Later stage cyanobacterial cell death visualized by the TUNEL assay.

<p>The TUNEL assay emits green fluorescence in response to DNA fragmentation. (<b>A</b>) Hormogonium filaments at the plant apex, with groups of cells showing a positive TUNEL reaction (arrows). Non-affected cells exhibit red autofluorescence. (<b>B</b>) Chains of pro-akinetes in a megasporocarp which exhibits green fluorescence in apoptotic-like (arrow) and necrotic-like (double arrows) cells, the latter showing leaking cytoplasms. (<b>C</b>) A heterocytous filament (leaf no. 15) showing green TUNEL fluorescence in vegetative cells entering the later stages of the cellular death process: i) ∼10% of the DNA is fragmented (dots) and faintly stained in green-red, due to the red autofluorescence of the background pigmentation; ii) ∼50% of the DNA is stained and the fluorescing dots are larger; iii) ∼80% of the DNA is intensively stained; and ix) the DNA fragmentation is complete and the green fluorescence smeared (i.e., the cells are dead). (<b>D</b>) Positive control: homogonium filaments pre-treated with DNase I show a positive TUNEL reaction in all cells. (<b>E</b>) Negative control: chains of pro-akinetes with all cells displaying red fluorescence after omitting TdT, which detects the nicked DNA. Scale bars: A-E 10 µm; i-iv in <b>C</b> 1 µm.