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Phosphate (Pi) is an essential nutrient for all organisms. Roots are underground organs, but the majority of the root biology studies have been done on root systems growing in the presence of light. Root illumination alters the Pi starvation response (PSR) at different intensities. Thus, we have analyzed morphological, transcriptional and physiological responses to Pi starvation in dark-grown roots. We have identified new genes and pathways regulated by Pi starvation that were not described previously. We also show that Pi-starved plants increase the cis-zeatin (cZ) : trans-zeatin (tZ) ratio. Transcriptomic analyses show that tZ preferentially represses cell cycle and PSR genes, whereas cZ induces genes involved in cell and root hair elongation and differentiation. In fact, cZ-treated seedlings show longer root system as well as longer root hairs compared with tZ-treated seedlings, increasing the total absorbing surface. Mutants with low cZ concentrations do not allocate free Pi in roots during Pi starvation. We propose that Pi-starved plants increase the cZ : tZ ratio to maintain basal cytokinin responses and allocate Pi in the root system to sustain its growth. Therefore, cZ acts as a PSR hormone that stimulates root and root hair elongation to enlarge the root absorbing surface and to increase Pi concentrations in roots.

Cytokinins (CKs) are well-established as important phytohormonal regulators of plant growth and development. An increasing number of studies have also revealed the function of these hormones in plant responses to biotic and abiotic stresses. While the function of certain CK classes, including trans-zeatin and isopentenyladenine-type CKs, have been studied in detail, the role of cis-zeatin-type CKs (cZs) in plant development and in mediating environmental interactions is less well defined. Here we provide a comprehensive summary of the current knowledge about abundance, metabolism and activities of cZs in plants. We outline the history of their analysis and the metabolic routes comprising cZ biosynthesis and degradation. Further we provide an overview of changes in the pools of cZs during plant development and environmental interactions. We summarize studies that investigate the role of cZs in regulating plant development and defence responses to pathogen and herbivore attack and highlight their potential role as ‘novel’ stress-response markers. Since the functional roles of cZs remain largely based on correlative data and genetic manipulations of their biosynthesis, inactivation and degradation are few, we suggest experimental approaches using transgenic plants altered in cZ levels to further uncover their roles in plant growth and environmental interactions and their potential for crop improvement.

Cytokinins (CKs) are plant hormones affecting numerous developmental processes. Zeatin and its derivatives are the most important group of isoprenoid CKs. Zeatin occurs as two isomers: while frans-zeatin (transZ) was found to be a bioactive substance, cis-zeatin (cisZ) was reported to have a weak biological impact. Even though cisZ derivatives are abundant in various plant materials their biological role is still unknown. The comprehensive screen of land plants presented here suggests that cisZ-type CKs occur ubiquitously in the plant kingdom but their abundance might correlate with a strategy of life rather than with evolutionary complexity. Changing levels of transZ and cisZ during Arabidopsis ontogenesis show that levels of the two zeatin isomers can differ significantly during the life span of the plant, with cisZ-type CKs prevalent in the developmental stages associated with limited growth. A survey of the bioassays employed illustrates mild activity of cisZ and its derivatives. No cis ↔ trans isomerization, which would account for the effects of cisZ, was observed in tobacco cells and oat leaves. Differences in uptake between the two isomers resulting in distinct bioactivity have not been detected. In contrast, cisZ and transZ have a different metabolic fate in oat and tobacco. Analysis of a CK-degrading enzyme, cytokinin oxidase/dehydrogenase (CKX), reveals that Arabidopsis possesses two isoforms, AtCKX1 expressed in stages of active growth, and AtCKX7, both of which have the highest affinity for the cisZ isomer. Based on the present results, the conceivable function of cisZ-type CKs as delicate regulators of CK responses in plants under growth-limiting conditions is hypothesized.

Organ-to-organ signal transmission is essential for higher organisms to ensure coordinated biological reactions during metabolism and morphogenesis. Similar to organs in animals, plant organs communicate by various signalling molecules. Among them, cytokinins, a class of phytohormones, play a key role as root-to-shoot long-distance signals, regulating various growth and developmental processes in shoots 1 , 2 . Previous studies have proposed that trans -zeatin-riboside, a type of cytokinin precursor, is a major long-distance signalling form in xylem vessels and its action depends on metabolic conversion via the LONELY GUY enzyme in proximity to the site of action 3 – 5 . Here we report an additional long-distance signalling form of cytokinin: trans -zeatin, an active form. Grafting between various cytokinin biosynthetic and transportation mutants revealed that root-to-shoot translocation of trans -zeatin, a minor component of xylem cytokinin, controls leaf size but not meristem activity-related traits, whereas that of trans -zeatin riboside is sufficient for regulating both traits. Considering the ratio of trans -zeatin to trans -zeatin-riboside in xylem and their delivery rate change in response to environmental conditions, this dual long-distance cytokinin signalling system allows plants to fine-tune the manner of shoot growth to adapt to fluctuating environments. After trans -zeatin riboside, the active cytokinin trans -zeatin is identified as the second major xylem cytokinin translocated from root to shoot. Interestingly, these two forms of root-derived cytokinins play different roles in regulating shoot development.

Plants growing at high densities can detect competitors through changes in the composition of light reflected by neighbours. In response to this far-red-enriched light, plants elicit adaptive shade avoidance responses for light capture, but these need to be balanced against other input signals, such as nutrient availability. Here, we investigated how Arabidopsis integrates shade and nitrate signalling. We unveiled that nitrate modulates shade avoidance via a previously unknown shade response pathway that involves root-derived trans -zeatin (tZ) signal and the BEE1 transcription factor as an integrator of light and cytokinin signalling. Under nitrate-sufficient conditions, tZ promotes hypocotyl elongation specifically in the presence of supplemental far-red light. This occurs via PIF transcription factors-dependent inhibition of type-A ARRs cytokinin response inhibitors. Our data thus reveal how plants co-regulate responses to shade cues with root-derived information about nutrient availability, and how they restrict responses to this information to specific light conditions in the shoot. The authors here investigate information integration in plants using light and nutrient stimuli and identify a novel regulation pathway towards photoreceptor-regulated stem growth.

Maintaining nitric oxide (NO) homeostasis is essential for normal plant physiological processes. However, very little is known about the mechanisms of NO modulation in plants. Here, we report a unique mechanism for the catabolism of NO based on the reaction with the plant hormone cytokinin. We screened for NO-insensitive mutants in Arabidopsis and isolated two allelic lines, cnu1-1 and 1–2 (continuous NO-unstressed 1), that were identified as the previously reported altered meristem program 1 (amp1) and as having elevated levels of cytokinins. A double mutant of cnu1-2 and nitric oxide overexpression 1 (nox1) reduced the severity of the phenotypes ascribed to excess NO levels as did treating the nox1 line with trans -zeatin, the predominant form of cytokinin in Arabidopsis . We further showed that peroxinitrite, an active NO derivative, can react with zeatin in vitro, which together with the results in vivo suggests that cytokinins suppress the action of NO most likely through direct interaction between them, leading to the reduction of endogenous NO levels. These results provide insights into NO signaling and regulation of its bioactivity in plants.

Pedicel length is a crucial agronomic trait of cucumbers. Fruit deformation can occur When the pedicel is too long or too short. Moreover, an appropriate pedicel length is advantageous for mechanized harvesting. Therefore, it is essential to investigate the molecular regulatory mechanisms underlying cucumber pedicel length. In the current study, we obtained a short pedicel mutant through EMS mutagenesis and discovered that the reduced cell number was the primary cause of the shortened pedicel. Upon analyzing the hormone content, we found that the level of trans zeatin in the long-pedicel material was significantly higher than that in the short-pedicel material. Further transcriptome sequencing analysis revealed that differentially expressed genes were enriched in cytokinin synthesis-related pathways. Based on these results, the present study concluded that cucumber pedicel length is regulated by genes related to the cytokinin synthesis pathway and that differences in length result from differences in zeatin content and cell number.

Cytokinins are a major group of phytohormones regulating plant growth, development and stress responses. However, in contrast to the well-defined polar transport of auxins, the molecular basis of cytokinin transport is poorly understood. Here we show that an ATP-binding cassette transporter in Arabidopsis , AtABCG14, is essential for the acropetal (root to shoot) translocation of the root-synthesized cytokinins. AtABCG14 is expressed primarily in the pericycle and stelar cells of roots. Knocking out AtABCG14 strongly impairs the translocation of trans -zeatin ( t Z)-type cytokinins from roots to shoots, thereby affecting the plant’s growth and development. AtABCG14 localizes to the plasma membrane of transformed cells. In planta feeding of C 14 or C 13 -labelled t Z suggests that it acts as an efflux pump and its presence in the cells directly correlates with the transport of the fed cytokinin. Therefore, AtABCG14 is a transporter likely involved in the long-distance translocation of cytokinins in planta. Cytokinins are a major group of plant hormones that control plant development and some of these hormone species are translocated from roots to shoots, but how they are transported in planta is unknown. Here, ABCG14 is shown to transport cytokinins from the roots of Arabidopsis to the shoots.

Studies of vitality/mortality of cortex cells, as well as of the concentrations of ethylene (ETH), gibberellins (GAs), indolic compounds/auxins (ICs/AUXs) and cytokinins (CKs), were undertaken to explain the hormonal background of kinetin (Kin)-regulated cell death (RCD), which is induced in the cortex of the apical parts of roots of faba bean ( Vicia faba ssp. minor ) seedlings. Quantification was carried out with fluorescence microscopy, ETH sensors, spectrophotometry and ultrahigh-performance liquid chromatography tandem mass spectrometry (UHPLC‒MS/MS). The results indicated that Kin was metabolized to the transport form, i.e., kinetin-9-glucoside (Kin9G) and kinetin riboside (KinR). KinR was then converted to cis -zeatin ( c Z) in apical parts of roots with meristems, to cis -zeatin riboside ( c ZR) in apical parts of roots without meristems and finally to cis -zeatin riboside 5’-monophosphate ( c ZR5’MP), which is indicated to be a ligand of cytokinin-dependent receptors inducing CD. The process may be enhanced by an increase in the amount of dihydrozeatin riboside (DHZR) as a byproduct of the pathway of zeatin metabolism. It seems that crosstalk of ETH, ICs/AUXs, GAs and CKs with the c ZR5’MP, the cis -zeatin-dependent pathway, but not the trans -zeatin-dependent pathway, is responsible for Kin-RCD, indicating that the process is very specific and offers a useful model for studies of CD hallmarks in plants.

Cytokinins are adenine derivatives serving as phytohormones, which are essential regulators of plant growth, development, and response to environmental factors. The transport process of cytokinins determines their spatial distributions and is critical to their functions. The AZA-GUANINE RESISTANT (AZG) family member AZG2 in Arabidopsis thaliana has been identified as a cytokinin and purine transporter. Here, we characterize the binding and transport of AZG2 towards the natural cytokinin, trans -zeatin (tZ), as well as adenine. AZG2 structures are determined in both the substrate-unbound, adenine-bound, and tZ-bound states, under both acidic and neutral pH. Key residues involved in substrate binding are identified. Two distinct conformations are observed in the tZ-bound state of AZG2 in the neutral pH. Structural analysis reveals the structural dynamics of AZG2 during cytokinin transport, which fit into the elevator-type transport model. These results provide insights into the molecular mechanism of cytokinin transport in plants. This study characterized the substrate binding and transport mechanisms of the plant purine/cytokinin transporter AZG2. Conformational dynamics were examined by determining AZG2 structures in the apo or substrate-bound states under different pH.