Cell division and subsequent radicle protrusion in tomato seeds are inhibited by osmotic stress but DNA synthesis and formation of microtubular cytoskeleton are not

We studied cell cycle events in embryos of tomato (Lycopersicon esculentum Mill. cv Moneymaker) seeds during imbibition in water and during osmocondit ioning ("priming") using both quantitative and cytological analysis of DNA synthesis and beta-tubulin accumulation. Most embryonic nuclei of dry, untr eated control seeds were arrested in the G(1) phase of the cell cycle. This indicated the absence of DNA synthesis (the S-phase), as confirmed by the absence of bromodeoxyuridine incorporation. In addition, beta-tubulin was n ot detected on western blots and microtubules were not present. During imbi bition in water, DNA synthesis was activated in the radicle tip and then sp read toward the cotyledons, resulting in an increase in the number of nucle i in G(2). Concomitantly, beta-tubulin accumulated and was assembled into m icrotubular cytoskeleton networks. Both of these cell cycle events preceded cell expansion and division and subsequent growth of the radicle through t he seed coat. The activation of DNA synthesis and the Formation of microtub ular cytoskeleton networks were also observed throughout the embryo when se eds were osmoconditioned. However, this preactivation of the cell cycle app eared to become arrested in the G(2) phase since no mitosis was observed. T he pre-activation of cell cycle events in osmoconditioned seeds appeared to be correlated with enhanced germination performance during re-imbibition i n water.