One of the major characteristics of chloroplast membranes is their enrichment in galactoglycerolipids, monogalactosyldiacylglycerol (MGDG), and digalactosyldiacylglycerol (DGDG), whereas phospholipids are poorly represented, mainly as phosphatidylglycerol (PG). All these lipids are synthesized in the chloroplast envelope, but galactolipid synthesis is also partially dependent on phospholipid synthesis localized in non-plastidial membranes. MGDG synthesis was previously shown essential for chloroplast development. In this report, we analyze the regulation of MGDG synthesis by phosphatidic acid (PA), which is a general precursor in the synthesis of all glycerolipids and is also a signaling molecule in plants. We demonstrate that under physiological conditions, MGDG synthesis is not active when the MGDG synthase enzyme is supplied with its substrates only, i.e. diacylglycerol and UDP-gal. In contrast, PA activates the enzyme when supplied. This is shown in leaf homogenates, in the chloroplast envelope, as well as on the recombinant MGDG synthase, MGD1. PG can also activate the enzyme, but comparison of PA and PG effects on MGD1 activity indicates that PA and PG proceed through different mechanisms, which are further differentiated by enzymatic analysis of point-mutated recombinant MGD1s. Activation of MGD1 by PA and PG is proposed as an important mechanism coupling phospholipid and galactolipid syntheses in plants.Chloroplast membrane lipids are mostly composed of nonphosphorous galactoglycerolipids i.e. monogalactosyldiacylglycerol (MGDG) 2 and digalactosyldiacylglycerol (DGDG) that represent more than 50 and 30% of thylakoid lipids, respectively. Phosphatidylglycerol is the main phospholipid in plastids, representing about 10% of thylakoid lipids. Each of those glycerolipid classes is represented by a range of molecular species differing in the acyl composition at sn-1 and sn-2 positions of the glycerol backbone. Based on the model of cyanobacterial lipids, the prokaryotic type of glycerolipids contains a 16-carbon fatty acid at the sn-2 position of glycerol. The eukaryotic type contains an 18-carbon fatty acid at the sn-2 position. Some plants, such as Arabidopsis or spinach, have both prokaryoticand eukaryotic-type MGDG, whereas other plants, such as pea or cucumber, have only eukaryotic-type MGDG. DGDG is mostly of eukaryotic type in all plants. Chloroplast PG contains exclusively a prokaryotic-type DAG moiety in contrast to nonplastidial PG. These different chloroplast lipids are assembled in the chloroplast envelope (1). Most enzymes have now been identified, but their functioning and regulation remain largely unknown.MGDG is synthesized by MGDG synthase (MGD), which transfers galactose from UDP-gal to DAG. In Arabidopsis, there are three MGDG synthases, and among them, MGD1, is necessary for synthesis of galactolipids and for development of photosynthetic membranes (2, 3). MGD1 utilizes DAG derived from two main sources, either from a DAG de novo synthesized in plastid envelope by double acylation of glycer...