Evidence for UVinduced PCD in a Plant

The only publication dealing with a case of clear-cut apoptosis induced by ultraviolet is that by Danon and Gallois (1998). They irradiated either seedlings of Arabidopsis thaliana, or protoplasts prepared from them, with 1-50 kJm-2 of 254 nm radiation, and observed fragmentation of DNA which increased both with UV exposure and with time after exposure up to 12 h. Southern analysis revealed the "ladder" of DNA fragments with sizes in multiples of 180 base pairs, typical of apoptosis. Using the TUNEL reaction, fragmentation of DNA in protoplasts could be shown as soon as 2 h after irradiation. Also morphological changes of the nuclei similar to those observed in animal cells took place. It should be noted that the radiation doses needed to induce apoptosis in these plant cells were several hundred times larger than those needed in experiments with animal cell cultures.

Danon and Gallois (1998) propose that the apoptosis is triggered by DNA damage. One should not read into this that the primary radiation target necessarily is DNA. This question would have to be decided by action spectroscopy. Scheuerlein et al. (1995) studied UV-B-induced DNA degradation in Euglena. If this degradation, as the authors seem to believe, depends on the same photoreaction as UV-inhibition of motility in the same organism, the action spectrum (Hader and Liu, 1990) rules out DNA as the primary target. The DNA degradation observed inEuglena has in common with apoptosis in animal cells that it depends on nucleases activated by calcium and other divalent metal ions, and elevation of cytoplasmic [Ca2+] is probably one step in signal transduction.

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