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At least two genera (
Amorpha
and
Dalea
) have indeterminate nodules (Corby, 1988),
making them distinct from those in the Dalbergioid clade. They can apparently be nodu-
lated by various
-rhizobia, but little detailed work has been carried out. However, it
is now being realised that some of the native plants of North America are being threat-
ened by agriculture and should be studied. Martir et al. (2007) looked at
Dalea purpurea
(purple prairie clover) and found it to be nodulated by several species of
Rhizobium
.
1.3.4 The Dalbergioid clade
The Dalbergioid clade is unique in the Leguminosae in that one of the characteristics
used to define it is nodule morphology (Lavin et al., 2001). Table 1.11 lists its gen-
era, excluding the anomalous ones discussed in section 1.3.2. There are three major
sub-clades, corresponding to the tribes that have been amalgamated to form the new
clade, except that the Dalbergia sub-clade also includes former tribe Aeschynomeneae.
The Adesmia and Pterocarpus groups are largely Central and South American. The
Dalbergia group has members from either side of the Atlantic and some that are pan-
tropical. All that have been described have aeschynomenoid nodules (Fig. 1.1), which
are located in the axils of lateral or adventitious roots and whose infected tissue does not
contain uninfected cells. Infection, where known, is via cracks in the epidermis where
roots emerge.
Brya
and
Cranocarpus
, which Corby (1988) noted had aeschynomenoid
nodules, were formerly placed in tribe Desmodieae, but are now in the Pterocarpus
group. Two closely related genera,
Nissolia
and
Chaetocalyx
(Adesmia group), appear
to have lost the ability to nodulate. Within genus
Pterocarpus
, Brazilian species appear
unable to nodulate, although species from elsewhere in South America and in Africa
can (Faria et al., 1989). The genus
Aeschynomene
, which is currently being revised, has
a number of species that can form nodules on stems. These are always associated with
adventitious root initials and are true stem nodules, in that they are plumbed into the
vascular system of the stem. There have been several reports of stem nodules on other
legumes (e.g.
Neptunia
spp., Mimoseae), but these have all been shown to be plumbed
into adventitious roots (Subba-Rao et al., 1995). It is common for trees and lianas in
rainforest to produce adventitious roots on their trunks and these can be profusely
nodulated (Fig. 1.2D in Sprent, 2001). Stem nodules are normally formed in wet con-
ditions, especially when the root system is flooded.
Discolobium pulchellum
, a species
from the large Pantanal freshwater region in Brazil, is unusual in that stem nodules
only form under water (Loureiro et al., 1994). Nodules in the Dalbergioid clade may
be formed in association with various
-rhizobia, usually slow-growing forms. The
report that
Machaerium
can be nodulated by
Burkholderia
(Moulin et al., 2001) is almost
certainly the result of incorrect identification of plant roots. Rasolomampianina et al.
(2005) isolated seven different genera of nodulating bacteria, both
-and
-, from
Dalbergia
species in Madagascar, but these remain to be fully authenticated.
1.3.5 The Mirbelioid clade
In Lewis et al. (2005), the Mirbelioid clade includes three tribes, Hypocalypteae, Mir-
belieae and Bossiaeeae. The first of these has only one genus, with three species and is
