The seeds of tomatoes transformed with a bacterial ACC deaminase gene under the transcriptional control of either the cauliflower mosiac virus 355 promoter, the rolD (root specific) promoter or the prb-1b (pathogenesis related) promoter, and the seeds of nontransformed (NT) tomato plants were germinated in the presence or absence of ACC in the dark, in the light, or in the presence of UV-B light for 12 days. Roots grew to a greater length under UV-B than in the light or dark, in the absence of ACC for all of the plant lines tested. In contrast, root growth was inhibited by ACC under all conditions tested. The 35S/ACC deaminase transgenic seedlings had the longest hypocotyl, the rolD/ACC deaminase had intermediate sized hypocotyls and the NT and the prb-1b/ACC deaminase had the shortest hypocotyl in the presence of ACC in the dark. Hypocotyl growth was inhibited by both light and UV-B in both the absence and presence of ACC in NT and transformed plants. Taken together these results indicate that root growth is more sensitive to applied ACC (the precursor of ethylene) than is hypocotyl growth under all of the light regimes tested.