Symbiotic Fungi: Principles and Practice (Soil Biology)

16 A. Das and A. Varma
fixed nitrogen is transferred in the reverse sequence from Nostoc to bryophytes
(Rashid 1998).
1.8 Symbiosis in Pteridophytes
Azolla, a floating fern, floats on the surface of water by means of numerous, small,
closely-overlapping scale-like leaves, with their roots hanging in the water. When
a section of Azolla is examined under a microscope, filaments of Anabaena living
within ovoid cavities inside the leaves Azolla can be seen. This association of Azolla
with the filaments of the blue–green alga Anabaena forms a symbiotic relationship,
which fixes atmospheric nitrogen, giving the plant access to the essential nutrient
(Peters 1977).
When free-living, the alga develops only 5–10% of its cells into heterocysts.
However, the number of heterocysts increases to 25–30% when it lives symbiotically
in association with Azolla. The epidermal cells inside the leaf cavity of Azolla
project inside, to form numerous multicellular hairs with warty outgrowths. These
probably serve as transfer cells for free exchange of nutrients between Azolla and
Anabaena. Azolla can absorb nitrates from the water; it can also absorb ammonia
secreted by Anabaena within the leaf cavities (http://waynesword.palomar.edu/
plnov98.htm).
Since Azolla is easy to maintain in aquarium cultures, it is an excellent source of
prokaryotic cells and heterocysts for general biology laboratory exercises on cell
structure and function. It also has an interesting heterosporous life cycle, and can
readily be adapted to laboratory exercises on symbiosis. In addition, this little fern
and its algal partner provide an important contribution toward the production of rice
for a hungry world.
A heterocyst is a differentiated cyanobacterial cell that carries out nitrogen
fixation. The heterocysts function as the sites for nitrogen fixation under aerobic
conditions. They are formed in response to a lack of fixed nitrogen (NH4 + or NO3 ).
The morphological differentiation is accompanied by biochemical alterations. The
mature heterocysts contain no functional photosystem II, and cannot produce
oxygen. Instead, they contain only photosystem I, which enables them to carry
out cyclic photophosphorylation and ATP regeneration. These changes provide the
appropriate conditions for the functioning of the oxygen-sensitive nitrogenase.
Azolla is grown in rice fields and ploughed as green manure, and this can supply
up to 50–75% of the nitrogen requirement of the crop. This practice is extensively
employed in China and Japan as a source of biologically fixed nitrogen in rice
cultivation.
1.9 Symbiosis in Gymnosperms
Cycas is a gymnosperm, and it belongs to Cycadales. Cycas produces two types of
roots. The normal roots grow downwards in the soil, and some of the branches
of lateral roots come just above the soil and become aerial. These roots are