Novel cytokinins: The predominant forms in mature buds of Pinus radiata

Cytokinin Perception in Ancient Plants beyond Angiospermae

Cytokinins (CKs) control many plant developmental processes and responses to environmental cues. Although the CK signaling is well understood, we are only beginning to decipher its evolution. Here, we investigated the CK perception apparatus in early-divergent plant species such as bryophyte Physcomitrium patens, lycophyte Selaginella moellendorffii, and gymnosperm Picea abies. Of the eight CHASE-domain containing histidine kinases (CHKs) examined, two CHKs, PpCHK3 and PpCHK4, did not bind CKs. All other CHK receptors showed high-affinity CK binding (K_D of nM range), with a strong preference for isopentenyladenine over other CK nucleobases in the moss and for trans-zeatin over cis-zeatin in the gymnosperm. The pH dependences of CK binding for these six CHKs showed a wide range, which may indicate different subcellular localization of these receptors at either the plasma- or endoplasmic reticulum membrane. Thus, the properties of the whole CK perception apparatuses in early-divergent lineages were demonstrated. Data show that during land plant evolution there was a diversification of the ligand specificity of various CHKs, in particular, the rise in preference for trans-zeatin over cis-zeatin, which indicates a steadily increasing specialization of receptors to various CKs. Finally, this distinct preference of individual receptors to different CK versions culminated in vascular plants, especially angiosperms.

The Hulks and the Deadpools of the Cytokinin Universe: A Dual Strategy for Cytokinin Production, Translocation, and Signal Transduction

Cytokinins are plant hormones, derivatives of adenine with a side chain at the N⁶-position. They are involved in many physiological processes. While the metabolism of trans-zeatin and isopentenyladenine, which are considered to be highly active cytokinins, has been extensively studied, there are others with less obvious functions, such as cis-zeatin, dihydrozeatin, and aromatic cytokinins, which have been comparatively neglected. To help explain this duality, we present a novel hypothesis metaphorically comparing various cytokinin forms, enzymes of CK metabolism, and their signalling and transporter functions to the comics superheroes Hulk and Deadpool. Hulk is a powerful but short-lived creation, whilst Deadpool presents a more subtle and enduring force. With this dual framework in mind, this review compares different cytokinin metabolites, and their biosynthesis, translocation, and sensing to illustrate the different mechanisms behind the two CK strategies. This is put together and applied to a plant developmental scale and, beyond plants, to interactions with organisms of other kingdoms, to highlight where future study can benefit the understanding of plant fitness and productivity.

Naturally Occurring and Artificial N9-Cytokinin Conjugates: From Synthesis to Biological Activity and Back

Cytokinins and their sugar or non-sugar conjugates are very active growth-promoting factors in plants, although they occur at very low concentrations. These compounds have been identified in numerous plant species. This review predominantly focuses on 9-substituted adenine-based cytokinin conjugates, both artificial and endogenous, sugar and non-sugar, and their roles in plants. Acquired information about their biological activities, interconversions, and metabolism improves understanding of their mechanisms of action and functions in planta. Although a number of 9-substituted cytokinins occur endogenously, many have also been prepared in laboratories to facilitate the clarification of their physiological roles and the determination of their biological properties. Here, we chart advances in knowledge of 9-substituted cytokinin conjugates from their discovery to current understanding and reciprocal interactions between biological properties and associated structural motifs. Current organic chemistry enables preparation of derivatives with better biological properties, such as improved anti-senescence, strong cell division stimulation, shoot forming, or more persistent stress tolerance compared to endogenous or canonical cytokinins. Many artificial cytokinin conjugates stimulate higher mass production than naturally occurring cytokinins, improve rooting, or simply have high stability or bioavailability. Thus, knowledge of the biosynthesis, metabolism, and activity of 9-substituted cytokinins in various plant species extends the scope for exploiting both natural and artificially prepared cytokinins in plant biotechnology, tissue culture, and agriculture.

Endogenous cytokinin and auxin profiles during in vitro organogenesis from vegetative buds of Pinus radiata adult trees

In Pinus radiata D. Don, the transition from the juvenile to the mature phase is characterized by a reduction in the tree's organogenic potential, which is usually reverted in breeding programs by reinvigoration procedures to enable vegetative propagation. In this work, we have determined the best culture conditions for in vitro reinvigoration of radiata pine buds, tested different cytokinin (CK) types [N⁶-benzyladenine (BA), meta-topolin (mT) and trans-zeatin] and concentrations (25 and 50 µM), and studied the effect of culture conditions on endogenous CK and indole-3-acetic acid (IAA) levels at different stages of the organogenic process. To this end, the levels of 43 CKs and IAA were determined in P. radiata buds before and during the reinvigoration process. When BA or mT was applied to the induction medium, we did not observe any significant increase or decrease in endogenous isoprenoid CK content. We also report for the first time the presence of O-glucosides in non-treated P. radiata explants from the field and remark the importance of O-glucosides as storage forms.

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

BACKGROUND

We have developed a new analytical approach for isolation and quantification of cytokinins (CK) in minute amounts of fresh plant material, which combines a simple one-step purification with ultra-high performance liquid chromatography-fast scanning tandem mass spectrometry.

RESULTS

Plant tissue samples (1-5 mg FW) were purified by stop-and-go-microextraction (StageTip purification), which previously has only been applied for clean-up and pre-concentration of peptides. We found that a combination of two reverse phases and one cation-exchange phase, was the best tool, giving a total extraction recovery higher than 80%. The process was completed by a single chromatographic analysis of a wide range of naturally occurring cytokinins (bases, ribosides, O- and N-glucosides, and nucleotides) in 24.5 minutes using an analytical column packed with sub-2-microne particles. In multiple reaction monitoring mode, the detection limits ranged from 0.05 to 5 fmol and the linear ranges for most cytokinins were at least five orders of magnitude. The StageTip purification was validated and optimized using samples of Arabidopsis thaliana seedlings, roots and shoots where eighteen cytokinins were successfully determined.

CONCLUSIONS

The combination of microextraction with one-step high-throughput purification provides fast, effective and cheap sample preparation prior to qualitative and quantitative measurements. Our procedure can be used after modification also for other phytohormones, depending on selectivity, affinity and capacity of the selected sorbents.

Evolution of cytokinin biosynthesis and degradation

Cytokinin hormones are important regulators of development and environmental responses of plants that execute their action via the molecular machinery of signal perception and transduction. The limiting step of the whole process is the availability of the hormone in suitable concentrations in the right place and at the right time to interact with the specific receptor. Hence, the hormone concentrations in individual tissues, cells, and organelles must be properly maintained by biosynthetic and metabolic enzymes. Although there are merely two active cytokinins, isopentenyladenine and its hydroxylated derivative zeatin, a variety of conjugates they may form and the number of enzymes/isozymes with varying substrate specificity involved in their biosynthesis and conversion gives the plant a variety of tools for fine tuning of the hormone level. Recent genome-wide studies revealed the existence of the respective coding genes and gene families in plants and in some bacteria. This review summarizes present knowledge on the enzymes that synthesize cytokinins, form cytokinin conjugates, and carry out irreversible elimination of the hormones, including their phylogenetic analysis and possible variations in different organisms.

6-benzyladenine metabolism during reinvigoration of mature Pinus radiata buds in vitro

Maturation or phase change is a serious challenge in the deployment of superior trees of Pinus radiata D. Don because of the difficulties associated with propagation of cuttings from mature trees. We used an in vitro system to study 6-benzyladenine (BA)-induced reinvigoration of the fascicle meristems of mature buds during in vitro culture. Anatomical examinations revealed that BA inhibited the development of secondary needle primordia and 'rejuvenated' the fascicle meristems of the mature bud to produce primary needles, which are characteristic of the juvenile phase in P. radiata. Without BA supplement in the culture media, fascicle primordia continued developing secondary needles and quiescent fascicle meristems. BA metabolite analysis showed that the novel cytokinin pathway reported previously in P. radiata (H. Zhang, K.J. Horgan, P.H.S. Reynolds, G.E. Norris and P.E. Jameson. 2001. Novel cytokinins: The predominant forms in mature buds of Pinus radiata. Physiol. Plant. 112: 127-134) was mirrored in vitro, with BA converted into a variety of metabolites including 6-benzylamino-9-glucopyranosylribosyl-purine and its novel phosphorylated form, 6-benzylamino-9-glucopyranosylribosyl-purine. The culture of mature buds in the presence of BA caused a reduction in the level of endogenous cytokinins, suggesting a direct action of BA itself. Similar correlations are noted between levels of certain metabolites and the maturation status of buds from field-grown trees and buds in culture, indicating that this in vitro system may be a good model for studying the processes of maturation and reinvigoration.

Caulogenic induction in cotyledons of stone pine (Pinus pinea): relationship between organogenic response and benzyladenine trends in selected families

Adventitious bud formation in stone pine cotyledons cultured in the presence of benzyladenine (BA) has been proposed as a model for the study of in vitro shoot organogenesis in conifers. This is because of its advantageous characteristics including the requirement of only one plant growth regulator (BA), the synchronous fashion of its induction, and the homogeneity and low degree of differentiation of cotyledons. Although optimal culture conditions have been developed and are currently in use, we still lack data for BA dynamics in cotyledons cultured under these conditions, and the morphological description of the early induction stages has not, until now, been approached from a histological perspective. Consequently, this is the focus of the present report. Additionally, we examined uptake and metabolism of BA in cotyledons from two selected families, previously characterized by, and selected for, the difference in the magnitude of their organogenic response. Media transfer experiments established that cotyledons should be in contact with 44.4 microM BA for at least 6h to obtain any caulogenic response (minimum shoot-induction period). Histological observations, carried out here for the first time in this species, determined that meristemoid structures had already begun to appear in explants within 12 h of culture. Moreover, results from the BA uptake and metabolism experiments indicated that the point at which explants reached the maximum concentration of active forms of BA (276.60 microM at 6 h) and the onset of the determination phase of shoot organogenesis were directly related. A direct relationship was also observed between the intensity of the caulogenic response in cotyledons from families 36 and 61 and the endogenous concentration of BA and its riboside at the start of the induction phase. Hence, family 36, characterized by its higher bud production, reached concentrations of 251.56 microM, while family 61, selected for its low bud-producing trait, only attained 175.80 microM. Finally, a correlation was observed between 6-benzylamino-9-[O-glucopyranosyl-(1-->3)ribofuranosyl]-purine values and the magnitude of the shoot organogenesis response.

Benzyladenine metabolism and temporal competence of Pinus pinea cotyledons to form buds in vitro

Germination negatively affects adventitious shoot formation induced by cytokinins in pine cotyledons. To investigate the causes of this decrease in the organogenic response, uptake and metabolism of benzyladenine (BA) were studied in stone pine cotyledons (Pinus pinea) isolated from in vitro germinating embryos and cultured in bud induction medium. As embryos grew, cotyledons showed a progressive decrease in the amount of BA taken up from the medium. BA was barely metabolized; however, a BA metabolite previously undescribed in conifers was found. It was identified as a glucoside of the BA riboside, a type of metabolite recently described in other gymnosperms. Data revealed that differences in the organogenic capacity of P. pinea cotyledons associated with embryo germination are related primarily to their ability to absorb BA from the bud induction medium.

Dynamic changes in concentrations of auxin, cytokinin, ABA and selected metabolites in multiple genotypes of Douglas-fir (Pseudotsuga menziesii) during a growing season

Changes in concentrations of several endogenous phytohormones and metabolites were analyzed in the long shoots of nine genotypes of coastal Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) at five developmental stages: (1) closed buds, (2) flushing buds, (3) rapidly elongating shoots, (4) growing shoots and (5) near full-length shoots during one growing season. When averaged across genotypes, indole-3-acetic acid (IAA) concentration was high at stages 1 and 3. The only pattern that correlated with cone productivity was the one that was unique to IAA, in which high concentrations at stages 3 and 4 were found in all genotypes with high female cone productivity. Concentrations of isopentenyl adenosine (iPA) decreased and zeatin riboside (ZR) concentrations increased as the buds initiated and differentiated; ZR was 30 and 28 ng g(-1) dry weight (DW) at stages 1 and 4, respectively, before increasing to 166 ng g(-1) DW at stage 5. Isopentenyl adenosine peaked at 92 ng g(-1) DW at stage 2 and declined to low concentrations at stages 4 and 5. Zeatin-O-glucoside was 30 ng g(-1) DW at stage 1, declined at stages 2 and 3 and increased at stages 4 and 5. High abscisic acid (ABA) concentrations were positively correlated with rapid shoot elongation (stages 1 and 2), but as growth slowed and terminated, ABA concentrations decreased. Abscisic acid was 7 microg g(-1) DW at stage 1, increased to 13 microg g(-1) DW at stage 2 and then declined. The glucosyl ester (GE) of ABA decreased rapidly in early summer, and increased inversely with an increase in ABA. Between stages 1 and 2, ABA-GE decreased from 10 to 0.2 microg g(-1) DW and then increased. Of the ABA catabolites studied, 7'-hydroxy-ABA was about 2 microg g(-1) DW at stage 1, declined at stages 2 and 3 and increased at stages 4 and 5; phaseic acid concentrations were low at all stages, whereas dihydrophaseic acid was detected only at stages 4 and 5.

Cytokinin profiling in plant tissues using ultra-performance liquid chromatography-electrospray tandem mass spectrometry

We have developed a simple, high-throughput batch immunoextraction (IAE) micropurification procedure for extracting a wide range of naturally occurring cytokinins (bases, ribosides, O- and N-glucosides, and nucleotides) from plant tissues in solutions that are compatible with ultra-performance liquid chromatography (UPLC), thereby facilitating sensitive subsequent analysis. The UPLC system was coupled to a tandem quadrupole mass spectrometer (MS/MS) equipped with an electrospray interface (ESI). Small (mg) amounts of tissues were purified by solid-phase extraction (SPE) followed by an immunoaffinity clean-up step and two fast chromatographic separations of most cytokinin metabolites (bases, ribosides, and 9-glucosides in the first, O-glucosides and nucleotides in the second). Using UPLC, the runs were up to 4-fold faster than in standard cytokinin analyses, and both retention times and injection volumes were less variable (RSDs, 0.15-0.3% and 1.0-5.5%, respectively). In multiple reaction monitoring (MRM) mode, the detection limit for most of the cytokinins analyzed was close to 1 fmol (5-25 fmol for O-glucosides and nucleotides) and the linear range spanned at least five orders of magnitude. The extraction and purification method was optimized using poplar (Populusxcanadensis Moench, cv Robusta) leaf samples, and the analytical accuracy was further validated using IAE-purified 10-day-old Arabidopsis thaliana plants spiked with 1 and 10 pmol of cytokinin derivatives. This approach can be used for rapid, sensitive qualitative and/or quantitative analysis of more than 50 natural cytokinins in minute amounts of plant tissues with high performance, robustness, and accuracy.

Down-regulation of S-adenosyl-L: -homocysteine hydrolase reveals a role of cytokinin in promoting transmethylation reactions

S-adenosyl-L: -homocysteine hydrolase (SAHH) is a key enzyme for maintenance of cellular transmethylation potential. Although a cytokinin-binding activity had been hypothesized for SAHH, the relation between cytokinin and transmethylation reactions has not been elucidated. Here we show that, of the two Arabidopsis thaliana SAHH genes, AtSAHH1 has a much higher expression level than AtSAHH2. A T-DNA insertion mutant of AtSAHH1 (sahh1-1) and the RNA interference (RNAi) plants (dsAtSAHH2) accumulated a higher level of cytokinins, exhibited phenotypic changes similar to those of cytokinin-overproducers, and their global DNA methylation status was reduced. On the other hand, cytokinins positively regulate the transmethylation pathway genes, including AtSAHH1, AtADK1 (for adenosine kinase), and this regulation involves the cytokinin activity. Furthermore, expression of three cytosine DNA methyltransferase genes examined was inducible by cytokinin treatment. Unlike adenine and adenosine which are SAHH inhibitors, the adenine-type cytokinins have no effect on SAHH activity at protein level. Changing of endogenous cytokinin levels by transgene expression resulted in alterations of DNA methylation status in the sahh1-1 background, suggesting that cytokinins promote DNA methylation, at least under transmethylation stringent conditions. These data demonstrate that the phytohormone cytokinin plays a role in promoting transmethylation reactions, including DNA methylation.

Batch immunoextraction method for efficient purification of aromatic cytokinins

A range of benzylaminopurines naturally occur in plants and exhibit high biological activity. Others have been synthesized, such as 6-(2-hydroxy-3-methoxybenzylamino)purine riboside (2OH3MeOBAPR), which has shown interesting anti-cancer activity under in vitro conditions. In order to study the biological activity of this interesting compound in more detail, a rapid and highly efficient method for its purification from complex samples (e.g. blood and plant extracts) is needed. Therefore, we prepared monoclonal antibodies against 2OH3MeOBAPR. The antibody had undetectable cross-reactivity with all natural isoprenoid cytokinins, but relatively high cross-reactivity with aromatic cytokinins as well as some synthetic di- and tri-substituted 6-benzylaminopurines and the corresponding ribosides. The antibody also showed strong responses and specificity in enzyme-linked immunoassays (ELISAs). In addition, it was used to prepare, for the first time, an immunoaffinity sorbent with high specificity and capacity for aromatic cytokinins. A batch immunoextraction method was then developed and optimized for the purification of 2OH3MeOBAPR from murine blood samples. The high efficacy and simplicity of this method (in off-line combination with HPLC-MS) for the isolation of target analytes from biological material is demonstrated in this study.

New cytokinin metabolites in IPT transgenic Arabidopsis thaliana plants

Two novel cytokinin metabolites were identified in transgenic Arabidopsis thaliana (L.) Heynh. plants containing the bacterial IPT gene under the transcriptional control of a heat-regulated promoter. After cyclic heat-shock treatment, the endogenous cytokinin concentrations were elevated up to 100-fold compared to the wild-type plants. More then 20 different cytokinin metabolites were found, with zeatin-type cytokinins being the most abundant. The metabolic inactivation of these compounds occurred predominantly through N-glucosylation. No significant accumulation of isopentenyladenine-type cytokinins, and only a small increase in dihydrozeatin metabolites, was observed. Subsequent studies of the abundant, unidentified conjugates revealed the presence of zeatin and dihydrozeatin diglucoside conjugates. Structural analysis, utilizing electrospray-liquid tandem mass spectrometry, identified these as a zeatin-O-glucoside-9-glucoside and dihydrozeatin-O-glucoside-9-glucoside, respectively. A third unknown metabolite, was tentatively identified as a phosphorylated form of zeatin-9-glucoside. The biological activity of these compounds in three cytokinin bioassays was low. A comparison of the cytokinin pattern in transgenic and wild-type plants indicates that these specific metabolites accumulate as a consequence of enhanced cytokinin biosynthesis, and are probably involved in the homeostatic mechanisms that control endogenous cytokinin levels.

Novel jasmonate amino acid conjugates in Asparagus officinalis during harvest-induced and natural foliar senescence

Five jasmonates, including novel tryptophan conjugates of jasmonic acid and dihydrojasmonic acid, were identified in extracts from spears of Asparagus officinalis L. by electrospray tandem mass spectrometry. Spears were harvested and were held dry or with bases immersed in water. The concentrations of jasmonic acid, dihydrojasmonic acid, their tryptophan conjugates, cucurbic acid and methyl jasmonate, were measured by ELISA in spears in the 10 d following harvest. A transient increase that occurred in all spear tips immediately following harvest in the concentration of jasmonates can be attributed to a wounding response. A second increase in the concentration of jasmonates occurred from 7 d after harvest but only in dry-treated spear tips indicating that jasmonates may have accumulated in response to water stress. Jasmonate levels were also monitored during natural foliar senescence. Increased levels of jasmonates occurred after the onset of senescence, implicating them as a consequence rather than a cause of senescence.