The Role of DNA Synthesis During Hypocotyl Elongation in Light and Dark

Abstract

Fluorodeoxyuridine, an inhibitor of DNA synthesis, did not affect germination and post-germinative growth in the aerial part of lettuce and Haplopappus gracilis seedlings when grown in the light. In the dark, however, elongation of the hypocotyl was inhibited by fluorodeoxyuridine, strikingly in lettuce and only slightly in Haplopappus gracilis. This could imply that the controlling mechanism of hypocotyl elongation is in some cases related to DNA synthesis, either because mitotic processes (often little taken into account in considering hypocotyl growth) are involved in the elongation of hypocotyls only when they are grown in the dark, or because DNA synthesis affects cell elongation directly, or through the production of a greater number of endopolyploid cells in the dark. Using mainly autoradiographic and cytofluorimetric methods, these possibilities were tested. Besides lettuce (Lactuca sativa L. var. Great Lakes) and H. gracilis (Nutt.) Gray, radish (Raphanus sativus L. var. Tondo rosso quarantino) and soybean (Soya hyspida Sieb. and Zucc. var. Tokyo) seedlings were also studied. Fluorodeoxyuridine drastically inhibits cell elongation only when it is preceded or accompanied by mitotic or endomitotic events. Need for DNA synthesis during hypocotyl elongation, as well as during early post-germinative growth, seems to be of particular importance when endomitotic processes are involved.