The Cyanobacteria Plant Symbioses

This chapter deals with the endophytic cyanobacteria-plant symbioses in which the cyanobiont is located within the host tissue or, in the case of the angiosperm Gunnera, within the host cells. However, many cyanobacteria, including nitrogen-fixing strains, grow as epiphytes on a wide range of plants, particularly in aquatic environments (Adams 2011). These "loose" associations won't be discussed here, as they are poorly studied and little is known about the extent to which the plant benefits, although the nitrogen-fixing cyanobacteria certainly contribute to the local nitrogen economy. By contrast, in the endophytic associations the plant host clearly benefits from the provision of combined nitrogen by the cyanobionts and there are clear, although not well understood, interactions between the cyanobiont and its host.

2.1 Bryophytes (Mosses, Hornworts and Liverworts)

The mosses, hornworts and liverworts are small, non-vascular land plants (Figs. 1 and 3 a), some of which can become infected with primarily heterocystous cyanobacteria (Adams 2002a, b; Meeks 2003; Solheim et al. 2004; Adams et al. 2006; Adams and Duggan 2008; Bergman et al. 2007a, 2008). The cyanobacteria in moss associations are mostly epiphytic (Solheim and Zielke 2002; Solheim et al. 2004; Gentili et al. 2005), except for two Sphagnum species in which they occupy water-filled, dead (hyaline) cells, where they may gain protection from the low pH of the bog environment (Solheim and Zielke 2002). The most common cyanobacterial epiphytes on mosses are members of the filamentous, heterocystous genera Nostoc, Stigonema and Calothrix (DeLuca et al. 2002, 2007; Gentili et al. 2005; Houle et al. 2006), although the non-heterocystous, filamentous genera Phormidium and Oscillatoria, and even the unicellular Microcystis, have also been reported (Solheim et al. 2004). These moss associations will not be discussed further here as they are almost exclusively epiphytic, and even in the seemingly endophytic Sphagnum symbioses the cyanobacteria occupy dead cells which are connected to the outside environment. Nevertheless, these nitrogen-fixing cyanobacteria-moss associations are of environmental significance as they are often the major source of combined nitrogen in local ecosystems in the Arctic, the Antarctic and forests of the northern hemisphere where mosses are abundant (Zielke et al. 2002, 2005; Solheim and Zielke 2002; Nilsson and Wardle 2005; DeLuca et al. 2008).

Fig. 1 The liverwort Blasia pusilla collected from the wild, showing the dark Nostoc colonies (~0.5-1.0 mm in diameter) bordering the thallus midrib. Reproduced with permission from Adams (2000)

Fig. 1 The liverwort Blasia pusilla collected from the wild, showing the dark Nostoc colonies (~0.5-1.0 mm in diameter) bordering the thallus midrib. Reproduced with permission from Adams (2000)

Cavicularia Images

Endophytic cyanobacterial symbioses are found in all 13 hornwort genera (Renzaglia et al. 2007), but in only two (Blasia and Cavicularia) of the greater than 340 liverwort genera, although a further two (Marchantia and Porella) form epiphytic associations (Rai et al. 2000; Adams 2000, 2002a, b; Adams et al. 2006; Adams and Duggan 2008). In the endophytic symbioses, cyanobacterial colonies can be seen as dark spots up to 0.5 mm in diameter within the flattened thallus of the plant (Fig. 1). The endophytes of liverworts and hornworts are almost exclusively members of the genus Nostoc (Costa et al. 2001; Rasmussen and Nilsson 2002; Adams 2002a, b; Adams et al. 2006; Bergman et al. 2007a; Adams and Duggan 2008), and a wide variety of strains can infect a single thallus in the field (West and Adams 1997; Costa et al. 2001; Adams 2002a, b; Adams and Duggan 2008), although some can be dominant and widespread (Rikkinen and Virtanen 2008). The ease with which liverworts and hornworts can be grown in the laboratory in shaken liquid culture (Fig. 2b), with or without symbiotic cyanobacteria, makes them an excellent model for studying the infection process (Adams 2002a, b; Meeks 2003; Duckett et al. 2004; Adams and Duggan 2008).

The structures housing the symbiotic cyanobacteria in hornworts (slime or mucilage cavities; Fig. 2a) and liverworts (auricles; Fig. 2c, d) are produced constitutively and not as a response to the presence of potential symbionts (Renzaglia et al. 2000; Adams 2002a, b; Meeks 2003; Adams et al. 2006; Villarreal and Renzaglia 2006; Bergman et al. 2008). The slime cavities of hornworts such as Anthoceros and Phaeoceros are formed within the thallus and are connected to the ventral surface by mucilage clefts (Fig. 2a) which superficially resemble stomata, but are not thought to be related (Villarreal and Renzaglia 2006). The cleft results from the separation of adjacent epidermal cells, after which the slime cavity develops beneath the cleft (Renzaglia et al. 2000). The slime cavities in the hornwort Leiosporoceros dussii also develop beneath mucilage clefts (Fig. 3c, d) but are elongated mucilage-filled "canals" (Fig. 3b) resulting from the separation of plant cell walls along their middle lamellae. These canals branch to form an integrated network enabling the Nostoc symbiont to spread throughout the thallus (Villarreal and Renzaglia 2006). By contrast, in the liverwort Blasia the cyanobacteria occupy dome-shaped auricles (Fig. 2c, d), which develop on the ventral surface of the thallus from a three-celled mucilage hair that undergoes extensive elaboration (Renzaglia et al. 2000).

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