Structural Characteristics Of The Cyanobacterium-Azolla Symbioses

Structure is a fundamental base for all life forms, whether plants or microbes. The development of special structures results in unique functions. Though structures of the mutualistic Azolla-cyanobacterial symbiosis are still largely un-explored, they have attracted attention of researchers in the past two decades. The occurrence of the leaf cavity and trichomes within the water-fern Azolla are hallmarks for the two cyanobacterial plant symbiotic systems. The trichomes and the multicellular. laments are suggested to be involved in metabolic exchange between the cyanobionts and host plants due to their cell wall ingrowths, i.e., transfer cell characteristics. At the apical region of the Azolla plant, the primary branched trichomes touch each other, thereby forming linked bridge-like structures that lead to partitioning of the cyanobacteria into the young leaf cavities, thus promoting horizontal transfer of the cyanobiont during vegetative growth and asexual reproduction via sporophyte fragmentation. The trichomes developing during the sexual reproduction stages of Azolla facilitate the partitioning of the motile cyanobiont into the sporocarps, thus promoting a vertical transfer of cyanobacteria between Azolla generations, a capacity unique among cyanobacterial-plant symbioses. The cyanobionts in Azolla, Blasia and Anthoceros undergo pronounced morphological, physiological and molecular modi. cations to keep a synchronized development with the plant partner and to meet needs for maintaining a mutualistic symbiosis.