INHIBITION OF CELL-DIVISION DURING CABBAGE (BRASSICA-OLERACEA L.) SEED-GERMINATION

Links between germination, DNA replication and beta-tubulin accumulati on were studied with cabbage (Brassica oleracea L.) seeds, by using fl ow cytometric analysis of nuclear DNA content and immunodetection of b eta-tubulin levels. The seeds were incubated in water or 0.1-500 mM hy droxyurea solutions. Radicle tips isolated from dry cabbage seeds reve aled most 2C (G(0) or G(1) stage) and some 4C (G(2) stage) signals of nuclear DNA contents and a constitutive level of beta-tubulin. The ons et of DNA replication in the radicle tip was observed between 12 and 2 4 h of imbibition in water and was preceded by an increase of beta-tub ulin levels. Incubation of the seeds in 1 mM hydroxyurea retarded DNA replication, whereas an arrest of DNA replication occurred upon incuba tion in 10 mM hydroxyurea or higher concentrations. The arrest of DNA replication and cell division did not block radicle protrusion and all owed some extension of the radicle. This demonstrated that DNA replica tion is not a prerequisite for radicle protrusion and initial extensio n. However, further seedling development, including root growth and ro ot hair development, was dependent on DNA replica ion. Accumulation of beta-tubulin was not affected by hydroxyurea. Thus, it can be deduced that both DNA replication and beta-tubulin accumulation are two paral lel and independent cell cycle events during seed germination.