SYMBIOSIS - Cyanobacteria - page 362

fl uorescein-labelled ABP from X . parietina could bind to the cell walls of the phycobiont only when
the phycobiont possessed urease activity in its cell walls. Due to the binding there is an inhibition
in both the arginase activity of the ABP and the urease activity of phycobiont cell wall. Moreover,
when purifi ed ABP is added to the cultures of the phycobiont with urease activity, no cellular
disorganization occurred (Molina et al ., 1996, 1998b).
E . prunastri (a lichen belonging to Parmeliaceae) has been shown to produce both ABP and
SAE (a secreted arginase from Evernia ). But the polysaccharide moiety of the latter is composed of
fructose, mannose and glucose (Planelles and Legaz, 1987). The arginase activity of both SAE and
SAX has been shown to be dependent on Mn 2+ . Although Ca 2+ could not replace Mn 2+ , its addition
in presence of Mn 2+ signifi cantly inhibited arginase activity. The lectin function of both SAE and
SAX has been demonstrated by their binding to the polysaccharide moiety of the urease on the
cell walls of the phycobionts. However, binding of SAE to the cell walls of the phycobiont of E .
prunastri has been found to be specifi c as it could not bind to the cell walls of the phycobiont of X .
parietina . On the other hand, SAX is non-specifi c in its binding activity as it could bind to the cell
walls of the phycobionts of both E . prunastri and X . parietina . The polysaccharide moiety of the urease
signifi cantly contained α-D-galactose which serves as a ligand for lectin binding. So the important
requirement for binding of ABP to the cell walls of homologous and heterologous phycobionts is
the presence of the galactose residues as the ligand molecule. This indeed has been demonstrated
to be the mechanism for ensuring specifi city of the phycobiont in the formation of lichen symbiosis
(Legaz et al ., 2004). Hydrolysis of the α-1,4-polygalactoside moiety of urease located in the cell walls
of the phycobiont with α-1,4-galactosidase caused the release of high amounts of D-galactose. This
resulted in an inhibition of lectin binding due to the absence of the ligand for binding but if the
hydrolysis is performed with β-4-galactosidase there was no effect on binding as very low amounts
of D-galactose could be released (Sacristán et al ., 2006). Partial amino acid sequences of SAE and
SAX have been determined. The former showed an undecapeptide (a peptide with 11 amino acid
residues) which is homologous to the Mn 2+ -binding site where as the latter contained a heptapeptide
and an undecapeptide (Sacristán et al ., 2008). The cell recognition model of chlorolichens has now
been extended to cyanolichens. Leptogium corniculatum is shown to secrete an arginase that could
bind to the cell wall ligand of the cyanobiont Nostoc strain. Furthermore, the secreted arginase could
also bind to some extent to the cell walls of Trebouxia from E . prunastri (Sacristán et al ., 2007; Vivas et
al ., 2010). P . canina , a cyanolichen, is shown to produce and secrete an arginase that acts as a lectin in
binding to the cell surface urease present on the cell wall of the cyanobiont Nostoc (Díaz et al ., 2009).
The secreted lectin from the thallus of P . canina has been suggested to act as a chemoattractant and
induces chemotropism in compatible Nostoc cells by contriction-relaxation pulses similar to that of
myosin II (Díaz et al ., 2011).
x) Role of hydrophobins in symbiosis : Proteins that occur on the surface of mycelial (asmomycetous
and basidiomycetous) fungi are known as hydrophobins (Wösten, 2001). These confer water repellent
properties to conidia, hyphae and multicellular structures. As many as 50 hydrophobins have so far
been described from a wide variety of fungi including saprophytic moulds, edible mushrooms, plant
pathogens and fungi forming mycorrhizal associations (Whiteford and Spanu, 2002). They enable
the fungi to escape their aquatic environment and come in contact with air and play a greater role
in fungal pathogenesis (Carpenter et al ., 1992; Zhang et al ., 1994; Spanu, 1997; Arntz and Tudzynski,
1997), ectomycorrhizal associations (Tagu et al ., 1996) and symbiotic association with green algae
and/or cyanobacteria (Honegger, 1991; Scherrer et al ., 2000). Hydrophobins thus constitute a group
of morphogenetic proteins that can be classifi ed into two groups, i.e. class I and class II based on