Agriculture Reference
In-Depth Information
Because of the different locations of developing nodule meristems and hence the
progress of infection threads, hormones may have different effects in different types
of nodule. For example, ethylene is a negative regulator in indeterminate nodules,
because it affects pre-infection thread formation. It also has a negative effect in
Lotus
,
where cell divisions occur in the mid-cortex, but has no effect on nodule development
in
Phaseolus
beans and soybeans, where divisions occur in the hypodermis (Table 5.1,
Fig. 5.1) and no pre-infection threads are formed (Manoury et al., 2008). All the positive
regulators are necessary for nodule organogenesis, although exactly how they func-
tion is not entirely clear. Possibilities are discussed by Oldroyd and Downie (2008) and
include effects on expression of early nodulation (
ENOD
) genes. Local accumulation
of cytokinin (the 'secret agent' of symbiosis, Frugier et al., 2008) may stimulate nodule
organogenesis at the expense of lateral root development. Supporting this idea, is the
conclusion of Mathesius (2008) that high auxin:cytokinin ratios are needed for lateral
root development, but low ratios for nodule development. How the negative regula-
tors function is even less clear, except in the case of some of the effects of ethylene (see
above). Its effects on
Sesbania
nodulation will be discussed later.
Plant hormones are produced by rhizobia as well as by plant cells and may be
involved in nodulation. The possible role of rhizobial cytokinins in crack infection in
stem nodulating species of
Aeschynomene
has already been mentioned, but rhizobia are
also sources of auxin. Pii et al. (2007) showed that auxin production by
Sinorhizobium
meliloti
can increase nodulation in indeterminate (
Medicago sativa
) but not determinate
(
Phaseolus vulgaris
) nodules, and that this effect may be mediated by plant-produced
nitric oxide (NO), which is also known to be involved in lateral root production in
various non-legume plants.
5.3 Autoregulation
Nodules are metabolically very active and a drain on the host plant's resources. Thus
it is essential that the plant can control the number of nodules formed. Work on this
topic has been aided by the discovery of supernodulating mutants in a number of
legumes. Other critical experiments involve use of split-root systems and grafting.
Using split-root systems of soybeans for example, it was possible to divide the effects
of nitrate into direct effects on nodule formation and indirect effects via the shoot
system, which apparently can send signals to the nodulated root, lowering activity of
pre-formed nodules (Hinson, 1975, cited by Kinkema et al., 2006). Although most work
on the effects of combined nitrogen have been carried out using nitrate, as mentioned
elsewhere, in the real world legumes encounter other forms of nitrogen. Ruffel et al.
(2008) found that the functional response of
M. truncatula
to nitrogen was different
for nitrate and ammonium. It is likely that autoregulation acts in different ways in
indeterminate and determinate nodules, because of the different origins of their nodule
meristems (Kinkima et al., 2006). Evidence for this comes from grafting experiments
between shoots and roots of the two model legumes. These suggested that a shoot-
derived factor may be needed for development of nodules in
Medicago truncatula
but
not
Lotus japonicus
(Lohar & Vandenbosch, 2005). Roots and shoots certainly speak
