SYMBIOSIS - Cyanobacteria - page 387

from apex to the base of the coralloid roots (Grilli-Caiola, 1980). The cyanobiont of Z . skinneri does
not show any sub-cellular modifi cations in the coralloid roots and healthy vegetative cells of the
symbiont show thylakoids with phycobilisomes, carboxysomes, cyanophycin and glycogen granules.
Furthermore, the ultrastructural details of the coralloid roots of Z . skinneri revealed the presence of
characteristic transfer cells that play a role in the transfer of solutes between the symbiont and the
host (Lindblad et al ., 1985). The presence of akinetes as noted by Lindblad et al . (1985) in Z. skinneri
has also been substantiated earlier in C . revoluta (Zhu, 1982), Dioon edule (Grilli-Caiola, 1975b) and
M . communis (Grilli-Caiola, 1974). Baulina and Lobakova (2003) observed vegetative cells with
reduced cell walls in the cyanobionts of C . circinalis , Ceratozamia mexicana and Encephalartos villosus
growing in the intercellular spaces of cyanobacterial zone of cortex. These cells resembled protoplasts
and sphaeroplasts and overproduced mucilage-like polysaccharide- and protein-like substances
that hastend the death of the cells.
Morphological and physiological properties of the symbionts residing in the coralloid roots of
cycads are shown to be different with those isolated and grown in culture (Grilli, 1963; Grilli-Caiola,
1974, 1975a,b). One of the most distinguishing features of the symbionts is the continuous increase
in the frequency of heterocysts from apex to the base of the coralloid roots (Grilli-Caiola, 1980).
Besides the increase in the percentage of heterocysts from the growing tip of the coralloid roots (as
noted in Zamia ) towards basal, older parts, there was increase in the frequency of multiple (double,
triple and quadruple) heterocysts (Lindblad et al ., 1985). This feature was also noted in other cycad
species (Spratt, 1911; Grilli-Caiola, 1975a, 1980; Zhu, 1982).
An inverse relationship has been noted between ARAs and the total heterocyst frequency
suggesting that it is likely that nitrogenase is active only in single heterocysts and in one of the
heterocysts occurring in a group (Lindblad, 1984; Lindblad et al. , 1985).
Lindblad and Bergman (1989) showed the localization of phycoerythrin associated with
thylakoids of vegetative cells of the symbiotic Nostoc growing in the coralloid roots of C . revoluta
with the help of immunocytochemistry but phycoerythrin was absent in the heterocysts. Nitrogenase
activity assayed by ARAs and 15 N fi xation by cyanobionts freshly isolated from coralloid roots of
Macrozamia riedlei showed a direct correlation with the frequency of heterocysts and these depended
mostly on the substrates supplied by the host (Lindblad et al ., 1991).
iii) Diversity of Nostoc strains : A detailed study on the diversity of cyanobionts has been conducted
by Grobbelaar et al . (1987) on the basis of light microscopy. They identifi ed N . commune , N . punctiforme ,
N . ellipsosporum , N . paludosum , N . muscorum and Calothrix sp. as the cyanobionts from the coralloid
roots of 31 species of Encephalartos indigenous to S. Africa. Subsequent studies on the genetic diversity
of Nostoc strains occurring in the coralloid roots of cycads have been made by the application of
molecular techniques such as RFLP, PCR fi ngerprinting and sequencing of tRNA Leu (UAA) intron
but the results obtained in each of these are not comparable. Lindblad et al . (1989) used Southern blot
technique and cloned Anabaena sp. strain PCC 7120 nif K and gln A genes as probes and compared
RFLPs of cyanobionts freshly isolated from coralloid roots of Ceratozamia mexicana , C . robusta , Dioon
spinulosum , Z. furfuracea and Z . skinneri . Differences in the sizes of their DNA fragments hybridizing
with both probes indicated that different cyanobacterial species and/or strains were associated with
the symbiosis. Alternatively, when symbionts freshly isolated from roots of E. altensteinii and three
independently isolated strains from the same coralloid root were compared, they turned out to be
the same organism. It is thus likely that a mixture of Nostoc strains can be associated with a single
cycad species (Lindblad et al ., 1989).