Involved in plant growth, and salt and osmotic tolerance (PubMed:10652125). Catalyzes the decarboxylation of 4'-phosphopantothenoylcysteine to 4'-phosphopantetheine, a key step in coenzyme A biosynthesis (PubMed:11279129, PubMed:11923307, PubMed:12860978, PubMed:16415216). The enzyme is also able to decarboxylate pantothenoylcysteine to pantothenoylcysteamine (PubMed:11923307). 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. Homotrimer. Expressed in roots, shoots, leaves, flowers, developing siliques and seeds with highest expression in seed embryos and phloem. Induced by salt stress. No visible phenotype under normal growth conditions, but homozygous double mutants hal3a-1 and hal3b are embryonic lethal. Belongs to the HFCD (homooligomeric flavin containing Cys decarboxylase) superfamily.