Lateral root formation was examined following the expression of a fission yeast mitotic regulator gene, cdc25, under the control of a tetracycline-inducible promoter in cultured roots of tobacco. Over expression of cdc25 in fission yeast results in premature cell division at a reduced cell size. Our aim was to examine whether cdc25 expression would affect cell size in the tobacco roots, and what effect this would have on lateral root morphogenesis. Transgene integration was confirmed by Southern blotting; it was inherited as a dominant Mendelian trait. Conditions for optimal expression, determined using plants transformed with gus under the control of the same promoter, were: addition of tetracycline (5 micrograms/ml) every 72 h, to cultured roots in Murashige-Skoog liquid medium in darkness at 27 degrees C. After the addition of tetracycline, cdc25 transcripts were detected using RT-PCR, initially after 48 h, and more strongly after 72 h. Appearance of cdc25 transcripts was followed by major changes in the roots. Compared with controls, lateral root primordia were initiated more frequently, were significantly smaller and comprised smaller cells at mitosis. However, cdc25 expression did not perturb normal development of the lateral roots. The data are consistent with cdc25 expression leading to a greater frequency of lateral root primordium formation and establishing a new threshold size for cell division in the primordia which was then maintained throughout subsequent development.