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.

Diversity, Phylogeny and Antagonistic Activity of Fungal Endophytes Associated with Endemic Species of Cycas (Cycadales) in China

“…C. debaoensis was distributed at the open karst areas near the border of Guangxi and Yunnan provinces, while C. fairylakea grew naturally in moist closed forests in the East Guangdong province [ 30 , 31 ]. Environmental conditions with extreme drought and poor nutrients are the characteristics of the extant cycads’ habitats millions of years ago [ 32 ]. Cycads are then believed to evolve a differentiated organ called coralloid roots, which are morphologically distinguished by dichotomous branching, coral-like forms and apogeotropic growth [ 32 ].…”

“…Environmental conditions with extreme drought and poor nutrients are the characteristics of the extant cycads’ habitats millions of years ago [ 32 ]. Cycads are then believed to evolve a differentiated organ called coralloid roots, which are morphologically distinguished by dichotomous branching, coral-like forms and apogeotropic growth [ 32 ]. These special roots were found to shelter a diverse group of endosymbiotic microorganisms.…”

“…These special roots were found to shelter a diverse group of endosymbiotic microorganisms. In fact, it is most recognized with its relationship to the symbiotic cyanobacteria manifesting a macroscopically visible green zone around its cortex [ 32 ]. An increase in attention relative to genomic research and conservation studies is particularly given for Cycas due to its rarity, taxonomic and evolutionary uniqueness, high degree of endangerment and extinction risk [ 33 , 34 , 35 ].…”

Ecology and evolution of cycad‐feeding Lepidoptera

“…β‐Methylamino‐ l ‐alanine (BMAA) is a non‐protein amino acid found in cycad tissues, but is also produced by cyanobacteria in aquatic, marine and terrestrial environments (Cox et al ., 2005; Metcalf et al ., 2015). All cycads engage in endosymbioses with cyanobacteria, which are housed in specialised corraloid roots and are thought to provision plants with fixed nitrogen (and potentially other specialised metabolites) in exchange for carbon and physical protection (Gutiérrez‐García et al ., 2019; Chang et al ., 2019). Given that BMAA is produced by free‐living cyanobacteria in other habitats, its biosynthetic source in cycads has been debated (Cox et al ., 2003; Marler et al ., 2010).…”

Genetic, Genomics, and Responses to Stresses in Cyanobacteria: Biotechnological Implications

“…By colonizing aquatic ecosystems (fresh, brackish, and marine waters) and soils (including deserts) of our planet, cyanobacteria are inevitably exposed to multiple stresses such as solar ultraviolet radiations and variations in light intensity and quality, inorganic-nutrients availabilities, temperatures (high and low), salinity, pH (acidic and basic), drought, and pollutants (herbicides and heavy-metals). In addition, cyanobacteria are involved in numerous interactions with competitors, predators, or symbiotic hosts [ 9 ]. Consequently, it is not surprising that cyanobacteria have evolved as a widely diverse organisms, which are of high interest for basic and applied research [ 10 ].…”