In animal cells, regulated cell death (RCD) is a well defined and molecularly characterised process that is orchestrated by proteolytic action of various types of caspase enzymes. RCD plays a vital role in organism's development and in response to a vast array of different environmental stress conditions. Similar also applies to plants, despite the fact that these organisms lack caspase enzymes. Instead, plants in their genomes carry genes which encode metacaspases. Like caspases these proteases are cysteine proteases as well but they cleave different types of substrates. Metacaspases are believed to be also involved in plant RCD, however their precise role has not been fully characterised yet.
We induced cell death in the unicellular algal model organism Chlamydomonas reinhardtii using two different stress agents, hydrogen peroxide (H2O2) and salt (NaCl). After treatment we monitored different parameters of cell response such as: changes in growth, cell viability, genomic DNA stability and metacaspase protease activity. Cell growth and viability were both negatively correlated with concentration and time of exposure to stress agents. H2O2-treated cells exhibited orderly and continuous fragmentation of DNA. This occurence is termed laddering. DNA of cells exposed to NaCl displayed DNA smear on gel or it remained intact. Cells exposed to NaCl also gradually exhibited decline of metacaspase-like enzyme activity, whereas those that were exposed to H2O2 displayed some metacaspase-like activity, however without any obvious correlation to concentration. Based on literature we deduced that NaCl-treated cells underwent necrotic cell death, whereas H2O2-treated cells experienced RCD.
When a unicellular organism undergoes RCD this leads to what is effectively suicide. Since such behaviour is not in line with the idea of selfish individual, it therefore makes sense only if it is beneficial to the survival of the population as a whole. After treatment of cell cultures with stress agents we transferred cells into fresh TAP medium and monitored their growth or formation of colonies. Since our cultures got contaminated, the results are inconclusive, however regardless of the stress agent, Chlamydomonas cells were not able to reestablish a healthy culture. Growth on TAP plates displayed more definite results, enabling the growth of cells that were treated with H2O2 but not of those that were treated with salt.
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