Biology Reference
In-Depth Information
Figure 7:
Confocal laser scanning electron micrograph of leaf cavity of
Azolla fi liculoides
with
Anabaena
inside. Photograph
courtesy E. S. Pierson
1
, L. Masselink
2
and M. M. L. van Kempen
2
, Departments of General Instruments
1
and Aquatic Ecology
& Environmental Biology
2
, Faculty of Science, Mathematics and Computing Science, Radboud University, Huygensgebouw,
Heyendaalseweg 135, NL-6525, AJ Nijmegen, The Netherlands
Color image of this figure appears in the color plate section at the end of the topic.
assume a reddish colour due to enhanced accumulation of deoxyanthocyanins (Pieterse
et al
., 1977;
Wagner, 1997). It is signifi cant that aqueous extracts of
A
.
pinnata
and
A
.
fi liculoides
induced
hrmA
-
luxB
genes in
N
.
punctiforme
strain UCD 328 releasing the HRF. This was compared with the levels
of deoxyanthocyanins that repressed hormogonia formation in
Nostoc
sp. Thus this constitutes the
fi rst report on deoxyanthocyanins serving as molecules of plant recogntion by symbionts (Cohen
et
al
., 2002). Moreover, the induction of
hrmA
-
luxAB
genes by
Azolla
spp. is due to a synergistic action
of deoxyanthocyanins and other plant-derived components and the percentage of induction by
these substances is far more higher than the fl avonoid naringin (Cohen and Yamasaki, 2000) and
by extracts of
A
.
punctatus
(Cohen and Meeks, 1997).
i)
Isolation and cultivation of the symbiont
:
A number of workers have reported the isolation
and cultivation of the symbiont from
Azolla
(Newton and Herman, 1979; Gates, 1980; Tel-Or
et al
.,
1983; Subramanian and Malliga, 1988). The inoculum in these studies has been derived from the
microdissection or homogenization of surface sterilized megasporocarps of
Azolla
or
Azolla
fronds or
algal packets isolated from the fronds. Cultures thus propagated exhibited morphological resemblances
to species of
Anabaena
or
Nostoc
but one distinctive feature that has been noted is that the cultured
isolates are morphologically distinct from the fi laments of symbionts freshly isolated from
Azolla
.
This has raised a number of questions. The fi rst is that whether the isolated strains do really
represent the symbiont located in the leaf cavities of
Azolla
or represent some other strains that are
epiphytic. Secondly, is it possible for the existence of more than one strain in the leaf cavities of
Azolla
and if so is it that one strain is variable which is facultative and amenable for cultivation and
not the other strain that is obligate in nature. This obligate symbiont is metabolically defi cient and
could not be cultured so far. Thirdly, the taxonomic identity of the symbiont whether it is a species
of
Anabaena
or
Nostoc
has been questioned. The following studies have provided answers to these
questions.
