeckia
as described by van Wyk
(2005) has now been separated into three,
Calobota, Lebeckia
and
Wiborgiella
(Boatwright
et al., 2008b). All are African, with most species in the CFR. The first two genera
can be nodulated by
- rhizobia (Phallane et al., 2008), but the endophyte of
Wiborgiella
spp. is not yet known. Nodules are indeterminate and can be extensively
branched, a type of morphology described as crotalarioid by Corby (1988). Few detailed
structural studies have been made, but those that have suggest that the infected tissue
does not contain uninfected cells (summarised in Sprent, 2007). The recent study on
Lotononis angolensis
(Yates et al., 2007) clearly shows this and also that in this case nodule
meristems may grow around the subtending root forming a type of nodule called
-and
