SYMBIOSIS - Cyanobacteria - page 395

The cyanobiont can support the growth and can meet the nitrogen demand of the host as
evidenced by its growth in nitrogen-defi cient media. The rates of nitrogen fi xation by the Gunnera -
Nostoc symbiosis have been reported to be of the order of 72 kg N ha -1 a -1 (Silvester and Smith, 1969).
However, some investigators concluded that this symbiosis has a decreased capacity for nitrogen
fi xation due to the existence of low frequencies of heterocysts in G . tinctoria (Silvester, 1975) or the
presence of large number of degenerate cells (Towata, 1985). Osborne et al . (1992) showed that
nitrogen fi xed by the cyanobiont could meet the complete nitrogen requirements of G . tinctoria . The
cyanobiont occupies less than 1% of the total biomass of the host and cyanobiont put together. The
ability of the host to utilize nitrate or ammonium has been shown to be very much limited.
Nitrogen fi xation in relation to gland development has been studied in G . megallanica by the
assay of total and specifi c nitrogenase activity and the frequency of heterocysts both in intact glands
and sections of the gland apex downwards. The apical portion of the gland consists of vegetative
cells of the cyanobiont loaded with high density of storage granules. Gradually downwards the
cyanobiont differentiated heterocysts where the frequency ranged from zero to 30% with toal and
specifi c nitrogenase activity being maximum and nitrogenase protein was localized only in the
heterocysts. Progressively down the gland although the heterocyst frequency increased to as high
as 60%, the nitrogenase activity was lower. However, the decrease in nitrogenase activity may be
due to certain other factors as the nitrogenase protein remained at the same level (Söderback et al .,
1990). Transcription and protein profi les of genes related to heterocyst differentiation and dinitrogen
fi xation have been examined in Nostoc strain 0102 (isolated from Gunnera) under simulated symbiotic
conditions in G . megallanica and G . manicata . The expression of hetR gene correlated positively with
high frequency of heterocysts. The expression of genes ntcA and nifH was also high whereas the
expression of gln B showed decreased expression. These studies have been successfully conducted
by using RT-PCR and Western blot analysis (Wang et al ., 2004). A comparison of protein expression
profi les of freshly isolated cyanobiont from the glands of G . manicata with cultures of the same strain
has revealed that a signifi cant number of proteins are abundant whereas certain other proteins
are either down-regulated or entirely missing in a symbiotic state. For example, nitrogenase and
enzymes of OPP pathway are highly expressed while enzymes of Calvin's cycle are down-regulated
(Ekman et al ., 2006).
By the use of 15 N, Silvester et al. (1996) confi rmed that only 12% of the fi xed nitrogen is retained
by the freshly isolated cyanobiont from G . megallanica whereas the rest 88% is released as NH 3 outside
the cells. Within the intact glands, the cyanobiont retained only 2-5% of the fi xed N 2 and up to 30% of
the extracellular N is in the form of asparagine after one hour. The release of ammonia by the excised
glands suggests that the excised glands because of their surrounding Gunnera envelope/membrane
also are akin to the intact glands and in these cases rates of recovery of NH 3 corresponded well with
C 2 H 2 reduction rates. The overall evidences suggest that Gunnera regulates the activity of GS of the
cyanobiont but not at the level of its synthesis as noted in case of Anthoceros - Nostoc symbiosis (Joseph
and Meeks, 1987). A metabolic model has been proposed that envisages the nitrogen fi xation by the
cyanobiont supported by either light or carbohydrates derived from the host. The assimilation of
fi xed nitrogen seems to require the input of 1 mol oxaloacetate for the export of 1 mol of asparagine
(Silvester et al ., 1996).
In root nodule symbiosis, there is ample evidence for the transport of nitrogen compounds
through xylem from the nodule. The importance of phloem has been indicated in the transport
of carbon and also in the retranslocation of nitrogen when it reaches the leaves. Nitrogen fi xation
by the cyanobionts in Gunnera seems to be supported by the translocation of carbon from the host
presumably by way of phloem. G . monoica is a stoloniferous plant with 4-6 leaves at each node and