We have previously shown that gibberellin (GA) is required for tissue-reunion in the cortex of cut hypocotyls in cucumber and tomato seedlings, and that cotyledons are necessary for the tissue-reunion in hypocotyls. In this study, we analyzed expression of GA biosynthesis genes and endogenous GA contents to examine the contribution of cotyledons to providing GAs for the tissuereunion in hypocotyls. Quantitative RT-PCR analysis indicated that transcripts of LeGA_<20>ox1 and LeGA3ox2 genes (encoding GA 20-oxidase and GA 3-oxidase, respectively) accumulate at relatively high levels in cotyledons in tomato seedlings. The levels of LeGA20ox1 and LeGA3ox2 transcripts in the hypocotyl increased after the removal of cotyledons. To study the role of cotyledons in GA content in the hypocotyl directly, cotyledons were removed from 7-day-old cucumber seedlings, and endogenous GA levels in the hypocotyls were determined. Relative to intact seedlings, those without cotyledons contained reduced amounts of bioactive GA_4, and its precursors. Accumulation of ent-kaurene after uniconazole treatment was also reduced in the hypocotyl when cotyledons were removed. Feeding experiments showed that a local application of uniconazole to cotyledons was effective to inhibit the hypocotyl elongation. Simultaneous application of ent-kaurenoic acid or GA_4, but not ent-kaurene, canceled the inhibitory effect of uniconazole on hypocotyl elogation. These results suggest that cotyledons play a role in determining GA content in cucumber and tomato hypocotyls.