HETEROCYST - Cyanobacteria - page 176

down) undergo differentiation of heterocysts simultaneously at regularly spaced pattern resembling
the pattern in nitrogen-free medium (Talpasayi and Kale, 1967; Kulasooriya et al ., 1972).
Heterocyst differentiation in A . ambigua has been reported to be stimulated by red light
(600-700 nm) whereas green light (450-550 nm) has been found to be inhibitory. A linear increase
in heterocyst frequency with increase in light intensity with a maximum frequency at 1300 lux was
noted (Kale and Talpsayi, 1969; Kale, 1972). Kaushik and Kumar (1970) reported similar results on the
intensity and quality of light required for heterocyst formation in A . doliolum . A photomorphogenetic
control in heterocyst differentiation has largely been ruled out on the basis that the low light has not
been stimulatory and the involvement of a phytochrome-like pigment in the differentiation process
seems to be remote because of requirement of (i) high light intensity for longer duration, (ii) CO 2 in
presence of light, (iii) red light for differentiation followed by yellow and blue lights and complete
inhibition in green light and (iv) white light for attainment of maximum frequency of heterocysts.
The recognition of incipient heterocyst-like structures in cultures of Nostoc muscorum and C . fritschii
grown in dark with granular inclusions and packed thylakoids and their subsequent differentiation
into mature heterocysts upon exposure to light suggests that light is required for completion of the
differentiation process probably in order to meet the photoautotrophic requirements (Whitton and
Peat, 1967; Ginsberg and Lazaroff, 1973).
The differentiation of heterocysts after nitrogen shift-down is generally completed within 12 h as
in A . doliolum (Tyagi, 1973a), 16 h as in A . cylindrica (Wilcox et al ., 1973a) and about 24 h in A . variabilis
(Ogawa and Carr, 1969), Anabaena catenula (Wilcox et al ., 1973a), 24-36 h in Nostoc punctiforme (Wong
and Meeks, 2001) and 36 to 48 h in Anabaena sp. strain PCC 7120 (Hebbar and Curtis, 2000). During
this period, a molecular network of events takes place in an orderly step-by-step programme. Cell
division takes place preceding heterocyst development and the selection of one of the products of
division by competition and commitment leads to fi rst a proheterocyst development by the end of
6-12 h. The proheterocysts are recognizable by their pale colour and slightly enlarged size. These
events are followed by middle and late developmental changes leading to a mature heterocyst. The
early, middle and late phases are associated with the expression of a number of genes (or clusters
of genes) and the interplay of the respective gene products that govern the development and
maintenance of a pattern of heterocysts. The initiation of the events very much depends on the global
nitrogen regulator NtcA which perceives the signals from the signalling molecule (2-oxoglutarate)
regarding the nitrogen status of the cells. According to status of nitrogen suffi ciency or defi ciency
being a transcriptional regulator NtcA regulates the expression of the concerned set of genes. Under
nitrogen-defi cient conditions, NtcA triggers the expression of the master regulator of heterocyst
differentiation, HetR. The mutual interaction of these two regulators decides the future course of
differentiation process. The laying down of outer wall layers over the pre-existing cell wall of the
vegetative cell such as an outermost fi brous layer, middle homogeneous layer and the innermost
laminated layer are the morphological changes associated with middle and late phases of heterocyst
differentiation which generally occurs between 18 to 24 h of differentiation. The outermost fi brous and
middle homogeneous wall layers are composed of polysaccharides and since these are in the form of
an envelope it is known as heterocyst envelope polysaccharide (Hep) layer which is synthesized and
deposited under the regulation of hep genes. The innermost laminated layer is composed of heterocyst-
specifi c Hgls synthesized and transported by hgl genes. At the narrow point of contact between the
vegetative cell and a heterocyst the walls get constricted and constriction-specifi c genes regulate
this process. The junctional part of the septum of a vegetative cell with that of the heterocyst forms
microplasmodesmata whose formation is regulated by the synthesis and their close integration brings
into picture the means of transport channels between the two cells. The deposition of cyanophycin