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

Colonization of Cycad Coralloid Roots by Cyanobacteria

Figure 25

Colonization is an intricate coordination of action and reaction by endophyte and plant alike. The first step toward symbiotic existence is contact between the two individuals involved. For cycads and cyanobacteria, this occurs in the light-available soil region inhabited by both precoralloid roots and photosynthesizing cyanobacteria. There is a specific window of opportunity for colonization, during which time precoralloid roots and cyanobacteria of the proper type must be present. Many organisms occupy this part of the soil and symbiotic partners must somehow recognize one another. Communication between partners is also crucial to coordinate developmental changes necessary to support symbiosis (Rai et al, 2000).

More on Recognition Between Partners

There have been very few investigations of the initiation and process of coralloid colonization by cyanobacteria. Most studies have focused on changes occurring in cycad tissues after cyanobacteria have entered the roots. The exact site and method of entry by the endosymbiont has remained obscure. Several routes of entry into precoralloids have been proposed.

More on Mode of Colonization

The duration of a single cyanobacterial colony is entirely dependant on the life of the coralloid root within which it is contained. New coralloid growth is colonized directly through the connecting cyanobacterial zone as it differentiates. Growing apices of coralloids appear pale green until eventually becoming enclosed in the periderm, as they become dormant (Ahern and Staff, 1994). Large masses of coralloids are maintained over several seasons and, according to the needs of the plant, may originate new growth of normal secondary roots as well (Halliday and Pate, 1976). Once a colony is established, the cycad controls cyanobacterial growth. A balance must be maintained between expenditure of carbon resources and receipt of nitrogen. This may involve nutrient distribution or chemical signaling (Rai et al, 2000)

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