Agriculture Reference
In-Depth Information
HAIR INFECTION
New host wall
synthesis
Localised hair
wall digestion,
bacteria penetrate
Plasmalemma
invaginates
Bacteria in
matrix
Tip of infection thread (IT)
remains unwalled
Limited
meristematic
activity
DESMODIOID
NODULES
IC + UC
Trans - and
intercellular IT
infect some
meristem cells
in outer cortex
CRACK ENTRY
IT penetrate to
meristem in inner
cortex, some
cells invaded
Meristem remains
active
INDETERMINATE
NODULES
IC + UC
Bacteria enter
through breaks
where roots
emerge
IT
formed
IT not
formed
Meristematic activity
limited
AESCHYNOMENOID
NODULES
IC only
Few cells invaded,
and divided
repeatedly
EPIDERMAL ENTRY
Meristem remains
activite
INDETERMINATE
AND LUPINOID
NODULES
IC only
Between epidermal
cells, often at or near
root hair bases
Figure 5.1
Summary of the different pathways leading from rhizobia in soil to formation of different
types of nodules. (IC, infected cells; UC, uninfected cells, in infected tissue.) Note that there are two
routes from infection to indeterminate nodules, with similar external morphology, but differing internal
structure. See also Fig. 3.2.
is the 'shepherd's crook' structure that follows when growth is redirected towards the
colony of rhizobia developing below the original apical dome. This process, which
can be simulated by local application of Nod factor, is an essential prerequisite for
infection thread formation. It is accompanied by the reorganisation of actin filaments
in the root hair and root cortex that will become invaded and also by the secretion of
glycoproteins (extensins) into the space under the curl. These are cemented by per-
oxide and thus confine the bacteria to an enclosed space in which they continue to
divide (Fig. 5.1). This could build up a pressure that aids penetration against the root
hair turgor. The actin filaments form a cytoskeleton (sometimes called the pre-infection
thread), which is involved in directing cellulose biosynthesis down the root hair to-
wards the cortex, allowing the bacteria to grow down an emerging infection thread.
Between these two events, the original hair wall must be breached and this is likely to
