During my thesis, I explored the function of an anionic lipid, phosphatidylinositol 4-phosphate (PI4P), in Arabidopsis thaliana. I focused on the function of SUPPRESSOR OF ACTIN7 (SAC7), a PI4P phosphatase. I found that SAC7 plays a pivotal role in regulating tissue-specific PI4P levels in root. I showed that SAC7 localizes in the cortical endoplasmic reticulum (ER), in close proximity with the plasma membrane (PM). ER/PM contact sites are maintained thanks to the action of tethering proteins that are embedded in the ER and contact the PM through lipid binding. I showed that in plants, PI4P plays a critical role in the establishment of ER/PM contacts via its interaction with two families of tethering proteins, the SYNAPTOTAGMINs (SYT) and MULTIPLE C2 DOMAINS PROTEIN (MCTPs). In a first study, I showed that SAC7 controls the density and dynamics of SYT1-mediated ER/PM contacts. Furthermore, through its tissue-specific expression, SAC7 contributes to shape very different ER morphologies in the different cell types of the root epidermis. In a second study, I discovered SAC7's involvement in plasmodesmata biology. Plasmodesmata are membrane lined channels that are unconventional ER/PM contact sites. These structures create cytoplasmic sleeves between neighboring cells, enabling cell-to-cell diffusion of developmental regulators. I found that SAC7 regulates MCTP stabilization at plasmodesmata, likely affecting ER/PM tethering in these structures. As such, SAC7 affects cytosolic diffusion in a tissue specific manner. Together, my work and a companion study from the Bayer lab, propose that the strength of MCTP/PI4P interaction controls the ER/PM apposition in plasmodesmata and thus the opening or closing of the cytoplasmic sleeve. This mode of regulation is independent of the well-known plasmodesmata regulator callose and provides a new paradigm involving membrane contact sites in the regulation of intercellular communication.