Agriculture Reference
In-Depth Information
Curled root hair
Infection thread
Pericycle cells
Inner cortex cells
Figure 2.2
Schematic drawing of a part of a root cross section undergoing nodulation. Bacterial
infection induces root hair curling and formation of an infection thread that carries the bacteria into
inner cell layers. ENOD40 expression is first detected in the pericycle cells, which undergo limited cell
division following the infection. The inner cortex cells dedifferentiate to enter the cell cycle, leading to
formation of a nodule primordium. After Geurts and Bisseling (2002).
terminal sugar is replaced by an acyl chain (Geurts & Bisseling, 2002). Nod factors
made by different rhizobial species are modified in a variety of ways and are a major
determinant of host specificity.
Nod factors are the signaling molecules secreted by rhizobia to the root surface
where they are recognized by specific receptors of host cells. They induce many host
genes termed nodulin or nodule-specific genes and initiate a series of coordinated
events, leading to reprogramming of differentiated root cells and formation of a
nodule primordium (Fig. 2.2) (Geurts & Bisseling, 2002).
Rhizobia enter plants through root hairs. Rhizobia infection causes root hair to
deform by reorienting tip growth (root hair curling). An infection thread containing
the bacteria is then formed within the curled root hair. The infection thread grows
toward the base of the root hair cell and eventually to the nodule primordium. The
response of the epidermal cells to rhizobia is induced within minutes after Nod
factor application. The response of inner cell layers to Nod factors can be detected
within an hour. Following the rhizobial infection, pericycle cells undergo a limited
number of cell divisions. The inner cortex cells then dedifferentiate to enter the
