Agriculture Reference
In-Depth Information
view. The genus
Sesbania
has been taken out of tribe Robinieae and put in a tribe of
its own (Lavin & Schrire, 2005). Some species are able to form nodules located on
stems, but plumbed into adventitious root initials.
S. rostrata
has its own nodulating
genus,
Azorhizobium caulinodans
, the only genus of the
-rhizobia so far shown to
fix and assimilate nitrogen ex planta (Chapter 5). Stem nodules superficially appear
aeschynomenoid, and have a crack infection, but later form infection threads and are
structurally unlike aeschynomenoid nodules. Under certain conditions they become
indeterminate and nodules on roots (these can be induced by rhizobia other than
A.
caulinodans
) are infected via root hairs. These features are discussed in more detail in
Chapter 5.
Tribe Robinieae (Table 1.16) is confined to the Americas and the Caribbean. Some
species of
Coursetia
have been transferred to
Poissonia
and
Sphinctospermum
is now
placed here rather than in Millettieae. The most widely studied species is
Robinia
pseudoacacia
, which is widely grown as an ornamental, but which suckers badly and
can become very invasive, for example in parts of Europe.
Gliricidia sepium
is widely
used in agroforestry in Africa and elsewhere. It and other species in this tribe can
nodulate with a wide variety of bacteria, with varying degrees of effectiveness (Bala &
Giller, 2001). Nodules are generally indeterminate with no known unusual features.
Molecular and morphological data have now firmly placed former tribe Coronilleae
in tribe Loteae, which is a pity from a nodulating point of view since genera in it have
indeterminate and in the former Loteae determinate nodules (Corby, 1988; Sprent,
2001). In the revised Loteae, there are two main groups and these do not correspond to
the former tribes (Sokoloff & Lock, 2005). The current tribe Loteae is thus an anomaly
in its nodule characteristics. However, although some are determinate, they do not
export ureides, as in the determinate nodules of the Phaseoloid group (see section
1.3.6). Some, such as
Lotus uliginosus
, can be found in the far north of Europe, where
indeterminate nodules are the norm. These aspects will be discussed again in Chapters
2 and 5. The overall distribution of the tribe is north temperate, but extending down
to South America and some species of
Lotus
are also found in Australia.
L. japonicus
is
one of the 'model' legumes under intensive study (Chapter 3). Apart from this species,
little is known of the rhizobia nodulating members of tribe Loteae, except that they are
usually fast growing.
1.3.8 The inverted repeat lacking clade (IRLC)
The final group of tribes (together with some anomalous parts of other tribes) form
the IRLC clade (Fig. 1.5), whose members have lost an inverted duplicated insertion
in the chloroplast genome. This is a major genetic change and if it has only happened
once, then those genera that have it should be segregated from those that do not. This
would also apply to the third group of the Milletteae, discussed above. The tribes
within it are Galegeae, Hedysareae, Cicereae, Trifoleae and Fabeae. Almost all genera
are temperate or boreal, and all have indeterminate nodules, usually nodulated by fast
growing rhizobia, with large variations in degrees of specificity (Chapters 4 and 5).
There are no known genera that lack the ability to nodulate, suggesting that nodula-
tion is particularly important in these geographical regions (Chapter 2). The genera are
