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in the formation of their infected tissue, which involves infection threads; and third, in
their possession of an apical meristem. The common feature is the infection by rhizobia
through cracks where the adventitious roots occur, although there are differences here
too. In a detailed study D'Haeze et al. (2003) showed that Nod factors induce the
formation of ethylene and reactive oxygen species, leading to localised cell death. These
dead cells forman infection pocket, where bacteria proliferate and fromwhich infection
threads are formed, infecting cells in the usual way for indeterminate nodules, but with
an arrested apical meristem so that superficially the nodules appear aeschynomenoid.
Roots, when grown under non-flooded conditions, produce hairs that are infected
by rhizobia with the production of normal indeterminate nodules. However, when
grown hydroponically, roots produce fewhairs and nodulation occurs via cracks where
lateral roots are intiated, in exactly the same way as described for nodules formed
on stems (Goormachtig et al., 2004). Nodules formed on the stem, but not the root,
contain chloroplasts with photosynthetic capacity, but not all of them receive sufficient
light to photosynthesise in natural conditions (James et al., 1998). Thus, basically,
Sesbania nodules are indeterminate, but when grown inmoist conditions show arrested
development and are superficially aeschynomenoid. Because of their ability to show
variations in growth and infection processes, S. rostrata nodules have been a very
fruitful experimental tool to study nodule developmental processes.
5.6 Other variations in nodule structure
The surface of a nodule is the interface with the root (or sometime the aerial) envi-
ronment. Some nodules, most notably the desmodioid ones, have a distinct pattern
of lenticels through which gaseous exchange occurs (Fig. 1.2A). These lenticels are
produced, as in other parts of the plant, by a phellogen (cork cambium). Activity of
this phellogen varies in response to environment. For example, in soybeans grown
under waterlogged conditions it produces copious aerenchyma and under water stress
lenticels collapse (Pankhurst & Sprent, 1975). The phellogens of nodules of many
legumes native to dry environments can produce cork. This protects the nodules from
desiccation, but also depresses nitrogen fixation (Sprent, 1988). Recovery from this state
is only possible for indeterminate nodules, whose meristems resume growth when ad-
equate water supply becomes available (e.g. Trifolium repens , Engin & Sprent, 1973).
Yet other nodules in woody legumes from various tribes produce one or more tightly
packed layers of sclereids in their cortices (Sprent et al., 1989). Such nodules are very
brittle and difficult to section, but the function of these sclereids is unclear. It could pro-
tect against invasion of pathogens, but one of the many intriguing things about legume
nodules is that when developing and when actively fixing nitrogen they appear to be
resistant to invasion by pathogens or even mycorrhizal hyphae (Scheublin & Heijden,
2006), although senescent nodules may be colonised.
All legume nodules have their vascular system in the inner cortex, often with a nod-
ule endodermis (that may take the form of a sclereid layer, as in soybeans), but always
with an endodermis around each vascular bundle. Individual bundles vary greatly in
size and in number of elements within them. Some of the smallest are in phaseoloid
nodules, such as in Vigna , where the cross-sectional area of an entire bundle can be the
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