Perturbation of cytokinin and ethylene-signalling pathways explain the strong rooting phenotype exhibited by Arabidopsis expressing theSchizosaccharomyces pombemitotic inducer,cdc25

BackgroundEntry into mitosis is regulated by cyclin dependent kinases that in turn are phosphoregulated. In most eukaryotes, phosphoregulation is through WEE1 kinase and CDC25 phosphatase. In higher plants a homologous CDC25 gene is unconfirmed and hence the mitotic inducerSchizosaccharomyces pombe(Sp)cdc25has been used as a tool in transgenic plants to probe cell cycle function. Expression ofSpcdc25in tobacco BY-2 cells accelerates entry into mitosis and depletes cytokinins; in whole plants it stimulates lateral root production. Here we show, for the first time, that alterations to cytokinin and ethylene signaling explain the rooting phenotype elicited bySpcdc25expression in Arabidopsis.ResultsExpressingSpcdc25in Arabidopsis results in increased formation of lateral and adventitious roots, a reduction of primary root width and more isodiametric cells in the root apical meristem (RAM) compared with wild type. Furthermore it stimulates root morphogenesis from hypocotyls when cultured on two way grids of increasing auxin and cytokinin concentrations. Microarray analysis of seedling roots expressingSpcdc25reveals that expression of 167 genes is changed by >2-fold. As well as genes related to stress responses and defence, these include 19 genes related to transcriptional regulation and signaling. Amongst these was the up-regulation of genes associated with ethylene synthesis and signaling. Seedlings expressingSpcdc25produced 2-fold more ethylene than WT and exhibited a significant reduction in hypocotyl length both in darkness or when exposed to 10 ppm ethylene. Furthermore inSpcdc25expressing plants, the cytokinin receptorAHK3was down-regulated, and endogenous levels of iPA were reduced whereas endogeous IAA concentrations in the roots increased.ConclusionsWe suggest that the reduction in root width and change to a more isodiametric cell phenotype in the RAM inSpcdc25expressing plants is a response to ethylene over-production. The increased rooting phenotype inSpcdc25expressing plants is due to an increase in the ratio of endogenous auxin to cytokinin that is known to stimulate an increased rate of lateral root production. Overall, our data reveal important cross talk between cell division and plant growth regulators leading to developmental changes.