Functional stress responses in Glaucophyta: evidence of ethylene and abscisic acid functions inCyanophora paradoxa

Abstract

AbstractGlaucophytes, an enigmatic group of freshwater algae, occupy a pivotal position within the Archaeplastida, providing insights into the early evolutionary history of plastids and their host cells. These algae possess unique plastids, known as cyanelles, that retain certain ancestral features, enabling a better understanding of the plastid transition from cyanobacteria. In this study, we investigated the role of ethylene, a potent hormone used by land plants to coordinate stress responses, in the glaucophyte algaCyanophora paradoxa. We demonstrate thatC. paradoxaconverts 1-aminocyclopropane-1-carboxylic acid (ACC), the ethylene precursor in land plants, into gaseous ethylene. In addition, we show that cells produce ethylene natively in response to abiotic stress, and another plant hormone, abscisic acid (ABA), negatively interferes with ACC-dependent ethylene synthesis, while positively regulating reactive oxygen species (ROS) accumulation. ROS synthesis also occurred following abiotic stress and ACC treatment, leading to growth inhibition inC. paradoxa. Using transcriptomics, we also reveal that ACC treatment induces the upregulation of senescence-associated proteases, consistent with the observation of growth inhibition. This is the first report of hormone usage in a glaucophyte alga, extending our understanding of hormone-mediated stress response coordination to the base of Archaeplastidae. By unraveling hormone signaling pathways in glaucophytes, we pave the way for a more comprehensive understanding of the evolution and diversification of hormone-mediated communication across the tree of life, ultimately enhancing our knowledge of the adaptive strategies employed by diverse lineages.