Expression levels of the Band-7 protein FLOTILLIN modulate salt tolerance, growth and development in the moss Physcomitrium patens

The Band-7 proteins, known as FLOTILLINs (FLOT), are present at the plasma membranes of most land plants. They function in clathrin-independent endocytosis and contribute to nodule formation following symbiotic infections. This study reveals that the single FLOT variant in Physcomitrium patens is located at the thylakoid membranes in chloroplasts, serving an unanticipated function. Phenotypic analysis of knockout and overexpression lines demonstrates that PpFLOT overexpression significantly impairs the high salinity tolerance of P. patens. Additionally, liquid protonema cultures of PpFLOT-OEX lines exhibited a distinct color change due to necrotic events and developed brachycyte-like cells. These changes correlate with the strength of PpFLOT expression and do not occur when these lines are cultivated on solid medium. Our study found that PpFLOT-OEX lines display increased chlorophyll and H2O2 production. We also discovered that PpFLOT is regulated by ABA and light, and its high expression can potentially affect retrograde signaling. Metabolomics and proteomics analyses revealed changes in the pigment and lipid composition as well as differentially accumulated proteins in PpFLOT mutant lines. We also observed changes in the expression of ion-transport related genes, accumulation of lipids crucial during pathogen defense, and differentially accumulated proteins taking part in multiple metabolomic pathways. Consequently, our study suggests a novel role for chloroplastic PpFLOT in plant terrestrialization, as it is putatively involved in Ca2+ and reactive oxygen species (ROS) signaling in response to abiotic and biotic stress, along with the light-dependent regulation of chlorophyll biosynthesis.