Salt-stress-induced association of phosphatidylinositol 4,5-bisphosphate with clathrin-coated vesicles in plants

Plants exposed to hyperosmotic stress undergo changes in membrane dynamics and lipid composition to maintain cellular integrity and avoid membrane leakage. Various plant species respond to hyperosmotic stress with transient increases in PtdIns(4,5)P₂; however, the physiological role of such increases is unresolved. The plasma membrane represents the outermost barrier between the symplast of plant cells and its apoplastic surroundings. In the present study, the spatio-temporal dynamics of stress-induced changes in phosphoinositides were analysed in subcellular fractions of Arabidopsis leaves to delineate possible physiological roles. Unlabelled lipids were separated by TLC and quantified by gas-chromatographic detection of associated fatty acids. Transient PtdIns(4,5)P₂ increases upon exposure to hyperosmotic stress were detected first in enriched plasmamembrane fractions, however, at later time points, PtdIns(4,5)P₂ was increased in the endomembrane fractions of the corresponding two-phase systems. When major endomembranes were enriched from rosette leaves prior to hyperosmotic stress and during stimulation for 60 min, no stress-induced increases in the levels of PtdIns(4,5)P₂ were found in fractions enriched for endoplasmic reticulum, nuclei or plastidial membranes. Instead, increased PtdIns(4,5)P₂ was found in CCVs (clathrin-coated vesicles), which proliferated several-fold in mass within 60 min of hyperosmotic stress, according to the abundance of CCV-associated proteins and lipids. Monitoring the subcellular distribution of fluorescence-tagged reporters for clathrin and PtdIns(4,5)P₂ during transient co-expression in onion epidermal cells indicates rapid stress-induced co-localization of clathrin with PtdIns(4,5)P₂ at the plasma membrane. The results indicate that PtdIns(4,5)P₂ may act in stress-induced formation of CCVs in plant cells, highlighting the evolutionary conservation of the phosphoinositide system between organismic kingdoms.