HETEROCYST - Cyanobacteria - page 215

1967; Wilcox, 1970). Proheterocysts are slightly enlarged cells that are similar to vegetative cells in
their shape and pigmentation, fi lled with granular inclusions and in some cases even gas vacuoles.
Wilcox et al . (1973b) have identifi ed seven stages of development of mature heterocysts from
proheterocysts in case of A . cylindrica and A . catenula supported by ultrastructural studies. The fi rst
signs of development are the laying down of the outer fi brous layer of the heterocyst envelope that
constitutes stage I. At stage II, the fi brous layer is completely laid down with the junction between
the developing heterocyst and the vegetative cells gets demarcated. This progressively increases
through stages III and IV at which time the disorganization of photosynthetic lamellae takes place
with increased intracellular spaces. The deposition of inner laminated layer of the envelope becomes
apparent by stage V and with this the developing heterocysts appear to be quite distinct from the
vegetative cells. The condensation of the contorted lamellae to the two poles and the formation of
polar plug at the two junctions with vegetative cells constitute the stage VI. Heterocyst maturation
and aging constitutes the stage VII. The regression of proheterocysts into vegetative cells is quite
possible up to stage V but not those which have crossed this stage and entered stage VI. Once the
fi laments complete the same pattern of heterocyst development after nitrogen shift-down, futher
growth of the organism in nitrogen-defi cient medium requires the continuation of the same pattern.
Two important things that have been taken into consideration are cell division leading to increased
fi lament length in between two heterocysts and the localization of the new site of proheterocyst in
between the existing mature heterocysts, establishing a regular spacing of the new heterocysts. What
triggers cell division before a proheterocyst differentiates? This has been discussed at length by
various hypotheses put forward from time to time. Fogg (1949) put forward the view that there exist
nutritional gradients in the fi laments caused by the decreasing concentration of specifi c inhibitory
substance probably ammonia that falls below a critical level and such a cell is triggered to undergo
development into a proheterocyst, thus maintaining the same distance between two heteocysts. The
hypothesis of Fritsch (1951) though on similar lines envisages the existence of growth-promoting
substances probably derivatives of ammonia contribute to the growth of the intervening portion of
vegetative cells in between two heterocysts and where the concentration of this growth-promoting
substance falls below a critical level such cells differentiate into proheterocysts and then they mature
into heterocysts. It means that the inhibitory substances or the growth-promoting substances released
by the heterocysts prevent the adjacent vegetative cells from getting differentiated into heterocysts.
Wolk (1967) demonstrated that the fi laments of A . cylindrica when fragmented show increased
heterocyst frequency due probably to the rate of cell division and not to any of the stimulatory
substances or as a result of injury. But he suggested that the normal pattern shown by the fi laments
in nitrogen-free medium is due to the inhibitory substance released by the heterocyst itself. Diffusion
of this compound away from the heterocyst, its loss or degradation from vegetative cells would
cause a gradient of concentration and where the concentration of the inhibitor falls below a threshold
level only then another proheterocyst would develop (Wolk, 1975, 1989). Wilcox et al . (1973a) advanced
and extended the same theory and in support of this they showed that as the gradient of the inhibitory
substance decreases and reaches a certain critical or threshold level, such cells undergo an asymmetric
division producing few cells and there exists a competition in between the new products of division
and only the smallest daughter cell is resolved to differentiate into a proheterocyst (Mitchison and
Wilcox, 1972). Adams and Carr (1981) also found the existence of asymmetric cell divisions but
observed the production of double heterocysts in A . cylindrica due to an increase in the intensity of
illumination to 500 µE cm -2 s -1 , 10-fold higher than normal intensity. This is explained by the
occurrence of symmetric cell division due to 12 h at high light intensity and it took nearly 8 h at
normal light intensity to return to normal pattern. This brings out two issues, one is that the