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or possibly both in the same species (Boivin et al., 1997); and (3) in the rhizobia that
nodulate them, some of which are photosynthetic (Giraud & Fleischman, 2004). It
is possible that coccoid bacteroids are associated with photosynthetic bacteria, but
definitive evidence is not available. Infection thread-like structures are occasionally
seen, as they may be in other nodules with uniform infected tissue, such as lupin, but
they are not involved in the infection process.
5.5.2 Genisteae and Crotalarieae
Inmembers of tribeGenistae that have been studied, infection is via the epidermis, often
at the base of root hairs. Species of
Lupinus
have been most widely studied, but nodules
of several other genera are similar. Bacteria divide repeatedly as they pass between cells
and infect one or a few dividing cells in the root cortex. These infected cells divided
repeatedly to form an infected region containing no uninfected cells. Sometimes when
several infections occur in the same region, files of uninfected cells remain between
segments of infected cells (Plate 2.13). In this group the meristem remains active,
either as a single or branched entity, resulting in an indeterminate nodule, similar in
external appearance to those on legumes with a hair infection (Fig. 1.1). Evidence from
five other genera of Genisteae suggests that these are tribal features (Sprent, 2007). In
Chamaecytisus
(syn.
Cytisus
, Chapter 1), infection threads may be seen in root hairs, but
these abort and take no part in the nodulation process (Vega-Hernandez et al., 2001).
Lupin nodules often have two lateral meristems that grow round and encircle the
root, forming the collar nodules that were a favourite experimental tool for some early
studies on nodule physiology and biochemistry, because they were easy to remove and
harvest in the bulk needed in those days.
In tribe Crotalarieae, evidence is sparse. No records have been found of infection
processes, but where nodule structure has been studied, the infected tissue is uniform
and infection threads are not seen (Rothschild ,1963, for
Crotalaria juncea
and Yates
et al., 2007, for
Lotononis angolensis
). In that there are no known examples of nodules
with uniform infected tissue having a root hair infection, it may be inferred that this
holds true for at least some of the Crotalarieae.
5.5.3 The special case of
Sesbania
Sesbania
is genus that is often groupedwith
Aeschynomene
because of its apparent ability
to form stem nodules (Boivin et al., 1997). However, taxonomically (Chapter 1) and on
grounds on nodule structure it is quite distinct. Of the 60 species, half of which are
in Africa-Madagascar and the rest distributed in mainly tropical regions around the
world, 37 have been recorded as nodulated (Chapter 1). Of these, 5 have been reported
as having stem nodules, all from Africa-Madagascar or Asia (Boivin et al., 1997). Of
the latter,
S. rostrata
has been extensively studied, but where information is available
other species are similar (e.g., James et al., 2001). How are they different from stem
nodules on
Aeschynomene
(Plate 5.1), when they are clearly situated on the stem, in
rows corresponding to the positions of adventitious root initials (Plate 5.2)? First, when
examined in detail they are plumbed into the adventitious root, not the stem; second,
