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.

Flavin mononucleotide regulated photochemical isomerization and degradation of zeatin.

cis-Zeatin (cZ), a cytokinin often overlooked compared to trans-zeatin (tZ), can now be controlled in live cells and plants through a new biocompatible reaction. Using flavin photosensitizers, cZ can be isomerized to tZ or degraded, depending on the presence of a reducing reagent. This breakthrough offers a novel approach for regulating plant growth through chemical molecules.

Shoot Multiplication and Callus Induction of Labisia pumila var. alata as Influenced by Different Plant Growth Regulators Treatments and Its Polyphenolic Activities Compared with the Wild Plant.

This study aims to investigate whether the in vitro-cultured L. pumila var. alata has higher antioxidant activity than its wild plant. An 8-week-old L. pumila var. alata nodal segment and leaf explants were cultured onto Murashige and Skoog (MS) medium supplemented with various cytokinins (zeatin, kinetin, and 6-benzylaminopurine (BAP)) for shoot multiplication and auxins (2,4-dichlorophenoxyacetic acid (2,4-D) and picloram) for callus induction, respectively. The results showed that 2 mg/L zeatin produced the optimal results for shoot and leaf development, and 0.5 mg/L 2,4-D produced the highest callus induction results (60%). After this, 0.5 mg/L 2,4-D was combined with 0.25 mg/L cytokinins and supplemented to the MS medium. The optimal results for callus induction (100%) with yellowish to greenish and compact texture were obtained using 0.5 mg/L 2,4-D combined with 0.25 mg/L zeatin. Leaves obtained from in vitro plantlets and wild plants as well as callus were extracted and analyzed for their antioxidant activities (DPPH and FRAP methods) and polyphenolic properties (total flavonoid and total phenolic content). When compared with leaf extracts of in vitro plantlets and wild plants of L. pumila var. alata, the callus extract displayed significantly higher antioxidant activities and total phenolic and flavonoid content. Hence, callus culture potentially can be adapted for antioxidant and polyphenolic production to satisfy pharmaceutical and nutraceutical needs while conserving wild L. pumila var. alata.

Micropropagation and Secondary Metabolites Content of White-Purple Varieties of Orthosiphon aristatus Blume Miq.

<b>Background and Objective:</b> The cat whiskers plant (<i>Orthosiphon aristatus</i> Blume Miq) is a plant that has been widely used as raw material for traditional medicine. The population of white-purple varieties of <i>O. aristatus</i> is decreasing efforts to maintain the white-purple <i>O. aristatus</i> need to be done keeping in mind its potential as raw material for traditional medicine. This study aims to determine the composition of a suitable medium in growing plantlet <i>O. aristatus</i> white-purple varieties and the content of its secondary metabolites. <b>Materials and Methods:</b> The internode explants were induced on MS medium added by various combinations of zeatin and 2,4-Dichlorophenoxyacetic acid (2,4-D). Root induction was carried out on shoots formed on MS medium with Indole-3-Butyric Acid (IBA). The acclimatization process was carried out using soil media. Determination of secondary metabolite levels was carried out on <i>O. aristatus</i> (<i>in vitro</i> culture) and wild-type plants aged ten months using high-performance liquid chromatography (HPLC). <b>Results:</b> MS+BAP 2ppm+NAA3 ppm media was the optimal medium for growing shoots in leaf explants. Media MS+zeatin 3 ppm+2,4-D 2 ppm produced good shoot growth on internode explants. The best root induction occurred in MS+IBA media of 0.75 ppm. The acclimatization process was successful on shoots originating from the internode, while those from leaf explants had not succeeded in growing and developing. <b>Conclusion:</b> The levels of rosmarinic acid and sinensetin in the white-purple variety <i>O. aristatus</i> (<i>in vitro</i> culture) were 1.08 and 1.62% w/w and higher than those of wild varieties.

Structure of mistletoe lectin I from Viscum album in complex with the phytohormone zeatin.

The crystal structure of mistletoe lectin I (ML-I) isolated from the European mistletoe Viscum album in complex with the most active phytohormone zeatin has been analyzed and refined to 2.54 A resolution. X-ray suitable crystals of ML-I were obtained by the counter-diffusion method using the Gel-Tube R crystallization kit (GT-R) onboard the Russian Service Module on the international space station ISS. High quality hexagonal bipyramidal crystals were grown during 3 months under microgravity conditions. Selected crystals were soaked in a saturated solution of zeatin and subsequently diffraction data were collected applying synchrotron radiation. A distinct F(o)-F(c) electron density has been found inside a binding pocket located in subunit B of ML-I and has been interpreted as a single zeatin molecule. The structure was refined to investigate the zeatin-ML-I interactions in detail. The results demonstrate the ability of mistletoe to protect itself from the host transpiration regulation by absorbing the most active host plant hormones as part of a defense mechanism.

Influence of cadmium and selenate on the interactions between hormones and phospholipids.

In this study, the influence of plant hormones, negatively charged indolilo-3-acetic acid (IAA) and positively charged zeatin, on lipid membranes was studied. As models of negatively and positively charged biological membranes, monolayers of 1,2-dimyristoyl-sn-glycero-3-[phospho-l-serine] (DMPS) and 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP) at the water/air interface were used, respectively. Additionally, the effect of cadmium and selenium ions on the interactions between hormones and lipids was studied. Surface pressure and surface potential measurements, Brewster angle microscopy (BAM), and grazing incidence X-ray diffraction (GIXD) were used for that purpose. Both IAA and zeatin led to an expansion of the lipid monolayer caused by electrostatic interactions between oppositely charged groups: negatively charged polar group of DMPS and positively charged zeatin or positive DPTAP headgroup and negative IAA. The addition of ions to the subphase containing hormones causes competitive interactions of both solutes with oppositely charged lipid polar heads. The largest effect was observed for IAA. While zeatin does not change the domain shape of DMPS, IAA causes the complete disappearance of characteristic DPTAP domains. Addition of SeO(4)(2-) ions causes restoration of DPTAP domains observed on pure water.

Trans-Zeatin inhibits UVB-induced matrix metalloproteinase-1 expression via MAP kinase signaling in human skin fibroblasts.

Ultraviolet (UV) irradiation induces the expression of matrix metalloproteinases (MMPs), disturbing the metabolism of extracellular matrix (ECM), and causes the characteristic changes of photoaging in skin. Inhibition of induction of MMPs is suggested to alleviate photoaging induced by UV irradiation. Zeatin, purified from Zea mays, is a member of the cytokinin group of plant growth factors, the activity of which is attributed to its more stable trans form. In this study, we investigated the effect of trans-Zeatin on UVB-induced matrix metalloproteinase-1 (MMP-1) expression in cultured human skin fibroblasts (HSFs) and studied the mechanisms of its actions. We found that pretreatment with trans-Zeatin significantly inhibits UVB-induced MMP-1 expression and c-Jun activation in a dose-dependent manner. We also observed that trans-Zeatin inhibits UVB-induced phosphorylation of ERK, JNK and p38 MAP kinases (MAPKs) dose-dependently. As expected, PD98059, an ERK inhibitor, SP600125, a JNK inhibitor and SB203580, a p38 MAPK inhibitor effectively inhibit UVB-induced phosphorylation of ERK, JNK and p38 MAPKs, respectively. Moreover, the inhibitory mechanism of trans-Zeatin was further demonstrated in MMP-1 secretion using MAPK-specific inhibitors. PD98059, SP600125 and SB203580 suppressed UVB-induced MMP-1 secretion, which is consistent with the above results. Collectively, our results suggest that trans-Zeatin inhibits UVB-induced MMP-1 expression, which may be mediated by inhibition of ERK, JNK and p38 MAPKs signaling pathways in HSFs. Trans-Zeatin is a potential agent for the management of skin photoaging.

The type IV secretion system component VirB5 binds to the trans-zeatin biosynthetic enzyme Tzs and enables its translocation to the cell surface of Agrobacterium tumefaciens.

VirB5 is a minor component of the extracellular T pilus determined by the Agrobacterium tumefaciens type IV secretion system. To identify proteins that interact with VirB5 during the pilus assembly process, we purified VirB5 as a recombinant fusion protein and, by using a gel overlay assay, we detected a 26-kDa interacting protein in Agrobacterium cell lysates. The VirB5-binding protein was purified from A. tumefaciens and identified as the cytokinin biosynthetic enzyme Tzs. The VirB5-Tzs interaction was confirmed using pulldown assays with purified proteins and the yeast two-hybrid system. An analysis of the subcellular localization in A. tumefaciens showed that Tzs was present in the soluble as well as the membrane fraction. Tzs was extracted from the membranes with the mild detergent dodecyl-beta-D-maltoside in complexes of different molecular masses, and this association was strongly reduced in the absence of VirB5. Using immunoelectron microscopy, we also detected Tzs on the Agrobacterium cell surface. A functional type IV secretion system was required for efficient translocation to the surface, but Tzs was not secreted into the cell supernatant. The fact that Tzs localizes on the cell surface suggests that it may contribute to the interaction of Agrobacterium with plants.

Multiplication, morphogenesis and foliar abscision of Schinopsis brasiliensis Engl. in vitro / Multiplicación, morfogénesis y abscisión foliar de Schinopsis brasiliensis Engl. in vitro

The objective was to evaluate plant growth regulators and ethylene inhibitors on the development and leaf abscission of Schinopsis brasiliensis Engl. Zeatin (ZEA) was evaluated in concentrations combined with concentrations of indolacetic acid (IAA), naphthalene acetic acid (NAA) and indolbutyric acid (IBA). ZEA and 6-benzylamino purine (BAP) were evaluated in concentrations plus a control. Ethylene inhibitors, silver nitrate and cobalt chloride were evaluated in four concentrations. The addition of 0.2 µL-1 of NAA to 0.4 µL-1 of ZEA promotes a greater number of baraúna sprouts. At concentrations of 5 and 10 µM, cobalt chloride is more efficient than silver nitrate for reducing leaf abscission in baraúna. IAA is the most suitable auxin to be associated with ZEA for higher shoot length and number of buds. Silver nitrate from a concentration of 20 µM completely avoids leaf abscission whilecobalt chloride has a maximum reduction in abscission at a concentration of 40 µM.

El objetivo fue evaluar reguladores de crecimiento e inhibidores de etileno sobre el desarrollo y abscisión foliar en Schinopsis brasiliensis Engl. La zeatina (ZEA) se evaluó en concentraciones combinadas con concentraciones de ácido indolacético (IAA), ácido naftaleno acético (NAA) y ácido indolbutírico (IBA). Se evaluaron ZEA y 6-bencilamino purina (BAP) en concentraciones más un control. Se evaluaron inhibidores de etileno, nitrato de plata y cloruro de cobalto, en cuatro concentraciones. La adición de 0.2 µL-1 de NAA a 0.4 µL-1 de ZEA promueve un mayor número de brotes de baraúna. A concentraciones de 5 y 10 µM, el cloruro de cobalto es más eficaz que el nitrato de plata para reducir la abscisión de las hojas en baraúna. IAA es la auxina más adecuada para asociar con ZEA para una mayor longitud de brotes y número de brotes. El nitrato de plata a partir de una concentración de 20 µM evita completamente la abscisión de las hojas, mientras que el cloruro de cobalto tiene una reducción máxima en la abscisión a una concentración de 40 µM.

Systemic transport of trans-zeatin and its precursor have differing roles in Arabidopsis shoots.

Organ-to-organ signal transmission is essential for higher organisms to ensure coordinated biological reactions during metabolism and morphogenesis. Similar to organs in animals, plant organs communicate by various signalling molecules. Among them, cytokinins, a class of phytohormones, play a key role as root-to-shoot long-distance signals, regulating various growth and developmental processes in shoots1,2. Previous studies have proposed that trans-zeatin-riboside, a type of cytokinin precursor, is a major long-distance signalling form in xylem vessels and its action depends on metabolic conversion via the LONELY GUY enzyme in proximity to the site of action3-5. Here we report an additional long-distance signalling form of cytokinin: trans-zeatin, an active form. Grafting between various cytokinin biosynthetic and transportation mutants revealed that root-to-shoot translocation of trans-zeatin, a minor component of xylem cytokinin, controls leaf size but not meristem activity-related traits, whereas that of trans-zeatin riboside is sufficient for regulating both traits. Considering the ratio of trans-zeatin to trans-zeatin-riboside in xylem and their delivery rate change in response to environmental conditions, this dual long-distance cytokinin signalling system allows plants to fine-tune the manner of shoot growth to adapt to fluctuating environments.

Structures of Michaelis and product complexes of plant cytokinin dehydrogenase: implications for flavoenzyme catalysis.

Cytokinins form a diverse class of compounds that are essential for plant growth. Cytokinin dehydrogenase has a major role in the control of the levels of these plant hormones by catalysing their irreversible oxidation. The crystal structure of Zea mays cytokinin dehydrogenase displays the same two-domain topology of the flavoenzymes of the vanillyl-alcohol oxidase family but its active site cannot be related to that of any other family member. The X-ray analysis reveals a bipartite architecture of the catalytic centre, which consists of a funnel-shaped region on the protein surface and an internal cavity lined by the flavin ring. A pore with diameter of about 4A connects the two active-site regions. Snapshots of two critical steps along the reaction cycle were obtained through the structural analysis of the complexes with a slowly reacting substrate and the reaction product, which correspond to the states immediately before (Michaelis complex) and after (product complex) oxidation has taken place. The substrate displays a "plug-into-socket" binding mode that seals the catalytic site and precisely positions the carbon atom undergoing oxidation in close contact with the reactive locus of the flavin. A polarising H-bond between the substrate amine group and an Asp-Glu pair may facilitate oxidation. Substrate to product conversion results in small atomic movements, which lead to a planar conformation of the reaction product allowing double-bond conjugation. These features in the mechanism of amine recognition and oxidation differ from those observed in other flavin-dependent amine oxidases.

Receptor of trans-zeatin involved in transcription activation by cytokinin.

Zeatin-binding protein (67 +/- 2 kDa) was isolated from the cytosol of the first leaf of 10-day-old barley plants. The protein fits to all requirements for a zeatin receptor: (i) it binds [3H]trans-zeatin reversibly and specifically, (ii) it is recognized by anti-idiotype antibodies from antiserum raised against trans-zeatin, (iii) in concert with 10(-8) M trans-zeatin it activates rRNA synthesis in vitro in a transcription elongation system containing chromatin from barley leaves associated with RNA-polymerase I. In the presence of trans-zeatin, the protein activates also RNA synthesis directed by RNA-polymerase I and RNA-polymerase II in isolated nuclei from barley leaves.

Inhibitory effect of zeatin, isolated from Fiatoua villosa, on acetylcholinesterase activity from PC12 cells.

Acetylcholinesterase (AChE) inhibitors, which enhance cholinergic transmission by reducing the enzymatic degradation of acetylcholine, are the only source of the compound that is currently approved for the treatment of Alzheimer's disease (AD). The methanol extract from Fiatoua villosa among 100 traditional edible plants that were tested, showed the most potent inhibitory effect (51%) on acetylcholinesterase in vitro. After the sequential solvent fractionation of the methanol extract of Fiatoua villosa, the active fraction was repeatedly subjected to open-column chromatography on silica gel. From the highest inhibitory fraction, the chloroform fraction (75%) on AChE, the single compound, was obtained by the Sep-Pak Cartridge (C18: reverse phase column). This compound was finally purified by HPLC (micro-bondapack C18 reverse phase column: 19 x 300 mm). According to the electron impact mass spectrometry (EI-MS), we confirmed that the molecular mass was 219 m/z. The structure of this compound was identified as zeatin [2-methyl-4-(1H-purine-6-ylamino)-2-buten-1-ol], one of the derivatives of purine adenine. The concentration that was required for 50% enzyme inhibition (IC50 value) was 1.09 x 10(-4) M. This study demonstrated that the zeatin from Fiatoua villosa appeared to be the most potent AChE inhibitor in AD.

Movement to bark and metabolism of xylem cytokinins in stems of Lupinus angustifolius.

Following uptake of [(3)H]zeatin riboside and [(3)H]dihydrozeatin riboside by girdled lupin (Lupinus angustifolius L.) stems via the transpiration stream, rapid lateral movement of the radioactivity from xylem to bark was observed. Short-term studies with intact stems, and other studies with excised stem tissues, revealed that the ribosides and/or the corresponding nucleotides were the cytokinin forms which actually moved into the bark tissues. Relative to cytokinin metabolism in xylem plus pith, metabolism in bark was both more rapid and more complex. Riboside cleavage and formation of the O-acetylzeatin and O-acetyldihydrozeatin ribosides and nucleotides were almost completely confined to bark tissues. Exogenous (3)H-labelled O-acetylzeatin riboside was converted to zeatin riboside in bark tissue, but the presence of the acetyl group suppressed degradation to adenine metabolites. The sequestration and modification of xylem cytokinins by stem tissues probably contributes significantly to the cytokinin status of the shoot. New cytokinins identified by mass spectrometry in lupin were: O-acetyldihydrozeatin 9-riboside, a metabolite of exogenous dihydrozeatin riboside in stem bark; O-methylzeatin nucleotide and O-methyldihydrozeatin 9-riboside, metabolites of endogenous cytokinins in stem bark; O-methylzeatin nucleotide and O-methylzeatin 9-riboside, metabolites of exogenous zeatin riboside in excised pod walls.

Conformational preferences of anticodon 3'-adjacent hypermodified nucleic acid base cis-or trans-zeatin and its 2-methylthio derivative, cis-or trans- ms(2)zeatin.

Conformational preferences of the hypermodified nucleic acid bases N6-(Delta(2)-cis-hydroxyisopentenyl)adenine, cis-io(6)Ade also known as cis-zeatin, and N(6)-(Delta(2)-trans-hydroxyisopentenyl)adenine, trans-io(6)ade or trans-zeatin, and 2-methylthio derivatives of these cis-ms(2)io(6)Ade or cis-ms(2)zeatin, and trans-ms(2)io6Ade or trans-ms(2)zeatin have been investigated theoretically by the quantum chemical Perturbative Configuration Interaction with Localized Orbitals (PCILO) method. Automated geometry optimization using quantum chemical MNDO, AM1 and PM3 methods has also been made to compare the salient features. The predicted most stable conformation of cis-io(6)Ade, trans-io(6)Ade, cis-ms(2)io(6)Ade and trans-ms(2)io(6)Ade are such that in each of these molecules the isopentenyl substituent spreads away (has "dista" conformation) from the five membered ring imidazole moiety of the adenine. The atoms N(6), C(10) and C(11) remain coplanar with the adenine ring in the predicted preferred conformation for each of these molecules. In cis-io(6)Ade as well as cis-ms(2)io(6)Ade the hydroxyl oxygen may participate in intramolecular hydrogen bonding with the H-C(10)-H group. In trans-io(6)Ade the hydroxyl group is oriented towards the H-C(2) instead. This orientation is retained in trans-ms(2)io(6)Ade, possible O-H...S hydrogen bonding may be a stabilizing factor. In all these four modified adenines C(11)-H is favourably placed to participate in intramolecular hydrogen bonding with N(1). In cis-ms(2)io(6)Ade as well as trans-ms(2)io(6)Ade the 2-methylthio group preferentially orients on the same side as C(2)-N(3) bond, due to this non-obstrusive placing, orientation of the hydroxyisopentenyl substituent remains unaffected by 2-methylthiolation. Thus the N(1) site remains shielded irrespective of the 2-methylthiolation status in these various cis-and trans-zeatin analogs alike. Firmly held orientation of hydroxyisopentenyl substituent in zeatin isomers and derivatives, in contrast to adaptable orientation of isopentenyl substituent in i(6)Ade and ms(2)i(6)Ade, may account for the increased efficiency of suppressor tRNA and reduced codon context sensitivity accompanied with the occurrence of ms(2)-zeatin (ms(2)io(6)Ade) modification.

Hormonal changes throughout maturation and ageing in Pinus pinea.

Phytohormones, which are responsible for certain age-related changes in plants, play a major role throughout maturation and ageing. Previous results dealing with this topic allowed us to describe an ageing and vigour index in Pinus radiata based on a ratio between different forms of cytokinins (Cks). The aim of the present study was to extend the studies on the changes in the hormonal status throughout maturation and ageing to Stone pine (Pinus pinea L.). With this aim in mind, a number of Cks were analysed in addition to indole-3-acetic acid (IAA) and abscisic acid (ABA) in terminal buds, axillary buds and in the apical portion of needles collected from trees at different stages of development. The results showed an increasing pattern in the levels of various Cks similar to that found in previous studies on P. radiata. Although the maintenance of the same ratio as an ageing and vigour index was not ratified, these results seem to point to Cks as major hormones throughout maturation and related processes in conifers. The distribution of hormones between the two parts of the needle is also discussed.

[A new micromethod for differential quantitative assay of zeatin and zeatin riboside]. / Novyi mikrometod differentsial'nogo kolichestvennogo opredeleniia zeatina i zeatinribozida.

A new method is proposed for differential quantitative assay of two major endogenous cytokinin forms. It is based on determination of two effective parameters-concentrations of zeatin and zeatin riboside--with the use of appropriate antigens as standards. The method can be used for determining cytokinins in small samples of plant tissues without extract fractionation. This study pioneers in quantitation of changes in the hormonal status of ovules and ovaries of Triticum aestivum L. at early stages of embryogeny. A gradual increase in the content of the active and storage forms of the hormones from the ovary to the ovule was revealed.

Endogenous cytokinin profiles of tissue-cultured and acclimatized 'Williams' bananas subjected to different aromatic cytokinin treatments.

Endogenous cytokinin (CK) levels of in vitro-cultured and greenhouse-acclimatized 'Williams' bananas treated with six aromatic CKs were quantified using UPLC-MS/MS. The underground parts had higher endogenous CK levels than the aerial parts. Control plantlets had more isoprenoid CKs while the aromatic-type CKs were predominant in all other regenerants. Following acclimatization of the control and 10 µM CK regenerants, there was a rapid decline in both isoprenoid and aromatic CK in the greenhouse-grown plants. Apart from the control and 6-(3-Methoxybenzylamino)-9-tetrahydropyran-2-ylpurine (MemTTHP) treatment with higher level of isoprenoid CK, aromatic CK remain the predominant CK-type across all CK treatments. The most abundant CK forms were meta-topolin (mT) and benzyladenine (BA) in the micropropagated and acclimatized plants, respectively. Micropropagated plantlets had cis-Zeatin (cZ) as the major isoprenoid CK-type which was in turn replaced by isopentenyladenine (iP) upon acclimatization. On a structural and functional basis, 9-glucoside, a deactivation/detoxicification product was the most abundant and mainly located in the underground parts (micropropagation and acclimatization). The results establish the wide variation in metabolic products of the tested aromatic CKs during micropropagation and acclimatization. The findings are discussed with the possible physiological roles of the various CK constituents on the growth and development of banana plants.

An improve method for somatic embryogenesis of Schisandra chinensis (Turcz.) Baillon.

A somatic embryogenesis protocol for plant regeneration of Schisandra chinensis (Turcz.) Baillon was established. Increased callus induction was obtained from mature zygotic embryos on Murashige and Skoog (MS) medium supplemented with 1.0 mg L(-1) N-phenyl-1,2,3-thidiazol-5-yl-urea (TDZ) or 2.0 mg L(-1) 2,4-dichlorophenoxyacetic acid (2,4-D). Addition of Zeatin (Zt) promoted the formation of embryogenic calli. To induce somatic embryogenesis, 2,4-D, TDZ and Zt were incorporated in the medium alone or in combination. Development of the maximum number of somatic embryos (81 globular, 37 heart, 52 torpedo and 37 cotyledon-stage) and germination of the highest number of embryos (50%) was observed on 1/2 MS medium supplemented with 1.0 mg L(-1) TDZ and 0.2 mg L(-1) Zt. Further development of somatic embryos into plantlets was completed in 1/2 MS medium free of plant growth regulators.

Effects of cytokinins, cytokinin ribosides and their analogs on the viability of normal and neoplastic human cells.

We examined the effects of some cytokinins and cytokinin ribosides including a series of adenosine analogs differently substituted in the N(6) position, along with some hypoxanthine derivatives on the viability of normal and neoplastic human cells. Cytokinins such as trans-zeatin, isopentenyladenine and benzyladenine do not show any effect, while cytokinin ribosides such as trans-zeatin riboside, isopentenyladenosine, and benzylaminopurine riboside impair the viability of normal and neoplastic cells, apart from colon carcinoma LoVo cells.