The RSI-1 gene is expressed in pericycle cells just prior to the first round of cell division for lateral root development in tomato. We transformed tomato plants with the RSI-1 gene promoter linked to a GUS reporter gene. GUS activity was detected not only at the sites of initiation for lateral and adventitious roots, but also at the primary roottip. Expression of the fusion gene was also regulated at various stages of tissue development: in particular, during the formation of reproductive organs such as pollen and fruit. Overexpression of the RSI-1 gene in either the sense or antisense orientation resulted in arrest of fruit development and seed germination. The RSI-1 gene product, therefore, may play a role in auxin-induced cell division in various developing tissues. Inter- and intramolecular disculfide bridges between cysteines rich in the RSI-1 protein might be involved in cell-wall modifications that are essential for new cell division. These hypotheses for the role of the RSI-1 gene in lateral-root and reproductive-organ development remain to be tested.