Symbiotic Fungi: Principles and Practice (Soil Biology)

8 A. Das and A. Varma
all the root hairs which get infected show curling (root curling factor). It is therefore
not a necessary prerequisite for infection.
The root hair is covered with a mucilaginous substance in which the bacterium
first becomes embedded. The cell wall of the root hair dissolves at the point of
bacterial attachment, probably because of the secretion of enzymes like pectinases
and polygalactouronases by the bacteria. Such dissolution of the root hair cell wall
facilitates the entry of the bacterium into the root hair cell. This is the normal and
common mode of infection. At times for instance in peanut, the bacteria may infect
the root at the point of emergence of lateral root.
1.3.3.3 Infection Thread Formation
The area on the plant cell wall where the bacteria are attached begins to grow
inwards (invaginates), which forms a growing tube (infection thread) made of plant
cell wall material and membrane with the rhizobia inside the tube (Dazzo and
Wopereis 2000). When once the bacterium enters the root hair, certain structural
and physiological changes occur in the root hair cell. Cytoplasmic streaming and
respiration increase. The nucleus also increases in size. A new wall is laid down
around the bacterium so that it is separated from the contents of the surrounding
host cell. The new cell wall extends like a tube (infection thread), and the bacterium
inside the tube starts dividing (Fig. 1.3). The infection thread further extends from
the root hair cell toward the underlying cortical cells of the root (Devlin and
Witham 1986).
The cells of the root cortex develop into large cells, each with an enlarged central
nucleus. The infection thread intrudes, settles, and liberates its contents in cortical
cells which are always polyploid (tetraploid) (Brewin 1991).
Bacterial cell surface polysaccharides appear to play a crucial role in infection
thread growth. The bacteria divide as they move down the infection thread, either
by gliding motility (since the bacterium does not have flagella inside the infection
Fig. 1.3 Infection thread formation on establishment of the mutualistic relationship between
rhizobia and legume