| Programmed cell death (PCD), as a conscious and cell-suicide activity, is controled by genes in the cells. There are several pathways of PCD, such as apoptosis, necrosis, and autophagy, were found in the animals and higher plants. Over the past years, a couple of labs worldwide have discovered that PCD-like phenomenons also occurred in the microorganisms, such as bacteria, including E.coli, Pseudomonas, free-living cyanobacteria etc.In present study, an attempt have been made to investigate PCD in the symbiotic cyanobacteria associated with a water fern Azolla microphylla Using the fluorescence microscope, transmission electron microscope as well as PCD related genes cloning and sequencing, we have, at morphological, cytological and molecular levels, demonstrated that some of cyanobacteiral cells within the leaf cavities and the megasporocarps of Azolla died through PCD process by apoptosis-like and autolysis pathways.Morphologically some of the symbiotic cyanobacteria cells were shown to appear typically ultrastructural characteristics of PCD, including cell shrinkage with membrane integrity. And some of inclusions and membrane systems within the cyanobacterial cells were degradated and lysed, providing cellular evidences of apoptosis-like and autolysis-like PCD in the symbiotic cyanobacterial cells.Through TUNEL assay and PCD-related gene PCR and sequencing, DNA fragmentation, the important characteristics of apoptosis, was confirmed to happen in the symbiotic cyanobacterias and the genes encoding metacaspase and responsible for autolysis were amplified successfully with 3 pairs of designed primers (Cas202, Cas203, Hcw3 / 4 ) from the cyanobactrial cells. The amplicons were sequenced and uploaded to GenBank, the accession numbers are FJ541186, FJ541185 and FJ541183.The results obtained from the present study suggested that PCD events originated from prokaryotes, no mater the free-living one or symbiont, which has evolved at least two PCD pathways: apoptosis-like and autolysis-like.
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