SYMBIOSIS - Cyanobacteria - page 368

schreberi , a species of feather moss forms symbiotic association with cyanobacteria (DeLuca et al .,
2002) that has great impact on nitrogen cycle in mature boreal forests (Zackrisson et al ., 2004).
The gametophytes of liverworts and hornworts are prostrate, few cm long and attached to
the substrata by rhizoids. The location of the cyanobionts varies in the host plants. In case of the
widely distributed B . pusilla , Nostoc is restricted to specialized auricles that develop on the ventral,
three-celled slime hairs. Nostoc is attracted. The symbiotic association of Nostoc with Blasia is worth
noting because of the two types of asexual propagules produced by it. On the dorsal side of the
thallus, stellate gemmae are produced while small, ovoid gemmae are produced in long-necked
fl ask-shaped receptacles. The stellate gemmae are of particular interest because these are equipped
with two lobes and two auricles containing symbiotic Nostoc colonies (Renzaglia, 1982; Duckett and
Renzaglia, 1993). It is a miniature Blasia gametophyte as upon dispersal it germinates to give rise to a
new thallus with the cyanobiont. In contrast, C . densa is a rare plant endemic to Japan and produces
gemmae in crescent-shaped receptacles near the mature lobes. Two types of gemmae are produced
by the thallus. The fi rst type is small and pale gemmae which upon dispersal get infected with
endosymbiont. The second type is the larger, stellate gemmae which get infected by the endosymbiont
even while attached to the plant. The latter infact represent miniature thalli and are equipped with
two symbiotic auricles and it is into these auricles the symbiont enters (Fig. 6 A-D).
In hornworts, the invasion of cyanobacteria takes place through the ventral mucilage clefts that
are formed throughout the life of the plants. The middle lamella between internal cells separates
to form a schizogenous space and Nostoc gets established as a globose colony. In Anthoceros and
Phaeoceros slime cavities within the thallus open to the ventral surface by means of pores. The
only exception in hornworts is Leiosporoceros dussii , a monotypic genus considered to be the most
genetically and morphologically divergent hornwort (Duff et al ., 2004; Villarreal and Renzaglia,
2006). The plants form large rosettes. The male and female plants grow intermixed. The female
gametophytes are strap-shaped and fl eshy with dorsal outgrowths. In transverse section the thallus
is 9-15 celled-thick in the middle and taper to margins with 2-4 cells. Multiple schizogenous
cavities (2-6) are some times present. This hornwort is distinguishable from others by the absence
of ventral clefts and the cyanobacteria are present in ventrally developed schizogenous canals that
are dichotomously branched and run parallel to the main axis of the thallus (Villarreal and Renzaglia
2006). Exceptionally, in case of P . screberi besides Nostoc sp. a species each of Calothrix and Stigonema
are also associated. Villarreal and Renzaglia (2006) described the ultrastructure and development of
schizogenous canals in L . dussii and the ultrastructure of Nostoc fi laments. Young plants (80-90 day
old) develop mucilage clefts that are randomly scattered along the fl attened anterior rim of lobes.
Small intercellular spaces or cavities that extend to several cell lengths are scattered throughout the
interior of the thallus. These irregular cavities are schizogenous, i.e. formed by gradual separation
of middle lamella. These cavities contain mucilage, often interconnected and are in close proximity
to apical cells. The mucilage strands elongate and branch synchronously with apical growth of the
thallus. Although a single invasion of Nostoc is required for strand production, non-specifi city is
suggested on the basis of the presence of two distinct Nostoc species.
Campbell and Meeks (1989) identifi ed early events in the initiation of Anthoceros punctatus and
Nostoc symbiosis. A hormogonium-inducing factor (HIF) is released by A . punctatus in a nitrogen-
limited or growth conditioned medium devised for it. HIF transforms Nostoc fi laments into
hormogonia at a high frequency. Since gametophytic tissue of A . punctatus requires a source of fi xed
nitrogen for its continuous growth, the release of HIF and the subsequent hormogonia so produced at
high frequency would colonize the slime cavities in the newly formed gametophytic tissue. The HIF
is suggested to be heat labile substance, 12-14 kD in size and complexed by polyvinylpyrrolidone.