Agriculture Reference
In-Depth Information
evidence of PHB granules in the determinate nodules of
Lotus
(E.K. James, personal
communication).
5.5 Nodules lacking root hair infection
As briefly outlined in Chapter 3, nodule formation without root hair infection is found
in tribes Dalbergieae (sensu Lavin et al., 2001), Genisteae and at least some of tribe
Crotalarieae, with the formation of either determinate (Dalbergieae) or indeterminate
(Genisteae, Crotalarieae) nodules.
5.5.1 Dalbergioid legumes
In Dalbergieae, there are good early studies on
Arachis hypogea
(Chandler, 1978) and
Stylosanthes
spp. (Chandler et al., 1982), which are important grain and forage legumes,
respectively. Apart from general observations about the nature of the infected tissue,
and the location of nodules in axils of lateral or adventitious roots (Fig. 5.1), all of which
are diagnostic characteristics (Sprent, 2000), there are no new data on the details of the
infection processes in this group, with the notable exception of the recent work on some
stem nodulating
Aeschynomene
species. The presence and function of oleosomes (lipid
bodies) in
Arachis
nodules has been of interest in view of the use of the host plant as an
oil crop. These studies are summarised in Sprent (1994), with some more recent work
in Khetmala and Bal (2005).
Aeschynomene
Aeschynomene is a particularly interesting genus from a nodulation point of view.
There are approximately 200 species, distributed almost equally between the Old and
New World tropics, with
A. aspera
endemic to Asia and Australia. Recent molecular
evidence suggests that theremay be two sub-genera (Lavin et al., 2001), although others
have suggested three (Klitgaard & Lavin, 2005). All appear to be able to nodulate on
roots, but not on stems, using different bacteria (Giraud & Fleischman, 2004; see also
Section 4.1.4). A subdivision, based on stem nodulation characteristics was made by
Boivin et al. (1997); this classification was extended to other stem-nodulating genera,
including
Sesbania
. Three categories were recognised: (1) species where nodulationmay
occur anywhere on the stem; (2) species where nodulation is largely, but not exclusively
on the submerged part of the stem; and (3) species where nodulation is restricted to
the lower and submerged part of the stem. Studies have concentrated on species from
moist areas, althoughmany are found in areas that are at least seasonally dry (Plate 5.1).
In all species, nodulation occurs where adventitious root initials occur, where rhizobia
can invade intercellularly. It has not been possible to align the nodule classification of
Boivin et al. (1997) with the host species classification of Lavin et al. (2001). However,
all nodules are of the aeschynomenoid type, with three variations: (1) in
A. sensitiva
nodules may grow out laterally, forming a collar, as in some species of
Lupinus
and
Lotononis
(Fig. 1.1E); (2) the shape of bacteroids may be coccoid (see above), rod shaped
