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Cycads In Symbiosis Relationship With Cyanobacteria: Cycads form symbiosis relationships with cyanobacteria. The symbiosis relationship maximize nutrient exchange and balance partner growth of both cyanobacteria and cycads.

Nitrogen is most abundant gas in the atmosphere ^[1]. However, living organisms have no direct access to this nitrogen. Microorganisms must fix N2 before it becomes accessible to other life forms. In plants, nitrogen is an important component of chlorophyll ^[2]. Healthy plants usually contain 3 to 4 percent of nitrogen in their shoot. Plants need nitrogen to make proteins. Without proteins, plants weaken and die. In addition, nitrogen can act as a signaling to regulate gene expression and metabolism ^[2]. Plants cannot take or use dinitrogen(N2) because it is relatively inert. Plants can take and use nitrogen that is in the fixed forms (organic forms). Microorganisms like cyanobacteria can fix N2 using the enzyme nitrogenase. This capability has allowed them to form symbiotic relationships with the cycads and other plants.

Cyanobacteria fix nitrogen for cycads; at the same time, cycads provide fixed carbon and a good environment to cyanobacteria. Cycads and cyanobacteria both undergo physiological modifications to enable their symbiosis. Cyanobacteria are endosymbionts; they live within the roots of cycads.

Cyanobacteria can live within the root of cycads because of the development of specialized symbiotic organs called precoralloid roots ^[3]. Precoralloid roots develop at young age of cycad’s life. Upon successful colonization by cyanobacteria, precoralloids begin an irreversible transformation into coralloid roots. Not all cyanobacteria live in symbiosis. Free-living cyanobacteria are among the most ancient of prokaryotic autotrophs. Free-living cyanobacteria can photosynthesize (fix carbon by photosynthesis), but when they are incorporated into cycad roots, they need fixed carbon to be provided by their hosts to fix nitrogen^[3]. All cyanobacteria in mutualistic association with cycads can fix nitrogen. Therefore, all known extant species of cycads are capable of forming nitrogen fixing symbiose, while there are only a few types of possible cyanobacteria partners.

All known extant cycads (160 species in 2009^[3]) are capable of forming coralloid roots which are specialized structures that provide protection and habitat to their endosymbionts. The endosymbionts are cyanobacteria from the genus Nostoc ^[4]. Different strains of Nostoc have been identified in cycads from different geographical areas ^[5].

In optimal environments, cycads can grow and produce offspring in the absence of cyanobacteria. However, when cycads grow in a nitrogen-limited environment, they develop symbiotic nitrogen fixing relationships with cyanobacteria ^[3]. Under nitrogen-rich conditions, these associations do not develop at all. However, cycads usually grow in harsh environments where there is a lack of fixed nitrogen ^[3]. Cycads can also grow in nutrient poor environments like sand and even rock, in anoxic environments, and in harsh deserts or semi-deserts ^[3]. Under these conditions, completion of the cycad life cycle is difficult if not impossible without supplemental nitrogen provided by cyanobacteria. Today only a few of species still exist, and many are facing possible extinction in the wild^[3].

In low nitrogen environments, cycad roots secrete hormogonia-inducing factor (HIF), a chemical that attracts cyanobacteria. ^[4] Hormogonia are the motile form of cyanobacteria.^[5] They respond by moving toward the source of HIF, the cycad roots.

In general, cycads develop three types of roots: a tap root, which is the primary root system found in most types of plants, lateral roots, and coralloid roots, which are highly specialized lateral roots to house cyanobionts ^[3].

Precoralloid roots form early in cycad development, growing from a lateral root or the taproot^[6]. According to Ahern and Staff (1994)^[6], they are apogeotrophic, growing upward toward the surface of the soil, instead of growing downward, and they have a papillose sheath, which may help cyanobacteria enter the root.

When cyanobacteria come into contact with a precoralloid root, they enter through a break in the dermal tissue and proceed to the cyanobacteria zone. The process of how cyanobacteria penetrate the precoralloid roots is still unknown, but it has been proposed to be through: lenticels, papillae, or other breaks in the epidermis^[3]. This penetration initiates the transformation of cycad precoralloid roots into their coralloid form.

Upon infection by cyanobacteria, precoralloid roots develop geotrophic growth and a subcortical cyanobacterial zone ^[4]. Within the cyanobacteria zone, cyanobacteria inhabit a wide, mucilage-filled intercellular space system generated by partial separation and circular elongation of host cells. It is located between the inner cortex and the loose outer cortex. This region becomes green when inhabited by cyanobacteria and is visible to the naked eye as a hollow green cylinder.

Coralloid roots can grow up to 10 cm in diameter and 500 g in weight, and provide an important contribution to the nitrogen metabolism of the plants as well as the nitrogen budget of the ecosystem.

It is possible for cycads to harbor more than one strain of cyanobacteria a single coralloid root ^[7].

Inside the coralloid roots, the filamentous cyanobacteria differentiate to form more heterocysts than in their free-living form, but they also retain RuBisCO to fix carbon ^[3]. Heterocysts are specialized cells that have thickened walls to create an anaerobic environment to contain the nitrogenase enzyme, which is inactivated by oxygen. The fixed nitrogen is transferred to cycad hosts as a combination of citrulline and glutamine in some cycads, and as glutamine only in others ^[3].

Cyanobacteria within the coralloid roots are chemoheterotrophic. Free-living cyanobacteria can photosynthesize, but when they are in plant roots without access to sunlight, they need fixed carbon to be provided by their hosts.

Adams DG (2000) Symbiotic interactions. In: Whitton BA, Potts M (eds) The Ecology of Cyanobacteria. Kluwer Academic Publishers, Dordrecht, pp 523–561.

Bergman, Birgitta, Andrey Matveyev, and Ulla Rasmussen. "Chemical signalling in cyanobacterial-plant symbioses." Trends in Plant Science 1.6 (1996): 191-197.

Dehgan, Bijan, and Nancy B. Dehgan. "Comparative pollen morphology and taxonomic affinities in Cycadales." American Journal of Botany (1988): 1501-1516.

Hoffman, Brian M., Dennis R. Dean, and Lance C. Seefeldt. "Climbing nitrogenase: toward a mechanism of enzymatic nitrogen fixation." Accounts of chemical research 42.5 (2009): 609-619.

Hoffman, Brian M., et al. "Mechanism of nitrogen fixation by nitrogenase: the next stage." Chemical reviews 114.8 (2014): 4041-4062.

Loconte, Henry, and Dennis W. Stevenson. "Cladistics of the Spermatophyta." Brittonia 42.3 (1990): 197-211.