Martina Válková

Cytokinins modulate auxin-induced organogenesis in plants via regulation of the auxin efflux

Postembryonic de novo organogenesis represents an important competence evolved in plants that allows their physiological and developmental adaptation to changing environmental conditions. The phytohormones auxin and cytokinin (CK) are important regulators of the developmental fate of pluripotent plant cells. However, the molecular nature of their interaction(s) in control of plant organogenesis is largely unknown. Here, we show that CK modulates auxin-induced organogenesis (AIO) via regulation of the efflux-dependent intercellular auxin distribution. We used the hypocotyl explants-based in vitro system to study the mechanism underlying de novo organogenesis. We show that auxin, but not CK, is capable of triggering organogenesis in hypocotyl explants. The AIO is accompanied by endogenous CK production and tissue-specific activation of CK signaling. CK affects differential auxin distribution, and the CK-mediated modulation of organogenesis is simulated by inhibition of polar auxin transport. CK reduces auxin efflux from cultured tobacco cells and regulates expression of auxin efflux carriers from the PIN family in hypocotyl explants. Moreover, endogenous CK levels influence PIN transcription and are necessary to maintain intercellular auxin distribution in planta. Based on these findings, we propose a model in which auxin acts as a trigger of the organogenic processes, whose output is modulated by the endogenously produced CKs. We propose that an important mechanism of this CK action is its effect on auxin distribution via regulation of expression of auxin efflux carriers.

Effects of Conditional IPT-Dependent Cytokinin Overproduction on Root Architecture of Arabidopsis Seedlings

Cytokinin (CK) has been known to inhibit primary root elongation and suggested to act as an auxin antagonist in the regulation of lateral root (LR) formation. While the role of auxin in root development has been thoroughly studied, the detailed and overall description of CK effects on root system morphology, particularly that of developing lateral root primordia (LRPs), and hence its role in organogenesis is still in progress. Here we examine the effects of conditional endogenous CK overproduction on root architecture and consider its temporal aspect during the early development of Arabidopsis thaliana. We employed the pOp/LhGR system to induce ectopic ipt overexpression with a glucocorticoid dexamethasone at designated developmental points. The transient CaMV 35S>GR>ipt transactivation greatly enhanced levels of biologically active CKs of zeatin (Z)-type and identified a distinct developmental interval during which primary root elongation is susceptible to increases in endogenous CK production. Long-term CK overproduction inhibited primary root elongation by reducing quantitative parameters of primary root meristem, disturbed a characteristic graded distribution pattern of auxin response in LRPs and impaired their development. Our findings indicate the impact of perturbed endogenous CK on the regulation of asymmetric auxin distribution during LRP development and imply that there is cross-talk between auxin and CK during organogenesis in A. thaliana.

Cytokinin-induced photomorphogenesis in dark-grown Arabidopsis: a proteomic analysis

High concentrations of cytokinins (CKs) in the cultivation medium can induce partial photomorphogenesis in dark-grown Arabidopsis seedlings. However, no significant increases in endogenous CK levels have been found in de-etiolated mutants, suggesting that either parallel pathways are involved in the light and CK responses, or changes in the sensitivity to CKs occur during photomorphogenesis. Here it is shown that even modest increases in endogenous CK levels induced by transgenic expression of the CK biosynthetic gene, ipt, can lead to many typical features of light-induced de-etiolation, including inhibition of hypocotyl elongation and partial cotyledon opening. In addition, significant changes in expression of 37 proteins (mostly related to chloroplast biogenesis, a major element of light-induced photomorphogenesis) were detected by image and mass spectrometric analysis of two-dimensionally separated proteins. The identified chloroplast proteins were all up-regulated in response to increased CKs, and more than half are up-regulated at the transcript level during light-induced photomorphogenesis according to previously published transcriptomic data. Four of the up-regulated chloroplast proteins identified here have also been shown to be up-regulated during light-induced photomorphogenesis in previous proteomic analyses. In contrast, all differentially regulated mitochondrial proteins (the second largest group of differentially expressed proteins) were down-regulated. Changes in the levels of several tubulins are consistent with the observed morphological alterations. Further, 10 out of the 37 differentially expressed proteins detected have not been linked to either photomorphogenesis or CK action in light-grown Arabidopsis seedlings in previously published transcriptomic or proteomic analyses.

Antibodies against CKI1RD, a receiver domain of the sensor histidine kinase in Arabidopsis thaliana: from antigen preparation to in planta immunolocalization.

Immunodetection is a powerful tool in functional studies of all organisms. In plants, the gene redundancy and presence of gene families composed of highly homologous members often impedes the unambiguous identification of individual gene products. A family of eight sensor histidine kinases (HKs) mediates the transduction of diverse signals into Arabidopsis thaliana cells, thereby ensuring the initiation of appropriate adaptive responses. Antibodies recognizing specific members of the HK family would be valuable for studying their functions in Arabidopsis and other plant species including important crops. We have focused on developing and applying antibodies against CYTOKININ-INDEPENDENT 1 (CKI1), which encodes a constitutively active membrane-bound sensor HK that regulates the development of female gametophytes and vascular tissue in Arabidopsis. A coding sequence delimiting the C-terminal receiver domain of CKI1 (CKI1(RD)) was expressed in Escherichia coli using the IPTG-inducible expression system and purified to give a highly pure target protein. The purified CKI1(RD) protein was then used as an antigen for anti-CKI1(RD) antibody production. The resulting polyclonal antibodies had a detection limit of 10 ng of target protein at 1:20,000 dilution and were able to specifically distinguish CKI1, both in vitro and in situ, even in a direct comparison with highly homologous members of the same HK family AHK4, CKI2 and ETR1. Finally, anti-CKI1(RD) antibodies were able to selectively bind CKI1-GFP fusion protein in a pull-down assay using crude lysate from an Arabidopsis cell suspension culture. Our results suggest that the receiver domain is a useful target for the functional characterization of sensor HKs in immunological and biochemical studies.