SYMBIOSIS - Cyanobacteria - page 360

and cyanomorph lobes. The supporting evidences cited in favour of this are soredial fusion in the
asexual life cycle of P . didactyla (Stocker-Wörgötter and Türk, 1990) and the presence of chemical
hybrids between two varieties of this species (Goffi net and Hastings, 1995). The existence of similar
5.8S ITS sequences of the mycobionts of fused cyanomorph-chloromorph structures of Peltigera
and Nephroma prompted them to conclude that the same species of mycobiont is involved in the
symbiosis. Phylogenetic patterns among Nostoc cyanobionts within bi- and tripartite lichens of the
genus Pannaria have been deduced based on 16S rRNA gene sequences. The cyanobionts have been
derived from 21 Pannaria species from both Northern and Southern hemispheres and compared
with 69 free-living and symbiotic strains. Most of the Nostoc sequences from Pannaria are distributed
among the “ Nephroma -guild” and within two subgroups of “ Peltigera -guild”. Cyanobionts from
several tripartite lichens of Pannaria are grouped together with the Nostoc sequences of cyanobionts
of corticolous bipartite lichens from both the hemispheres (Arve et al ., 2008). The morphotypes of
the photobionts of tropical lichen genera Dictyonema , Acantholichen and Coccocarpia and some other
Stereocaulon lichens have been identifi ed microscopically under Scytonema and a unicellular member
Chroococcus . Phylogenetic analysis of the photobionts of the above lichens revealed the presence of a
previously unrecognized lineage of fi lamentous cyanobacteria clustered into a single clade known
as Rhizonema . But the Rhizonema clade did not cluster with Scytonema (Lücking et al ., 2009). Otálora et
al . (2010) investigated the symbiotic specifi city at an intercontinental spatial-scale among gelatinous
lichens belonging to Collemataceae (Lecanoromycetes). They conducted a phylogenetic study on
the basis of rbcLXS sequences of Nostoc strains sampled from 79 thalli belonging to 24 species of
Collemataceae. Most of the fungal species belonging to Collemataceae showed a generalist pattern
of association with Nostoc strains. However, only in case of fi ve species a one-one specifi city has been
noted. According to them, during the course of evolution these fi ve mycobiont species represent
independent transitions from a generalized pattern of photobiont association.
ix) Role of lectins in lichen symbiosis : Lectins are a class of proteins or glycoproteins which
specifi cally bind to cell surface carbohydrate moieties. Their presence has been detected in animals,
plants, fungi and bacteria. In most of the plants and legumes lectins have been detected mostly from
seeds. However, in a number of cases their presence has been demonstrated in other vegetative
parts such as leaves, stems, barks and roots. The important properties of lectins are that they: (i)
are of non-immune origin; (ii) are soluble or membrane bound; (iii) agglutinate erythrocytes with
a high specifi city (so also called as haemagglutinins); and (iv) stimulate resting lymphocytes to
actively divide. The role of lectins in rhizobium-legume symbiosis, in the formation of arbuscular
endomycorrhiza and parasitism has been clearly established (Albrecht et al ., 1999; Hirsch, 2004;
Brewin, 2004; Tikhonovich et al ., 2004). The mitogenic, antitumor, antiviral and immunity stimulating
potential of mushroom lectins have been summarized (Singh et al ., 2010).
In lichen symbiosis the mycobiont fi rst recuits a photobiont. In doing so the initial interaction
of the mycobiont with the photobiont is through the secretion of lectins that bind to the cell surface
carbohydrate moieties of the phycobiont. Two classes of lectins with different roles have been
detected. The fi rst type is the secreted lectins which promote the recruitment of algal cells to the
neighbourhood of the mycobiont. These intial interactions lead to the binding of the cells of the
photobiont to the mycobiont and also prevent the loss of the cells of photobiont. The second type
is the algal binding proteins (ABP) that have enzyme activity and promote the physical interaction
between the fungus and its specifi c algal partner. These two types are described here seaparately.
A number of phytohaemagglutinins have been isolated and characterized from various lichen
species. Haemagglutinins isolated from the cyanolichen thalli P . canina and P . polydactyla could bind