Ultrastructural changes of the endosymbionts of Azolla microphylla during megaspore germination and early plantlet development

Abstract

The ontological changes in cyanobacterial ultrastructure, including akinete germination and heterocyst differentiation, during the propagation of mature sporophytes from Azolla microphylla sporocarps were examined using transmission electron microscopy. Rehydrated Azolla megasporocarps contained cyanobacterial akinetes having a multilayered envelope and large amounts of cyanophycin and glycogen. Concomitant with the initial stages of sporocarp germination was the formation of vegetative cyanobacterial cells via germination of akinetes. Once fern plantlets had four visible leaves, vegetative cyanobacterial cells had begun to differentiate into heterocysts in the cavity of the oldest leaf; this timing was very similar to that observed in vegetatively reproducing sporophytes. Ultrastructural characteristics of vegetative cyanobacterial cells present at the apical meristem of plants generated either by sporocarp germination or vegetative propagation were indistinguishable. These cells remained in an undifferentiated state regardless of plant age. However, as in vegetatively propagated plants, the cyanobacterial population and ultrastructural characteristics of cells in older leaf cavities were exceedingly diverse. This is the first report that groups of pleomorphic eubacteria are contained beneath the indusium of rehydrated Azolla microphylla megasporocarps. Concomitant with the early morphological changes of germination of Azolla megasporocarps and endosymbiotic akinetes was the initiation of cell division by the eubacteria. Key words: akinetes, Azolla–Anabaena, endosymbionts, sporocarp.