On the biological activity of cytokinin free bases and their ribosides

Nitric oxide and cytokinin cross-talk and their role in plant hypoxia response

Nitric oxide (NO) and cytokinins (CKs) are known for their crucial contributions to plant development, growth, senescence, and stress response. Despite the importance of both signals in stress responses, their interaction remains largely unexplored. The interplay between NO and CKs emerges as particularly significant not only regarding plant growth and development but also in addressing plant stress response, particularly in the context of extreme weather events leading to yield loss. In this review, we summarize NO and CKs metabolism and signaling. Additionally, we emphasize the crosstalk between NO and CKs, underscoring its potential impact on stress response, with a focus on hypoxia tolerance. Finally, we address the most urgent questions that demand answers and offer recommendations for future research endeavors.

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.

Cytokinin Translocation to, and Biosynthesis and Metabolism within, Cereal and Legume Seeds: Looking Back to Inform the Future

Early in the history of cytokinins, it was clear that Zea mays seeds contained not just trans-zeatin, but its nucleosides and nucleotides. Subsequently, both pods and seeds of legumes and cereal grains have been shown to contain a complex of cytokinin forms. Relative to the very high quantities of cytokinin detected in developing seeds, only a limited amount appears to have been translocated from the parent plant. Translocation experiments, and the detection of high levels of endogenous cytokinin in the maternal seed coat tissues of legumes, indicates that cytokinin does not readily cross the maternal/filial boundary, indicating that the filial tissues are autonomous for cytokinin biosynthesis. Within the seed, trans-zeatin plays a key role in sink establishment and it may also contribute to sink strength. The roles, if any, of the other biologically active forms of cytokinin (cis-zeatin, dihydrozeatin and isopentenyladenine) remain to be elucidated. The recent identification of genes coding for the enzyme that leads to the biosynthesis of trans-zeatin in rice (OsCYP735A3 and 4), and the identification of a gene coding for an enzyme (CPN1) that converts trans-zeatin riboside to trans-zeatin in the apoplast, further cements the key role played by trans-zeatin in plants.

Zeatin: The 60th anniversary of its identification

While various labs had shown cell division-inducing activity in a variety of plant extracts for over a decade, the identification of zeatin (Z) in 1964, the first known naturally occurring cytokinin, belongs to Letham and co-workers. Using extracts from maize (Zea mays), they were the first to obtain crystals of pure Z and in sufficient quantity for structural determination by MS, NMR, chromatography, and mixed melting-point analysis. This group also crystallized Z-9-riboside (ZR) from coconut (Cocos nucifera) milk. However, their chemical contributions go well beyond the identification of Z and ZR and include two unambiguous syntheses of trans-Z (to establish stereochemistry), the synthesis of 3H-cytokinins that facilitated metabolic studies, and the synthesis of deuterated internal standards for accurate mass spectral quantification. Letham and associates also unequivocally identified Z nucleotide, the 7-and 9-glucoside conjugates of Z, and the O-glucosides of Z, ZR, dihydro Z (DHZ) and DHZR as endogenous compounds and as metabolites of exogenous Z. Their contributions to the role of cytokinins in plant physiology and development were also substantial, especially the role of cytokinins moving in the xylem. These biological advances are described and briefly related to the genetic/molecular biological contributions of others that established that plants have an absolute requirement for cytokinin.

Synthesis of active cytokinins mediated by LONELY GUY is associated with cell production during early fruit growth in peach [Prunus persica (L.) Batsch]

Early fruit growth in peach is characterized by cell production. Cytokinins have established roles in regulating cell division and may regulate cell production during early fruit growth. However, the role of active cytokinins and regulation of their metabolism are not well characterized in the peach fruit. In this study, fruit growth parameters, concentrations of active cytokinin bases and a cytokinin riboside, and expression of three key cytokinin metabolism-related gene families were determined during early fruit growth. Early fruit growth was associated with intensive cell production until around 40 days after full bloom. During the early stages of this period, trans-zeatin (tZ), isopentenyladenine (iP), dihydrozeatin (DHZ) and tZ-riboside (tZR), displayed higher abundance which declined rapidly by 3.5- to 16-fold during the later stages. Changes in concentration of active cytokinin bases were consistent with roles for them in regulating cell production. Expression analyses of members of cytokinin biosynthesis-related gene families, ISOPENTENYL TRANSFERASE (IPT) and LONELY GUY (LOG), further indicated that mechanisms of synthesis of cytokinin metabolites and their activation are functional within the fruit pericarp. Changes in expression of multiple members of the LOG family paralleled changes in active cytokinin concentrations. Specifically, transcript abundance of LOG3 and LOG8 were correlated with concentrations of tZ, and iP and DHZ, respectively, suggesting that the direct activation pathway is an important route for active cytokinin base synthesis during early fruit development. Transcript abundance of two CYTOKININ OXIDASE (CKX) genes, CKX1 and CKX2, was consistent with roles in cytokinin catabolism during later stages of early fruit growth. Together, these data support a role for active cytokinins synthesized in the fruit pericarp in regulating early fruit growth in peach.

From synthesis to the biological effect of isoprenoid 2'-deoxyriboside and 2',3'-dideoxyriboside cytokinin analogues

Isoprenoid cytokinins are a class of naturally occurring plant signaling molecules. A series of prepared compounds derived from isoprenoid cytokinins (isopentenyladenine, trans-zeatin and cis-zeatin) with attached 2'-deoxy-d-ribose or 2',3'-dideoxy-d-ribose at the N⁹ position of the purine were prepared and their biological activities were examined. Different synthetic approaches were employed. The final compounds were characterized with variety of physicochemical methods (TLC, HPLC-MS, and NMR) and their cytokinin activity was determined in classical bioassays such as Amaranthus, tobacco callus, detached wheat leaf senescence and Arabidopsis thaliana root elongation inhibition assay. In addition, compounds were screened for activation of the cytokinin signaling pathway (bacterial receptor, competitive ligand binding and ARR5::GUS assay) to provide a detailed assessment of CK structure-activity relationship. The prepared compounds were found to be non-toxic to human cells and the majority of assays exhibited the highest activity of free bases while 2',3'-dideoxyribosides had very weak or no activity. In contrast to the free bases, all 2'-deoxyriboside derivatives were not toxic to tobacco callus even at the highest tested concentration (10^-4 moL/l) and compound 1 (iPdR) induced betacyanin synthesis at higher concentration even stronger than iP free base in the Amaranthus bioassay. The general cytokinin activity pattern base > riboside >2'-deoxyriboside > 2',3'-dideoxyriboside was distinguished.

The Effect of a Biostimulant Based on a Protein Hydrolysate of Rainbow Trout (Oncorhynchus mykiss) on the Growth and Yield of Wheat (Triticum aestivum L.)

The research object was the liquid protein hydrolysate “AGROMOREE” from the rainbow trout, which was provided by the company “Russian Aquaculture LLC”. The purpose of this study was the evaluation of the effect of the hydrolysate “AGROMOREE” as a biostimulant on the growth and yield of wheat (Triticum aestivum L.). Biometric indicators of wheat (Triticum aestivum L.) growth were determined in the laboratory and in field tests. In the laboratory, the liquid concentrated hydrolysate was dried to facilitate its use. “AGROMOREE” promoted an increase in germination of 2–4% in all samples compared to the control samples, as well as an increase in the length and number of wheat roots. The biostimulant “AGROMOREE” was introduced in the soil in liquid form at about 3000 L/ha and 4000 L/ha in the field tests. This study showed that “ARGOMOREE” contributed to an increase in the length and quantity of wheat ears, the quantity of grains in the ear and the seed weight. At the same time, the quantity of productive stems increased, so that the biostimulant “AGROMOREE” increased the productivity by 3.9–6.3% with respect to the control sample. In general, using the biostimulant “AGROMOREE” on spring wheat seeds from 2019 in the growing season of 2021 provided an increase in yield from 0.21 t/ha to 0.28 t/ha. The maximum value of raw gluten content in the seed was 5.2%, higher than the content in the control. The content of the mass fraction of protein in the seed was in the range of 12.33–12.56%, i.e., 2% higher than that of the control sample. Thus, according to qualitative and quantitative indicators and the total productivity indicators, the biostimulant «AGROMOREE» can be used to increase wheat productivity and reduce the use of nitrogen fertilizers.

In Planta, In Vitro and In Silico Studies of Chiral N6-Benzyladenine Derivatives: Discovery of Receptor-Specific S-Enantiomers with Cytokinin or Anticytokinin Activities

Cytokinins, classical phytohormones, affect all stages of plant ontogenesis, but their application in agriculture is limited because of the lack of appropriate ligands, including those specific for individual cytokinin receptors. In this work, a series of chiral N6-benzyladenine derivatives were studied as potential cytokinins or anticytokinins. All compounds contained a methyl group at the α-carbon atom of the benzyl moiety, making them R- or S-enantiomers. Four pairs of chiral nucleobases and corresponding ribonucleosides containing various substituents at the C2 position of adenine heterocycle were synthesized. A nucleophilic substitution reaction by secondary optically active amines was used. A strong influence of the chirality of studied compounds on their interaction with individual cytokinin receptors of Arabidopsis thaliana was uncovered in in vivo and in vitro assays. The AHK2 and CRE1/AHK4 receptors were shown to have low affinity for the studied S-nucleobases while the AHK3 receptor exhibited significant affinity for most of them. Thereby, three synthetic AHK3-specific cytokinins were discovered: N6-((S)-α-methylbenzyl)adenine (S-MBA), 2-fluoro,N6-((S)-α-methylbenzyl)adenine (S-FMBA) and 2-chloro,N6-((S)-α-methylbenzyl)adenine (S-CMBA). Interaction patterns between individual receptors and specific enantiomers were rationalized by structure analysis and molecular docking. Two other S-enantiomers (N6-((S)-α-methylbenzyl)adenosine, 2-amino,N6-((S)-α-methylbenzyl)adenosine) were found to exhibit receptor-specific and chirality-dependent anticytokinin properties.

Effect of low-temperature stress on the growth of plants of Secale cereale (Poaceae) and endogenous cytokinin content in roots and shoots

Phytohormones play a key role in the regulation of plant acclimation to low temperature. To elucidate the role of cytokinins in rye plant response to chilling, we studied the dynamics of these hormones in shoots and roots under short-term and prolonged cold stress. The 7-day-old plants were exposed to cold stress (2 °C) for 2 h (alarm phase of response) or for 6 h for two days (acclimation phase of response). Endogenous content of cytokinins was analyzed by HPLC-MS method. Low temperature had a differential effect on the content of individual cytokinins and their localization in rye plants. During the short-term stress, a decrease in the content of active cytokinins (trans-zeatin and trans-zeatin riboside) in the roots and an increase in the shoots were shown. Prolonged low-temperature stress declined the amount of cytokinins except trans-zeatin riboside, which was detected in both roots and shoots. Significant rise in trans-zeatin riboside content in roots and shoots in this period evidenced an important role of this cytokinin during cold acclimation of rye plants.

The LONELY GUY gene family: from mosses to wheat, the key to the formation of active cytokinins in plants

LONELY GUY (LOG) was first identified in a screen of rice mutants with defects in meristem maintenance. In plants, LOG codes for cytokinin riboside 5'-monophosphate phosphoribohydrolase, which converts inactive cytokinin nucleotides directly to the active free bases. Many enzymes with the PGGxGTxxE motif have been misannotated as lysine decarboxylases; conversely not all enzymes containing this motif are cytokinin-specific LOGs. As LOG mutants clearly impact yield in rice, we investigated the LOG gene family in bread wheat. By interrogating the wheat (Triticum aestivum) genome database, we show that wheat has multiple LOGs. The close alignment of TaLOG1, TaLOG2 and TaLOG6 with the X-ray structures of two functional Arabidopsis thaliana LOGs allows us to infer that the wheat LOGs 1-11 are functional LOGs. Using RNA-seq data sets, we assessed TaLOG expression across 70 tissue types, their responses to various stressors, the pattern of cis-regulatory elements (CREs) and intron/exon patterns. TaLOG gene family members are expressed variously across tissue types. When the TaLOG CREs are compared with those of the cytokinin dehydrogenases (CKX) and glucosyltransferases (CGT), there is close alignment of CREs between TaLOGs and TaCKXs reflecting the key role of CKX in maintaining cytokinin homeostasis. However, we suggest that the main homeostatic mechanism controlling cytokinin levels in response to biotic and abiotic challenge resides in the CGTs, rather than LOG or CKX. However, LOG transgenics and identified mutants in rice variously impact yield, providing interesting avenues for investigation in wheat.