Symbiotic associations

Hornworts are well known for their endosymbiotic Nostoc colonies. Nostoc is a photosynthetic, filamentous, soil-dwelling cyanobacterium that can develop symbiotic associations with fungi, liverworts and even cycads. Nostoc filaments penetrate the hornwort thallus through clefts on the lower epidermis. Inside the gametophyte, it forms spherical colonies that are soon surrounded by mucilage secreted by the parenchyma cells. Nostoc is recruited for its ability to fix atmospheric nitrogen, which is highly abundant but not directly usable by most organisms. In Nostoc, nitrogen fixation takes place in specialized non-photosynthetic cells called heterocysts. Hornworts tap into this supply of a crucial macronutrient through haustoria-like cell extensions that penetrate the colony. Nostoc in return receives organic carbon from the hornwort

Fig. 5.14. The N-fixing symbiosis between cyanobacteria and hornworts. Example of the Nostoc-Leiosporoceros association (reproduced from Villareal & Renzaglia 2006 with permission of American Journal of Botany). (a) Dorsal view of the thallus showing bifurcating strands of Nostoc parallel to the main axis of the thallus (arrows). Note the mature sporophyte, S, with a basal involucre I. (b) and (c) Longitudinal (b) and transverse (c) section of the thallus with Nostoc canals (arrows) originating behind the apical cell A and scattered mucilage cells (M). Note the presence of an immersed archegonia Ar within the thallus.

Fig. 5.14. The N-fixing symbiosis between cyanobacteria and hornworts. Example of the Nostoc-Leiosporoceros association (reproduced from Villareal & Renzaglia 2006 with permission of American Journal of Botany). (a) Dorsal view of the thallus showing bifurcating strands of Nostoc parallel to the main axis of the thallus (arrows). Note the mature sporophyte, S, with a basal involucre I. (b) and (c) Longitudinal (b) and transverse (c) section of the thallus with Nostoc canals (arrows) originating behind the apical cell A and scattered mucilage cells (M). Note the presence of an immersed archegonia Ar within the thallus.

(Duckett et al. 1977, Rodgers & Stewart 1977). In Leiosporoceros, Nostoc must be acquired at the sporeling stage, since clefts are lacking in mature tissues. The Nostoc colonies typically form a globose mass inside the cavity, except in Leiosporoceros, wherein the cyanobacterium elongates within long more or less axial canals (Fig. 5.14). In symbiotic colonies, heterocysts compose nearly 50% of the Nostoc cells, as also occurs in other, well-documented cases of symbioses involving cyanobacteria, e.g. in the fern Azolla and cycads. Some hornworts also establish mycorrhizae-like associations with aseptate fungi of the Glomales (SchiiBler 2000). Whether such fungal partnerships are widespread among species and obligatory for the hornwort involved needs further study.

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