ABSTRACT
Members of the orders Nostocales (except Oscillatoriaceae) and Stigonematales corresponding to Sections IV and V of Rippka et al. (1979) differentiate specialized cells known as heterocysts during nitrogen-defi cient growth conditions. The importance of the presence or absence of heterocysts for taxonomic purposes was realized long time back by Bornet and Flahault (1886) who divided the order Hormogonales into Homocystaceae and Heterocystaceae. Although once considered to be a “botanical enigma” (Fritsch, 1951), the heterocysts have now emerged as a suitable prokaryotic model for understanding cellular differentiation and pattern formation in multicellular cyanobacteria and we are at a threshold of understanding the web of molecular events involved in their differentiation (Adams and Duggan, 1999; Adams, 2000; Wolk, 2000; Zhang et al., 2006; Kumar et al., 2010; Flores and Herrero, 2010). Thus the development of heterocyst from its antecedent vegetative cell proceeds through very important morphological, structural, biochemical and genomic changes culminating in the production of a slightly enlarged cell with microoxic internal atmosphere. Such a cell after formation can neither revert to a vegetative cell nor divide but can only fi x nitrogen. That is why it is often designated as a terminally differentiated cell.
