Cathepsin X is a conserved cell death protein involved in algal response to environmental stress

Abstract

AbstractPhytoplankton play a crucial role in global primary production and can form vast blooms in aquatic ecosystems. Bloom demise and the rapid turnover of phytoplankton are suggested to involve programmed cell death (PCD) induced by diverse environmental stressors. However, fundamental knowledge of the PCD molecular components in algae and protists in general remains elusive. Previously, we revealed that early oxidation in the chloroplast predicted subsequent cell death or survival in isogenic subpopulations that emerged following H2O2treatment in the diatomPhaeodactylum tricornutum. Here, we performed transcriptome analysis of sorted sensitive oxidized cells and resilient reduced cells, to discover genes linked to their contrasting fates. By cross-comparison with a large-scale mutant screen in the green algaChlamydomonas reinhardtii, we identified functionally relevant conserved PCD gene candidates, including the cysteine protease cathepsin X/Z (CPX).CPXmutants inP. tricornutum CPX1andC. reinhardtii CEP12both exhibited profound resilience to oxidative stress, supporting a conserved function in algal PCD.P. tricornutum cpx1mutants, generated using CRISPR-Cas9, also exhibited resilience to the toxic diatom-derived infochemical cyanogen bromide. Phylogenetic and predictive structural analyses show thatCPXis highly conserved in eukaryotes, and algae of the green and red lineages exhibit strong structural similarity to human cathepsinCTSZ.CPXis expressed by diverse algae across the oceans and during toxicPseudo-nitzschiablooms, supporting its ecological importance. Elucidating PCD components in algae sheds light on the evolutionary origin of PCD in unicellular organisms, and on the cellular strategies employed by the population to cope with stressful conditions.