Over expression of Spcdc25 and of ATWEE1 in Arabidopsis alter the sensitivity of hypocotyl explants to auxin and cytokinin in culture

The cell cycle is regulated by plant growth regulators (PGRs), mainly auxins and cytokinins. In eukaryotes, two genes regulate entry into mitosis: cdc25 and wee1, although a confirmed homologue of cdc25 in plants has not yet been identified. Expressing the fission yeast cdc25 (Spcdc25) in tobacco BY2 cells allows them to bypass a cytokinin block at G2/M. In tobacco and Arabidopsis roots, Spcdc25 expression led to an increased frequency of lateral roots. Conversely, over-expression of AtWEE1 in Arabidopsis induces a slower rate of root elongation and a reduced frequency of lateral roots. Wild type Arabidopsis hypocotyl explants respond to increasing levels of cytokinin with rapid proliferation, greening and formation of shoots, but are unable to form shoots in the absence of auxin and cytokinin. We tested if hypocotyl explants from Arabidopsis lines over-expressing ATWEE1 and Spcdc25 respond like WT to auxin (NAA) and cytokinin (Kinetin). We used a system of two-way grids in which we increased concentration of NAA and kinetin in X- and Y-axes, respectively. There were clear differences in the response between the transgenic and WT. All concentrations of auxin and cytokinin suppressed growth of calli in the AtWEE1OE line compared to WT. Moreover, in the AtWEE1OE, both organogenesis and formation of root hair structures were suppressed. At higher PGR levels neither WT nor Spcdc25 lines produced shoots but both produced roots and hairs. A minimum level of NAA was required for callus induction in both WT and Spcdc25 lines. However at low NAA/Kinetin there was growth of calli and more roots in Spcdc25 lines compared with WT. In an inducible Spcdc25 line plus inducer, roots formed at concentrations which suppressed roots in the inducible Spcdc25 line without inducer These results are discussed in the context of plant growth regulators and regulatory genes of the plant cell cycle.