The partners in the <i>Azolla</i> symbiosis.

A) Fronds of the Azolla filiculoides Lam. plant. B) Close up of an Azolla branch showing the apex and the alternating ‘stacked’ dorsal leaves, each containing a cavity in which the cyanobiont (NoAz) filaments reside. C) Light micrograph of the cyanobiont. The larger cells in the vegetative filaments represent the nitrogen-fixing heterocysts. Scale bar = 5 µm. D) Transmission electron micrograph of the cyanobiont. Note the thicker cell-walls and the electron dense polar nodules of the heterocyst (middle cell) at the interface to flanking vegetative cells, which function as combined N storage structures (cyanophycin). Scale bar = 5 µm E) A snap-shot in the vertical transmission process of the cyanobiont between Azolla plant generations, using fluorescence microscopy. Pairs of megasporocarps (blue) develop at the underside of the cyanobacterial colonized Azolla leaves. Filaments of the motile cyanobacterial cell stage (red), the hormogonia (h), are attracted to the sporocarps, gather at the base and subsequently move towards the tip, before entering the sporocarps via channels (white arrows). Once inside the sporocarp the hormogonia differentiate into individual thick walled resting spores (or akinetes; ak), seen as the intensively red fluorescing small inoculum on top of the megaspores (sp). For details see [15].