SYMBIOSIS - Cyanobacteria - page 392

synthesis of glucosamine that acts like a plant defence elicitor. So it is likely that Nostoc can also
produce a factor containing glucosamine that induces cell divisions in Gunnera gland channels. Nod-
factors activate cytokinin pathway, which include cell division and nodule development (Frugier et
al ., 2008). Exopolysaccharide governing exo gene sequences of Rhizobium , important for establishing
successful nodulation of legumes, are found in Nostoc .
vi) Intracellular localization : This is the only cyanobacterial symbiosis in which the cyanobiont
becomes intracellular. In order to enter the cells, the cyanobiont has to penetrate the cell walls. The
infection process and ultrastructure of Gunnera - Nostoc symbiosis have been investigated by Silvester
and McNamara (1976) who highlighted that the cyanobiont upon reaching the cavity, fi rst penetrates
the thin-walled meristematic cells situated at the base of the gland. This has also been confi rmed
by the work of Bonnett (1990). The cells of the symbiont become intracellular and are surrounded
by a host membrane. The question that has to be answered is whether the cyanobiont gains entry
by sheer mechanical force or through the invaginations/cell foldings or enzymic degradation of
cell wall. One of the theories put forward is that the cyanobiont gains entry through the cell wall
foldings/invaginations. It has been observed that the cell wall foldings/invaginations take place
independently even in the absence of the symbiont (Schmidt, 1901). During entry into the cell
the symbiont acquires a host membrane that separates it from the cytoplasm of the host cell (Von
Neumann et al ., 1970; Silvester and McNamara, 1976). There are no evidences for the production of
pectolytic or cellulolytic enzymes or the release of IAA by the cyanobiont that can trigger cellulase
activity. The second theory putforward is the cell wall dissolution theory. The observations of
Towata (1985) that the bacteria associated with the cyanobiont help in the degradation of cell wall
also received attention. The presence of large populations of other microorganisms in the fresh
mucilage secretions besides the symbiont has also been confi rmed (Flowers, 1998). Towata (1985)
further mentions that fungi such as Penicillium , Fusarium and Alternaria have been isolated from the
mucilage of G . kaalensis . The pectolytic and cellulolytic enzymes contributed by these fungi have
been suggested to bring about the dissolution of the host cell wall and/or middle lamellae. However,
the establishment of Gunnera - Nostoc symbiosis in vitro in the absence of the said fungi rules out the
probable role of these fungi (Chiu et al ., 2005). After the dissolution of cell wall, the cyanobiont is
taken in by the folding of the plasmolemma. This theory has received much support from the works
of Jönsson (1894), Miehe (1924), Schaede (1951) and Johansson and Bergman (1992). The absence
of wall rupture after the infection by the cyanobiont suggests that the initial dissolution must have
been repaired. Observations of Silvester and McNamara (1976) also confi rm this theory but in their
reconstitution experiments they did not come across any bacteria or other microorganisms that could
contribute to the dissolution of the cell wall. The presence of infoldings has fi nally been suggested
as individual variations that can be noted depending on the species of Gunnera used in the study.
The infected cell soon gets fi lled with the ensuing growth of the endosymbiont. The cytoplasm
of the cell gets occupied to a peripheral portion. The endosymbiont is shared between the daughter
cells as cell organelles during cell division and wall penetration does not seem to be a requirement
subsequently. However, the host plasma membrane around the symbiont serves as a selective barrier.
The symbiont exhibits an altered morphology as the cells are spherical rather than cylindrical,
aseriate or in short fi laments with altered distribution and shape of the thylakoids in the cells and
with a high heterocyst frequency.
vii) Specifi city : It is generally accepted that all the species of Gunnera have Nostoc spp. as endosymbiont.
In order to fi nd out specifi city of the Nostoc spp. a number of workers have established symbiosis in
vitro by a range of Nostoc spp. and closely related Anabaena spp. Reconstitution experiments have