Agriculture Reference
In-Depth Information
thinner and less cross-linked cell walls that allow for the re-arrangement of
underlying microtubules, the changing of vesicle trafficking to the growing tip
and the ease of subsequent penetration by the rhizobia. Attachment of the
rhizobia to the root hairs stimulates root hair deformation within 6-8 h (Yao
et
al
. 1969; Bhuvaneswari
et al
. 1981; Bhuvaneswari and Solheim 1985). It also
initiates root CCD (Calvert
et al.
1984). Rhizobia entry occurs through the
deformed root hair tip, which encapsulates a small proportion of the dividing
bacteria. This enclosed microcolony presumably has enriched Nod factor
concentration as well as cell wall degrading enzymes.
Penetration of the host cell wall, but not its plasma membrane, is followed
by re-synthesis and re-digestion. This re-occurring cycle, coupled with the
viscous extracellular matrix embedding of the microcolony and continued
bacterial growth produces a ‗forward' pressure that ‗pushes' against the root
hair tugour pressure. The dynamics of this process result in the formation of
the plant cell-wall derived infection thread (filled with proliferating bacteria
embedded in ever-hardening extracellular matrix). One presumes that the
invading bradyrhizobia, still capable of Nod factor production as evidenced by
NodC::LacZ
fusion expression, stimulate ever-increasing Nod factor levels
that lead to mitotic activation of cortical cells in the root. This eventually
results in the development of the nodule primordia. The radial position of the
cell divisions, and thus the primordium, is controlled by positional gradients in
the root (Heidstra
et al.
1997; Ferguson and Mathesius 2003; Gresshoff
et al
.
2009). This involves source-sink relations with hormones such as ethylene
(Lorteau
et al.
2001; Ferguson
et al.
2005b; Gresshoff
et al
. 2009; Ferguson
et
al
. 2011). Accordingly, most nodules develop close to xylem radial cells and
away from the phloem (Heidstra
et al.
1997; Penmetsa
et al
. 2003; Lohar
et al
.
2007).
The infection thread grows through the root hair into the root cortex and
the newly induced dividing cells. Bacteria are released from near the growing
tip of the infection thread into the host cell cytoplasm. This occurs through a
process resembling endocytosis where the bacteria are surrounded by a plant-
derived membrane, called the symbiosome membrane. The symbiosome
membrane-enveloped bacteria continue to divide within the host cells before
they differentiate into bacteroids and start to fix atmospheric nitrogen (Roth
and Stacey 1989a, b).
