A new approach for cytokinin isolation from Arabidopsis tissues using miniaturized purification: pipette tip solid-phase extraction

Diverse geotropic responses in the orchid family

In epiphytes, aerial roots are important to combat water-deficient, nutrient-poor, and high-irradiance microhabitats. However, whether aerial roots can respond to gravity and whether auxin plays a role in regulating aerial root development remain open-ended questions. Here, we investigated the gravitropic response of the epiphytic orchid Phalaenopsis aphrodite. Our data showed that aerial roots of P. aphrodite failed to respond to gravity, and this was correlated with a lack of starch granules/statolith sedimentation in the roots and the absence of the auxin efflux carrier PIN2 gene. Using an established auxin reporter, we discovered that auxin maximum was absent in the quiescent center of aerial roots of P. aphrodite. Also, gravity failed to trigger auxin redistribution in the root caps. Hence, loss of gravity sensing and gravity-dependent auxin redistribution may be the genetic factors contributing to aerial root development. Moreover, the architectural and functional innovations that achieve fast gravitropism in the flowering plants appear to be lost in both terrestrial and epiphytic orchids, but are present in the early diverged orchid subfamilies. Taken together, our findings provide physiological and molecular evidence to support the notion that epiphytic orchids lack gravitropism and suggest diverse geotropic responses in the orchid family.

High-Performance Liquid Chromatographic Quantification of the Plant Hormone Abscisic Acid at ppb Levels in Plant Samples after a Single Immunoaffinity Column Cleanup

High-performance liquid chromatography with ultraviolet detection (HPLC-UV) is a common analysis technique due to its high versatility and simple operation. In the present study, HPLC-UV detection was integrated with immunoaffinity cleanup (IAC) of the sample extracts. The matrix effect was greatly reduced, and the limit of detection was as low as 1 ng/g of free abscisic acid (ABA) in fresh plant tissues. A monoclonal antibody 3F1 (mAb 3F1) was developed to specifically recognize free ABA but not ABA analogues. The mAb 3F1-immobilized immunoaffinity column exhibited a capacity of 850 ng/mL and an elution efficiency of 88.8-105% for standards. The extraction recoveries of the column for ABA ranged from 80.4 to 108.9%. ABA content was detected in various plant samples with IAC-HPLC-UV. The results were verified with ultraperformance liquid chromatography-electrospray tandem mass spectrometry. IAC-HPLC-UV can be a sensitive and cost-efficient method for plant hormone analysis.

Cytokinin-deficient Chlamydomonas reinhardtii CRISPR-Cas9 mutants show reduced ability to prime resistance of tobacco against bacterial infection

Although microalgae have only recently been recognized as part of the plant and soil microbiome, their application as biofertilizers has a tradition in sustainable crop production. Under consideration of their ability to produce the plant growth-stimulating hormone cytokinin (CK), known to also induce pathogen resistance, we have assessed the biocontrol ability of CK-producing microalgae. All pro- and eukaryotic CK-producing microalgae tested were able to enhance the tolerance of tobacco against Pseudomonas syringae pv. tabaci (PsT) infection. Since Chlamydomonas reinhardtii (Cre) proved to be the most efficient, we functionally characterized its biocontrol ability. We employed the CRISPR-Cas9 system to generate the first knockouts of CK biosynthetic genes in microalgae. Specifically, we targeted Cre Lonely Guy (LOG) and isopentenyltransferase (IPT) genes, the key genes of CK biosynthesis. While Cre wild-type exhibits a strong protection, the CK-deficient mutants have a reduced ability to induce plant defence. The degree of protection correlates with the CK levels, with the IPT mutants showing less protection than the LOG mutants. Gene expression analyses showed that Cre strongly stimulates tobacco resistance through defence gene priming. This study functionally verifies that Cre primes defence responses with CK, which contributes to the robustness of the effect. This work contributes to elucidate microalgae-mediated plant defence priming and identifies the role of CKs. In addition, these results underscore the potential of CK-producing microalgae as biologicals in agriculture by combining biofertilizer and biocontrol ability for sustainable and environment-friendly crop management.

Profiling of 1-aminocyclopropane-1-carboxylic acid and selected phytohormones in Arabidopsis using liquid chromatography-tandem mass spectrometry

BACKGROUND

Gaseous phytohormone ethylene levels are directly influenced by the production of its immediate non-volatile precursor 1-aminocyclopropane-1-carboxylic acid (ACC). Owing to the strongly acidic character of the ACC molecule, its quantification has been difficult to perform. Here, we present a simple and straightforward validated method for accurate quantification of not only ACC levels, but also major members of other important phytohormonal classes - auxins, cytokinins, jasmonic acid, abscisic acid and salicylic acid from the same biological sample.

RESULTS

The presented technique facilitates the analysis of 15 compounds by liquid chromatography coupled with tandem mass spectrometry. It was optimized and validated for 10 mg of fresh weight plant material. The extraction procedure is composed of a minimal amount of necessary steps. Accuracy and precision were the basis for evaluating the method, together with process efficiency, recovery and matrix effects as validation parameters. The examined compounds comprise important groups of phytohormones, their active forms and some of their metabolites, including six cytokinins, four auxins, two jasmonates, abscisic acid, salicylic acid and 1-aminocyclopropane-1-carboxylic acid. The resulting method was used to examine their contents in selected Arabidopsis thaliana mutant lines.

CONCLUSION

This profiling method enables a very straightforward approach for indirect ethylene study and explores how it interacts, based on content levels, with other phytohormonal groups in plants.

Rapid profiling of cytokinins using supercritical fluid chromatography coupled with tandem mass spectrometry

BACKGROUND

The determination of plant hormones is still a very challenging analytical discipline, mainly due to their low concentration in complex plant matrices. Therefore, the involvement of very sensitive high-throughput techniques is required. Cytokinins (CKs) are semi-polar basic plant hormones regulating plant growth and development. Modern methods for CK determination are currently based on ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS), which enables the separation of CK isomeric forms occurring endogenously in plants. Here, ultra-high performance supercritical fluid chromatography coupled with tandem mass spectrometry (UHPSFC-MS/MS) was used for the simultaneous determination of 37 CK metabolites.

RESULTS

The chromatographic conditions were tested on three different columns with various retention mechanisms. Hybrid silica modified with 2-picolylamine was selected as the stationary phase. Several parameters such as column temperature, back pressure regulation, mobile phase composition and make-up solvent were investigated to achieve efficient separation of CK isomers and reasonable sensitivity. Compared to UHPLC-MS/MS, a 9-min chromatographic analysis using a mobile phase of supercritical CO2 and 5 mM ammonia in methanol represents a three-fold acceleration of total run time. The quantification limit of UHPSFC-MS/MS method was in the range of 0.03-0.19 fmol per injection and the method validation showed high accuracy and precision (below 15 % for most analytes). The method was finally applied to the complex plant matrix of the model plant Arabidopsis thaliana and the obtained profiles of CK metabolites were compared with the results from the conventional UHPLC-MS/MS method.

SIGNIFICANCE

The presented work offers a novel approach for quantification of endogenous CKs in plants. Compared to the conventional UHPLC-MS/MS, the total run time is shorter and the matrix effect lower for the key CK metabolites. This approach opens the opportunity to utilize UHPSFC-MS/MS instrumentation for targeted plant hormonomics including other plant hormone families.

Characteristics of turion development in two aquatic carnivorous plants: Hormonal profiles, gas exchange and mineral nutrient content

Turions are vegetative, dormant, and storage overwintering organs formed in perennial aquatic plants in response to unfavorable ecological conditions and originate by extreme condensation of apical shoot segments. The contents of cytokinins, auxins, and abscisic acid were estimated in shoot apices of summer growing, rootless aquatic carnivorous plants, Aldrovanda vesiculosa and Utricularia australis, and in developing turions at three stages and full maturity to reveal hormonal patterns responsible for turion development. The hormones were analyzed in miniature turion samples using ultraperformance liquid chromatography coupled with triple quadrupole mass spectrometry. Photosynthetic measurements in young leaves also confirmed relatively high photosynthetic rates at later turion stages. The content of active cytokinin forms was almost stable in A. vesiculosa during turion development but markedly decreased in U. australis. In both species, auxin content culminated in the middle of turion development and then decreased again. The content of abscisic acid as the main inhibitory hormone was very low in growing plants in both species but rose greatly at first developmental stages and stayed very high in mature turions. The hormonal data indicate a great strength of developing turions within sink-source relationships and confirm the central role of abscisic acid in regulating the turion development.

Fluorescence-activated multi-organelle mapping of subcellular plant hormone distribution

Auxins and cytokinins are two major families of phytohormones that control most aspects of plant growth, development and plasticity. Their distribution in plants has been described, but the importance of cell- and subcellular-type specific phytohormone homeostasis remains undefined. Herein, we revealed auxin and cytokinin distribution maps showing their different organelle-specific allocations within the Arabidopsis plant cell. To do so, we have developed Fluorescence-Activated multi-Organelle Sorting (FAmOS), an innovative subcellular fractionation technique based on flow cytometric principles. FAmOS allows the simultaneous sorting of four differently labelled organelles based on their individual light scatter and fluorescence parameters while ensuring hormone metabolic stability. Our data showed different subcellular distribution of auxin and cytokinins, revealing the formation of phytohormone gradients that have been suggested by the subcellular localization of auxin and cytokinin transporters, receptors and metabolic enzymes. Both hormones showed enrichment in vacuoles, while cytokinins were also accumulated in the endoplasmic reticulum.

Genetic architecture of source-sink-regulated senescence in maize

Source and sink interactions play a critical but mechanistically poorly understood role in the regulation of senescence. To disentangle the genetic and molecular mechanisms underlying source-sink-regulated senescence (SSRS), we performed a phenotypic, transcriptomic, and systems genetics analysis of senescence induced by the lack of a strong sink in maize (Zea mays). Comparative analysis of genotypes with contrasting SSRS phenotypes revealed that feedback inhibition of photosynthesis, a surge in reactive oxygen species, and the resulting endoplasmic reticulum (ER) stress were the earliest outcomes of weakened sink demand. Multienvironmental evaluation of a biparental population and a diversity panel identified 12 quantitative trait loci and 24 candidate genes, respectively, underlying SSRS. Combining the natural diversity and coexpression networks analyses identified 7 high-confidence candidate genes involved in proteolysis, photosynthesis, stress response, and protein folding. The role of a cathepsin B like protease 4 (ccp4), a candidate gene supported by systems genetic analysis, was validated by analysis of natural alleles in maize and heterologous analyses in Arabidopsis (Arabidopsis thaliana). Analysis of natural alleles suggested that a 700-bp polymorphic promoter region harboring multiple ABA-responsive elements is responsible for differential transcriptional regulation of ccp4 by ABA and the resulting variation in SSRS phenotype. We propose a model for SSRS wherein feedback inhibition of photosynthesis, ABA signaling, and oxidative stress converge to induce ER stress manifested as programed cell death and senescence. These findings provide a deeper understanding of signals emerging from loss of sink strength and offer opportunities to modify these signals to alter senescence program and enhance crop productivity.

Hormonal crosstalk controls cell death induced by kinetin in roots of Vicia faba ssp. minor seedlings

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 (cZ) in apical parts of roots with meristems, to cis-zeatin riboside (cZR) in apical parts of roots without meristems and finally to cis-zeatin riboside 5'-monophosphate (cZR5'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 cZR5'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.

CEP peptide and cytokinin pathways converge on CEPD glutaredoxins to inhibit root growth

C-TERMINALLY ENCODED PEPTIDE (CEP) and cytokinin hormones act over short and long distances to control plant responses to environmental cues. CEP and cytokinin pathway mutants share phenotypes, however, it is not known if these pathways intersect. We show that CEP and cytokinin signalling converge on CEP DOWNSTREAM (CEPD) glutaredoxins to inhibit primary root growth. CEP inhibition of root growth was impaired in mutants defective in trans-zeatin (tZ)-type cytokinin biosynthesis, transport, perception, and output. Concordantly, mutants affected in CEP RECEPTOR 1 showed reduced root growth inhibition in response to tZ, and altered levels of tZ-type cytokinins. Grafting and organ-specific hormone treatments showed that tZ-mediated root growth inhibition involved CEPD activity in roots. By contrast, root growth inhibition by CEP depended on shoot CEPD function. The results demonstrate that CEP and cytokinin pathways intersect, and utilise signalling circuits in separate organs involving common glutaredoxin genes to coordinate root growth.

The Role of a Cytokinin Antagonist in the Progression of Clubroot Disease

Plasmodiophora brassicae is an obligate biotrophic pathogen causing clubroot disease in cruciferous plants. Infected plant organs are subject to profound morphological changes, the roots form characteristic galls, and the leaves are chlorotic and abscise. The process of gall formation is governed by timely changes in the levels of endogenous plant hormones that occur throughout the entire life cycle of the clubroot pathogen. The homeostasis of two plant hormones, cytokinin and auxin, appears to be crucial for club development. To investigate the role of cytokinin and auxin in gall formation, we used metabolomic and transcriptomic profiling of Arabidopsis thaliana infected with clubroot, focusing on the late stages of the disease, where symptoms were more pronounced. Loss-of-function mutants of three cytokinin receptors, AHK2, AHK3, and CRE1/AHK4, were employed to further study the homeostasis of cytokinin in response to disease progression; ahk double mutants developed characteristic symptoms of the disease, albeit with varying intensity. The most susceptible to clubroot disease was the ahk3 ahk4 double mutant, as revealed by measuring its photosynthetic performance. Quantification of phytohormone levels and pharmacological treatment with the cytokinin antagonist PI-55 showed significant changes in the levels of endogenous cytokinin and auxin, which was manifested by both enhanced and reduced development of disease symptoms in different genotypes.

Cytokinin signaling regulates two-stage inflorescence arrest in Arabidopsis

To maximize reproductive success, flowering plants must correctly time entry and exit from the reproductive phase. While much is known about mechanisms that regulate initiation of flowering, end-of-flowering remains largely uncharacterized. End-of-flowering in Arabidopsis (Arabidopsis thaliana) consists of quasi-synchronous arrest of inflorescences, but it is unclear how arrest is correctly timed with respect to environmental stimuli and reproductive success. Here, we showed that Arabidopsis inflorescence arrest is a complex developmental phenomenon, which includes the arrest of the inflorescence meristem (IM), coupled with a separable "floral arrest" of all unopened floral primordia; these events occur well before visible inflorescence arrest. We showed that global inflorescence removal delays both IM and floral arrest, but that local fruit removal only delays floral arrest, emphasizing their separability. We tested whether cytokinin regulates inflorescence arrest, and found that cytokinin signaling dynamics mirror IM activity, while cytokinin treatment can delay both IM and floral arrest. We further showed that gain-of-function cytokinin receptor mutants can delay IM and floral arrest; conversely, loss-of-function mutants prevented the extension of flowering in response to inflorescence removal. Collectively, our data suggest that the dilution of cytokinin among an increasing number of sink organs leads to end-of-flowering in Arabidopsis by triggering IM and floral arrest.

Cytokinins and auxins in organs of aquatic carnivorous plants: what do they reflect?

BACKGROUND AND AIMS

Aquatic carnivorous plants have typical rootless linear shoots bearing traps and exhibit steep physiological polarity with rapid apical growth. The aim was to analyse auxin and cytokinin metabolites in traps, leaves/shoots and shoot apices in several species of genera Aldrovanda and Utricularia to elucidate how the hormonal profiles reflect the specific organ functions and polarity.

METHODS

The main auxin and cytokinin metabolites were analysed in miniature samples (>2 mg dry weight) of different organs of Aldrovanda vesiculosa and six Utricularia species using ultraperformance liquid chromatography coupled with triple quadrupole mass spectrometry.

KEY RESULTS

Total contents of biologically active forms (free bases, ribosides) of all four main endogenously occurring cytokinin types were consistently higher in traps than in leaves in four Utricularia species with monomorphic shoots and/or higher than in shoots in two Utricularia species with dimorphic shoots. In Aldrovanda traps, the total content of different cytokinin forms was similar to or lower than that in shoots. In U. australis leaves, feeding on prey increased all cytokinin forms, while no consistent differences occurred in Aldrovanda. In four aquatic Utricularia species with monomorphic shoots, the content of four auxin forms was usually higher in traps than in leaves. Zero IAA content was determined in U. australis leaves from a meso-eutrophic site or when prey-fed.

CONCLUSIONS

Different cytokinin and auxin profiles estimated in traps and leaves/shoots of aquatic carnivorous plants indicate an association with different dominant functions of these organs: nutrient uptake by traps versus photosynthetic function of traps. Interplay of cytokinins and auxins regulates apical dominance in these plants possessing strong polarity.

High-throughput interspecies profiling of acidic plant hormones using miniaturised sample processing

BACKGROUND

Acidic phytohormones are small molecules controlling many physiological functions in plants. A comprehensive picture of their profiles including the active forms, precursors and metabolites provides an important insight into ongoing physiological processes and is essential for many biological studies performed on plants.

RESULTS

A high-throughput sample preparation method for liquid chromatography-tandem mass spectrometry determination of 25 acidic phytohormones classed as auxins, jasmonates, abscisates and salicylic acid was optimised. The method uses a small amount of plant tissue (less than 10 mg fresh weight) and acidic extraction in 1 mol/L formic acid in 10% aqueous methanol followed by miniaturised purification on reverse phase sorbent accommodated in pipette tips organised in a 3D printed 96-place interface, capable of processing 192 samples in one run. The method was evaluated in terms of process efficiency, recovery and matrix effects as well as establishing validation parameters such as accuracy and precision. The applicability of the method in relation to the amounts of sample collected from distantly related plant species was evaluated and the results for phytohormone profiles are discussed in the context of literature reports.

CONCLUSION

The method developed enables high-throughput profiling of acidic phytohormones with minute amounts of plant material, and it is suitable for large scale interspecies studies.

Photolysis of caged cytokinin in single cells of Arabidopsis thaliana

BACKGROUND

Cytokinins are a class of phytohormone that play a crucial role in the development of plants. They are involved in the regulation of nearly every aspect of plant growth, from germination to senescence. The role of cytokinins in many developmental programs is complex and varies both spatially and temporally. Current techniques used to investigate the functions of cytokinins in plant development lack this spatial and temporal resolution required to observe cell-type specific effects.

RESULTS

To this end, we present a method of activating a caged cytokinin in single cells. A caged benzyladenine was synthesized, along with caged adenine as a negative control. In vitro testing confirmed ultraviolet light-mediated uncaging, and subsequent root growth assays demonstrated that uncaging produced a cytokinin phenotype. This uncaging was confined to single cells using multiphoton confocal microscopy. Using an Arabidopsis thaliana cytokinin reporter line expressing TCSn::GFP, the resulting GFP expression was confined to the uncaging region, including in single cells. This study presents a novel cell-targeted method of cytokinin delivery, which has the potential to elucidate a broad range of processes in plant development.

CONCLUSIONS

We combined multiphoton confocal microscopy and a caged cytokinin treatment, allowing cell type-specific uncaging of a cytokinin in Arabidopsis roots.

IPT9, a cis-zeatin cytokinin biosynthesis gene, promotes root growth

Cytokinin and auxin are plant hormones that coordinate many aspects of plant development. Their interactions in plant underground growth are well established, occurring at the levels of metabolism, signaling, and transport. Unlike many plant hormone classes, cytokinins are represented by more than one active molecule. Multiple mutant lines, blocking specific parts of cytokinin biosynthetic pathways, have enabled research in plants with deficiencies in specific cytokinin-types. While most of these mutants have confirmed the impeding effect of cytokinin on root growth, the ipt29 double mutant instead surprisingly exhibits reduced primary root length compared to the wild type. This mutant is impaired in cis-zeatin (cZ) production, a cytokinin-type that had been considered inactive in the past. Here we have further investigated the intriguing ipt29 root phenotype, opposite to known cytokinin functions, and the (bio)activity of cZ. Our data suggest that despite the ipt29 short-root phenotype, cZ application has a negative impact on primary root growth and can activate a cytokinin response in the stele. Grafting experiments revealed that the root phenotype of ipt29 depends mainly on local signaling which does not relate directly to cytokinin levels. Notably, ipt29 displayed increased auxin levels in the root tissue. Moreover, analyses of the differential contributions of ipt2 and ipt9 to the ipt29 short-root phenotype demonstrated that, despite its deficiency on cZ levels, ipt2 does not show any root phenotype or auxin homeostasis variation, while ipt9 mutants were indistinguishable from ipt29. We conclude that IPT9 functions may go beyond cZ biosynthesis, directly or indirectly, implicating effects on auxin homeostasis and therefore influencing plant growth.

Topolin cytokinins enhanced shoot proliferation, reduced hyperhydricity and altered cytokinin metabolism in Pistacia vera L. seedling explants

The effect of 10 µM meta-topolin (mT) and meta-topolin riboside (mTR) on in vitro proliferation and anomalies of Pistacia vera L. were evaluated and compared to that of 6-benzylaminopurine (BA). The highest proliferation rate (15.6) was recorded in the mT-medium, with a value 6 times higher than in BA-medium. Moreover, the lowest percentage of hyperhydric usable shoots (58,9%) and callus weight (46,9%) were found in mTR-treated shoots. Shoot tip as well as leaf necrosis were not influenced by cytokinin (CK) type. Image analysis was used to evaluate photosynthetic efficiency as well as anthocyanin index. Photosynthesis was more efficient with BA and mTR but the higher anthocyanin accumulation in BA-treated shoots suggests more stress. Endogenous CKs and their metabolites were determined in seedlings and, for the first time, the metabolism of exogenous BA, mT and mTR was studied in pistachio. The stimulating effect on cis-zeatin and its riboside and the appearance of BA and traces of ortho-topolin and para-topolin as natural CKs are discussed. The quantitative and qualitative CK metabolite analyses provides some initial clues as to why topolin would be superior to BA in terms of proliferation rate and avoiding hyperhydricity and allowed a better understanding of the effect of exogenous administration of CK.

Molecular framework integrating nitrate sensing in root and auxin-guided shoot adaptive responses

Mineral nutrition is one of the key environmental factors determining plant development and growth. Nitrate is the major form of macronutrient nitrogen that plants take up from the soil. Fluctuating availability or deficiency of this element severely limits plant growth and negatively affects crop production in the agricultural system. To cope with the heterogeneity of nitrate distribution in soil, plants evolved a complex regulatory mechanism that allows rapid adjustment of physiological and developmental processes to the status of this nutrient. The root, as a major exploitation organ that controls the uptake of nitrate to the plant body, acts as a regulatory hub that, according to nitrate availability, coordinates the growth and development of other plant organs. Here, we identified a regulatory framework, where cytokinin response factors (CRFs) play a central role as a molecular readout of the nitrate status in roots to guide shoot adaptive developmental response. We show that nitrate-driven activation of NLP7, a master regulator of nitrate response in plants, fine tunes biosynthesis of cytokinin in roots and its translocation to shoots where it enhances expression of CRFs. CRFs, through direct transcriptional regulation of PIN auxin transporters, promote the flow of auxin and thereby stimulate the development of shoot organs.

New Water-Soluble Cytokinin Derivatives and Their Beneficial Impact on Barley Yield and Photosynthesis

Solubility of growth regulators is essential for their use in agriculture. Four new cytokinin salts─6-benzylaminopurine mesylate (1), 6-(2-hydroxybenzylamino)purine mesylate (2), 6-(3-hydroxybenzylamino)purine mesylate (3), and 6-(3-methoxybenzylamino)purine mesylate (4)─were synthesized, and their crystal structures were determined to clarify structural influence on water solubility. The mesylates were several orders of magnitude more water-soluble than the parent CKs. The new salts significantly reduced chlorophyll degradation and impairment of photosystem II functionality in barley leaf segments undergoing artificial senescence and had pronounced effects on the leaves' endogenous CK pools, maintaining high concentrations of functional metabolites for several days, unlike canonical CKs. A foliar treatment with 1 and 3 increased the harvest yield of spring barley by up to 8% when compared to treatment with the parent CKs while also increasing the number of productive tillers. This effect was attributed to the higher bioavailability of the mesylate salts and the avoidance of dimethyl sulfoxide exposure.

Impairment of root auxin-cytokinins homeostasis induces collapse of incompatible melon grafts during fruit ripening

The factors underlying the plant collapse of certain melon-pumpkin graft combinations are not fully understood. Our working hypothesis was that impairment of photoassimilates transport in incompatible combinations induces an imbalance in the homeostasis of root auxin (indole-3-acetic acid; IAA) and of cytokinins, probably triggering plant collapse. Root IAA and cytokinins levels in the presence and absence of fruit and changes in root and scion metabolites were investigated in compatible and incompatible combinations. We showed elevated levels of IAA, 2-oxoindole-3-acetic acid (IAA catabolite), indole-3-acetylaspartate (IAA conjugate), and cis-zeatin-type cytokinins, but low levels of trans-zeatin-type cytokinins in the roots of plants of the incompatible combination during fruit ripening. Similarly, during fruit ripening, the expression of the YUCCA genes, YUC2, YUC6, and YUC11 (required for auxin biosynthesis), the GRETCHEN-HAGEN3 gene (required for auxin conjugation), and the cytokinin oxidase/dehydrogenase 7 (CKX7) gene (regulates the irreversible degradation of cytokinin) was enhanced in the roots of plants of the incompatible combination. Moreover, in the incompatible combination the fruiting process restricted transport of photoassimilates to the rootstock and induces their accumulation in the scion. In addition, high levels of hydrogen peroxide and malondialdehyde and reduced activity of antioxidant enzymes were observed in the roots of the incompatible graft. Our results showed that the collapse of the incompatible graft combination during fruit ripening is closely associated with a dramatic accumulation of IAA in the roots, which probably elicits oxidative damage and disturbs the balance of IAA and cytokinins that is of critical importance in melon-pumpkin graft compatibility.

CYTOKININ RESPONSE FACTOR 2 is involved in modulating the salt stress response

Cytokinin has strong connections to development and a growing role in the abiotic stress response. Here we show that CYTOKININ RESPONSE FACTOR 2 (CRF2) is additionally involved in the salt (NaCl) stress response. CRF2 promoter-GUS expression indicates CRF2 involvement in the response to salt stress as well as the previously known cytokinin response. Interestingly, CRF2 mutant seedlings are quite similar to the wild type (WT) under non-stressed conditions yet have many distinct changes in response to salt stress. Cytokinin levels measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS) that increased in the WT after salt stress are decreased in crf2, potentially from CRF2 regulation of cytokinin biosynthesis genes. Ion content measured by inductively coupled plasma optical emission spectrometry (ICP-OES) was increased in the WT for Na, K, Mn, Ca and Mg after salt stress, whereas the corresponding Ca and Mg increases are lacking in crf2. Many genes examined by RNA-seq analysis were altered transcriptionally by salt stress in both the WT and crf2, yet interestingly approximately one-third of salt-modified crf2 transcripts (2655) showed unique regulation. Different transcript profiles for salt stress in crf2 compared with the WT background was further supported through an examination of co-expressed genes by weighted gene correlation network analysis (WGCMA) and principal component analysis (PCA). Additionally, Gene Ontology (GO) enrichment terms found from salt-treated transcripts revealed most photosynthesis-related terms as only being affected in crf2, leading to an examination of chlorophyll levels and the efficiency of photosystem II (via the ratio of variable fluorescence to maximum fluorescence, F_v /F_m ) as well as physiology after salt treatment. Salt stress-treated crf2 plants had both reduced chlorophyll levels and lower F_v /F_m values compared with the WT, suggesting that CRF2 plays a role in the modulation of salt stress responses linked to photosynthesis.

CLAVATA modulates auxin homeostasis and transport to regulate stem cell identity and plant shape in a moss

The CLAVATA pathway is a key regulator of stem cell function in the multicellular shoot tips of Arabidopsis, where it acts via the WUSCHEL transcription factor to modulate hormone homeostasis. Broad-scale evolutionary comparisons have shown that CLAVATA is a conserved regulator of land plant stem cell function, but CLAVATA acts independently of WUSCHEL-like (WOX) proteins in bryophytes. The relationship between CLAVATA, hormone homeostasis and the evolution of land plant stem cell functions is unknown. Here we show that in the moss, Physcomitrella (Physcomitrium patens), CLAVATA affects stem cell activity by modulating hormone homeostasis. CLAVATA pathway genes are expressed in the tip cells of filamentous tissues, regulating cell identity, filament branching, plant spread and auxin synthesis. The receptor-like kinase PpRPK2 plays the major role, and Pprpk2 mutants have abnormal responses to cytokinin, auxin and auxin transport inhibition, and show reduced expression of PIN auxin transporters. We propose a model whereby PpRPK2 modulates auxin gradients in filaments to determine stem cell identity and overall plant form. Our data indicate that CLAVATA-mediated auxin homeostasis is a fundamental property of plant stem cell function, probably exhibited by the last shared common ancestor of land plants.

Control of cambium initiation and activity in Arabidopsis by the transcriptional regulator AHL15

Plant secondary growth, which is the basis of wood formation, includes the production of secondary xylem, which is derived from meristematic cambium cells embedded in vascular tissue. Here, we identified an important role for the Arabidopsis thaliana (Arabidopsis) AT-HOOK MOTIF CONTAINING NUCLEAR LOCALIZED 15 (AHL15) transcriptional regulator in controlling vascular cambium activity. The limited secondary xylem development in inflorescence stems of herbaceous Arabidopsis plants was significantly reduced in ahl15 loss-of-function mutants, whereas constitutive or vascular meristem-specific AHL15 overexpression produced woody inflorescence stems. AHL15 was required for enhanced secondary xylem formation in the woody suppressor of overexpression of constans 1 (soc1) fruitfull (ful) double loss-of-function mutant. Moreover, we found that AHL15 induces vascular cambium activity downstream of the repressing SOC1 and FUL transcription factors, most likely similar to how it enhances lateral branching by promoting biosynthesis of the hormone cytokinin. Our results uncover a novel pathway driving cambium development, in which AHL15 expression levels act in parallel to and are dependent on the well-established TDIF-PXY-WOX pathway to differentiate between herbaceous and woody stem growth.

Changing Temperature Conditions during Somatic Embryo Maturation Result in Pinus pinaster Plants with Altered Response to Heat Stress

Under the global warming scenario, obtaining plant material with improved tolerance to abiotic stresses is a challenge for afforestation programs. In this work, maritime pine (Pinus pinaster Aiton) plants were produced from somatic embryos matured at different temperatures (18, 23, or 28 °C, named after M18, M23, and M28, respectively) and after 2 years in the greenhouse a heat stress treatment (45 °C for 3 h/day for 10 days) was applied. Temperature variation during embryo development resulted in altered phenotypes (leaf histology, proline content, photosynthetic rates, and hormone profile) before and after stress. The thickness of chlorenchyma was initially larger in M28 plants, but was significantly reduced after heat stress, while increased in M18 plants. Irrespective of their origin, when these plants were subjected to a heat treatment, relative water content (RWC) and photosynthetic carbon assimilation rates were not significantly affected, although M18 plants increased net photosynthesis rate after 10 days recovery (tR). M18 plants showed proline contents that increased dramatically (2.4-fold) when subjected to heat stress, while proline contents remained unaffected in M23 and M28 plants. Heat stress significantly increased abscisic acid (ABA) content in the needles of maritime pine plants (1.4-, 3.6- and 1.9-fold in M18, M23, and M28 plants, respectively), while indole-3-acetic acid content only increased in needles from M23 plants. After the heat treatment, the total cytokinin contents of needles decreased significantly, particularly in M18 and M28 plants, although levels of active forms (cytokinin bases) did not change in M18 plants. In conclusion, our results suggest that maturation of maritime pine somatic embryos at lower temperature resulted in plants with better performance when subjected to subsequent high temperature stress, as demonstrated by faster and higher proline increase, lower increases in ABA levels, no reduction in active cytokinin, and a better net photosynthesis rate recovery.

Arabidopsis HIPP proteins regulate endoplasmic reticulum-associated degradation of CKX proteins and cytokinin responses

Eukaryotic organisms are equipped with quality-control mechanisms that survey protein folding in the endoplasmic reticulum (ER) and remove non-native proteins by ER-associated degradation (ERAD). Recent research has shown that cytokinin-degrading CKX proteins are subjected to ERAD during plant development. The mechanisms of plant ERAD, including the export of substrate proteins from the ER, are not fully understood, and the molecular components involved in the ERAD of CKX are unknown. Here, we show that heavy metal-associated isoprenylated plant proteins (HIPPs) interact specifically with CKX proteins synthesized in the ER and processed by ERAD. CKX-HIPP protein complexes were detected at the ER as well as in the cytosol, suggesting that the complexes involve retrotranslocated CKX protein species. Altered CKX levels in HIPP-overexpressing and higher-order hipp mutant plants suggest that the studied HIPPs control the ERAD of CKX. Deregulation of CKX proteins caused corresponding changes in the cytokinin signaling activity and triggered typical morphological cytokinin responses. Notably, transcriptional repression of HIPP genes by cytokinin indicates a feedback regulatory mechanism of cytokinin homeostasis and signaling responses. Moreover, loss of function of HIPP genes constitutively activates the unfolded protein response and compromises the ER stress tolerance. Collectively, these results suggests that HIPPs represent novel functional components of plant ERAD.

Non-cell autonomous and spatiotemporal signalling from a tissue organizer orchestrates root vascular development

During plant development, a precise balance of cytokinin is crucial for correct growth and patterning, but it remains unclear how this is achieved across different cell types and in the context of a growing organ. Here we show that in the root apical meristem, the TMO5/LHW complex increases active cytokinin levels via two cooperatively acting enzymes. By profiling the transcriptomic changes of increased cytokinin at single-cell level, we further show that this effect is counteracted by a tissue-specific increase in CYTOKININ OXIDASE 3 expression via direct activation of the mobile transcription factor SHORTROOT. In summary, we show that within the root meristem, xylem cells act as a local organizer of vascular development by non-autonomously regulating cytokinin levels in neighbouring procambium cells via sequential induction and repression modules.

Macro-microporous zeolitic imidazole framework-8/cellulose aerogel for semi-automated pipette tip solid phase extraction of fluoroquinolones in water

In this study, Zeolitic Imidazole Framework-8/cellulose aerogel (ZIF-8/CA) hybrid was successfully fabricated through a simple doping method and ZIF-8 acted as the major component for adsorption. In order to elucidate the adsorption mechanism deeply, molecular simulation was adopted to the expound the interaction modes between ZIF-8 and the fluoroquinolones (FQs). ZIF-8/CA was used as the adsorbent for semi-automated pipette tip solid phase extraction (PT-SPE). In combination with high performance liquid chromatography tandem fluorescence detector (HPLC-FLD), the established method was successfully employed to determine trace amount of FQs in water samples. Extraction parameters such as the content of ZIF-8, pH of sample solution, volume of sample, flow rate of sampling, type and volume of elution solvent were investigated. Under the optimized conditions, satisfactory linearity was achieved with the correlation coefficient (R²) ranging from 0.9954 to 0.9992. The limits of detection were in the range of 0.337-1.707 ng L^-1. And the recoveries varied from 75.9% to 96.8% with RSD less than 8.0%. The established method was demonstrated to be sensitive, efficient and convenient.

Plant Growth Regulators INCYDE and TD-K Underperform in Cereal Field Trials

Using plant growth regulators to alter cytokinin homeostasis with the aim of enhancing endogenous cytokinin levels has been proposed as a strategy to increase yields in wheat and barley. The plant growth regulators INCYDE and CPPU inhibit the cytokinin degrading enzyme cytokinin oxidase/dehydrogenase (CKX), while TD-K inhibits the process of senescence. We report that the application of these plant growth regulators in wheat and barley field trials failed to enhance yields, or change the components of yields. Analyses of the endogenous cytokinin content showed a high concentration of trans-zeatin (tZ) in both wheat and barley grains at four days after anthesis, and statistically significant, but probably biologically insignificant, increases in cisZ-O-glucoside, along with small decreases in cZ riboside (cZR), dihydro Z (DHZ), and DHZR and DHZOG cytokinins, following INCYDE application to barley at anthesis. We discuss possible reasons for the lack of efficacy of the three plant growth regulators under field conditions and comment on future approaches to manipulating yield in the light of the strong homeostatic mechanisms controlling endogenous cytokinin levels.

Jasmonate inhibits adventitious root initiation through repression of CKX1 and activation of RAP2.6L transcription factor in Arabidopsis

Adventitious rooting is a de novo organogenesis process that enables plants to propagate clonally and cope with environmental stresses. Adventitious root initiation (ARI) is controlled by interconnected transcriptional and hormonal networks, but there is little knowledge of the genetic and molecular programs orchestrating these networks. Thus, we have applied genome-wide transcriptome profiling to elucidate the transcriptional reprogramming events preceding ARI. These reprogramming events are associated with the down-regulation of cytokinin (CK) signaling and response genes, which could be triggers for ARI. Interestingly, we found that CK free base (iP, tZ, cZ, and DHZ) content declined during ARI, due to down-regulation of de novo CK biosynthesis and up-regulation of CK inactivation pathways. We also found that MYC2-dependent jasmonate (JA) signaling inhibits ARI by down-regulating the expression of the CYTOKININ OXIDASE/DEHYDROGENASE1 (CKX1) gene. We also demonstrated that JA and CK synergistically activate expression of the transcription factor RELATED to APETALA2.6 LIKE (RAP2.6L), and constitutive expression of this transcription factor strongly inhibits ARI. Collectively, our findings reveal that previously unknown genetic interactions between JA and CK play key roles in ARI.

Beyond transport: cytokinin ribosides are translocated and active in regulating the development and environmental responses of plants

Riboside type cytokinins are key components in cytokinin metabolism, transport, and sensitivity, making them important functional signals in plant growth and development and environmental stress responses. Cytokinin (CKs) are phytohormones that regulate multiple processes in plants and are critical for agronomy, as they are involved in seed filling and plant responses to biotic and abiotic stress. Among the over 30 identified CKs, there is uncertainty about the roles of many of the individual CK structural forms. Cytokinin free bases (CKFBs), have been studied in great detail, but, by comparison, roles of riboside-type CKs (CKRs) in CK metabolism and associated signaling pathways and their distal impacts on plant physiology remain largely unknown. Here, recent findings on CKR abundance, transport and localization, are summarized, and their importance in planta is discussed. The history of CKR analyses is reviewed, in the context of the determination of CK metabolic pathways, and research on CKR affinity for CK receptors, all of which yield essential insights into their functions. Recent studies suggest that CKR forms are a lot more than a group of transport CKs and, beyond this, they play important roles in plant development and responses to environmental stress. In this context, this review discusses the involvement of CKRs in plant development, and highlight the less anticipated functions of CKRs in abiotic stress tolerance. Based on this, possible mechanisms for CKR modes of action are proposed and experimental approaches to further uncover their roles and future biotechnological applications are suggested.

Concurrent overexpression of amino acid permease AAP1(3a) and SUT1 sucrose transporter in pea resulted in increased seed number and changed cytokinin and protein levels

Using pea as our model crop, we sought to understand the regulatory control over the import of sugars and amino acids into the developing seeds and its importance for seed yield and quality. Transgenic peas simultaneously overexpressing a sucrose transporter and an amino acid transporter were developed. Pod walls, seed coats, and cotyledons were analysed separately, as well as leaves subtending developing pods. Sucrose, starch, protein, free amino acids, and endogenous cytokinins were measured during development. Temporal gene expression analyses (RT-qPCR) of amino acid (AAP), sucrose (SUT), and SWEET transporter family members, and those from cell wall invertase, cytokinin biosynthetic (IPT) and degradation (CKX) gene families indicated a strong effect of the transgenes on gene expression. In seed coats of the double transgenics, increased content and prolonged presence of cytokinin was particularly noticeable. The transgenes effectively promoted transition of young sink leaves into source leaves. We suggest the increased flux of sucrose and amino acids from source to sink, along with increased interaction between cytokinin and cell wall invertase in developing seed coats led to enhanced sink activity, resulting in higher cotyledon sucrose at process pea harvest, and increased seed number and protein content at maturity.

Interactive effects of plant growth-promoting rhizobacteria and a seaweed extract on the growth and physiology of Allium cepa L. (onion)

Detrimental effects caused by the overuse of synthetic agrochemicals have led to the development of natural biostimulants such as seaweed extracts and plant growth-promoting rhizobacteria (PGPR) being used as an alternative, environmentally-friendly technology to improve crop growth and increase agricultural yields. The present study aimed to investigate the interactions between PGPR and a commercial seaweed extract on the growth and biochemical composition of onion (Allium cepa). A pot trial was conducted under greenhouse conditions where onion plants were treated individually with the two PGPR, namely Bacillus licheniformis (BL) and Pseudomonas fluorescens (PF) and a seaweed extract Kelpak® (KEL) and combinations of KEL + BL and KEL + PF. Growth and yield parameters were measured after 12 weeks. KEL-treated plants showed the best growth response and overcame the inhibitory effects of BL treatment. KEL-treated plants also had the highest chlorophyll content. PGPR application improved the mineral nutrition of onion with these plants having the highest mineral content in the leaves and bulb. All biostimulant treatments increased the endogenous cytokinin and auxin content with the highest concentrations generally detected in the PF-treated plants. These results suggest that co-application of different biostimulant classes with different modes of action could further increase crop productivity with an improvement in both growth and nutrition content being achieved in onion with the co-application of a seaweed extract and PGPR.

Cytokinins are involved in drought tolerance of Pinus radiata plants originating from embryonal masses induced at high temperatures

Vegetative propagation through somatic embryogenesis is an effective method to produce elite varieties and can be applied as a tool to study the response of plants to different stresses. Several studies show that environmental changes during embryogenesis could determine future plant development. Moreover, we previously reported that physical and chemical conditions during somatic embryogenesis can determine the protein, hormone and metabolite profiles, as well as the micromorphological and ultrastructural organization of embryonal masses and somatic embryos. In this sense, phytohormones are key players throughout the somatic embryogenesis process as well as during numerous stress-adaptation responses. In this work, we first applied different high-temperature regimes (30 °C, 4 weeks; 40 °C, 4 days; 50 °C, 5 min) during induction of Pinus radiata D. Don somatic embryogenesis, together with control temperature (23 °C). Then, the somatic plants regenerated from initiated embryogenic cell lines and cultivated in greenhouse conditions were subjected to drought stress and control treatments to evaluate survival, growth and several physiological traits (relative water content, water potential, photosynthesis, stomatal conductance and transpiration). Based on those preliminary results, even more extreme high-temperature regimes were applied during induction (40 °C, 4 h; 50 °C, 30 min; 60 °C, 5 min) and the corresponding cytokinin profiles of initiated embryonal masses from different lines were analysed. The results showed that the temperature regime during induction had delayed negative effects on drought resilience of somatic plants as indicated by survival, photosynthetic activity and water- use efficiency. However, high temperatures for extended periods of time enhanced subsequent plant growth in well-watered conditions. High-temperature regime treatments induced significant differences in the profile of total cytokinin bases, N6-isopentenyladenine, cis-zeatin riboside and trans-zeatin riboside. We concluded that phytohormones could be potential regulators of stress-response processes during initial steps of somatic embryogenesis and that they may have delayed implications in further developmental processes, determining the performance of the generated plants.

Soil nutrient status of KwaZulu-Natal savanna and grassland biomes causes variation in cytokinin functional groups and their levels in above-ground and underground parts of three legumes

Cytokinins (CKs) are involved in several developmental stages in the life-cycle of plants. The CK content in plants and their respective organs are susceptible to changes under different environmental conditions. In the current study, we profiled the CK content in the above and underground organs of three legumes (Lessertia frutescens, Mucuna pruriens and Pisum sativum) grown in soils collected from four locations (Ashburton, Bergville, Hluhluwe and Izingolweni) in KwaZulu-Natal province, South Africa. The quantified CK contents in the three legumes were categorized on the basis of their side chains (isoprenoid, aromatic and furfural) and modifications (e.g. free bases and glucosides). Legume and soil types as well as their interaction significantly influenced the concentrations of CKs. Lessertia frutescens, Mucuna pruriens and Pisum sativum had CK content that ranged from 124-653, 170-670 and 69-595 pmol/g DW, respectively. Substantial quantity (> 600 pmol/g DW) of CK were observed in plants grown in Bergville (above-ground part of Lessertia frutescens) and Izingolweni (underground part of Mucuna pruriens) soils. A total of 28 CK derivatives observed in the legumes comprised of isoprenoid (22), aromatic (5) and furfural (1) side-chain CKs. However, the 16 CK derivatives in Mucuna pruriens were isoprenoid-type based on the side-chain. Generally, a higher ratio of cis-zeatin (cZ) relative to the trans-zeatin (tZ) was evident in the above-ground part of Lessertia frutescens and Pisum sativum for the four soil treatments. In terms of functional and physiological importance of the CKs, the free bases (active form) and ribosides (translocation form) were the most abundant CK in Lessertia frutescens and Pisum sativum. However, N-glucoside, a deactivation/detoxicification product was the most dominant CK in Mucuna pruriens from Hluhluwe and Izingolweni soils. The total CKs in the underground parts of the legumes had a positive significant correlation with the total phosphorus and nitrogen content in the plant as well as the soil nitrogen. Overall, the CK profiles of the legumes were strongly influenced by the soil types.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01021-2.

Does scion-rootstock compatibility modulate photoassimilate and hormone trafficking through the graft junction in melon-pumpkin graft combinations?

The effect of the rootstock on the acropetal and basipetal transport of photoassimilates and hormones was studied in the 'Kiran' (Ki) melon cultivar grafted onto pumpkin rootstocks with different degrees of compatibility. A complementary experiment was performed to compare the incompatible combination (as evidenced by plant collapse at the fruit ripening stage), designated Ki/r53, with self-grafted r53/r53 as a model compatible combination. Both experiments showed the accumulation of a number of amino acids, sugars, and sugar alcohols in the scion of the incompatible Ki/r53 grafts. Additionally, they showed a marked reduction of trans-zeatin-type cytokinins and an elevated content of cis-zeatin-type cytokinins in the rootstock, and the opposite pattern in the scion, hinting at the possible involvement of a hormonal signal for graft compatibility. There was no direct evidence of a blockage at the graft union, since hormone acropetal and basipetal trafficking was demonstrated for all combinations. Dye uptake experiments did not show xylem flow impairment. A possibly significant finding in the incompatible combination was the deposition of undifferentiated cells in the hollow space that replaces the pith region in melon and pumpkin. The link between the above findings and the collapse of the plants of the incompatible combination remains unclear.

Clade III cytokinin response factors share common roles in response to oxidative stress responses linked to cytokinin synthesis

Cytokinin response factors (CRFs) are transcription factors that are involved in cytokinin (CK) response, as well as being linked to abiotic stress tolerance. In particular, oxidative stress responses are activated by Clade III CRF members, such as AtCRF6. Here we explored the relationships between Clade III CRFs and oxidative stress. Transcriptomic responses to oxidative stress were determined in two Clade III transcription factors, Arabidopsis AtCRF5 and tomato SlCRF5. AtCRF5 was required for regulated expression of >240 genes that are involved in oxidative stress response. Similarly, SlCRF5 was involved in the regulated expression of nearly 420 oxidative stress response genes. Similarities in gene regulation by these Clade III members in response to oxidative stress were observed between Arabidopsis and tomato, as indicated by Gene Ontology term enrichment. CK levels were also changed in response to oxidative stress in both species. These changes were regulated by Clade III CRFs. Taken together, these findings suggest that Clade III CRFs play a role in oxidative stress response as well as having roles in CK signaling.

Early stages of legume-rhizobia symbiosis are controlled by ABCG-mediated transport of active cytokinins

Growing evidence has highlighted the essential role of plant hormones, notably, cytokinins (CKs), in nitrogen-fixing symbiosis, both at early and late nodulation stages^1,2. Despite numerous studies showing the central role of CK in nodulation, the importance of CK transport in the symbiosis is unknown. Here, we show the role of ABCG56, a full-size ATP-binding cassette (ABC) transporter in the early stages of the nodulation. MtABCG56 is expressed in roots and nodules and its messenger RNA levels increase upon treatment with symbiotic bacteria, isolated Nod factor and CKs, accumulating within the epidermis and root cortex. MtABCG56 exports bioactive CKs in an ATP-dependent manner over the plasma membrane and its disruption results in an impairment of nodulation. Our data indicate that ABCG-mediated cytokinin transport is important for proper establishment of N-fixing nodules.

Function of the pseudo phosphotransfer proteins has diverged between rice and Arabidopsis

The phytohormone cytokinin plays a significant role in nearly all aspects of plant growth and development. Cytokinin signaling has primarily been studied in the dicot model Arabidopsis, with relatively little work done in monocots, which include rice (Oryza sativa) and other cereals of agronomic importance. The cytokinin signaling pathway is a phosphorelay comprised of the histidine kinase receptors, the authentic histidine phosphotransfer proteins (AHPs) and type-B response regulators (RRs). Two negative regulators of cytokinin signaling have been identified: the type-A RRs, which are cytokinin primary response genes, and the pseudo histidine phosphotransfer proteins (PHPs), which lack the His residue required for phosphorelay. Here, we describe the role of the rice PHP genes. Phylogenic analysis indicates that the PHPs are generally first found in the genomes of gymnosperms and that they arose independently in monocots and dicots. Consistent with this, the three rice PHPs fail to complement an Arabidopsis php mutant (aphp1/ahp6). Disruption of the three rice PHPs results in a molecular phenotype consistent with these elements acting as negative regulators of cytokinin signaling, including the induction of a number of type-A RR and cytokinin oxidase genes. The triple php mutant affects multiple aspects of rice growth and development, including shoot morphology, panicle architecture, and seed fill. In contrast to Arabidopsis, disruption of the rice PHPs does not affect root vascular patterning, suggesting that while many aspects of key signaling networks are conserved between monocots and dicots, the roles of at least some cytokinin signaling elements are distinct.

SEEDSTICK Controls Arabidopsis Fruit Size by Regulating Cytokinin Levels and FRUITFULL

Upon fertilization, the ovary increases in size and undergoes a complex developmental process to become a fruit. We show that cytokinins (CKs), which are required to determine ovary size before fertilization, have to be degraded to facilitate fruit growth. The expression of CKX7, which encodes a cytosolic CK-degrading enzyme, is directly positively regulated post-fertilization by the MADS-box transcription factor STK. Similar to stk, two ckx7 mutants possess shorter fruits than wild type. Quantification of CKs reveals that stk and ckx7 mutants have high CK levels, which negatively control cell expansion during fruit development, compromising fruit growth. Overexpression of CKX7 partially complements the stk fruit phenotype, confirming a role for CK degradation in fruit development. Finally, we show that STK is required for the expression of FUL, which is essential for valve elongation. Overall, we provide insights into the link between CKs and molecular pathways that control fruit growth.

Priming Maritime Pine Megagametophytes during Somatic Embryogenesis Improved Plant Adaptation to Heat Stress

In the context of global climate change, forest tree research should be addressed to provide genotypes with increased resilience to high temperature events. These improved plants can be obtained by heat priming during somatic embryogenesis (SE), which would produce an epigenetic-mediated transgenerational memory. Thereby, we applied 37 °C or 50 °C to maritime pine (Pinus pinaster) megagametophytes and the obtained embryogenic masses went through the subsequent SE phases to produce plants that were further subjected to heat stress conditions. A putative transcription factor WRKY11 was upregulated in priming-derived embryonal masses, and also in the regenerated P37 and P50 plants, suggesting its role in establishing an epigenetic memory in this plant species. In vitro-grown P50 plants also showed higher cytokinin content and SOD upregulation, which points to a better responsiveness to heat stress. Heat exposure of two-year-old maritime pine plants induced upregulation of HSP70 in those derived from primed embryogenic masses, that also showed better osmotic adjustment and higher increases in chlorophyll, soluble sugars and starch contents. Moreover, ϕPSII of P50 plants was less affected by heat exposure. Thus, our results suggest that priming at 50 °C at the SE induction phase is a promising strategy to improve heat resilience in maritime pine.

Elucidation of molecular and hormonal background of early growth cessation and endodormancy induction in two contrasting Populus hybrid cultivars

BACKGROUND

Over the life cycle of perennial trees, the dormant state enables the avoidance of abiotic stress conditions. The growth cycle can be partitioned into induction, maintenance and release and is controlled by complex interactions between many endogenous and environmental factors. While phytohormones have long been linked with dormancy, there is increasing evidence of regulation by DAM and CBF genes. To reveal whether the expression kinetics of CBFs and their target PtDAM1 is related to growth cessation and endodormancy induction in Populus, two hybrid poplar cultivars were studied which had known differential responses to dormancy inducing conditions.

RESULTS

Growth cessation, dormancy status and expression of six PtCBFs and PtDAM1 were analyzed. The 'Okanese' hybrid cultivar ceased growth rapidly, was able to reach endodormancy, and exhibited a significant increase of several PtCBF transcripts in the buds on the 10th day. The 'Walker' cultivar had delayed growth cessation, was unable to enter endodormancy, and showed much lower CBF expression in buds. Expression of PtDAM1 peaked on the 10th day only in the buds of 'Okanese'. In addition, PtDAM1 was not expressed in the leaves of either cultivar while leaf CBFs expression pattern was several fold higher in 'Walker', peaking at day 1. Leaf phytohormones in both cultivars followed similar profiles during growth cessation but differentiated based on cytokinins which were largely reduced, while the Ox-IAA and iP7G increased in 'Okanese' compared to 'Walker'. Surprisingly, ABA concentration was reduced in leaves of both cultivars. However, the metabolic deactivation product of ABA, phaseic acid, exhibited an early peak on the first day in 'Okanese'.

CONCLUSIONS

Our results indicate that PtCBFs and PtDAM1 have differential kinetics and spatial localization which may be related to early growth cessation and endodormancy induction under the regime of low night temperature and short photoperiod in poplar. Unlike buds, PtCBFs and PtDAM1 expression levels in leaves were not associated with early growth cessation and dormancy induction under these conditions. Our study provides new evidence that the degradation of auxin and cytokinins in leaves may be an important regulatory point in a CBF-DAM induced endodormancy. Further investigation of other PtDAMs in bud tissue and a study of both growth-inhibiting and the degradation of growth-promoting phytohormones is warranted.

Dynamic cytokinin signalling landscapes during lateral root formation in Arabidopsis

By forming lateral roots, plants expand their root systems to improve anchorage and absorb more water and nutrients from the soil. Each phase of this developmental process in Arabidopsis is tightly regulated by dynamic and continuous signalling of the phytohormones cytokinin and auxin. While the roles of auxin in lateral root organogenesis and spatial accommodation by overlying cell layers have been well studied, insights on the importance of cytokinin is still somewhat limited. Cytokinin is a negative regulator of lateral root formation with versatile modes of action being activated at different root developmental zones. Here, we review the latest progress made towards our understanding of these spatially separated mechanisms of cytokinin-mediated signalling that shape lateral root initiation, outgrowth and emergence and highlight some of the enticing open questions.

Cytokinin regulates the activity of the inflorescence meristem and components of seed yield in oilseed rape

The number of flowers and seed-bearing structures formed by the inflorescence meristem and the formation of ovules in the female reproductive part of the flowers are important yield-related traits of crop plants. It has been shown that cytokinin is a pivotal factor regulating these traits. Here, we explore the impact of mutation of CYTOKININ OXIDASE/DEHYDROGENASE (CKX) genes encoding cytokinin-degrading enzymes on these yield-related traits in oilseed rape (Brassica napus L.). We describe the identification of four BnCKX3 and two BnCKX5 genes as regulators of reproductive development in the allotetraploid B. napus. RNA-seq analysis and in situ hybridization showed expression of these genes in reproductive organs. Loss-of-function mutants for each of these CKX gene copies were identified by targeting induced local lesions in genomes (TILLING) and combined by crossing. Sextuple ckx3 ckx5 mutants showed an increased cytokinin concentration and larger and more active inflorescence meristems. They also produced up to 72% more flowers with gynoecia containing 32% more ovules and up to 54% more pods on the main stem. The weight of seeds harvested from the main stem of plants grown in the greenhouse or in the field was increased by 20-32%. Our results show that cytokinin regulates inflorescence meristem and placenta activity in oilseed rape. The work demonstrates the potential to achieve yield enhancement in a dicot crop plant by modulating the cytokinin status through mutagenesis of specific CKX genes.

Reaction Wood Anatomical Traits and Hormonal Profiles in Poplar Bent Stem and Root

Reaction wood (RW) formation is an innate physiological response of woody plants to counteract mechanical constraints in nature, reinforce structure and redirect growth toward the vertical direction. Differences and/or similarities between stem and root response to mechanical constraints remain almost unknown especially in relation to phytohormones distribution and RW characteristics. Thus, Populus nigra stem and root subjected to static non-destructive mid-term bending treatment were analyzed. The distribution of tension and compression forces was firstly modeled along the main bent stem and root axis; then, anatomical features, chemical composition, and a complete auxin and cytokinin metabolite profiles of the stretched convex and compressed concave side of three different bent stem and root sectors were analyzed. The results showed that in bent stems RW was produced on the upper stretched convex side whereas in bent roots it was produced on the lower compressed concave side. Anatomical features and chemical analysis showed that bent stem RW was characterized by a low number of vessel, poor lignification, and high carbohydrate, and thus gelatinous layer in fiber cell wall. Conversely, in bent root, RW was characterized by high vessel number and area, without any significant variation in carbohydrate and lignin content. An antagonistic interaction of auxins and different cytokinin forms/conjugates seems to regulate critical aspects of RW formation/development in stem and root to facilitate upward/downward organ bending. The observed differences between the response stem and root to bending highlight how hormonal signaling is highly organ-dependent.

A metabolite roadmap of the wood-forming tissue in Populus tremula

Wood, or secondary xylem, is the product of xylogenesis, a developmental process that begins with the proliferation of cambial derivatives and ends with mature xylem fibers and vessels with lignified secondary cell walls. Fully mature xylem has undergone a series of cellular processes, including cell division, cell expansion, secondary wall formation, lignification and programmed cell death. A complex network of interactions between transcriptional regulators and signal transduction pathways controls wood formation. However, the role of metabolites during this developmental process has not been comprehensively characterized. To evaluate the role of metabolites during wood formation, we performed a high spatial resolution metabolomics study of the wood-forming zone of Populus tremula, including laser dissected aspen ray and fiber cells. We show that metabolites show specific patterns within the wood-forming zone, following the differentiation process from cell division to cell death. The data from profiled laser dissected aspen ray and fiber cells suggests that these two cell types host distinctly different metabolic processes. Furthermore, by integrating previously published transcriptomic and proteomic profiles generated from the same trees, we provide an integrative picture of molecular processes, for example, deamination of phenylalanine during lignification is of critical importance for nitrogen metabolism during wood formation.

Cytokinin isopentenyladenine and its glucoside isopentenyladenine-9G delay leaf senescence through activation of cytokinin-associated genes

Cytokinins (CKs) are well-known as a class of phytohormones capable of delaying senescence in detached leaves. However, CKs are often treated as a monolithic group of compounds even though dozens of CK species are present in plants with varied degrees of reported biological activity. One specific type of CK, isopentenyladenine base (iP), has been demonstrated as having roles in delaying leaf senescence, inhibition of root growth, and promoting shoot regeneration. However, its N-glucosides isopentenyladenine-7- and -9-glucoside (iP7G, iP9G) have remained understudied and thought of as inactive cytokinins for several decades, despite their relatively high concentrations in plants such as the model species Arabidopsis thaliana. Here we show that iP and one of its glucosides, iP9G, are capable of delaying senescence in leaves, though the glucosides having little to no activity in other bioassays. Additionally, we performed the first transcriptomic study of iP-delayed cotyledon senescence which shows that iP is capable of upregulating photosynthetic genes and downregulating catabolic genes in detached cotyledons. Transcriptomic analysis also shows iP9G has limited effects on gene expression, but that the few affected genes are CK-related and are similar to those seen from iP effects during senescence as seen for the type-A response regulator ARR6. These findings suggest that iP9G functions as an active CK during senescence.

ETHYLENE RESPONSE FACTOR 115 integrates jasmonate and cytokinin signaling machineries to repress adventitious rooting in Arabidopsis

Adventitious root initiation (ARI) is a de novo organogenesis program and a key adaptive trait in plants. Several hormones regulate ARI but the underlying genetic architecture that integrates the hormonal crosstalk governing this process remains largely elusive. In this study, we use genetics, genome editing, transcriptomics, hormone profiling and cell biological approaches to demonstrate a crucial role played by the APETALA2/ETHYLENE RESPONSE FACTOR 115 transcription factor. We demonstrate that ERF115 functions as a repressor of ARI by activating the cytokinin (CK) signaling machinery. We also demonstrate that ERF115 is transcriptionally activated by jasmonate (JA), an oxylipin-derived phytohormone, which represses ARI in NINJA-dependent and independent manners. Our data indicate that NINJA-dependent JA signaling in pericycle cells blocks early events of ARI. Altogether, our results reveal a previously unreported molecular network involving cooperative crosstalk between JA and CK machineries that represses ARI.

ACAULIS5 Is Required for Cytokinin Accumulation and Function During Secondary Growth of Populus Trees

In the primary root and young hypocotyl of Arabidopsis, ACAULIS5 promotes translation of SUPPRESSOR OF ACAULIS51 (SAC51) and thereby inhibits cytokinin biosynthesis and vascular cell division. In this study, the relationships between ACAULIS5, SAC51 and cytokinin biosynthesis were investigated during secondary growth of Populus stems. Overexpression of ACAULIS5 from the constitutive 35S promoter in hybrid aspen (Populus tremula × Populus tremuloides) trees suppressed the expression level of ACAULIS5, which resulted in low levels of the physiologically active cytokinin bases as well as their direct riboside precursors in the transgenic lines. Low ACAULIS5 expression and low cytokinin levels of the transgenic trees coincided with low cambial activity of the stem. ACAULIS5 therefore, contrary to its function in young seedlings in Arabidopsis, stimulates cytokinin accumulation and cambial activity during secondary growth of the stem. This function is not derived from maturing secondary xylem tissues as transgenic suppression of ACAULIS5 levels in these tissues did not influence secondary growth. Interestingly, evidence was obtained for increased activity of the anticlinal division of the cambial initials under conditions of low ACAULIS5 expression and low cytokinin accumulation. We propose that ACAULIS5 integrates auxin and cytokinin signaling to promote extensive secondary growth of tree stems.

Root-derived trans-zeatin cytokinin protects Arabidopsis plants against photoperiod stress

Recently, a novel type of abiotic stress caused by a prolongation of the light period-coined photoperiod stress-has been described in Arabidopsis. During the night after the prolongation of the light period, stress and cell death marker genes are induced. The next day, strongly stressed plants display a reduced photosynthetic efficiency and leaf cells eventually enter programmed cell death. The phytohormone cytokinin (CK) acts as a negative regulator of this photoperiod stress syndrome. In this study, we show that Arabidopsis wild-type plants increase the CK concentration in response to photoperiod stress. Analysis of cytokinin synthesis and transport mutants revealed that root-derived trans-zeatin (tZ)-type CKs protect against photoperiod stress. The CK signalling proteins ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN 2 (AHP2), AHP3 and AHP5 and transcription factors ARABIDOPSIS RESPONSE REGULATOR 2 (ARR2), ARR10 and ARR12 are required for the protective activity of CK. Analysis of higher order B-type arr mutants suggested that a complex regulatory circuit exists in which the loss of ARR10 or ARR12 can rescue the arr2 phenotype. Together the results revealed the role of root-derived CK acting in the shoot through the two-component signalling system to protect from the negative consequences of strong photoperiod stress.

Red Light Controls Adventitious Root Regeneration by Modulating Hormone Homeostasis in Picea abies Seedlings

Vegetative propagation relies on the capacity of plants to regenerate de novo adventitious roots (ARs), a quantitative trait controlled by the interaction of endogenous factors, such as hormones and environmental cues among which light plays a central role. However, the physiological and molecular components mediating light cues during AR initiation (ARI) remain largely elusive. Here, we explored the role of red light (RL) on ARI in de-rooted Norway spruce seedlings. We combined investigation of hormone metabolism and gene expression analysis to identify potential signaling pathways. We also performed extensive anatomical characterization to investigate ARI at the cellular level. We showed that in contrast to white light, red light promoted ARI likely by reducing jasmonate (JA) and JA-isoleucine biosynthesis and repressing the accumulation of isopentyl-adenine-type cytokinins. We demonstrated that exogenously applied JA and/or CK inhibit ARI in a dose-dependent manner and found that they possibly act in the same pathway. The negative effect of JA on ARI was confirmed at the histological level. We showed that JA represses the early events of ARI. In conclusion, RL promotes ARI by repressing the accumulation of the wound-induced phytohormones JA and CK.