Sucrose-phosphatase (SPP; EC 3.1.3.24) catalyzes the final step in the pathway of sucrose biosynthesis and higher plants contain multiple isoforms of the enzyme encoded by different genes. The genome of the dicotyledonous plant Arabidopsis thaliana (thale cress) contains four SPP-like genes on chromosomes 1 (AtSPP1), 2 (AtSPP2) and 3 (AtSPP3a and AtSPP3b), all of which are expressed. The genome of the monocotyledonous plant rice (Oryza sativa) also contains four SPP-like genes, which have very similar exon-intron structures to those from A. thaliana. Two cDNA clones that encode catalytically active SPP enzymes have been isolated from maize (Zea mays), showing that this species contains at least two functional SPP genes. Multiple SPP-like cDNA clones have also been identified from wheat (Triticum aestivum), barley (Hordeum vulgare) and tomato (Lycopersicon esculentum). The genomes of two cyanobacteria, Synechocystis sp. PCC 6803 and Anabaena sp. PCC 7120, contain single spp genes. The cyanobacterial SPPs and the N-terminal region of the higher plant enzyme share significant similarity with members of the haloacid dehalogenase (HAD) superfamily of hydrolases/phosphatases. In addition to the HAD phosphatase domain, SPP from higher plants also contains a shorter, C-terminal domain of unknown function. An SPP-like sequence from the bryophyte (moss) Physcomitrella patens also contains this C-terminal domain, indicating that its acquisition was an early event in the evolution of higher plants.