BACKGROUND In the root apical meristem, which contains the stem cells that feed into root development, the phytohormones auxin and cytokinin play opposing roles, with auxin promoting cell division and cytokinin promoting cell differentiation. Cytokinin acts in the root tip in part by modulating auxin transport through regulation of the level of the PIN auxin efflux carriers. Auxin plays a key r...

The plant hormone cytokinin is perceived by membrane-located sensor histidine kinases. Arabidopsis (Arabidopsis thaliana) possesses three cytokinin receptors: ARABIDOPSIS HISTIDINE KINASE2 (AHK2), AHK3, and CYTOKININ RESPONSE1/AHK4. The current model predicts perception of the cytokinin signal at the plasma membrane. However, cytokinin-binding studies with membrane fractions separated by two-ph...

The developmental programme of the pistil is under the control of both auxin and cytokinin. Crosstalk between these factors converges on regulation of the auxin carrier PIN-FORMED 1 (PIN1). Here, we show that in the triple transcription factor mutant cytokinin response factor 2 (crf2) crf3 crf6 both pistil length and ovule number were reduced. PIN1 expression was also lower in the triple mutant...

The plant hormone cytokinin realizes at least part of its signaling output through the regulation of gene expression. A great part of the early transcriptional regulation is mediated by type-B response regulators, which are transcription factors of the MYB family. Other transcription factors, such as the cytokinin response factors of the AP2/ERF family, have also been shown to be involved in th...

Climate change with increasing periods of drought is expected to reduce the yield of biomass crops such as poplars. To combat yield loss, it is important to better understand the molecular mechanisms that control growth under drought. Here, the goal was to resolve the drought-induced changes of active cytokinins, a main growth hormone in plants, at the tissue level in different cell types and o...

Excised roots of Bryophyllum calycinum require for growth both auxin and cytokinin. This is demonstrated by the poor growth of 2-mm root tips in a basal medium of mineral salts, sucrose, and vitamins supplemented with either an auxin or a cytokinin, and much better growth when the basal medium is supplemented with both auxin and cytokinin. However, both substances are synthesized by the root, a...

While the molecular basis for cytokinin action is quite well understood in flowering plants, little is known about the cytokinin signal transduction in early diverging land plants. The genome of the bryophyte Physcomitrella patens (Hedw.) B.S. encodes three classical cytokinin receptors, the CHASE domain-containing histidine kinases, CHK1, CHK2, and CHK3. In a complementation assay with protopl...

SPINDLY (SPY) is a negative regulator of gibberellin (GA) responses; however, spy mutants exhibit various phenotypic alterations not found in GA-treated plants. Assaying for additional roles for SPY revealed that spy mutants are resistant to exogenously applied cytokinin. GA also repressed the effects of cytokinin, suggesting that there is cross talk between the two hormone-response pathways, w...

Ultra-performance liquid chromatography-tandem mass spectrometry was used to establish the cytokinin profile of the bryophyte Physcomitrella patens (Hedw.) B.S.G.; of 40 analyzed cytokinins, 20 were detected. cis-Zeatin-riboside-O-glucoside, N(6)-(Delta(2)-isopentenyl)adenosine-5'-monophosphate (iPRMP), and trans-zeatin-riboside-O-glucoside were the most abundant intracellular cytokinins. In ad...

The cytokinin-active ribonucleosides present in tRNA from etiolated Phaseolus vulgaris L. seedlings have been isolated and identified as cis-ribosylzeatin, 2-methylthio-ribosylzeatin, and N(6)-(Delta(2)-isopentenyl)-adenosine. The structures of the compounds were established on the basis of their chromatographic properties and the mass spectra of their permethylated and perdeuteromethylated der...