The Nostoc–Gunnera symbiosis

Abstract

Gunnera L. develops a complex and intimate symbiosis with Ng-fixing cyanobacteria of the genus Nostoc, which renders the plant independent of combined nitrogen. The Nostoc-Gunnera symbiosis exhibits unique features compared to other cyanobacterial-plant symbioses: it is for example the only one that involves a flowering plant (angiosperm), the cyanobacterium infects specialized gland organs located on the stems of the host and once it has passed into the interior of the gland the cyanobacterium also enters the Gunnera cells where it starts to differentiate the highest frequency of heterocysts (the N^-fixing cells) recorded in any cyanobacterial population. Gunnera has attracted scientific attention also for the following reasons: the genus has a peculiar geographic distribution of its subgenera and species in the Southern Hemisphere. It differs morphologically and anatomically from related plants and also shows an anomalous polystelic vascular system (polystely). This review gives an updated account of present knowledge concerning the Nostoc-Gunnera symbiosis. Emphasis will be on the development of the symbiotic tissue (the gland), the structure and function of the prokaryotic Ng-fixing cyanobacterium, the infection process and on the relationship between the pro-and eukaryotic partners prior to and following the establishment of symbiosis.

From Traditional Usage to Pharmacological Evidence: Systematic Review of Gunnera perpensa L.

“…The genus was named in honour of the Norwegian bishop and botanist Johan Ernst Gunnerus (1718–1773) [1]. According to Qiu et al [2] and Soltis et al [3], there is a close relationship between genus Gunnera and Myrothamnus flabellifolia Welw., a morphologically different taxon confined to dry habitats belonging to the monogeneric family Myrothamnaceae.…”

“…are among some of the popular Gunnera species in the world [6–8]. They are natives of South America but also widely cultivated in temperate, tropical, and subtropical countries as ornamental and medicinal plant species [1, 6–11]. Both species have escaped from cultivation in some countries; for example, these species are now naturalized and considered potential invasive aliens in Australia, Ireland, and New Zealand [8, 12].…”

“…Its roots are 30 cm thick, fleshy, dark brown or blackish on the outside but yellow or pinkish-red inside [1, 45]. All the leaves arise from a central tuft near the top of the apex, just above the soil level.…”

Nitrogen fixation in eukaryotes – New models for symbiosis

“…For example, symbioses of filamentous heterocyst-forming Nostoc sp. have been reported for bryophytes, pteridophytes ( Azolla ), gymnosperms (cycads) and angiopsperms ( Gunnera ) [78-81]. In all plant hosts, with the exception of Gunnera , symbiotic Nostoc filaments are localised extracellularly in different locations depending on the host species.…”

“…More specialised is the mutualistic intracellular Gunnera - Nostoc symbiosis. Here the process begins with invasion of the petiole glands, followed by intracellular establishment within the meristematic cells of this tissue [60,78]. …”