Agriculture Reference
In-Depth Information
be by localised cellulose digestion. Robledo et al. (2008) showed that
Rhizobium legu-
minosarum
produced a highly specific cellulase that could breach the root hair wall
of
Trifolium repens
,butnotof
Medicago sativa
at a localised point beneath the hair tip:
thus infection could be coupled to specificity. Specificity at this level could also avoid
the induction of host defence responses, elicited by wall-degrading enzymes (Iannetta
et al., 1997). Bacteria inside an infection thread are really in an extracellular compart-
ment, surrounded by host cell wall material. Glycoproteins, similar to those in the
root hair curl, continue to be secreted into the lumen of the infection thread and to be
cemented, probably by H
2
O
2
(Jamet et al., 2007) except towards the apex of the thread,
where bacteria divide and make their own contribution to the matrix by forming ex-
tracellular polysaccharides.
At this stage there are differences between indeterminate and determinate nodules
(Table 5.1). In indeterminate papilionoid nodules, cell division is initiated in the inner
cortex and some of the cells behind the resulting meristem are infected by branches
of the transcellular infection thread. These cells develop into the nitrogen-fixing tissue
(see below). In determinate nodules, the meristem develops in the hypodermal region
(or middle cortex in the case of
Lotus
) and some cells become invaded by branches of the
infection thread and they remain meristematic. Nitrogen fixation commences before
the final enlargement of these cells and the initial products are used for nodule growth,
unlike those of indeterminate nodules where they are exported (Sprent & Thomas,
1984). Fig. 5.1 outlines these processes. A further difference that is still unexplained is
that determinate, ureide-exporting nodules of at least seven genera have crystal cells
containing calcium oxalate in their cortices, whereas nodules of five genera with inde-
terminate nodules do not (Sutherland & Sprent, 1984). Functional differences between
these two groups will be discussed later.
5.2 The roles of hormones
In their recent review, Oldroyd & Downie (2008), divide hormonal effects into two
types, giving positive and negative regulation. Table 5.2 lists these. Other substances,
such as salicylic acid (SA), have also been invoked and suggested to have different
effects in indeterminate (pea) and determinate (
Lotus
) nodules (van Spronson et al.,
2003). However, Stacey et al. (2006), using a series of plant mutants, showed that SA
is involved in the development of both types of nodule, but has pleiotropic effects.
Tab l e 5 . 2
Hormones involved in nodulation processes.
Data from Oldroyd & Downie (2008)
Positive regulators
Negative regulators
Auxins
Abscisic acid
Brassinosteroids
Ethylene
Cytokinins
Jasmonic acid
Gibberellins
