Evidence for p53-like-mediated stress responses in green algae

The diel disconnect between cell growth and division in Aureococcus is interrupted by giant virus infection

Viruses of eukaryotic algae have become an important research focus due to their role(s) in nutrient cycling and top-down control of algal blooms. Omics-based studies have identified a boon of genomic and transcriptional potential among the Nucleocytoviricota, a phylum of large dsDNA viruses which have been shown to infect algal and non-algal eukaryotes. However, little is still understood regarding the infection cycle of these viruses, particularly in how they take over a metabolically active host and convert it into a virocell state. Of particular interest are the roles light and the diel cycle in virocell development. Yet despite such a large proportion of Nucleocytoviricota infecting phototrophs, little work has been done to tie infection dynamics to the presence, and absence, of light. Here, we examine the role of the diel cycle on the physiological and transcriptional state of the pelagophyte Aureococcus anophagefferens while undergoing infection by Kratosvirus quantuckense strain AaV. Our observations demonstrate how infection by the virus interrupts the diel growth and division of this cell strain, and that infection further complicates the system by enhancing export of cell biomass.

An RNA thermometer in the chloroplast genome of Chlamydomonas facilitates temperature-controlled gene expression

Riboregulators such as riboswitches and RNA thermometers provide simple, protein-independent tools to control gene expression at the post-transcriptional level. In bacteria, RNA thermometers regulate protein synthesis in response to temperature shifts. Thermometers outside of the bacterial world are rare, and in organellar genomes, no RNA thermometers have been identified to date. Here we report the discovery of an RNA thermometer in a chloroplast gene of the unicellular green alga Chlamydomonas reinhardtii. The thermometer, residing in the 5' untranslated region of the psaA messenger RNA forms a hairpin-type secondary structure that masks the Shine-Dalgarno sequence at 25°C. At 40°C, melting of the secondary structure increases accessibility of the Shine-Dalgarno sequence to initiating ribosomes, thus enhancing protein synthesis. By targeted nucleotide substitutions and transfer of the thermometer into Escherichia coli, we show that the secondary structure is necessary and sufficient to confer the thermometer properties. We also demonstrate that the thermometer provides a valuable tool for inducible transgene expression from the Chlamydomonas plastid genome, in that a simple temperature shift of the algal culture can greatly increase recombinant protein yields.

The Genetics of Fitness Reorganization during the Transition to Multicellularity: The Volvocine regA-like Family as a Model

The evolutionary transition from single-celled to multicellular individuality requires organismal fitness to shift from the cell level to a cell group. This reorganization of fitness occurs by re-allocating the two components of fitness, survival and reproduction, between two specialized cell types in the multicellular group: soma and germ, respectively. How does the genetic basis for such fitness reorganization evolve? One possible mechanism is the co-option of life history genes present in the unicellular ancestors of a multicellular lineage. For instance, single-celled organisms must regulate their investment in survival and reproduction in response to environmental changes, particularly decreasing reproduction to ensure survival under stress. Such stress response life history genes can provide the genetic basis for the evolution of cellular differentiation in multicellular lineages. The regA-like gene family in the volvocine green algal lineage provides an excellent model system to study how this co-option can occur. We discuss the origin and evolution of the volvocine regA-like gene family, including regA-the gene that controls somatic cell development in the model organism Volvox carteri. We hypothesize that the co-option of life history trade-off genes is a general mechanism involved in the transition to multicellular individuality, making volvocine algae and the regA-like family a useful template for similar investigations in other lineages.

Lipid ROS- and Iron-Dependent Ferroptotic Cell Death in Unicellular Algae Chlamydomonas reinhardtii

The phenomenon of heat stress leading to ferroptosis-like cell death has recently been observed in bacteria as well as plant cells. Despite recent findings, the evidence of ferroptosis, an iron-dependent cell death remains unknown in microalgae. The present study aimed to investigate if heat shock could induce reactive oxygen species (ROS) and iron-dependent ferroptotic cell death in Chlamydomonas reinhardtii in comparison with RSL3-induced ferroptosis. After RSL3 and heat shock (50 °C) treatments with or without inhibitors, Chlamydomonas cells were evaluated for cell viability and the induction of ferroptotic biomarkers. Both the heat shock and RSL3 treatment were found to trigger ferroptotic cell death, with hallmarks of glutathione-ascorbic acid depletion, GPX5 downregulation, mitochondrial dysfunction, an increase in cytosolic calcium, ROS production, lipid peroxidation, and intracellular iron accumulation via heme oxygenase-1 activation (HO-1). Interestingly, the cells preincubated with ferroptosis inhibitors (ferrostatin-1 and ciclopirox) significantly reduced RSL3- and heat-induced cell death by preventing the accumulation of Fe²⁺ and lipid ROS. These findings reveal that ferroptotic cell death affects the iron homeostasis and lipid peroxidation metabolism of Chlamydomonas, indicating that cell death pathways are evolutionarily conserved among eukaryotes.

A personal cost of cheating can stabilize reproductive altruism during the early evolution of clonal multicellularity

Understanding how cooperation evolved and is maintained remains an important and often controversial topic because cheaters that reap the benefits of cooperation without paying the costs can threaten the evolutionary stability of cooperative traits. Cooperation-and especially reproductive altruism-is particularly relevant to the evolution of multicellularity, as somatic cells give up their reproductive potential in order to contribute to the fitness of the newly emerged multicellular individual. Here, we investigated cheating in a simple multicellular species-the green alga Volvox carteri, in the context of the mechanisms that can stabilize reproductive altruism during the early evolution of clonal multicellularity. We found that the benefits cheater mutants can gain in terms of their own reproduction are pre-empted by a cost in survival due to increased sensitivity to stress. This personal cost of cheating reflects the antagonistic pleiotropic effects that the gene coding for reproductive altruism-regA-has at the cell level. Specifically, the expression of regA in somatic cells results in the suppression of their reproduction potential but also confers them with increased resistance to stress. Since regA evolved from a life-history trade-off gene, we suggest that co-opting trade-off genes into cooperative traits can provide a built-in safety system against cheaters in other clonal multicellular lineages.

A life-history trade-off gene with antagonistic pleiotropic effects on reproduction and survival in limiting environments

Although life-history trade-offs are central to life-history evolution, their mechanistic basis is often unclear. Traditionally, trade-offs are understood in terms of competition for limited resources among traits within an organism, which could be mediated by signal transduction pathways at the level of cellular metabolism. Nevertheless, trade-offs are also thought to be produced as a consequence of the performance of one activity generating negative consequences for other traits, or the result of genes or pathways that simultaneously regulate two life-history traits in opposite directions (antagonistic pleiotropy), independent of resource allocation. Yet examples of genes with antagonistic effects on life-history traits are limited. This study provides direct evidence for a gene-RLS1, that is involved in increasing survival in nutrient-limiting environments at a cost to immediate reproduction in the single-celled photosynthetic alga, Chlamydomonas reinhardtii. Specifically, we show that RLS1 mutants are unable to properly suppress their reproduction in phosphate-deprived conditions. Although these mutants have an immediate reproductive advantage relative to the parental strain, their long-term survival is negatively affected. Our data suggest that RLS1 is a bona fide life-history trade-off gene that suppresses immediate reproduction and ensures survival by downregulating photosynthesis in limiting environments, as part of the general acclimation response to nutrient deprivation in photosynthetic organisms.

The evolution of multicellularity and cancer: views and paradigms

Conceptually and mechanistically, the evolution of multicellularity required the integration of single cells into new functionally, reproductively and evolutionary stable multicellular individuals. As part of this process, a change in levels of selection occurred, with selection at the multicellular level overriding selection at the cell level. The stability of multicellular individuals is dependent on a combination of mechanisms that supress within-group evolution, by both reducing the occurrence of somatic mutations as well as supressing somatic selection. Nevertheless, mutations that, in a particular microenvironment, confer mutant lineages a fitness advantage relative to normal somatic cells do occur, and can result in cancer. This minireview highlights several views and paradigms that relate the evolution of multicellularity to cancer. As a phenomenon, cancer is generally understood as a failure of multicellular systems to suppress somatic evolution. However, as a disease, cancer is interpreted in different frameworks: (i) a breakdown of cooperative behaviors underlying the evolution of multicellularity, (ii) a disruption of molecular networks established during the emergence of multicellularity to impose constraints on single-celled units, or (iii) an atavistic state resulting from reactivating primitive programs that originated in the earliest unicellular species. A number of assumptions are common in all the views relating cancer as a disease to the evolution of multicellularity. For instance, cancer is considered a reversal to unicellularity, and cancer cells are thought to both resemble unicellular organisms and benefit from ancestral-like traits. Nevertheless, potential limitations of current paradigms should be acknowledged as different perspectives can provide novel insights with potential therapeutic implications.

Programmed cell death of Chlamydomonas reinhardtii induced by three cyanobacterial volatiles β-ionone, limonene and longifolene

β-Ionone, limonene and longifolene are 3 main components in cyanobacterial volatile organic compounds, which are formed through different pathways and can poison and even kill other algae. To uncover their toxic mechanism from programmed cell death (PCD), the photosynthetic pigments, chlorophyll fluorescence, caspase-like activities, cell size, nuclear variations and DNA ladders were investigated in Chlamydomonas reinhardtii treated with β-ionone (0.2 mM), limonene (0.2 mM) and longifolene (0.4 mM) at lethal concentration during 24 h. In the treatments with the 3 compounds, the photosynthetic pigments in C. reinhardtii cells gradually degraded, and Fv/Fm gradually decreased and disappeared at 24 h, suggesting that the cell death might be a PCD, due to the physiological activities gradually disappearing. During the cell death, the activities of caspase-9-like and caspase-3-like significantly increased, with the highest at 1 h. With prolonging the treatment time, C. reinhardtii cells gradually shrank, and the nuclei concentrated firstly following by a broken process, with moving to the cell edge. For DNA, obvious ladders were detected at 1 h, and then they gradually degraded to fragments of 100-250 bp at 24 h. These hallmarks suggested that β-ionone, limonene and longifolene may poison other algae by inducing PCD.

The genetic basis for the evolution of soma: mechanistic evidence for the co-option of a stress-induced gene into a developmental master regulator

In multicellular organisms with specialized cells, the most significant distinction among cell types is between reproductive (germ) cells and non-reproductive/somatic cells (soma). Although soma contributed to the marked increase in complexity of many multicellular lineages, little is known about its evolutionary origins. We have previously suggested that the evolution of genes responsible for the differentiation of somatic cells involved the co-option of life history trade-off genes that in unicellular organisms enhanced survival at a cost to immediate reproduction. In the multicellular green alga, Volvox carteri, cell fate is established early in development by the differential expression of a master regulatory gene known as regA. A closely related RegA-Like Sequence (RLS1) is present in its single-celled relative, Chlamydomonas reinhardtii. RLS1 is expressed in response to stress, and we proposed that an environmentally induced RLS1-like gene was co-opted into a developmental pathway in the lineage leading to V. carteri. However, the exact evolutionary scenario responsible for the postulated co-option event remains to be determined. Here, we show that in addition to being developmentally regulated, regA can also be induced by environmental cues, indicating that regA has maintained its ancestral regulation. We also found that the absence of a functional RegA protein confers increased sensitivity to stress, consistent with RegA having a direct or indirect role in stress responses. Overall, this study (i) provides mechanistic evidence for the co-option of an environmentally induced gene into a major developmental regulator, (ii) supports the view that major morphological innovations can evolve via regulatory changes and (iii) argues for the role of stress in the evolution of multicellular complexity.

Toxic mechanism of eucalyptol and β-cyclocitral on Chlamydomonas reinhardtii by inducing programmed cell death

Eucalyptol and β-cyclocitral are 2 main compounds in cyanobacterial volatile organic compounds and can poison other algae. To uncover the toxic mechanism of the 2 compounds, the cell growth, photosynthetic abilities, H₂O₂ production, caspase-like activities, nuclear variation and DNA laddering were investigated in Chlamydomonas reinhardtii treated with eucalyptol and β-cyclocitral. Eucalyptol at ≥ 0.1 mM and β-cyclocitral at ≥ 0.05 mM showed toxic effects on C. reinhardtii cells, and 1.2 mM eucalyptol and 0.4 mM β-cyclocitral killed the whole of the cells during 24 h. During the death process, the photosynthetic pigment gradually degraded, and Fv/Fm gradually declined, indicating that the death is not a necrosis due to the gradual disappearance of the physiological process. In the treatments with 1.2 mM eucalyptol and 0.4 mM β-cyclocitral, H₂O₂ content burst at 10 min and 30 min, respectively. Caspase-9-like and caspase-3-like were activated, and cell nucleuses concentrated firstly and then broke with prolonging the treatment time. Meanwhile, DNA showed laddering after 1 h, and was gradually cleaved by Ca²⁺-dependent endonucleases to mainly about 100-250 bp fragments. These hallmarks indicated that eucalyptol and β-cyclocitral may poison other algal cells by inducing programmed cell death triggered by the increased H₂O₂.

Integrative omics analysis in Pandanus odorifer (Forssk.) Kuntze reveals the role of Asparagine synthetase in salinity tolerance

Pandanus odorifer (Forssk) Kuntze grows naturally along the coastal regions and withstands salt-sprays as well as strong winds. A combination of omics approaches and enzyme activity studies was employed to comprehend the mechanistic basis of high salinity tolerance in P. odorifer. The young seedlings of P. odorifer were exposed to 1 M salt stress for up to three weeks and analyzed using RNAsequencing (RNAseq) and LC-MS. Integrative omics analysis revealed high expression of the Asparagine synthetase (AS) (EC 6.3.5.4) (8.95 fold) and remarkable levels of Asparagine (Asn) (28.5 fold). This indicated that salt stress promoted Asn accumulation in P. odorifer. To understand this further, the Asn biosynthesis pathway was traced out in P. odorifer. It was noticed that seven genes involved in Asn bisynthetic pathway namely glutamine synthetase (GS) (EC 6.3.1.2) glutamate synthase (GOGAT) (EC 1.4.1.14), aspartate kinase (EC 2.7.2.4), pyruvate kinase (EC 2.7.1.40), aspartate aminotransferase (AspAT) (EC 2.6.1.1), phosphoenolpyruvate carboxylase (PEPC) (EC 4.1.1.31) and AS were up-regulated under salt stress. AS transcripts were most abundant thereby showed its highest activity and thus were generating maximal Asn under salt stress. Also, an up-regulated Na+/H+ antiporter (NHX1) facilitated compartmentalization of Na+ into vacuoles, suggesting P. odorifer as salt accumulator species.

Growth of Cyanobacterium aponinum influenced by increasing salt concentrations and temperature

The increasing requirement of food neutral biofuels demands the detection of alternative sources. The use of non-arable land and waste water streams is widely discussed in this regard. A Cyanobacterium was isolated on the area of a possible algae production side near a water treatment plant in the arid desert region al-Wusta. It was identified as Cyanobacterium aponinum PB1 and is a possible lipid source. To determine its suitability of a production process using this organism, a set of laboratory experiments were performed. Its growth behavior was examined in regard to high temperatures and increasing NaCl concentrations. A productivity of 0.1 g L-1 per day was measured at an alga density below 0.75 g L-1. C. aponinum PB1 showed no sign of altered growth behavior in media containing 70 g L-1 NaCl or less. Detection of a negative effect of NaCl on the growth using Pulse-Amplitude-Modulation chlorophyll fluorescence analysis was not more sensitive than optical density measurement.

Mastoparan-induced programmed cell death in the unicellular alga Chlamydomonas reinhardtii

BACKGROUND AND AIMS

Under stress-promoting conditions unicellular algae can undergo programmed cell death (PCD) but the mechanisms of algal cellular suicide are still poorly understood. In this work, the involvement of caspase-like proteases, DNA cleavage and the morphological occurrence of cell death in wasp venom mastoparan (MP)-treated Chlamydomonas reinhardtii were studied.

METHODS

Algal cells were exposed to MP and cell death was analysed over time. Specific caspase inhibitors were employed to elucidate the possible role of caspase-like proteases. YVADase activity (presumably a vacuolar processing enzyme) was assayed by using a fluorogenic caspase-1 substrate. DNA breakdown was evaluated by DNA laddering and Comet analysis. Cellular morphology was examined by confocal laser scanning microscopy.

KEY RESULTS

MP-treated C. reinhardtii cells expressed several features of necrosis (protoplast shrinkage) and vacuolar cell death (lytic vesicles, vacuolization, empty cell-walled corpse-containing remains of digested protoplast) sometimes within one single cell and in different individual cells. Nucleus compaction and DNA fragmentation were detected. YVADase activity was rapidly stimulated in response to MP but the early cell death was not inhibited by caspase inhibitors. At later time points, however, the caspase inhibitors were effective in cell-death suppression. Conditioned medium from MP-treated cells offered protection against MP-induced cell death.

CONCLUSIONS

In C. reinhardtii MP triggered PCD of atypical phenotype comprising features of vacuolar and necrotic cell deaths, reminiscent of the modality of hypersensitive response. It was assumed that depending on the physiological state and sensitivity of the cells to MP, the early cell-death phase might be not mediated by caspase-like enzymes, whereas later cell death may involve caspase-like-dependent proteolysis. The findings substantiate the hypothesis that, depending on the mode of induction and sensitivity of the cells, algal PCD may take different forms and proceed through different pathways.

Acetic acid-induced programmed cell death and release of volatile organic compounds in Chlamydomonas reinhardtii

Acetic acid widely spreads in atmosphere, aquatic ecosystems containing residues and anoxic soil. It can inhibit aquatic plant germination and growth, and even cause programmed cell death (PCD) of yeast. In the present study, biochemical and physiological responses of the model unicellular green algae Chlamydomonas reinhardtii were examined after acetic acid stress. H(2)O(2) burst was found in C. reinhardtii after acetic acid stress at pH 5.0 for 10 min. The photosynthetic pigments were degraded, gross photosynthesis and respiration were disappeared gradually, and DNA fragmentation was also detected. Those results indicated that C. reinhardtii cells underwent a PCD but not a necrotic, accidental cell death event. It was noticed that C. reinhardtii cells in PCD released abundant volatile organic compounds (VOCs) upon acetic acid stress. Therefore, we analyzed the VOCs and tested their effects on other normal cells. The treatment of C. reinhardtii cultures with VOCs reduced the cell density and increased antioxidant enzyme activity. Therefore, a function of VOCs as infochemicals involved in cell-to-cell communication at the conditions of applied stress is suggested.

On Programmed Cell Death in Plasmodium falciparum: Status Quo

Conflicting arguments and results exist regarding the occurrence and phenotype of programmed cell death (PCD) in the malaria parasite Plasmodium falciparum. Inconsistencies relate mainly to the number and type of PCD markers assessed and the different methodologies used in the studies. In this paper, we provide a comprehensive overview of the current state of knowledge and empirical evidence for PCD in the intraerythrocytic stages of P. falciparum. We consider possible reasons for discrepancies in the data and offer suggestions towards more standardised investigation methods in this field. Furthermore, we present genomic evidence for PCD machinery in P. falciparum. We discuss the potential adaptive or nonadaptive role of PCD in the parasite life cycle and its possible exploitation in the development of novel drug targets. Lastly, we pose pertinent unanswered questions concerning the PCD phenomenon in P. falciparum to provide future direction.

On the paradigm of altruistic suicide in the unicellular world

Altruistic suicide is best known in the context of programmed cell death (PCD) in multicellular individuals, which is understood as an adaptive process that contributes to the development and functionality of the organism. After the realization that PCD-like processes can also be induced in single-celled lineages, the paradigm of altruistic cell death has been extended to include these active cell death processes in unicellular organisms. Here, we critically evaluate the current conceptual framework and the experimental data used to support the notion of altruistic suicide in unicellular lineages, and propose new perspectives. We argue that importing the paradigm of altruistic cell death from multicellular organisms to explain active death in unicellular lineages has the potential to limit the types of questions we ask, thus biasing our understanding of the nature, origin, and maintenance of this trait. We also emphasize the need to distinguish between the benefits and the adaptive role of a trait. Lastly, we provide an alternative framework that allows for the possibility that active death in single-celled organisms is a maladaptive trait maintained as a byproduct of selection on pro-survival functions, but that could-under conditions in which kin/group selection can act-be co-opted into an altruistic trait.

Salt stress-induced cell death in the unicellular green alga Micrasterias denticulata

Programmed cell death (PCD) is a key element in normal plant growth and development which may also be induced by various abiotic and biotic stress factors including salt stress. In the present study, morphological, biochemical, and physiological responses of the theoretically immortal unicellular freshwater green alga Micrasterias denticulata were examined after salt (200 mM NaCl or 200 mM KCl) and osmotic stress induced by iso-osmotic sorbitol. KCl caused morphological changes such as cytoplasmic vacuolization, extreme deformation of mitochondria, and ultrastructural changes of Golgi and ER. However, prolonged salt stress (24 h) led to the degradation of organelles by autophagy, a special form of PCD, both in NaCl- and KCl-treated cells. This was indicated by the enclosure of organelles by ER-derived double membranes. DNA of NaCl- and KCl-stressed cells but not of sorbitol-treated cells showed a ladder-like pattern on agarose gel, which means that the ionic rather than the osmotic component of salt stress leads to the activation of the responsible endonuclease. DNA laddering during salt stress could be abrogated by addition of Zn(2+). Neither cytochrome c release from mitochondria nor increase in caspase-3-like activity occurred after salt stress. Reactive oxygen species could be detected within 5 min after the onset of salt and osmotic stress. Respiration, photosynthetic activity, and pigment composition indicated an active metabolism which supports programmed rather than necrotic cell death in Micrasterias after salt stress.