Carrier of the growing fatty acid chain in fatty acid biosynthesis. A number of isoforms are produced. According to EST sequences. By sucrose in the dark. Down-regulated by starvation. 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). Belongs to the acyl carrier protein (ACP) family.