Apoptosis-like cell death in unicellular photosynthetic organisms — A review

Abstract

The study of cell death in higher plants and animals has revealed the existence of an active (‘programmed’) suicidal process and similarities in this machinery between these two kingdoms suggest an evolutionarily ancient origin. Genetic, molecular and biochemical approaches have begun to reveal candidate regulators in plants that show both similarities and novel properties compared with their animal counterparts. In recent years, it has also been convincingly shown that several unicellular organisms from bacteria to ciliates possess the ability to undergo programmed death. In particular, the unicellular algal cells Chlamydomonas reinhardtii, Micrasterias denticulata, Chlorella saccharophila and Dunaliella tertiolecta as well as several protozoans, die in an apoptotic-like manner in response to exposure to various stress agonists. And, these organisms share several apoptotic hallmarks with metazoans. To date, three related issues that remain elusive in these unicellular organisms are, the signalling pathway and the key molecular players involved in this process as well as the precise physiological role (or requirement) of this cellular mechanism. Currently, there seem no convincing arguments about the evolutionary significance of such a death pathway of unicellular organisms. The past decade has seen a rise in the number of reports on PCD-like process in unicellular photosynthetic organisms. This review provides a historical perspective of the occurrence of algal PCD and lists out the molecular players while comparing them with metazoans and multicellular plants.