BETA-TUBULIN ACCUMULATION AND DNA-REPLICATION IN IMBIBING TOMATO SEEDS

The activation of the cell cycle in embryo root tips of imbibing tomat o (Lycopersicon esculentum Mill. cv Lerica) seeds was studied by flow cytometric analyses of the nuclear DNA content and by immunodetection of beta-tubulin. With dry seeds, flow cytometric profiles indicated th at the majority of the cells were arrested at the G(1) phase of the ce ll cycle. In addition, beta-tubulin was not detectable on western blot s. Upon imbibition of water, the number of cells in G(2) started to in crease after 24 h, and a 55-kD beta-tubulin signal was detected betwee n 24 and 48 h. Two-dimensional immunoblots revealed at least three dif ferent beta-tubulin isotypes. Thus, beta-tubulin accumulation and DNA replication were induced during osmotic priming. These processes, as w ell as seed germination rate, were enhanced upon subsequent imbibition of water, compared with control seeds that imbibed but were not prime d. By contrast, when aged seeds imbibed, DNA replication, beta-tubulin accumulation, and germination were delayed. In all cases studied, bot h DNA replication and beta-tubulin accumulation preceded visible germi nation. We suggest that activation of these cell-cycle-related process es is a prerequisite for tomato seed germination. Furthermore, beta-tu bulin expression can be used as a parameter for following the initial processes that are activated during seed imbibition.