SYMBIOSIS - Cyanobacteria - page 391

v) Infection process : The infection of each gland represents an independent event and everytime
Nostoc fi laments growing outside need to gain entry into the new differentiating gland. The manner
in which the symbiont is brought nearer to the gland is not clearly known. It is likely that during
the emergence of the seedling of Gunnera , Nostoc fi laments present in the soil may get entrapped
in the mucilage or fi laments of the symbiont may stick to the trichomes/hairs located on the gland
surface. The fi laments of Nostoc may be attracted towards gland by the chemoattractants present in
the mucilage. The presence of fi mbriare on the cell surface of the symbiont may also be helpful in
the attachment to the gland surfaces (Dick and Stewart, 1980; Lindblad and Bergman, 1990). Thus
mucilage serves as an important communication channel between the symbionts prior to infection.
How the symbiont survives in the acidic pH of the mucilage secreted by the glands of Gunnera (in
case of G . manicata the pH is 5.0; and in case of G . chilensis the pH is 4-5) is not understood. But it
has been confi rmed by many workers that the symbiont grows rapidly in the mucilage and forms
a thin fi lm over the gland surface. Two observations of Rasmussen et al . (1994) that (i) mucilage
secreted from the glands induces hormogonia formation in vegetative Nostoc fi laments and (ii)
mucilage contains certain factors that stimulate growth and induce protein synthesis in compatible
Nostoc strains are worth noting. Molecular characterization of the mucilage secreted by Gunnera
stem gland revealed arabinose and galactose in molar proportions of 1.00:0.25 with an additional
0.13 parts of glucoronic acid. Dot blot analysis confi rmed the existence of arabinogalactan proteins
(Rasmussen et al ., 1996).
Schaede (1951, 1962) suggested that the symbiont penetrates into the root cells where the
formation of canals allows free passsage for the algal fi laments. This mode of entry of the symbiont
has not received much support. However, it is now generally accepted that the entry of hormogonia
into the interior of the gland is affected via the mucilage channels (Batham, 1943; Silvester and
McNamara, 1976; Towata, 1985). In order to reach the interior and fi nally the cavity, the hormogonia
have to travel against the fl ow of mucilage and indeed it requires a great force. This may be achieved
by the chemotaxis of the hormogonia due to the presence of chemoattractants in high concentration in
the cavity of the gland. In otherwords, the cyanobiont responds to the signals of the host by showing
chemotaxis. Indeed chemotaxis has been shown to be possible in N. punctiforme PCC 73102 with
exudates or crushed extracts of G . manicata and a number of other hosts ( Cycas revoluta and Blasia
pusila ) and non-host plants ( Trifolium repens , Arabidopsis thaliana and Oryza sativa ) as well (Nilsson et
al ., 2006). In other cyanobacterial symbioses such as lichens, lectins are known to play a role in the
recognition process (described above in detail under the section on lichens) but in case of Gunnera -
Nostoc symbiosis the role of lectins in the recognition of partners is yet to be established. However,
Khamar et al . (2010) identifi ed multiple role of soluble sugars in the establishment of Gunnera - Nostoc
endosymbiosis. Prior to the establishment of the cyanobiont, the mucilage contained higher levels
of galactose and arabinose that had little effect on hormogonia formation. After the establishment
of the cyanobiont, the mature glands accumulated higher concentrations of glucose and fructose
that favoured the vegetative growth of the cyanobiont. This is supported by the accumulation of
starch in the cortical cells of the nitrogen-starved plants and simultaneously the expression of host
genes encoding enzymes involved in starch hydrolysis took place.
It is well known that nod genes of Rhizobium are involved in the production of Nod-factors, chitin
oligomers with an acyl chain at the non-reducing end that have numerous effects on the host plant.
Although common nod gene sequences ( nod ABC or the regulatory gene nod D or the host specifi c
gene nod L ) are not present in Nostoc but Nod-D binding part of the nod promoter and nod box showed
hybridization with specifi c nod genes such as nodEF . The presence of gene sequences homologous
to nodMN in Nostoc genome is of interest because NodM has been reported to be involved in the