Agriculture Reference
In-Depth Information
Tab l e 5 . 1
Major differences between indeterminate and determinate nodules of advanced papilionoid
legumes with a root hair infection. Where features differ between determinate nodules of the
phaseoloid tribes and
Lotus
this is indicated
Feature
Indeterminate
Determinate
Reference
Initiation of cell division
1
Outer cortex
2
Inner cortex
Brewin (2004 )
Meristematic growth
Apical, continuous
Limited
Sprent (2001)
Division of infected cells
No
Yes
Life span
Potentially perennial
Limited
Final shape of nodules
Cylindrical, often
Spherical, with
branched
lenticels
Vascular transfer cells
Present in some
Absent
Pate et al. (1969)
Bacteroids in symbiosomes
Single, rods or
Several, usually
Mergaert et al.(2006)
(rarely) pleomorphic,
rod shaped
large, increased DNA
Bacteroid viability
Absent
Present
Sprent (2001)
PHB
3
in bacteroids
Low or absent
Often high
Trainer & Charles (2006)
Ureides
4
Export product
Amides
Sprent (2001)
1
In
Lotus
it is middle cortex.
2
May be controlled by auxin levels (Kondorosi et al., 2005).
3
Poly
-hydroxybutyrate.
4
Except in
Lotus
and other members of tribe Loteae with determinate nodules.
defence responses is not yet clear (Oldroyd &Downie, 2008). Nod factors are perceived
by the host root epidermis, in the region of production of root hairs. Here they lead to
induction of the early nodulation (
ENOD
) genes as well as oscillations in calcium lev-
els in the cytosol of hairs (calcium spiking). Calcium spiking is the subject of intensive
research and is a complex process involving root hair nuclei and eventually leading to
transcription of
ENOD
genes. The current state-of play is discussed by Oldroyd and
Downie (2008). The situation in legumes without root hair infection is unknown.
5.1 Root hair infection
The overall process of root hair infection is described in most elementary textbooks,
having been known for over a century. However, the finer details of the process are
only now being unravelled. The first problem is how do bacteria manage to breach the
hair cell wall and enter against its turgor pressure? This has been excellently reviewed
by Brewin (2004), with a short update in Brewin et al. (2008). The process appears to be
very similar in all papilionoid legumes that have been studied in sufficient detail and
there is no reason to suppose that it is different in recently described root hair infections
by
-rhizobia in mimosoid nodules (Elliott et al., 2007a). It is initiated by Nod factors
thought to be produced by rhizobia adhering to the hair, and the first response is that
the root hair tip ceases to grow and usually becomes swollen. Growth is resumed at
a point below the tip and often results in a branched hair. Most commonly described
