Agriculture Reference
In-Depth Information
to each other all the time and between them control nodulation and other processes.
Of the latter, production of nematode galls has been widely studied and many of the
cell processes, such as rearrangement of the cytoskeleton and endo-reduplication, seen
in nodule formation, are also seen in nematode gall formation (reviewed in Kinkema
et al., 2006, but see also Mathesius 2003 and Chapter 3).
5.4 Formation of symbiosomes
Whatever the mode of infection or the passage of bacteria through the nodule cortex, in
all cases except some primitive legumes bacteria are 'released' into membrane-bound
symbiosomes (Fig. 1.2 B). When infection threads are involved, the tip of the thread de-
velops 'unwalled' droplets and bacteria are surrounded by a plant-derived membrane,
equivalent to the plasma membrane (Brewin, 2004), meaning that they remain topolog-
ically extracellular. The symbiosome membrane (called the peribacteroid membrane
in earlier literature) controls the exchange of material between the two symbionts.
Vance (2008) has reviewed the exchange of metabolites between host cells and bac-
teroids, describing nodules as 'carbon and nitrogen factories' Although the data are
mainly derived from model legumes and crop species, there is no reason to suppose
that the general metabolism of nodules in other species of legume is fundamentally
different, with one possible exception. This concerns the important role of uninfected
cells, worked out in most detail for their function in ureide-exporting legumes. How
do nodules without such uninfected cells in their infected regions manage? The only
definitive evidence for ureide export from nodules comes from the phaseoloid group
of legumes (Chapter 1). For these it forms the basis of a possible assay for nitrogen
fixation (Unkovich et al., 2008). Reports of ureide export as a measure of nitrogen
fixation in other legumes (e.g. Izaguirre-Mayoral & Vivas, 1996) urgently need to be
validated, as ureides can be formed in plants in processes not associated with nitrogen
fixation. Symbiosomes in host cells are bathed in haemoglobin, which, inter alia acts
as an oxygen transport agent, ensuring that the oxygen flux rate to bacteroids is high
enough to permit ATP formation, but low enough to avoid inactivation of nitrogenase.
In some primitive legumes (Caesalpinioideae and some Papilionoideae; see Chapters 1
and 3), bacteria are not released into symbiosomes, instead remaining in modi-
fied infection threads, known as fixation threads. Here they synthesise nitrogenase
(E.K. James, personal communication) and fix nitrogen (Sprent et al., 1996).
5.4.1 Bacteroid size and shape
Bacteroid size and shape varies between groups of legumes. In indeterminate nod-
ules of advanced papilionoid legumes, bacteria may undergo considerable endo-
reduplication of DNA, leading to enlarged size and loss of viability. Such bacteroids
occur singly in symbiosomes. On the other hand, in determinate nodules bacteroids are
rod shaped, similar in size to their free-living counterparts, retain at least some viability
and usually occur in groups of up to eight per symbiosome (Mergaert et al., 2006 and
