Agriculture Reference
In-Depth Information
this colouration has also been widely studied, as has its role in facilitating oxygen
diffusion and, more recently for interacting with nitric oxide metabolism (reviewed
in Minchin et al., 2008). Haemoglobins are of ancient origin (over 1500 Ma), and dur-
ing evolution genes coding for them have separated into many branches within a
superfamily (Wittenberg et al., 2002). They appear to occur in all angiosperms in two
gene/protein families, containing symbiotic and non-symbiotic forms, respectively.
The latter may have a role in AM as well as nodules (Vieweg et al., 2004). Most legumes
have several forms of symbiotic haemoglobin and recent work has found that these
are produced sequentially, coupled with the induction of genes associated with nitro-
gen fixation (Downie, 2005; Ott et al., 2005). A possible intermediate form, found in a
species of
Chamaecrista
, is discussed in Section 3.5.
Comparisons have been made between the growth of a pollen tube down a style
and the formation of an infection thread in root hairs. Both are a form of tip growth,
involving enzymes that degrade cell walls, such as polygalacturonase and cellulose,
and it has been suggested that the processes may involve genes that have been dupli-
cated in the past (Rodriguez-Llorente et al., 2004). Manoury et al. (2008) point out the
difficulties of taking this analogy too far. These include the fact that infection thread
growth occurs against the turgor pressure of the root hair (see Section 5.1), whereas
there is no pressure barrier for pollen tube growth.
Relatively early in the evolution of papilionoid legumes (about 50 Ma) the whole
genome was duplicated (Pfeil et al., 2005). Did this genome duplication event intro-
duce the flexibility that enabled the evolution of the different types of nodule growth
(determinate, indeterminate) and different types of nodule metabolism (amide versus
ureide export) to evolve? There have subsequently been other gene duplication events,
for example in soybean at about 3 to 5 Ma (Schauser et al., 2008), but their significance
for nodulation processes (if any) is unclear. It is known that the information for the
formation of both determinate and indeterminate nodules in the absence of rhizobia is
found in the genomes of four species of papilionoid legume, including the two model
legumes,
Medicago truncatula
and
Lotus japonicus
(Gleason et al., 2006; Tiricine et al.,
2006; see also Sprent & James, 2007).
One developmental process that has been demonstrated for lupinoid (
Lupinus albus
)
desmodioid (
Lotus japonicus
) and indeterminate (
Medicago truncatula, M. sativa
) nodules
is a high degree of endoreduplication in the infected cells (reviewed in Manoury
et al., 2008). Endoreduplication is also a feature of the giant cells of galls formed on
plants by nematodes and may be, at least in part, under the same host genetic control
(Mathesius, 2003). This paper also gives a comprehensive comparison of the role of
hormones and other factors in lateral root, root nodule and nematode gall formation.
Giant cells of nematode galls, unlike the infected cells in legume nodules, are also
transfer cells (i.e. their walls have many projections facilitating solute exchange with
neighbouring cells). Transfer cells in legume nodules are relatively uncommon, only
occurring in some tribes and associated with vascular bundles (see Section 5.6). Like
bacteria, nematodes tend to have a bad press, because of the diseases they cause. It has
been suggested that plant parasitic nematodes may have developed from saprophytic
forms, after acquiring the genetic information for producing phytohormones (Bird &
Kaloshian, 2003). Further, some of the genes associated with gall formation appear to
have been acquired from soil bacteria including rhizobia (Bird et al., 2003).
