Catalyzes the decarboxylation of 4'-phosphopantothenoylcysteine to 4'-phosphopantetheine, a key step in coenzyme A biosynthesis (PubMed:19232050, PubMed:19543273). Involved in salt and osmotic tolerance, and light-regulated plant growth (PubMed:19543273). Trimerization of HAL3 recruits and activates the E3 ubiquitin-protein ligase HIP1, which leads to the degradation of cell cycle suppressors, resulting in enhancement of cell division and plant growth (PubMed:19543273). HAL3 function in cell division seems to be independent from its PPC decarboxylase activity (PubMed:19543273). Acts as a positive regulator of flowering by binding to HD1 in the dark (PubMed:26537047). H(+) + N-[(R)-4-phosphopantothenoyl]-L-cysteine = (R)-4'-phosphopantetheine + CO2 Binds 1 FMN per subunit. Cofactor biosynthesis; coenzyme A biosynthesis; CoA from (R)-pantothenate: step 3/5. Forms homotrimers (PubMed:19543273). Interacts with HIP1 (PubMed:19543273). Interacts with HD1 in the dark (PubMed:26537047). Expressed in root meristem, shoot apical meristem (SAM), intercalary meristem, floral meristem, embryo and tip of the coleoptile before true leaf emergence. Induced by dark. Seedlings over-expressing HAL3 show enhanced growth rate under normal light/dark cycles, enhanced salt tolerance and Na(+)/K(+) homeostasis. Enhanced growth is due to increased cell number of root meristematic zone and root cap (PubMed:19543273). Belongs to the HFCD (homooligomeric flavin containing Cys decarboxylase) superfamily.