Biology Reference
In-Depth Information
Certain chemoattractants released by the host plant enable the motile hormogonia to glide effi ciently
towards the gametophytic tissue and colonize the slime cavities of
A
.
punctatus
(Knight and Adams,
1996). The association of type IV pili on the surface of the hormogonia has been shown to be essential
for symbiotic competence, since mutation of pil-like genes resulted in altered surface piliation and
reduced symbiotic competence (Duggan
et al
., 2007). DNA microarray analysis of 6,893 genes of
N
.
punctiforme
revealed that as many as 1827 genes are differentially transcribed in 24-h hormogonia,
majority of which were associated with genes encoding proteins for signal transduction and
transcriptional regulation (Campbell
et al
., 2007). The formation of hormogonia greatly correlated
with the loss of nitrogen fi xation and decrease of photosynthetic carbon fi xation and ammonia
assimilation by 30 and 40%, respectively. However, the ability of nitrogen and carbon fi xation was
restored back within 96 h of infection by hormogonia.
i)
Genetic diversity of Nostoc strains
:
A comparison of 31 cyanobionts of hornwort
Phaeoceros
(isolated from several closely spaced locations) by PCR amplifi cation of short arbitrary primers or
primers specifi c for the regions fl anking the 16S-23S rRNA internal transcribed spacer revealed that
(i) a diversity of symbiotic cyanobacteria infect
Phaeoceros,
(ii) the same thallus could be infected
with many different cyanobacterial strains, (iii) identical symbiotic strains found in different thalli
at the same place were never found to be in a free-living state and (iv) one of the cyanobionts was
a species of
Calothrix
while the rest belonged to
Nostoc
(West and Adams, 1997). Studies on the
genetic diversity of
Nostoc
strains, based on tRNA
Leu
(UAA) intron sequence, revealed that many
different
Nostoc
strains are involved in the symbiotic associations with
Anthoceros fusiformis
and
B
.
pusilla
(Costa
et al.
, 2001). A phylogenetic analysis of tRNA
Leu
(UAA) intron sequences of cyanobionts
derived from
B
.
pusilla
(78 sequences) and
C
.
densa
(12 sequences) revealed a great homogeneity in
the symbiotic
Nostoc
strains. These belonged to a specifi c group of symbiotic strains that bear closer
resemblance to those of hornworts and cycads on the one hand and terricolous cyanolichens on the
other (Rikkinen and Virtanen, 2008).
ii)
Reconstitution of symbiosis in vitro
:
Rodgers and Stewart (1977) fi rst provided information on
the morphological and physiological features of the symbiosis of
A
.
punctatus
and
B
.
pusilla
with
N
.
sphaericum
. Reconstitution of the symbioses by a range of symbiotic and free-living
Nostoc
spp.
in case of
Blasia
(Rodgers and Stewart, 1977; Meeks, 1998; Wong and Meeks, 2002) and
Anthoceros
(Enderlin and Meeks, 1983; Kimura and Nakano, 1990) has been successful. In the studies on
Blasia
,
a number of free-living and symbiotic strains of
Nostoc
,
Chlorogloea fritschii
and
Fischerella ambigua
were unable to reconstitute the symbiosis. Likewise, free-living and symbiotic strains of
Anabaena
and
Nostoc
were unable to reconstitute the symbiosis with
Anthoceros
. Reconstitution experiments
with
Phaeoceros
, on the other hand, revealed that not only free-living and symbiotic species of
Nostoc
but also a species of
Calothrix
and
Chlorogloeopsis
could establish symbioses, although
Chlorogloeopsis
has never been found to be a natural symbiont of any liverwort or hornwort (West and Adams, 1997).
A.
punctatus
-
Nostoc
(
N
.
punctiforme
) interactions can be defi ned in two stages. The fi rst is the infective
stage during which the pre-existing cavities in
A
.
punctatus
are invaded by motile hormogonia leading
to the symbiotic association. The stimulus is provided by the HIF and chemoattractants released by
the host. Once inside the mucilaginous cavity, the cyanobiont is probably regulated by the host to the
extent further hormogonia are no longer formed. In the second stage, the hormogonium gets deeply
entrenched and divides to form fi laments with high frequency of heterocysts (Enderlin and Meeks,
1983), equipped with higher rates of nitrogen fi xation (Steinberg and Meeks, 1989) and releases the
fi xed nitrogen to the host (Meeks
et al.
, 1985; Joseph and Meeks, 1987). Once hormogonia are formed
and infection of the gametophytic tissue takes place, there should be a mechanism to repress the
