The suppression subtractive hybridization technique was used to identify differentially expressed genes between hormone-autotrophic and hormone-dependent Panax ginseng callus lines. A tonoplast intrinsic protein cDNA (PgTIP1) was found to be highly and specifically expressed in hormone-autotrophic ginseng cells, which was slightly up-regulated by cytokinin while significantly down-regulated when treated with auxin. PgTIP1 encodes a polypeptide of 250 amino acids which shows sequence and structure similarity with tonoplast aquaporins in plants. The water channel activity of PgTIP1 was demonstrated by its expression in Xenopus laevis oocytes. When over-expressed in Arabidopsis thaliana, PgTIP1 substantially altered the plant's vegetative and reproductive growth and development. Arabidopsis plants over-expressing PgTIP1 showed significantly enhanced seed size and seed mass plus greatly increased growth rate compared with those of the wild type. Moreover, the seeds from PgTIP1 over-expressing Arabidopsis had 1.85-fold higher fatty acid content than the wild-type control. These results demonstrate a significant function of PgTIP1 in the growth and development of plant cells.