HETEROCYST - Cyanobacteria - page 168

( Nostochopsis lobatus , Brachytrichia balani, and Mastigocladus laminosus ) the presence of intercalary
heterocysts with two and three polar nodules has been demonstrated, besides lateral ones with single
polar nodule (Iyengar and Desikachary, 1953; Venkataraman, 1957; Desikachary, 1959). Materials
growing in nature or from nitrogen-defi cient cultures show detached free heterocysts intermingled
with normal fi laments. The detachment of intercalary heterocysts generally leads to fragmentation
of the fi laments and they grow once again to exhibit the normal pattern of differentiation. Likewise,
if terminal heterocysts get detached as in species of Anabaena , Nostoc , Cylindrospermum and in the
Rivulariaceae, the terminal cells undergo differentiation replacing the older ones.
2) ULTRASTRUCTURE
Of the four methods employed, i.e. disruption by French Press, sonication, osmotic shock and
lysozyme treatment for the isolation of intact structurally complete heterocysts of Anabaena
cylindrica , Fay and Lang (1971) favoured the use of lysozyme treatment over the other three methods.
Different degrees of damage involving disruption of the heterocyst cell wall and plasmolemma
have been generally observed in the heterocysts derived from the other methods. Most of the
ultrastuctural studies conducted on heterocysts of Anabaena azollae (Lang, 1965, 1968) and
A . cylindrica Lemm. (Wildon and Mercer, 1963a; Lang and Fay, 1971; Fay and Lang, 1971; Winkenbach
et al ., 1972) have clearly brought to light that of the two walls of the heterocyst seen in the light
microscope, it is the outer wall that it is deposited external to the inner wall that is contiguous with
the adjacent vegetative cells. These observations thus have cleared the misapprehensions held earlier
that it is the outer wall that belongs to the vegetative cell and it is the inner wall that is secreted from
cell membrane during the heterocyst development (Fritsch, 1945). The inner wall that is contiguous
with the adjacent vegetative cells is composed of four layers (L I -L IV ) and the outer wall is composed
of three distinct layers, i.e. the outer most fi brous layer, middle homogeneous and the inner laminated
layer in contact with L IV of the inner wall. Extensive reorganization of thylakoids in the heterocyst
takes place as revealed by the presence of coiled and densely layered structures. The deposition of
an opaque, electron dense material near the narrow neck of the junction between vegetative cell and
heterocyst results in the formation of a polar nodule. At this place, the thylakoid membranes undergo
suffi cient disintegration (Lang and Fay, 1971). Heterocysts are devoid of the granular inclusions such
as carboxysomes, cyanophycin, glycogen and polyphosphate granules which are present in large
numbers in vegetative cells (Wildon and Mercer, 1963b; Lang, 1968).
The three layers of the outer wall specifi c to an individual heterocyst are deposited one after
another. These are absent at the narrow point of contact with the heterocyst and the adjacent vegetative
cell. It is at this junction that the plasmolemma of vegetative cell and heterocyst are interconnected.
These structures have been variously designated in the literature. For example, pore-like structures
(Drawert and Metzner, 1956; Metzner, 1955), plasmodesmata (Hagedorn, 1960; Pankratz and Bowen,
1963) and microplasmodesmata (Lang and Fay, 1971; Giddings and Staehelin, 1978); of which, the
last one is generally in use. The number of microplasmodesmata decreases three to fi ve-fold (Fay,
1992) and these constitute the conduit for intercellular transport of metabolites.
Anabaena variabilis ATCC 29413, a cyanobacterium that exhibits heterotrophic growth (Wolk and
Schaffer, 1976), differentiates heterocysts with thin wall layers and lacking distinct polar nodules
although heterocyst-specifi c laminated layer is present (Haury and Spiller, 1981). It also formed
larger, more oblong heterocysts (when compared to rectangular ones in control cultures) fi lled with
glycogen granules and with a precocious maturation of the envelope layers in presence of fructose
(40 mM) (Lang et al ., 1987). Heterocyst differentiation in the branched M . laminosus UTEX1931