Perspectives on Endosymbiosis in Coralloid Roots: Association of Cycads and Cyanobacteria

Abstract

Past endosymbiotic events allowed photosynthetic organisms to flourish and evolve in terrestrial areas. The precursor of chloroplasts was an ancient photosynthetic cyanobacterium. Presently, cyanobacteria are still capable of establishing successful symbioses in a wide range of hosts. One particular host plant among the gymnosperms is cycads (Order Cycadales) in which a special type of root system, referred to as coralloid roots, develops to house symbiotic cyanobacteria. A number of studies have explained coralloid root formation and cyanobiont invasion but the questions on mechanisms of this host-microbe association remains vague. Most researches focus on diversity of symbionts in coralloid roots but equally important is to explore the underlying mechanisms of cycads-Nostoc symbiosis as well. Besides providing an overview of relevant areas presently known about this association and citing putative genes involved in cycad-cyanobacteria symbioses, this paper aims to identify the limitations that hamper attempts to get to the root of the matter and suggests future research directions that may prove useful.

Keywords: coralloid roots; cyanobacteria; cyanobionts; cycads; endosymbiosis; heterocyst; hormogonia; symbiosis.

FIGURE 1

Coralloid roots of Cycas fairylakea (A) growing apogeotropically at Shenzhen Fairy Lake Botanical Garden, (B) showing dichotomous branching of coralloid roots, (C) cross-section of coralloid root showing a distinct green layer – the cyanobacterial zone (the green ring pointed by the red and while arrows) and (D) microscopic image of filamentous cyanobacteria residing in the cyanobacterial zone. Heterocysts are marked with white arrows.

FIGURE 2

General mechanism of symbiosis based on other host plant-cyanobiont models. (1) HIF is released by host and (2) triggers cyanobacteria in surrounding soil to (3) transition into motile filaments. (4) Through chemotaxis, host attracts hormogonia following cyanobacterial entry. (5) Host releases HRF and (6) signals hormogonia that entered the host successfully to (7) transition into heterocysts-forming cells. (8) Symbiosis at this stage becomes irreversible.