Agriculture Reference
In-Depth Information
back at least 400 Ma (Remy et al., 1994), and it is thought that AM were needed by
plants to enable them to obtain nutrients from the inhospitable terrestrial environment
that they were beginning to colonise. These early plants had no roots and AM were
formed on both above and below ground axes (Nicolson, 1975). It has been known for
many years that some aspects of the signalling process leading to AM have common
features with early stages of nodulation (Szczyglowski &Amyot, 2003) as well as other
colonizing rhizobacteria (Sanchez et al., 2004). The plant gene SYMRK is involved in this
process. Versions of this gene, which is found in all AMplants tested, generate products
that vary in length, with some parts being intra- and others extracellular. Markmann
et al. (2008) analysed the products of this gene in plants of the Eurosid 1 clade (Fig. 3.1),
including actinorhizal and legume nodulating species and also non-nodulating species
from the same clade. All had a longer SYMRK product than plants from outside the
clade. By making a series of transgenic constructs between different species of plant
they showed that this longer product was necessary for nodulation in both actinorhizal
and legume plants. A shorter product was sufficient for AM formation. This important
observation paves the way for further understanding the particular properties of the
Eurosid 1 (nodulating) clade. SYMRK acts on an early stage of nodulation and is
independent of type of nodule formed or specific host-rhizobial combinations. A pair
of genes known as CASTOR and POLLUX are also implicated in both AM and nodule
formation, being involved in the entry of symbionts into plant cells. These genes are
unusual in being located in root plastids (Imaizumi-Anraku et al., 2005). In view of the
relations between nodules and roots, is there anything special about root development
in the Eurosid I clade? In an extensive analysis of the organisation of apical meristems
of roots in 132 families of angiosperms, none was apparent (Heimsch & Seago, 2008;
J.M. Seago, personal communication).
One difference between legume and actinorhizal nodules that has fascinated people
for many years is the fact that the former have some shoot-like features, most notably a
peripheral vascular system, whereas the latter aremore root-like with a central vascular
system. Pawlowski and Sprent (2007) have made a detailed comparison between these
two types of nodule. It has been known for many years that roots can grow out
of legume nodules, usually older ones (Sprent, 2001). More recently, Ferguson & Reid
(2005) studied a mutant of pea that could formhybrid nodule/root structures, in which
both the root and nodule parts were functional. The LATD gene of Medicago truncatula
is necessary for proper development of both lateral roots and nodules (Bright et al.,
2005). Considering the known role of plant hormones in the development of both
of these structures, sometimes acting in opposite directions (e.g. cytokinins, Lohar
et al., 2004) alterations in hormone balance are a likely cause of switching the balance
between formation of nodule and root structures. Kondorosi et al. (2005) point out that
lateral roots and nodules are formed on different regions of the growing tap root and
thus may be expected to have different hormone levels.
3.3.2 Gene duplication
Pink colouration in the centre of legumes nodules has been used for many years as a
likely indication of active nitrogen fixation (Chapter 5). (Leg)haemoglobin that causes
Search WWH ::




Custom Search