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

The activation of the cell cycle in embryo root tips of imbibing tomato (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 that the majority of the cells were arrested at the G(1) phase of the cell cycle. In addition, beta-tubulin was not detectable on western blots. Upon imbibition of water, the number of cells in G(2) started to increase after 24 h, and a 55-kD beta-tubulin signal was detected between 24 and 48 h. Two-dimensional immunoblots revealed at least three different beta-tubulin isotypes. Thus, beta-tubulin accumulation and DNA replication were induced during osmotic priming. These processes, as well as seed germination rate, were enhanced upon subsequent imbibition of water, compared with control seeds that imbibed but were not primed. By contrast, when aged seeds imbibed, DNA replication, beta-tubulin accumulation, and germination were delayed. In all cases studied, both DNA replication and beta-tubulin accumulation preceded visible germination. We suggest that activation of these cell-cycle-related processes is a prerequisite for tomato seed germination. Furthermore, beta-tubulin expression can be used as a parameter for following the initial processes that are activated during seed imbibition.