The biosynthesis of phytochelatins (PCs) and homophytochelatins (hPCs) has been studied in nodulated plants of the model legume Lotus (Lotus japonicus). In the first 6-24 h of treatment with cadmium (Cd), roots started to synthesize elevated amounts of both polypeptides, with a concomitant increase of glutathione and a decrease of homoglutathione, indicating the presence of active phytochelatin synthase (PCS) genes. Screening of transformation artificial competent chromosome libraries allowed the identification of a cluster of three genes, LjPCS1, LjPCS2, and LjPCS3, which were mapped at 69.0 centimorgans on chromosome 1. The genes differ in exon-intron composition and responsiveness to Cd. Gene structures and phylogenetic analysis of the three protein products, LjPCS1-8R, LjPCS2-7N, and LjPCS3-7N, are consistent with two sequential gene duplication events during evolution of vascular plants. Two sites for alternative splicing in the primary transcripts were identified. One of them, involving intron 2 of the LjPCS2 gene, was confirmed by the finding of the two predicted mRNAs, encoding LjPCS2-7R in roots and LjPCS2-7N in nodules. The amino acid sequences of LjPCS2-7R (or LjPCS2-7N) and LjPCS3-7N share 90% identity, but have only 43-59% identity with respect to the typical PCS1 enzymes of Lotus and other plants. The unusual LjPCS2-7N and LjPCS3-7N proteins conferred Cd tolerance when expressed in yeast cells, whereas the alternatively spliced form, LjPCS2-7R, differing only in a five amino acid motif (GRKWK) did not. These results unveil complex regulatory mechanisms of PCS expression in legume tissues in response to heavy metals and probably to other developmental and environmental factors.This work was supported by the Ministerio de Educación y Ciencia-Fondos Europeos de Desarrollo Regional (grant no. AGL2005-01404), the European Commission (grant no. FP6-2003-INCO-DEV2-517617), and Gobierno de Aragón-Fondo Social Europeo (group E33).Peer reviewed