Phosphatidic acid (PA) is a precursor metabolite for phosphoglycerolipids and also for galactoglycerolipids, which are essential lipids for formation of plant membranes. PA has in addition a main regulatory role in a number of developmental processes notably in the response of the plant to environmental stresses. We review here the different pools of PA dispatched at different locations in the plant cell and how these pools are modified in different growth conditions, particularly during plastid membrane biogenesis and when the plant is exposed to phosphate deprivation. We analyze how these modifications can affect galactolipid synthesis by tuning the activity of MGD1 enzyme allowing a coupling of phospho- and galactolipid metabolisms. Some mechanisms are considered to explain how physicochemical properties of PA allow this lipid to act as a central internal sensor in plant physiology. ► Phosphatidic acid is an intermediate in both phospholipid and galactolipid syntheses. ► We review the pools of phosphatidic acid dispatched in the plant cell membranes. ► We examine how modification of PA pools affects plant cell biology. ► PA regulates the essential MGDG synthase in formation of photosynthetic membranes.