Effects of the removal of cotyledons on endogenous gibberellin levels in hypocotyls of young cucumber and tomato seedlings

We have previously shown that gibberellin (GA) is required for tissue-reunion in the cortex of cut hypocotyls in cucumber (Cucumis sativus) and tomato (Solanum lycopersicum) seedlings, and that intact cotyledons are also necessary for this process. These results suggested that cotyledons might play an important role in controlling GA levels in the hypocotyl in these plant species. In this study, we found that a local application of a GA biosynthesis inhibitor uniconazole to cotyledons was effective to inhibit hypocotyl elongation, and that simultaneous application of GA canceled this inhibition. To study the role of cotyledons in GA content in the hypocotyl directly, cotyledons were removed from 7-days-old seedlings and endogenous GA levels in the hypocotyl were determined. Our results demonstrated that the cotyledon-removed seedlings contained lower levels of bioactive GAs and their precursors in the hypocotyls than did intact seedlings. Quantitative RT-PCR analysis indicated that transcript levels of LeGA20ox1 and LeGA3ox2 genes were elevated in the hypocotyl after the removal of cotyledons, suggesting that the reduced bioactive GA levels caused upregulation of these genes via the feedback regulation mechanism. Taken together, our results suggest that cotyledons are necessary for maintaining normal GA levels in young cucumber and tomato hypocotyls.