beta-tubulin accumulation and DNA synthesis are sequentially resumed in embryo organs of cucumber (Cucumis sativus L.) seeds during germination

Resumption of DNA synthetic activities and beta-tubulin accumulation was st udied in embryo organs of germinating cucumber (Cucumis sativus L.) seeds. Flow-cytometric analysis indicated the existence of 2C, 4C, and 8C nuclei i n the radicle of mature embryos, whereas in cotyledons most of the cells co ntained nuclei with 2C DNA content. Upon imbibition of water, nuclear DNA r eplication was initiated in the radicle within 15 h, subsequently spreading towards the cotyledons. Bromodeoxyuridine incorporation preceded detectabl e changes in the relative amounts of DNA, implying the occurrence of putati ve DNA repair. Organellar DNA synthesis occurred independently of the nucle ar DNA synthetic cycle. Western blotting and immunohistochemical localizati on demonstrated that the constitutive level of beta-tubulin originated from preserved P-tubulin granules. During imbibition, disappearance of fluoresc ent tubulin granules, accumulation of beta-tubulin, and formation of microt ubular cytoskeleton were found in the radicle, but not in the cotyledon are as. Mitosis only occurred after radicle protrusion at 21 h of imbibition. I t is concluded that the differences in the initiation and progress of these cellular and molecular events are associated with the discrete behaviors o f the radicle and the cotyledons upon imbibition. The formation of cortical microtubular cytoskeleton and the accumulation of tubulins are important f eatures in preparation of radicle protrusion, whereas DNA synthesis may con tribute to postgerminative growth.