SYMBIOSIS - Cyanobacteria - page 390

N . gunnerae because of its close resemblance to free-living N . punctiforme the cyanobiont has been
redesignated as N . punctiforme (Harder, 1917; Winter, 1935).
ii) Nature of symbiosis : The nature of Gunnera - Nostoc symbiosis is facultative. The two partners upon
separation can lead an independent existence. Reinke made the initial observations on the Gunnera -
Nostoc symbiosis. The true nature of the symbiosis was for the fi rst time confi rmed by Silverster and
Smith (1969). All the species of Gunnera form the glands at the base of their leaves and possess the
endosymbiont Nostoc . Even with the cyanobiont Nostoc , the symbiosis is relatively non-specifi c as a
number of strains and species can establish symbiosis with Gunnera (Bonnett and Silvester, 1981). Of
the many cyanobacterial symbioses, Gunnera - Nostoc symbiosis represents the most advanced type
because of the cyanobiont being in the intracellular state (Bergman et al ., 1992b).
iii) The glands : The host plant possesses peculiar glands at the base of each leaf stalk. Reinke (1873)
fi rst described the glands containing algae in Gunnera and their development has been described
subsequently in G . macrophylla (Miehe, 1924). The glands have also been designated as “stem glands”
by Merker (Solereder, 1908), “rosette organs” (Batham, 1943), “transformed root primordia” (Schaede,
1951) and nodules (Silvester and McNamara, 1976) in the literature.
The formation of the glands under sterile conditions in the absence of the symbiont signifi es that
their development is governed by the plant genes. However, it is universally accepted that (i) for
the initiation and further maintenance of the symbiosis the glands are required and (ii) symbiosis-
specifi c genes may also operate in Gunnera - Nostoc symbiosis but these are suggested to be required for
establishing contact between the two partners but are not involved in the formation of the gland.
iv) Morphology and development of the gland : The general structure and development of the
stem glands in Gunnera spp. have been studied by a number of workers. Bonnett (1981) provided
information on the development of glands in G . chilensis . The development of two unequal
meristematic masses at the base of the developing cotyledons constitutes the fi rst step. These soon
assume the shape of papillae. There are 6-9 papillae of which 5-8 papillae are arranged in a circle and
one is located in the center. The outer papillae in the circle are slightly bent outwards whereas the
central one is straight. The papillae grow and undergo division perpendicular to the hypocotyls. These
are visible to outside as hemispherical protuberances on the outside of the stem. Due to dissolution of
the middle lamellae of the cell walls running perpendicular to the hypocotyl, a number of channels
are formed between the papillae. At the bottom of these channels, the cells get separated forming
a cavity. Due to the production copious mucilage, the epidermis ruptures and the mucus fl ows out
through the channels. The chemical nature of mucus reveals the presence of carbohydrates, tannic
acids and other polyphenols. It is acidic and believed to be antifungal in nature.
New glands continue to be differentiated at the base of each leaf primordium under nitrogen-
limited conditions (Chiu et al ., 2005). The nutritional factors that govern the development of the
glands in vitro have been studied by Chiu et al . (2005) who suggested that (i) nitrogen status of
the plant is the main determinant for gland development; (ii) the presence of exogenous carbon
sources like sucrose accelerated gland development under nitrogen-limited conditions; (iii) under
nitrogen replete conditions gland development did not take place; and (iv) a high concentration of
sucrose under nitrogen enriched conditions stimulated the development of a callus-like outgrowth
instead of a gland. After sometime, growth and cell divisions are slowed down with a cessation in
the production of mucilage. Due to the formation of new cell layers, the glands turn brownish and
develop a cork-like surface. At this stage, infection is no longer possible. In the absence of infection,
disintegration of glands ensues afterwards. So during the production of copious mucilage infection
is a probable event.