Calcium deprivation enhances non-selective fluid-phase endocytosis and modifies membrane lipid profiles in Arabidopsis roots

Ecological studies have revealed significant decreases in calcium (Ca) levels in various soils, and widely occurring physiological Ca deficits worldwide. These changes may cause decreases in plant diversity and increases in plant vulnerability to environmental stress, but the underlying cellular mechanism is not well understood. In this study, we found that in Arabidopsis thaliana roots, deprivation of Ca2+, but not other minerals, dramatically enhanced plasma membrane invagination, endosome formation, and trafficking to the vacuole through the trans-Golgi network and pre-vacuole compartment, a typical pathway of endocytosis. Antagonist and cellular tracing analyses using non-bioactive, membrane-impermeable fluorescent probes indicated that this type of endocytosis is not regulated by receptors, instead representing a non-selective, non-specific fluid phase-based process. We performed lipid-profiling analysis of roots in response to Ca2+ deprivation, finding increased phosphatidylcholine (PC), Lyso-PC, phosphatidylethanolamine (PE), Lyso-PE, phosphatidylinositol (PI) and triacylglycerols (TAG) biosynthesis but deceased phosphatidic acid (PA) and diacylglycerols (DAG) biosynthesis. The increased TAG contents and molar ratio of PC/PE in these roots might reflect a cellular response to maintain membrane stability and the balance between the membrane and storage lipids. This study demonstrates the essential role of Ca in maintaining plasma membrane stability and the selectivity of plant root cells, and it highlights the potential deleterious effect of decreased Ca levels in surface soil on plant growth.