Terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) is a method for detecting DNA fragmentation by labelling the 3' terminal end of nucleic acids. This method can be used both in animal and plant tissues. In animal tissues, the use of Proteinase K is sufficient for permeabilizing the cells and to obtain optimal labelling, but in plant tissues, the presence of the cell wall, does not allow proper labelling. For this reason, we carried out several modifications to the original TUNEL protocol (ApoAlert® DNA Fragmentation Assay Kit, Clontech) to obtain an optimal labelling. These modifications were additional treatments with cellulase, Triton X-100 and Proteinase K. Also, we describe the optimization of the positive controls by adjusting the units of DNase used. The PI concentration for counterstaining has been also specifically adjusted to avoid excessive background noise and hence to correctly observe both labeled and unlabelled nuclei. This work also describes an additional protocol to collect, store and include samples (specifically stigmatic arms) in such a way that they do not interfere with the TUNEL labelling.
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