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The Cycad Pages
Figure 23
CORALLOID ROOTS AND NITROGEN FIXATION
Juliana Medeiros and Dennis Stevenson

Introduction

All known Cycads form mutualistic, facultative symbioses with nitrogen fixing cyanobacteria (formerly known as the blue-green algae). Cycads provide fixed carbon and a stable environment to the cyanobacteria in exchange for fixed nitrogen. These cyanobacteria are endosymbionts, living within the roots of Cycads. Symbiosis is "the living together of differently named organisms". There are many different types of symbioses. Some symbiotic associations afford mutual benefits to both partners.

In addition to normal roots, cycads develop specialized symbiotic organs at a young age called precoralloid roots. These roots are apogeotropic, that is they grow upward toward the surface of the soil, instead of downward into its depths. Upon successful colonization by cyanobacteria, precoralloids begin an irreversible transformation into coralloid roots, so named for their resemblance to coral. Within the coralloid root is the cyanobacterial zone, which is the region inhabited by cyanobacteria. This has unique characteristics which facilitate a close relationship between cycad and cyanobacteria.

Free-living cyanobacteria are among the most ancient of prokaryotic autotrophs, which fix carbon by photosynthesis. They are extremely hardy and have adapted to life in various inhospitable environments. Many have the ability to fix atmospheric nitrogen using the enzyme nitrogenase. This capability, as well as their flexible nature, has allowed them to form symbiotic relationships with a few plants from all of the major land plant groups. Cyanobacteria within the coralloid roots of Cycads are chemoheterotrophic and specifically adapted to life in symbiosis. Not all cyanobacteria live in symbiosis, and only a few species form associations with Cycads.

All Cyanobacteria in symbiosis with Cycads have the ability to fix nitrogen. Biologically fixed nitrogen makes up 90% of the nitrogen available globally for use by plants and animals. Nitrogen is an important component of many plant compounds and it is also one of the most limited nutrients. The complex physiological process of nitrogen fixation is a capability unique to prokaryotes, and is a key feature of the relationship between Cycads and Cyanobacteria.

The process of Cyanobacterial colonization of Cycad coralloid roots involves intricate coordination of action and reaction by both organisms. Once they come into contact with a precoralloid root, the cyanobacteria enter through a break in the dermal tissue and proceed to the cyanobacterial zone. Their entrance causes permanent changes in the symbiotic tissue, initiating the final transformation of precoralloids into symbiotic coralloid roots. Once established, colonies grow and develop under the direction of the Cycad. Over the lifetime of a Cycad coralloid masses grow and degenerate according to environmental conditions and the needs of the plant.

Every aspect of the symbiosis between Cycads and Cyanobacteria is aimed at the balanced growth of the partners and efficient exchange of nutrients between them. Fixed carbon and nitrogen resources flow from one partner to the other in a coordinated effort to thrive. Increased knowledge of the symbiosis between Cycads and Cyanobacteria has many possible applications in various areas from agriculture to medicine. Little is known about the physiology of their relationship, and there are many possibilities for future research.

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© 1998-2012 Royal Botanic Gardens Sydney
Written and maintained by Ken Hill 1998-2010
Maintained by Leonie Stanberg and Dennis Stevenson 2010-2012
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