Role Of Endogenous Cytokinins Research Articles

Role of endogenous cytokinins and ethylene in adventitious shoot regeneration in lemon (Citrus limon)

Role of endogenous cytokinins and ethylene in adventitious shoot regeneration in lemon (Citrus limon)

Interpreting Cytokinin Action as Anterograde Signaling and Beyond.

Among the major phytohormones, the cytokinin exhibits unique features for its ability to positively affect the developmental status of plastids. Even early on in its research, cytokinins were known to promote plastid differentiation and to reduce the loss of chlorophyll in detached leaves. Since the discovery of the components of cytokinin perception and primary signaling, the genes involved in photosynthesis and plastid differentiation have been identified as those directly targeted by type-B response regulators. Furthermore, cytokinins are known to modulate versatile cellular processes such as promoting the division and differentiation of cells and, in concert with auxin, initiating the de novo formation of shoot apical meristem (SAM) in tissue cultures. Yet how cytokinins precisely participate in such diverse cellular phenomena, and how the associated cellular processes are coordinated as a whole, remains unclear. A plausible presumption that would account for the coordinated gene expression is the tight and reciprocal communication between the nucleus and plastid. The fact that cytokinins affect plastid developmental status via gene expression in both the nucleus and plastid is interpreted here to suggest that cytokinin functions as an initiator of anterograde (nucleus-to-plastid) signaling. Based on this viewpoint, we first summarize the physiological relevance of cytokinins to the coordination of plastid differentiation with de novo shoot organogenesis in tissue culture systems. Next, the role of endogenous cytokinins in influencing plastid differentiation within the SAM of intact plants is discussed. Finally, a presumed plastid-derived signal in response to cytokinins for coupled nuclear gene expression is proposed.

Methyl Jasmonate and Cytokinin Mitigate the Salinity-Induced Oxidative Injury in Wheat Seedlings

Application of methyl jasmonate (MeJA) has been shown to be effective in improving plant tolerance to various environmental stresses. Present research was conducted to explore the possible role of endogenous cytokinins (CK) in MeJA-mediated alleviation of oxidative stress in wheat plants (Triticum aestivum L.) subjected to sodium chloride salinity. We have compared the effects of 0.1 µM MeJA and 0.04 µM cytokinin 6-benzylaminopurine (BAP), both causing a twofold increase in the level of endogenous cytokinins, on the reactive oxygen species generation, antioxidant enzymes activities, levels of lipid peroxidation, and electrolytes leakage, as well as on the contents of proline and dehydrins under normal and salinity conditions. Salt stress significantly increased generation of superoxide anion and activity of both superoxide dismutase and peroxidase resulting in accelerated lipid peroxidation and electrolyte leakage and also elevated the contents of proline and dehydrins in wheat seedlings. Pretreatment with either MeJA or BAP reduced the salt-induced pro-oxidants/antioxidants imbalance as well as membrane damages. Meanwhile, both hormones promoted additional accumulation of proline and dehydrins under stress and enhanced the barrier properties of the cell walls in seedling roots by accelerating the lignin deposition. Similarity in the protective action of MeJA and cytokinin together with previous data on prevention of the decline in the cytokinin content in MeJA-pretreated salt-stressed wheat seedlings suggest the involvement of endogenous CK in MeJA-induced increase in salinity tolerance of wheat plants.

Role of Endogenous Cytokinins in Osmotic Stress Response

Role of Endogenous Cytokinins in Osmotic Stress Response

Cytokinin as a negative effector of phospho enolpyruvate carboxylase induction in Mesembryanthemum crystallinum

Summary The possible role of cytokinin in the signal transduction pathway leading to phospho enol pyruvate carboxylase (PEPCase; EC 4.1.1.31) induction in the halophilic species Mesembryanthemum crystallinum L. was investigated. PEPCase is known to be induced by water stress in this species. When 6-benzylaminopurine (BA) was applied to M. crystallinum plants, water deficit developed as indicated by a decrease in leaf water content and by accumulation of proline. This effect was least pronounced when BA was applied to the shoots. Application of BA to the roots resulted in stronger symptoms of water stress, and simultaneous application to the roots and the shoots was most effective, leading to a 40% decrease in leaf water content and a 20-fold increase in proline within 3 days. PEPCase mRNA and enzyme activity were increased in the root-treated plants but an additional application of BA to the leaves could prevent PEPCase induction efficiendy. We therefore investigated the possible role of endogenous cytokinins in CAM induction. Fourteen species of endogenous cytokinins could be identified and were measured quantitatively. Endogenous cytokinin content was negatively correlated with mRNA levels specific for PEPCase but showed only weak correlation with PEPCase activity. In excised, wilting leaves, cytokinins decreased and PEPCase mRNA strongly increased. In rehydrating excised leaves derived from pre-stressed plants, PEPCase mRNA was down-regulated, but cytokinins were essentially unchanged. Taken together, the data support a negative regulatory function of leaf cytokinins in PEPCase induction.

Cytokinin signalling systems

A role of endogenous cytokinins as the hormonal signal, by which the roots regulate leaf metabolism and prevent leaf senescence in particular, is discussed. Cytokinin signal perception and transduction in leaf cells were studied on fully expanded first leaves of 10–13-day-old barley plants (Hordeum vulgare L. cv. Viner). Their high sensitivity to exogenous cytokinins depends on dramatic decrease in endogenous cytokinin content during leaf growth. Cytokinin-binding protein of 28–30 kD was isolated from barley leaf cytosol by its affinity to synthetic cytokinin benzyladenine (BA). The 30-kD protein was involved in cytokinin-dependent activation of RNA synthesis in vitro in the system containing chromatin-bound RNA polymerase I from barley leaves. Multistage purification of the protein included affinity chromatography on trans-zeatin-Sepharose or zeatin riboside-Sepharose resulted in isolation of barley leaf cytosol 67-kD protein up to electrophoretic homogeneity. The protein was revealed as a single band in Western blot analysis developed with anti-idiotype antibodies from antiserum to zeatin. In concert with trans-zeatin, the 67-kD protein activated transcription elongation directed by RNA polymerase I (in the system containing chromatin-bound RNA polymerase I from barley leaves) and by RNA polymerase II (in nuclei isolated from barley leaves). The protein effect strongly depended on cytokinin concentration. The maximum activation was observed at trans-zeatin concentration of 10−8M. cis-Zeatin had no effect. These results together with data on reversible [3H]zeatin-binding moiety of the 67-kD protein [8] provide a definitive proof to consider this protein as one of the cytokinin receptors in barley leaf cells which is responsible for cytokinin activation of transcription elongation directed by both RNA polymerase I and RNA polymerase II.

The role of endogenous cytokinins in correlation between cotyledon and its axillary bud and in hypocotyl regeneration of flax

When flax seedlings are decapitated above cotyledons and three days later one of the two cotyledons is removed then the remaining cotyledon stimulates in four to five days growth of its axillary bud. It has been found that content of endogenous cytokinins was higher in the stimulated bud as compared with the other one already 12 h after the cotyledon removal. Flax seedlings decapitated under cotyledons regenerate adventitious buds on thy hypocotyl stump during 5–6 days. The endogenous fytohormonal preparation of this regeneration was investigated in the 20 mm apical part of the hypocotyl stump. Decrease in auxin and increase in gibberellins was already found during the first day after decapitation while the level of cytokinins increased as late as three days after the apex removal.

Water relations and keeping-quality of cut Gerbera flowers. V. Role of endogenous cytokinins

The objective of the present work was to investigate if differences in internal water relations, due to differences in membrane permeability, between Gerbera petals of inflorescences ageing in a vase and on the plant could be ascribed to differences in cytokinin activities. Moreover, cytokinin activities in petal-extracts of 3 cultivars, differing in their keeping-quality, were compared. Cytokinin activities (“free” and “bound”) in petals decreased during the first 6 days of the experiment (both in the vase and on the plant). On Day 8, a very high activity was found in almost all the R f-fractions of the chromatogram. After Day 8, the activity decreased again. For young inflorescences, developing on the plant, a peak level was reached for “free” cytokinins at a very early bud stage and for “bound” cytokinins when the petals were just fully expanded. There were no correlations between petal-cytokinin activities at day of harvest of 3 Gerbera cultivars and their keeping-quality. It is concluded that changes in membrane permeability, which occur during ageing of petals of cut Gerbera inflorescences, are not triggered by changes in cytokinin activities.