Biology Reference
In-Depth Information
Although C3212 and C1068 exhibited similar growth potential in a nitrogen-defi cient medium as the
wild-type but formed hormogonia 12 h earlier than the wild-type. When co-cultured with
B
.
pusilla
tissue, C3212 showed higher symbiotic competence (67.12% infected colonies in 28 days) compared
to the reduced symbiotic competence of C1068 (8.5% infected colonies in 28 days).
Flavonoids, secreted by legumes, have been suggested to be the signalling molecules for the
rhizobia (Fisher and Long, 1992) and these substances also are suspected to play a role in the symbiotic
association of mycorrhizal fungi (Xie
et al
., 1995; Shirley, 1996; Stafford, 1997). In an attempt to fi nd
out the universality of the communication/signalling mechanism in between host and its symbiont,
Rasmussen
et al
. (1996) reported the induction of
nod
genes in
Rhizobium
by the seed exudates of
Gunnera
, a host that harbours the cyanobiont
Nostoc
sp. Alternatively, the fl avonoid naringin induced
the expression of
hrmA
gene in
N
.
punctiforme
(Cohen and Yamasaki, 2000). Transcription of specifi c
sets of genes in bacteria is regulated by alteration of the sigma subunit of RNA polymerase. Campbell
et al
. (1998) examined the role of alternative group 2 sigma factor in the development and symbiotic
interaction of
N
.
punctiforme
with
A
.
punctatus
. Thus the induction of transcription of
SigH
was noted
within 1.5 h of exposure of
N
.
punctiforme
with
A
.
punctatus
. This is the second genetic target in
N
.
punctiforme
that responds to chemical signals from
A.
punctatus
.
iii)
Structural and physiological changes in the symbiont
:
Once the symbiont enters the mucilaginous
cavity of the host, it is subjected to a number of structural and physiological changes. In general, it
has been observed that the growth rate of the symbiont is slowed down but how this is regulated is
not known. In addition, the cell size of the symbiont also increases. One important structural change
is the differentiation of heterocysts with high frequency. The physiological changes mainly relate to
photosynthetic and nitrogen metabolism. An eight-fold decrease in light-dependent CO
2
fi xation
and RuBisCO specifi c activity in reconstituted symbiotic tissues of
A
.
punctatus
with
Nostoc
sp. strain
UCD 7801 has been reported. Studies with enzyme-linked immunosorbent assays with polyclonal
antibodies against RuBisCO revealed that the regulation of RuBisCO activity in the symbiotic state
is by a posttranslational mechanism rather than by an alteration in RuBisCO protein synthesis
(Steinberg and Meeks, 1989). The RuBisCO protein and phycobiliprotein content of vegetative
cells of the symbiont and the free-living cyanobacterium were found to be the same (Steinberg and
Meeks, 1989; Rai
et al
., 1989; Meeks, 1990). The heterocyst frequency increased to 43-45% with a
concomitant enhancement in nitrogenase activity to 23.5-185.7 (nmol. min. mg
-1
). Nearly 80% of
the fi xed nitrogen is released as ammonia by the symbiont. The activity of GS is reduced to 15% to
that of cultures of the symbiont (Hill, 1975; Rodgers and Stewart, 1977; Meeks
et al
., 1985). However,
the studies of Joseph and Meeks (1987) revealed a reduction of three to four-fold in GS activity in
Nostoc
sp. strain 7801 grown in symbiotic association with
A.
punctatus
. A correlation between the
level of GS expression and the extent of symbiotic heterocyst differentiation has not been noticed.
According to these workers, the regulation of GS takes place by a posttranslational mechanism
in
A
.
punctatus
associated
Nostoc
sp. strain 7801. Steinberg and Meeks (1991) demonstrated that
symbiotic association of
Nostoc
with
A
.
punctatus
derived at least one-third of the reductant required
for nitrogenase activity by the photosynthates of the symbiont. However, steady state levels of
nitrogenase activity of symbiotic
Nostoc
depended largely on the endogenous carbohydrate reserves
of either
A
.
punctatus
or
Nostoc
strain.
The explanation given to the differentiation of heterocysts at a high frequency by the symbiont
is that it has evolved a different regulatory mechanism for the differentiation of heterocysts even
in presence of ammonia or fi xed nitrogen
in situ
. A symbiotic sensing and signalling pathway for
the differentiation of heterocysts in symbionts (different from the one that regulates heterocyst
