Elsevier

Algal Research

Volume 12, November 2015, Pages 126-133
Algal Research

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

https://doi.org/10.1016/j.algal.2015.07.016Get rights and content

Highlights

  • Focuses on PCD pathways and signalling molecules involved in unicellular organisms

  • Provides a comprehensive insight into the early stages of PCD evolution

  • Elucidates PCD in C. reinhardtii, a unicellular model organism

  • Points towards the existence of different cell death pathways in this alga

  • Clearly shows the presence of metazoan PCD homologues in C. reinhardtii

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.

Section snippets

Introduction to apoptosis as a term and discovery

Every cell has a time to live and a time to die; death could be accidental due to injury or otherwise. Cell death was observed as a part of the physiological process of multicellular organisms such as plants and animals since the mid of the 19th century [1]. However, a form of non-accidental yet, controlled cell deletion/removal observed in the year 1964 was termed programmed cell death [2], [3]. Originally discovered in 1972 by Kerr and co-workers [4], if cells are no more needed, they die by

Apoptosis in metazoans: morphological features of the process

Programmed cell death in metazoans is characterized by several morphological and biochemical changes, typically referred to as hallmarks; these being, shrinkage of the cytoplasm, nuclear condensation, membrane blebbing, externalization of phoshatidylserine, mitochondrial membrane potential breakdown with the release of Cytochrome c, formation of apoptotic bodies with their engulfment by phagocytes, release of (at least in mammalian cells) ATP and UTP, and induction of cysteine proteases and

Comparison of the process between animals and plants

It is now well-established that parts of the apoptotic process are conserved across worms, insects, vertebrates [9] and plants [16], [17], [18], [19]. PCD in plants encompasses a diverse set of mechanisms from initiation of the trigger to cell death itself [20], [21], [22], [23]. Although the differences in the details of the mechanism between plants and animals are now clear, many cellular and molecular features still remain the same. The common morphological and biochemical features are loss

PCD in non-photosynthetic unicellular organisms

The first study on the presence of PCD in a unicellular organism was reported during the in vitro differentiation of the proliferating epimastigote stage into the G0/G1 arrested trypomastigote stage of the parasite Trypanosoma cruzi [37]. Subsequent reports have shown ConA-induced apoptotic death in Trypanosoma brucei rhodesiense [38], [39], [40], heat shock-induced apoptosis in Leishmania amazonenis [41], a variety of stress conditions (serum deprivation, heat shock and nitric oxide exposure)

Caspases and caspase-like proteases

Current literature shows the existence of three types of caspases viz., inflammatory, initiator and executioner caspases. The first ever caspase to be discovered was ICE, an inflammatory caspase. Immediately after this discovery, the role of caspases in PCD was sought, their multiplicity and the diverse pathways in which they participate followed next; and then, came the search for caspase and caspase-like proteases in lower organisms. In 2000, Uren and his associates [100] found several

Conclusion

Although unicellular organisms are now used as experimental model systems to study PCD, it is unclear why these own a suicide programme. The unicellular organism is a cell and organism; and, therefore, death by a suicidal process is indeed ironical. The moot question is whether these programmes contribute in any way to their evolutionary fitness. For the sake of simplicity and to understand the reasons of PCD evolution in single-celled organisms, the only way of explanation is speculation and

Acknowledgement

This work was funded by the Department of Atomic Energy, India.

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