Symbiotic Fungi: Principles and Practice (Soil Biology)

10 A. Das and A. Varma
1. Such polyploidy cells may be already present in the root along with the diploid
cell. However, with the infection by rhizobia these polyploid cells divide and
form the nodule.
2. The other view is that upon infection by rhizobia, probably because of the
secretion of some hormones, endomitosis occurs and the cells become
polyploidy.
Usually the nodules originate opposite the protoxylem elements in the root. In this
respect, these resemble the lateral roots. However, ontogenetically, lateral roots and
nodules are quite different. While the roots originate in the pericycle, the nodules
develop from the cells of the inner cortex. Therefore, the nodules are not analogous
to lateral roots.
The nodules of different legumes differ in their anatomy and shape. As the
nodule meristem cells divide, some arrest their cell division and begin to differentiate,
leading to the zones of specialized cells and peripheral tissues. Depending on
the host plant, the nodule can develop as the determinate or indeterminate type
(Table 1.1).
Within the host cell cytoplasm, the bacteria become surrounded by the plasmalemma.
This means that the bacterium is separated from the cytoplasm of the host
cell. This plasmalemma surrounding the bacteria is known as peribacteroid membrane
(symbiosome membrane), which forms a small vesicle called a symbiosome.
The symbiosome is the active unit of nitrogen fixation. The bacteria divide and each
bacterium in turn is surrounded by a separate peribacteroid membrane. Often within
the same peribacteroid membrane there may be more than one bacterium, i.e.,
several bacteria may be surrounded by a common peribacteroid membrane.
After release, the bacteria stop dividing. Usually they enlarge in size and become
pea-shaped. Such transformed bacteria present within the host cell are called
bacteroids. Plant-derived environmental and/or bacterial-produced chemical
signals are likely to be involved in triggering this differentiation process.
Table 1.1 Difference between determinate and indeterminate nodule
Determinate nodule Indeterminate nodule
Does not possess a meristem and hence is
Presence of a meristem facilitates
capable of only limited growth
continuous growth of the nodule
Cells of the cortex divide only after being
The bacteria are released only after the
infected by bacteria
cell has ceased to divide
Nodules are spherical Nodules are elongated and branched
In the nodules, either the infection threads
are not formed or, if formed, they do
not branch extensively. Consequently,
spread of infection is by the division of
infected cell itself. The vasculature of
the nodule fuses at the tip
e.g., soybean, French bean, mungbean e.g., pea
The vasculature of the nodule remains
open at the tip