Agriculture Reference
In-Depth Information
Infected tissue uniform: only Papilionoideae
Infected tissue includes uninfected cells
P
Determinate nodules, amide-
exporting, some Loteae
Determinate dalbergiod
nodules associated with
lateral roots
Indeterminate nodules
not associated with
lateral roots
Indeterminate nodules,
amide-exporting
Determinate nodules,
ureide-exporting
P
M
Indeterminate nodules,
some millettioids
& others (?)
Many genistoids
P
P
Root hair infection
Bacteria released into
symbiosomes
Bacteria released into
symbiosomes
Bacteria retained in
modified infection
threads
A few host cells infected:
these divide repeatedly
and enlarge
Formation of infection threads,
which enter some but not all
daughter cells of meristem
C
P
Infection via wounds
or between epidermal
cells
Figure 3.2 Possible stages in the evolution of extant legume nodules. Encircled upper-case letters indicate
the sub-family where the feature is found. Modified from Sprent (2007).
a time of repeated wide fluctuations in temperature and rapid divergence of legumes
(Schrire et al., 2005). All nodulated legumes having their origin later than 40 Ma have a
root hair-based infection process, even though this may not always be expressed. The
large phaseoloid group (tribes Desmodieae, Phaseoleae and Psoraleae, Table 1.15) had
its origin about 28 Ma. With a few exceptions its members are herbaceous, lianes or
small shrubs. Extant species are spread more or less evenly across tropical continental
regions, the majority being in the grass biome (Schrire et al., 2005). Here their unique
determinate, ureide-exporting nodules (Chapter 5) appear to have given them a dis-
tinct advantage, although it is unclear what this is. It has been suggested that allantoin
and allantoic acid (the ureides) have the benefit of being economical with carbon, com-
paredwith amides, but the disadvantage of being poorly soluble at lower temperatures
(Sprent, 1980).
3.2 Where did nodulation first occur, and where are nodulated
legumes going?
The earliest idea was that legumes evolved in the humid tropics when Africa and South
America were still joined, enabling taxa to move readily across continents. However,
these two continents are now known to have separated before legumes evolved, so
an alternative was proposed in which legumes moved between South America and
Africa via a northerly land bridge (the boreal hypothesis). These ideas have been well
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