HETEROCYST - Cyanobacteria - page 216

proheterocysts inhibit adjacent vegetative cells from differentiation and the second one is that it
should also inhibit its own development. Wilcox et al . (1973a,b) provided evidences for these points
by breaking points of contact between the proheterocysts and adjacent vegetative cells in such a
manner that proheterocysts gradually are left with one, two or more adjacent vegetative cells. In
cases where single cells are left attached with heterocysts, the proheterocysts had undergone
regression into vegetative state in more than 75% such cases. Gradually, the percentage of regression
decreased to zero as the number of adjacent vegetative cells increased in number suggesting that
the regression of proheterocysts very much depended upon an inhibitory substance produced by
it. When percentage of regression is high it means the proheterocysts accumulated the inhibitory
substance beyond the threshold level and so further process of differentiation is discontinued and
regression is the outcome. It means the inhibitor has to be destroyed in the proheterocyst or it should
be lost to the adjacent vegetative cells in order to establish a gradient so that again the same pattern
of proheterocyst formation is continued. So it is possible that the inhibitory zone extends by 4-5 cells
on either side of the proheterocyst and that the number of intervening vegetative cells always are
kept constant (Mitchison et al ., 1976). That specifi c gradients exist under normal growth conditions
in heterocyst formation was also confi rmed in the genus Cylindrospermum sp. which possesses two
terminal heterocysts one on each side of the fi lament. Breakage of heterocysts on either side of the
fi lament leads to formation of a new heterocyst in its place only after the required cell divisions have
taken place. If both heterocysts are detached then a heterocyst would be formed fi rst at one of the
ends that is farthest from the heterocyst in the original fi lament and the second heterocyst would
be formed after a certain vegetative growth, keeping the interheterocyst distance constant (Reddy
and Taplasayi, 1974). Synchronous induction of heterocyst formation occurred after 24 h of nitrogen
step-down and heterocyst-specifi c characteristics were discernible following fi lament fragmentation
in C . licheniforme (Van De Water and Simon, 1982). The pattern of basal heterocyst differentiation
and polarity in Calthrix brevissima are maintained during diazotrophic growth conditions but in
presence of combined nitrogen sources heterocyst differentiation is completely suppressed with
retention of polarity (in nitrate medium) and complete loss of polarity and development of false
branches (in ammonium medium). After a nitrogen shift-down from ammonium medium, the
differentiation of heterocysts occurred at regularly spaced intervals with breakage of fi laments at
the site of differentiation with restoration of polarity (Rai et al ., 1978). In symbiotic cyanobacteria
such as N . punctiforme the normal pattern seems to be disturbed as the heterocyst frequency
considerably increases from 25% to 60% of the total cells and the interheterocyst distance also
decreases to a great extent (Meeks et al ., 2002). Nierzwicki-Bauer et al . (1984b) proposed a method
for the identifi cation of proheterocysts and heterocysts from complex branched fi lamentous types
such as M . laminosus by staining with toluidine blue. Fixed and embedded materials that are sectioned
could be readily examined under the light as well as electron microscope.
Modifi ers of the pattern : In order to further emphasize their point that proheterocyst regression
is dependent on the concentration of the inhibitor, Mitchison and Wilcox (1973) showed that
incorporation of 7-azatryptophan (AT), an analogue of tryptophan, signifi cantly reduced the
proheterocyst regression and caused differentiation of pairs of heterocysts in A . catenula and A .
cylindrica . Agrawal and Kumar (1978) reported that in A . doliolum the addition of AT resulted in
proheterocyst regression with a frequency of 100%, 35% and 10% in case of ammonium-, nitrite-
and nitrate-containing media, respectively. A screening of twelve amino acid analogues and related
compounds by Rogerson (1979) revealed that AT, MSX and β-2-thionyl-DL-alanine considerably
reduced the interheterocyst distance and enhanced the frequency of heterocysts in A . variabilis .