SYMBIOSIS - Cyanobacteria - page 379

Figure 9: The partners in the Azolla symbiosis.
A) Fronds of the Azolla fi liculoides Lam. plant. B) Close up of an Azolla branch showing the apex and the alternating 'stacked'
dorsal leaves, each containing a cavity in which the cyanobiont ( Nostoc azollae 0708) fi laments reside. C) Light micrograph
of the cyanobiont. The larger cells in the vegetative fi laments represent the nitrogen-fi xing heterocysts. Scale bar=5 µm. D)
Transmission electronmicrograph of the cyanobiont. Note the thicker cell-walls and the electron dense polar nodules of
the heterocyst (middle cell) at the interface to fl anking vegetative cells, which function as combined N storage structures
(cyanophycin). Scale bar=5 µm. E) A snap-shot in the vertical transmission process of the cyanobiont between Azolla plant
generations, using fl uorescence microscopy. Pairs of megasporocarps (blue) develop at the underside of the cyanobacterial
colonized Azolla leaves. Filaments of the motile cyanobacterial cell stage (red), the hormogonia (h), are attracted to the
sporocarps, gather at the base and subsequently move towards the tip, before entering the sporocarps via channels (white
arrows). Once inside the sporocarp the hormogonia differentiate into individual thick walled resting spores (or akinetes; ak),
seen as the intensively red fl uorescing small inoculum on top of the megaspores (sp). With the kind permission of B. Bergman,
Department of Botany, Stockholm University, Stockholm, Sweden. [Ran et al . (2010) PLoS ONE 5(7): e11486. doi:10.1371/
journal.pone.0011486] doi:10.1371/journal.pone.0011486.g001.
Color image of this figure appears in the color plate section at the end of the topic.
by S. Nierzwicki-Bauer, USA and R. Fisher, USA and one isolate from A. fi liculoides isolated by
E. Tel-Or, Israel) showed closer resemblances to A. variabilis ATCC 29413 at the level of electrophoretic
enzymatic analysis of nearly 12 important enzymes. On the other hand, all the Nostoc strains were
distinct from one another morphologically but unrelated enzymatically.
The symbionts from Azolla formed hormogonia only upon transfer to fresh medium while in the
symbionts from other hosts hormogonia are generally present in their populations. Secondly, Anabaena
strains from Azolla species do not produce typical punctiforme stage, a feature that is characteristic of
Nostoc symbiont. Moreover, these Anabaena strains prefer fructose as the carbon source (Vagnoli et al .,
1992). Four isolates of A. azollae from different species of Azolla that showed hormogonia production
have been assigned to the genus Nostoc though they do not possess punctiforme -stage (Tomaselli
et al ., 1988). However, it is suggested that Azolla possesses a Nostoc species as the major symbiont.
This symbiont has not so far been isolated as it is not amenable for laboratory cultivation because of
its obligate nature or certain defi ciency in its metabolism. Besides this Nostoc sp., minor symbionts
capable of free-living growth are also suggested to be present. It is these minor symbionts that have
been readily isolated into cultures (Peters, 1991). Ran et al . (2010) have conducted phylogenetic