Fatty acid desaturase involved in the production of chloroplast-specific phosphatidylglycerol molecular species. Catalyzes the formation of a trans double bond introduced close to the carboxyl group of palmitic acid, which is specifically esterified to the sn-2 glyceryl carbon of phosphatidylglycerol (By similarity). Lipid metabolism; fatty acid metabolism. Belongs to the fatty acid desaturase CarF family.