Wednesday, April 11, 2012

Anabaena azollae: Water Fern's Krantt
Uheda, Eiji, and Kazuhiro Maejima. "Inhibition of nitrogen-fixing activity of the cyanobiont
affects the localization of glutamine synthetase in hair cells of Azolla."
Journal of Plant Physiology 166. (2009): 1705-1709. Web. 9 Apr. 2012.
Anabaena is a genus of nitrogen fixing, blue green algae that associates with few plants and fungi.  The most common eukaryote associated with it is that of the genus Azollae, more commonly known as water ferns.  These ferns are found wherever they can have direct contact with water.  Anabaena azollae, although rarely free living, can begin as such, living on its own in an aquatic environment.  When it comes into contact with a water fern, A. azollae will live as an exosymbiont until a newly formed cell invagenates[1] and becomes an endosymbiont.
Now, most people, when reading the term endosymbiont, immediately think of root nodules on plants like clover.  Cyst that occur from fern cells that are inhabited by A. azollae form because the algae die from a lack of nitrogen.  To prevent this, the fern will send a steady supply of nitrogen to these cells so as to also prevent its own death [1].  One must remember that, since A. azollae inhabits the leaves of Azollae, formation of cysts would prevent photosynthesis, leading to the plant's death. 

Azollae-Anabaena leaves have two notable structures that are thought to have originated from co-evolution: leaf-hairs and plasmalemmasomes. 
1) The hairs on Azolla leaves have been proposed (Peters and Mayne 1974) to be "secretory or glandular and the cavities filled with a mucilaginous substance of unknown composition" [2].  However, it is more than likely that the hairs are to increase the surface area of the cell membrane for transport.  These hair cells resemble those of transfer cells with their dense cytoplasm, abundant ER, ribosomes, and numerous mitochondira [2].  If you wondered if these hairs may be the plant's way of helping A. azolla have access to the nitrogen found in the water, it would be an unlikely scenario.  One must think of the leaf as a large cell, for the plasmodesmada allows free movement of nutrients from one cell to the other, and since the leaves are in close contact to the aquatic environment, these hairs can be thought of as extra roots.
2) Plasmalemmasomes are infoldings inside the cell wall.  This is said to arise to help exchanges of metabolites between the symbiont and the host.  However, these manifestations are also found in plants infected by viruses and other diseases, indicating structurally the inbalance of membrane synthesis and general breakdown in structure [2].  A. azollae might be able to use this as a benefit, but this is obviously a sign of trauma from the symbiont's presence.

1) Anabaena azollae is normally inherited from the water fern's female parent plant.  This is because it does not only inhabit the dorsal side of the leaves, but also the magasporocap [3].  While free living stages of A. azollae exist, this situation helps prevent the need for that, and therefore the dangers of living in an open environment.  This association is the only known exception in which the symbiont is maintained in the host's life cycle during sporophyte and gametophyte cycles.  And, because of this, it is not possible to isolate and culture the A. azolla symbiont free from its host [3]. 

2) Anaabaena azollae is able to multiply independently of the fern.  Yet, it is able to detect and prevent overcrowding inside the cell it is inhabiting [1]. 

3) Leaf cells containing the algal cells are seven times larger than normal, and do not have thickened walls like that of spores [1].

4) Anabaena-free Azolla produced less biomass than the corresponding Azolla grown in both nitrogen freen and nitrogen supplemented conditions [4].

 1) As the lifespan of the algae and fern mature, carbon dioxide fixation and oxygen production ceases.  However, A. azollae (when removed) is still able to fix carbon dioxide though decreased from that of a young cell [1]. [Hill et al] believes this to be caused from photosystem II shutting down as the leaves mature. 

2) The fern that loses its A. azollae will die if not transferred to a medium containing both ammonia and ammonium [1][4].  So, once the fern has lived with the cyanobacteria, it will not survive without outside help.

Final tabulation:
With the mutualistic relationship being obligatory on both sides, and through the interpretation of the data, this relationship is worth continuing.

1. D, Hill J. "THE ROLE OF ANABAENA IN THE AZOLLA-ANABAENA SYMBIOSIS." New Phytology 78. (1977): 611-616. Web. 9 Apr. 2012.
2. J, Duckett G., R. Troth, and S. L. Soni. "AN ULTRASTRUCTURAL STUDY OF THE AZOLLA, ANABAENA AZOLLAE RELATIONSHIP." New Phytology 75. (1975): 111-118. Web. 9 Apr. 2012.

3. J, Ladha K., and I. Watanabe. "ANTIGENIC ANALYSIS OF ANABAENA AZOLLAE AND THE ROLE OF LECTIN IN THE AZOLLA-ANABAENA SYMBIOSIS." New Phytology 98. (1984): 295-300. Web. 9 Apr. 2012.

4.Watanabe, I, Chang Lin, and Terestia Santiago-Ventura. "RESPONSES TO HIGH TEMPERATURE OF THE AZOLLA-ANABAENA ASSOCIATION, DETERMINED IN BOTH THE FERN AND THE CYANOBACTERIUM." New Phytology 111. (1989): 625-630. Web. 9 Apr. 2012.

Uheda, Eiji, and Kazuhiro Maejima. "Inhibition of nitrogen-fixing activity of the cyanobiont affects the localization of glutamine synthetase in hair cells of Azolla." Journal of Plant Physiology 166. (2009): 1705-1709. Web. 9 Apr. 2012.

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