Targeting Cytokinin Homeostasis in Rapid Cycling Brassica rapa with Plant Growth Regulators INCYDE and TD-K

Beyond expectations: the development and biological activity of cytokinin oxidase/dehydrogenase inhibitors

Cytokinins are one of the main groups of plant hormones that regulate growth and development of plants. Cytokinin oxidase/dehydrogenase (CKX) is an enzyme that rapidly and irreversibly degrades cytokinins and thus directly affects their concentration and physiological effect. Genetically modified plants with reduced CKX activity in the shoot, i.e. with a higher concentration of cytokinins, showed e.g. increased tolerance to drought stress, formed larger inflorescences and had higher grain yield. For these reasons, chemical compounds capable of inhibiting the CKX activity (CKX inhibitors) were sought. First, they were identified among strong synthetic cytokinins, but their inhibitory activity was low. The trend has been to develop potent CKX inhibitors with minimal intrinsic cytokinin activity in the hope of avoiding the negative effect of cytokinins on root growth. Cloning CKX, production of key recombinant enzymes from Arabidopsis (AtCKX2) and maize (ZmCKX1 and ZmCKX4a), development of screening bioassays and progress in X-ray crystallography and synthetic organic chemistry led to extensive progress in the development of these compounds. Currently, the most suitable CKX inhibitors are seeking their application in research and the commercial sphere in two main areas - plant tissue cultures and agriculture. The key milestones that preceded it are summarized in this review.

Development of a New Micropropagation Protocol and Transfer of In Vitro Plants to In Vivo Conditions for Cascade Hop

The vegetative propagation of hops, despite being a reliable method, is not very common due to the unavailability of the plant material. In this study, the technique of in vitro propagation was applied to the Cascade variety of Humulus lupulus L. The plant material was collected from a private field in Sicily; the explants were subjected to sterilization before in vitro culture. Single-node explants were placed in in vitro culture in nine different culture media for multiplication. Thidiazuron (TDZ), Benzyladenine (BAP) and meta-Topoline (mT) were tested for multiplication phase. For the rooting phase, five types of different culture media were evaluated. Binodal cuttings coming from the previous multiplication test were placed in the culture. The rooting media differ from each other in the concentration and ratio of two auxin hormones: Indolo-3-acetic acid (IAA) and Indole-3-butyric acid (IBA). In vitro rooted plants obtained from the rooting phase were transferred to ex vitro conditions in a microbox with agri-perlite and a solution containing Murashige and Skoog (MS) basal medium at half concentration. With a culture medium containing the highest TDZ doses (H6) and combination with cytokinin (H8 and H9), the highest shoot percentage was obtained. After 3 months of in vitro culture, the highest shoot percentage was observed in the culture medium with 2 mL L-1 of BAP. The highest rooting percentage, roots numbers and root length were found when the culture medium was supplemented with 1 mL L-1 of IAA. The usage of agri-perlite and MS at half concentration, without PGR, allowed us to obtain a 99.1% survival rate. This micropropagation protocol is useful for obtaining virus-free plants and for the development of the brewery industry.

Dynamics of endogenous levels and subcellular localization of ABA and cytokinins during pollen germination in spruce and tobacco

We used the enzyme-linked immunosorbent assay (ELISA) to assess the level of endogenous hormones in spruce pollen, and immunolocalization and confocal microscopy to study hormone localization in spruce and tobacco pollen. During pollen activation, the levels of ABA, zeatin, and its riboside significantly decreased. After the initiation of polar growth, the levels of all cytokinins increased sharply; ABA level also increased. In dormant spruce pollen grains, zeatin and ABA were localized uniformly throughout the cytoplasm. Zeatin was not detected in the nuclei, and the antheridial cell showed higher levels than the vegetative cell; ABA signal was detected in the cytoplasm and the nuclei. In germinating pollen, both hormones were detected mainly in plastids. The similar pattern was found in growing pollen tubes; signal from ABA also had a noticeable level in the cytosol of the tube cell, and was weaker in the antheridial cell. Zeatin fluorescence, on the other hand, was more pronounced in the antheridial cell. In non-germinated grains of tobacco, zeatin was localized mainly in organelles. ABA in dormant pollen grains demonstrated uniform localization, including the nuclei and cytoplasm of both cells. After germination, zeatin was accumulated in the plasmalemma or cell wall. ABA signal in the cytoplasm decreased; in the nuclei, it remained high. In growing tubes, the strongest zeatin and ABA signals were observed at the plasma membrane. The differences in ABA and cytokinin localization between species and dynamic changes in their level in spruce pollen highlight the key spatial and temporal parameters of hormonal regulation of gymnosperm pollen germination.

The multipotent thidiazuron: A mechanistic overview of its roles in callogenesis and other plant cultures in vitro

Thidiazuron (TDZ) is an active substituted phenyl urea compound that has found a significant role as a plant growth regulator. The most exciting aspect of its function is that it can mimic auxins and cytokinin but is chemically different from these two. Many theories have been put forward, and experiments performed to understand the mode of action of TDZ in callogenesis. One suggested mechanism presents that it works by inhibiting the cytokinin degrading enzymes that compete with cytokinin for an active site on the enzyme. An example is the TDZ-induced suppressed expression of gibberellic acid (GA) biosynthesis genes encoding GA3 and GA20 oxidases. This is entailed with a slightly increased expression of GA catabolism genes encoding GA20 oxidase. Similarly, one of the recommendations is that TDZ induces the expression of specific genes and transcription regulatory sequences that are either responsible directly for callus formation or in turn induce other auxins or cytokinin for callogenesis. There is no concise review available that discusses the details of TDZ-induced callus, specifically and other in vitro cultures in general. This review is an attempt to explore all these pathways and mechanisms involved in callogenesis in plants stimulated by TDZ.

Cytokinin biosynthesis in cyanobacteria: Insights for crop improvement

Cytokinins, a type of phytohormones that induce division of cytoplasm, have considerable value in agriculture due to their influences on several physiological processes of plants such as morphogenesis, development of chloroplast, seed dormancy, leaf senescence, etc. Previously, it was assumed that plants obtain cytokinin from the soil produced by microbes as these hormones were first discovered in soil-inhabiting bacteria i.e., Agrobacterium tumefaciens. Later, the cytokinin biosynthesis gene, i.e., ipt gene, has been reported in plants too. Though plants synthesize cytokinins, several studies have reported that the exogenous application of cytokinins has numerous beneficial effects including the acceleration of plant growth and boosting economic yield. Cyanobacteria may be employed in the soil not only as the source of cytokinins but also as the source of other plant growth-promoting metabolites. These organisms biosynthesize the cytokinins using the enzyme isopentenyl transferases (IPTs) in a fashion similar to the plants; however, there are few differences in the biosynthesis mechanism of cytokinins in cyanobacteria and plants. Cytokinins are important for the establishment of interaction between plants and cyanobacteria as evidenced by gene knockout experiments. These hormones are also helpful in alleviating the adverse effects of abiotic stresses on plant development. Cyanobacterial supplements in the field result in the induction of adventitious roots and shoots on petiolar as well as internodal segments. The leaf, root, and stem explants of certain plants exhibited successful regeneration when treated with cyanobacterial extract/cell suspension. These successful regeneration practices mark the way of cyanobacterial deployment in the field as a great move toward the goal of sustainable agriculture.

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.