Salt affects structure, function and transcriptome in the giant cells of slime molds

Salt is essential for life, though excessive intake disrupts the balance of body fluids in multicellular organisms like humans. We wondered what happens when body fluids circulate in a single cell. To address this question, we studied the effects of excessive salt on the network-forming giant cell of the slime mold Physarum polycephalum. We analyse the phenotypic and transcriptomic responses during exposure of plasmodia to various concentrations of sodium chloride. Morphological observations revealed alterations in the tubular network architecture, including reduced network vein diameter and changes in typical peristaltic contraction frequency. Transcriptomic analysis identified more than 2000 differentially expressed genes, with a significant upregulation of ion transporters, membrane proteins and stress-related pathways, alongside with a downregulation of genes involved in metabolic degradation. These findings suggest that P. polycephalum mitigates salt stress by regulating ion homeostasis, adjusting cytoskeletal dynamics and modulating gene expression related to cellular defence mechanisms. Possible alterations of the secreted slime coat and corresponding mechanical feedback mechanisms are discussed. The insights into the salt stress responses of P. polycephalum help to understand the salt response of a protist that is evolutionary basal to animals and fungi.