Lichens to Gunnera — with emphasis on Azolla

Abstract

Summary N2-fJxing cyanobacteria occur in symbiotic associations with fungi (ascomycetes) as lichens and with a few green plants. The associated cyanobacterium is always a species of Nostoc or Anabaena. Only a small number of plant genera are involved but there is a remarkable range of host diversity. Associations occur with several bryophytes (e.g.Anthocero& Blasia, Cavicularia), a pteridophyte (Azolla), cycads (nine genera including Macrozamia and Encephalafros) and an angiosperm (Gunnera). Except for Gunnera, where the cyanobacterium penetrates the plant cells, the cyanobacteria are extracellular with specialized morphological modifications and/or structures of the host plant organs providing an environment which facilitates interaction with the prokaryote. Salient aspects of current knowledge pertaining to the establishment, perpetuation, and functioning of the individual symbioses are summarized. Where possible this includes information concerning recognition and specificity, mode(s) of infection, morphological modifications/ adaptations of the host plant and a synopsis of morphological, physiological and biochemical changes common to the symbiotic cyanobacteria. The latter encompasses heterocyst frequencies, enzymes involved in ammonia assimilation, photosynthetic capability and metabolic interaction with the host.The Azolla-Anabaena symbioses, which have potential agronomic significance as an alternative nitrogen source and maintain continuity with the endophyte through the sexual cycle, are emphasized.

Morphological and physiological properties of symbiotic cyanobacteria

“…Considering that the Nostoc strains originated from plants from different geographical sites, where, it seems, a greater variety of Nostoc spp. exists, it is probable tbat specific factors exclude incompetent cyanobacteria (Bonnett & Silvester, 1981 ;Peters et al, 1986). For the establishment of symbiotic associations, the production of actively gliding hormogonia and growth in heterotrophic conditions may be particularly important, together with the potential for the cyanobacterium to be affected by the regulatory control ofthe bost plant (Peters et al, 1986;Rippka, 1988).…”

“…exists, it is probable tbat specific factors exclude incompetent cyanobacteria (Bonnett & Silvester, 1981 ;Peters et al, 1986). For the establishment of symbiotic associations, the production of actively gliding hormogonia and growth in heterotrophic conditions may be particularly important, together with the potential for the cyanobacterium to be affected by the regulatory control ofthe bost plant (Peters et al, 1986;Rippka, 1988). It seems probable that tbe capacity to establisb symbiotic association witb plants is restricted to strains of genus Nostoc, even though some endosymbionts fronn cycads have been identified as Anabaena (Lindblad & Bergman, 1990).…”

“…In Azolla tbe cyanobiont never leaves the host because tbere is continuity of tbe symbiosis throughout the reproductive cycle of tbe fern (Peters et al, 1986); however, in all tbe other symbioses each new plant is infected de novo by the cyanobacterium. This raises tbe question of tbe specificity of tbe symbiosis.…”

N2-Fixation by Freshly Isolated Nostoc from Coralloid Roots of the Cycad Macrozamia riedlei (Fisch. ex Gaud.) Gardn.

“…Heterocystous, nonbranching cyanobacteria of the genus Nostoc and Anabaena form symbioses in which atmospheric N2 fixed by the prokaryotic partner is transferred to the host (22). These symbioses include those with fungi or with fungi and green algae in lichens (17,27), thalloid liverworts (26,33), the aquatic fern Azolla (21,22), the apogeotropic coralloid roots of cycads ( 15,16,19), and glands in the bases of the angiosperm Gunnera (5).…”

The role of hydrogen metabolism in photoheterotrophic cultures of the cyanobacterium Nostoc sp. strain Cc isolated from Cycas circinalis L.

“…It is well known that nitrogen assimilation by a cyanobacterium in symbiosis is regulated in such a way that the bulk of fixed nitrogen is diverted to the host (Peters et al, 1986). Any factor which enhances the efficiency of N2 fixation and assimilation in a cultured cyanobiont can therefore be considered potentially important for the increased supply of fixed nitrogen to the natural host of this strain in the symbiotic association.…”