Catalyzes the NADPH-dependent reduction of beta-ketoacyl-ACP substrates to beta-hydroxyacyl-ACP products, the first reductive step in the elongation cycle of fatty acid biosynthesis (Probable). Is capable of reducing acetoacetyl-CoA, but less well than its paralog PhaB (PubMed:26358291). a (3R)-hydroxyacyl-[ACP] + NADP(+) = a 3-oxoacyl-[ACP] + H(+) + NADPH Lipid metabolism; fatty acid biosynthesis. Mutation of key residues to match those found in its paralog PhaB increases it activity on acetoacetyl-CoA, not its usual substrate, which might help shift cellular metabolism to make poly(3-hydroxybutyrate) (PHB), a compound that has potential uses as a renewable, biodegradable thermoplastic (PubMed:26358291). Poly(3-hydroxyalkanoic acids) (PHA), of which PHB is among the most common compounds, are prokaryotic intracellular storage compounds with potential uses as a renewable, biodegradable thermoplastic. Cyanobacterial PHB synthesis is particularly attractive as cyanobacteria use CO(2) as the carbon source. Belongs to the short-chain dehydrogenases/reductases (SDR) family.