Catalyzes a base-exchange reaction in which the polar head group of phosphatidylethanolamine (PE) or phosphatidylcholine (PC) is replaced by L-serine. Is essential for phosphatidylserine (PS) biosynthesis and PE seems to be the most plausible substrate. Plays an important role in microspore maturation. a CDP-1,2-diacyl-sn-glycerol + L-serine = a 1,2-diacyl-sn-glycero-3-phospho-L-serine + CMP + H(+) Phospholipid metabolism; phosphatidylethanolamine biosynthesis; phosphatidylethanolamine from CDP-diacylglycerol: step 1/2. Mainly localized in nuclei and ER membranes during pollen development. A number of isoforms are produced. According to EST sequences. Expressed in trichomes, leaf veins and root vasculature. Expressed in anthers and microspores at floral stages 9 to 12, tapetum at floral stages 7 to 11, mature embryos at 5 days after flowering and dry seed embryos. Mostly embryonic lethality with low frequencies of dwarf infertile plants. Belongs to the CDP-alcohol phosphatidyltransferase class-I family.