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has been studied by PCR amplifi cation of STRR sequences of the symbiotic tissue of Gunnera . There
is great variation in the number of Nostoc strains establishing symbiosis both within and between
Gunnera populations. However, one Nostoc strain or closely related strains have been found within
an individual host plant (Guevara et al ., 2002).
16S rDNA sequence analysis has been regarded as one of the most valid criteria for unraveling
taxonomic relationships between closely related species or genera (Weisburg et al ., 1991). The diversity
of Nostoc strains establishing symbiosis with Gunnera has been analysed by this marker. Svenning
et al . (2005) conducted a phylogenetic analysis of symbiotic Nostoc strains establishing symbiosis
with species of Gunnera by RFLP of the 16S rDNA sequences and 16S-23S ITS regions. Cyanobionts
from Gunnera have been identifi ed as a distinct clade. They are of the opinion that it is not justifi ed
to assign all symbiotic Nostoc species to N . punctiforme .
viii) Nutrient exchange : The cyanobiont experiences a sudden change in its photosynthetic and
nitrogen fi xing capabilities. Although the cyanobiont possesses the pigments, RuBisCO enzyme and
other enzymes of the Calvin's cycle in the same proportion as that of the free-living cyanobacterium,
its autotrophic metabolism is shifted towards a heterotrophic metabolism. Alongside, the frequency
of heterocysts increases to 30% contributing towards an increase in nitrogen fi xation. The host
meets the carbon requirements of the cyanobiont that is transported in the form of sucrose. The
transport of carbon source from the host to the cyanobiont and the transport of fi xed nitrogen from
the cyanobiont to the apex of the plant body seem to be through the conducting elements. All these
aspects have been investigated in greater detail.
The following observations signify that the cyanobiont leads a heterotrophic existence. The
proportion of pigments such as chlorophyll a (Söderback et al ., 1990), phycobiliproteins and the
carboxylating enzyme (Söderback and Bergman, 1992) of the cyanobionts of G . megallanica are similar
to the free-living cyanobacteria. Söderback and Bergman (1993) observed that the photosynthetic
capabilities of G . chilensis and G . megallanica are better than their counterparts isolated into cultures.
The most important aspect is the high RuBisCO activity associated with low in vivo CO 2 fi xation.
14 CO 2 -incorporation revealed a fi xed carbon translocation from the leaves to the symbiotic tissue
resulting in high rates of nitrogen fi xation. Additionally, a major reduction in PSII activity results
when free-living Nostoc enters into symbiosis with Gunnera . This is evidenced by a smaller pool
size of electron acceptors (QA) and plastoquinone and a reduction in the ability to utilize light by
PSII units. These changes have been correlated with partial degradation of DI protein. It is not
that the PSII is entirely absent but the PSII effi ciency has been largely reduced which signals a
shift from autotrophic to a heterotrophic mode of nutrition (Black and Osborne, 2004). While the
photosynthetic ability of the cyanobiont is thus limited, the nitrogen fi xation potential is enhanced
due to the differentiation of heterocysts at a high frequency, enhancement in nitrogenase activity
and respiratory electron transport. Freshly isolated Nostoc clusters from the glands of G . megallanica
performed high rates of nitrogen fi xation in the presence of glucose, fructose and sucrose. When
nitrogenase activity of the symbiont was inhibited in vivo by high O 2 levels, the accumulation of
sucrose, glucose and fructose occurred (Parsons, 2002). Uptake of glucose analogue, 3-[ 14 C]-O-methyl-
glucose ( 14 C-OMG) by symbiotic and free-living Nostoc revealed that the uptake is mediated by a
hexose transporter and that the uptake slowed down with the increase of heterocyst frequency in
the symbiont signifying that the uptake process was specifi cally associated with vegetative cells.
Further, glucose also is shown to be metabolized through glycolysis as well as incomplete citric acid
cycle in symbiotic cells (Black et al ., 2002).
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