Carrier of the growing fatty acid chain in fatty acid biosynthesis. 4'-phosphopantetheine is transferred from CoA to a specific serine of apo-ACP by acpS. This modification is essential for activity because fatty acids are bound in thioester linkage to the sulfhydryl of the prosthetic group (By similarity). Plants over-expressing ACP1 show altered composition of fatty acids, with significant decrease in levels of hexadecatrienoic acid (16:3) and increase in linolenate (18:3) content. Belongs to the acyl carrier protein (ACP) family.