Histochemical analysis of root meristem activity in Arabidopsis thaliana using a cyclin:GUS (β-glucuronidase) marker line

We used a transgenic Arabidopsis line expressing a translational fusion between a mitotic cyclin and the reporter gene beta -glucuronidase (GUS) to investigate cell divisions in postembryonic root meristems. The fusion protein contains the cyclin destruction box (CDB) and this leads to a rapid degradation of the chimeric GUS-protein after mitosis. Hence, the staining pattern of the meristem marks dividing cells. We observed that upon germination the first cell divisions occur in epidermis cells at the junction with the hypocotyl. Moreover, the accelerated root growth on media supplemented with sucrose correlates with an increased number of dividing cells and an enlargement of the root meristematic zone. The conditional root expansion mutants pom pom1 and procuste1 (quill) suppress this sugar effect leading to a smaller meristematic zone. Simultaneous visualisation of the nucleus revealed that the CYCAT1:CDB:GUS expression is subcellularly localised around the nucleus. This particular staining starts at prophase and disappears after the completion of the new cell wall. In metaphase the staining invades the cytoplasm whereas in the telophase it concentrates again around the nucleus. This cell cycle-dependent distribution was used to characterise the two root specific cytokinesis mutants pleiade1 and hyade1. In both mutants, cells which fail to develop a complete cell wall during cytokinesis divide synchronously in further cell divisions leading to multinucleate cells. These experiments demonstrate the usefulness of the CYCAT1:CDB:GUS marker line for studying cell division of wild-type and mutants. Furthermore, this line can be used to analyse the influence of biotic and abiotic signals on the rate and spatial distribution of cell divisions.