SYMBIOSIS - Cyanobacteria - page 375

microsporocarp, the cyanobiont is released to outside environment and does not play any role
in the continuity of the symbiont (Calvert et al ., 1983; Peters and Calvert, 1983). On the contrary,
the symbiont is located inside the indusium at the tip and above the megaspore. It is during the
germination of the megaspore within the megasporophyte and during embryogenesis that the
Anabaena cells are able to re-establish the symbiosis. A number of workers have studied events leading
to the inoculation process and suggested processes for re-establishment of the symbiosis (Calvert
et al ., 1983; Becking, 1987). But none of these studies have focussed attention on the probable role
of epidermal trichomes in establishing symbiosis. Although studies of Campbell (1893), Dunham
and Fowler (1987) mention the existence of subepidermal trichomes associated with Anabaena but
no defi nite role has been assigned to them. Calvert et al . (1985) were the fi rst to have demonstrated
that the fi rst cells to contact the symbiont during the re-establishment of symbiosis are the epidermal
trichomes present on the developing embryo.
Establishment of symbiosis between A . mexicana-A . azollae has been investigated by Peters and
Perkins (1993) following gametogenesis and embryogenesis. By employing light, scanning electron and
transmission electron microscopy it was shown that (i) the cyanobiont is located in the space between
the indusium and apical membrane that is designated as inoculation chamber; (ii) Anabaena cells that
exist in the inoculation chamber are in the form of akinetes; (iii) embryo develops cotyledonary leaf
primordium possessing epidermal hairs during which the akinetes germinate to give rise to germlings;
(iv) the rapidly developing cotyledonary leaf with its four seriate trichomes engulfs the akinetes,
geminating akinetes and germlings and (v) the cells of the trichomes exhibit transfer cell ultrastructure
(TCU). The manner in which the indusial cap is generated is explained as follows.
Morphologically, the cotyledonary leaf is unlobed and vase shaped. By its faster uneven growth
rate, it rapidly surrounds the entire meristem. As the emerging sporophyte releases the indusial cap it
is completely surrounded by the cotyledonary leaf. Anabaena cells in the fi rst instance are associated
with hairs of cotyledonary leaf and later their association is shifted to the hairs of developing modifi ed
true leaf. These modifi ed true leaves differ from the leaves of the mature vegetative sporophyte by
the absence of bilobed character. These develop a cavity-like structure that extends from the dorsal
side to the ventral achlorophyllous portion. This cavity-like structure contains mostly the akinetes
of Anabaena . The inoculum for the fi rst true leaf cavities is provided by the cells of Anabaena in the
form of undifferentiated trichomes entrapped in the region of apical meristem of the cotyledonary
leaf, modifi ed true leaves and their associated hairs (Figs. 7 and 8). Thus these epidermal hairs
help in redistributing these fi laments into developing symbiotic cavities (Calvert et al. , 1985). It is
interesting to note that the nature and number of hairs associated with the different kinds of leaves
are different. For example, the cotyledonary leaf hairs are uniseriate and four in number whereas
the fi rst modifi ed true leaf possesses two branched hairs each of which consists of six to seven cells.
However, the second, third and fourth modifi ed true leaves contain a single multibranched hair
consisting of six to eight cells.
A correlation of leaf development in Azolla with nitrogen fi xation by the symbiont reveals that
the fi xed nitrogen content increases with maturity of the leaf. The role of hormogones for causing
fresh infection has been envisaged during the differentiation of leaf cavities in the developing leaves
from the apical meristem and it is at this juncture that specialized epidermal partitioning trichomes
play a role in the distribution of the symbiont. The concentration of HIF seems to decrease from
apex downwards where as the HRF seems to increase in its concentration from base to apex of the
stem axis. In other words, the repression in the formation of hormogonia is brought about by Azolla
in its leaf cavities enabling the symbiont to differentiate heterocysts at a high frequency. The role
of HRF has been discussed in case of A . punctatus - Nostoc symbiosis. In case of Azolla , the fronds