To cope with biotic and abiotic stress conditions, land plants have evolved several levels of protection, including delicate defense mechanisms to respond to changes in the environment. The benefits of inducible defense responses can be further augmented by defense priming, which allows plants to respond to a mild stimulus faster and more robustly than plants in the naïve (non-primed) state. Priming provides a low-cost protection of agriculturally important plants in a relatively safe and effective manner. Many different organic and inorganic compounds have been successfully tested to induce resistance in plants. Among the plethora of commonly used physicochemical techniques, priming by plant growth regulators (phytohormones and their derivatives) appears to be a viable approach with a wide range of applications. While several classes of plant hormones have been exploited in agriculture with promising results, much less attention has been paid to cytokinin, a major plant hormone involved in many biological processes including the regulation of photosynthesis. Cytokinins have been long known to be involved in the regulation of chlorophyll metabolism, among other functions, and are responsible for delaying the onset of senescence. A comprehensive overview of the possible mechanisms of the cytokinin-primed defense or stress-related responses, especially those related to photosynthesis, should provide better insight into some of the less understood aspects of this important group of plant growth regulators.

• Keywords
• ROS, chlorophyll fluorescence, cytokinin, photosynthesis, priming, stomata, stress,
• Publication type
• Journal Article MeSH
• Review MeSH

Leaf senescence, accompanied by chlorophyll breakdown, chloroplast degradation and inhibition of photosynthesis, can be suppressed by an exogenous application of cytokinins. Two aromatic cytokinin arabinosides (6-benzylamino-9-β-d-arabinofuranosylpurines; BAPAs), 3-hydroxy- (3OHBAPA) and 3-methoxy- (3MeOBAPA) derivatives, have recently been found to possess high anti-senescence activity. Interestingly, their effect on the maintenance of chlorophyll content and maximal quantum yield of photosystem II (PSII) in detached dark-adapted leaves differed quantitatively in wheat (Triticum aestivum L. cv. Aranka) and Arabidopsis (Arabidopsisthaliana L. (Col-0)). In this work, we have found that the anti-senescence effects of 3OHBAPA and 3MeOBAPA in wheat and Arabidopsis also differ in other parameters, including the maintenance of carotenoid content and chloroplasts, rate of reduction of primary electron acceptor of PSII (QA) as well as electron transport behind QA, and partitioning of absorbed light energy in light-adapted leaves. In wheat, 3OHBAPA had a higher protective effect than 3MeOBAPA, whereas in Arabidopsis, 3MeOBAPA was the more efficient derivative. We have found that the different anti-senescent activity of 3OHBAPA and 3MeOBAPA was coupled to different ethylene production in the treated leaves: the lower the ethylene production, the higher the anti-senescence activity. 3OHBAPA and 3MeOBAPA also efficiently protected the senescing leaves of wheat and Arabidopsis against oxidative damage induced by both H2O2 and high-light treatment, which could also be connected with the low level of ethylene production.

• Keywords
• Arabidopsis, chlorophyll fluorescence, cytokinin derivative, ethylene, oxidative stress, photosystem II, phytohormone, senescence, wheat,
• MeSH
• Arabidopsis drug effects   growth & development   metabolism   MeSH
• Cytokinins pharmacology   MeSH
• Ethylenes metabolism   MeSH
• Photosynthesis MeSH
• Plant Leaves drug effects   growth & development   metabolism   MeSH
• Triticum drug effects   growth & development   metabolism   MeSH
• Plant Growth Regulators pharmacology   MeSH
• Cellular Senescence * MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Cytokinins MeSH
• ethylene MeSH Browser
• Ethylenes MeSH
• Plant Growth Regulators MeSH

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.

• Keywords
• D-arabinoside, cytokinin nucleosides, cytokinin sugar conjugates, disaccharides, glucoside, meta-topolin, plant biotechnology, plant tissue culture, riboside, zeatin,
• MeSH
• Adenine chemistry   metabolism   MeSH
• Cytokinins biosynthesis   chemistry   metabolism   MeSH
• Molecular Structure MeSH
• Plants chemistry   metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Review MeSH
• Names of Substances
• Adenine MeSH
• Cytokinins MeSH

Cytokinins modulate a number of important developmental processes, including the last phase of leaf development, known as senescence, which is associated with chlorophyll breakdown, photosynthetic apparatus disintegration and oxidative damage. There is ample evidence that cytokinins can slow down all these senescence-accompanying changes. Here, we review relationships between the various mechanisms of action of these regulatory molecules. We highlight their connection to photosynthesis, the pivotal process that generates assimilates, however may also lead to oxidative damage. Thus, we also focus on cytokinin induction of protective responses against oxidative damage. Activation of antioxidative enzymes in senescing tissues is described as well as changes in the levels of naturally occurring antioxidative compounds, such as phenolic acids and flavonoids, in plant explants. The main goal of this review is to show how the biological activities of cytokinins may be related to their chemical structure. New links between molecular aspects of natural cytokinins and their synthetic derivatives with antisenescent properties are described. Structural motifs in cytokinin molecules that may explain why these molecules play such a significant regulatory role are outlined.

• Keywords
• antioxidant, antioxidant enzymes, antisenescent, cytokinin, derivative, genes, photosynthesis, plant defence, structure and activity relationship,
• MeSH
• Antioxidants chemistry   metabolism   MeSH
• Cytokinins chemistry   metabolism   MeSH
• Flavonoids analysis   MeSH
• Photosynthesis MeSH
• Plant Leaves chemistry   growth & development   physiology   MeSH
• Molecular Structure MeSH
• Plants chemistry   MeSH
• Plant Development MeSH
• Structure-Activity Relationship MeSH
• Publication type
• Journal Article MeSH
• Review MeSH
• Names of Substances
• Antioxidants MeSH
• Cytokinins MeSH
• Flavonoids MeSH

Plant hormones cytokinins (CKs) are one of the major mediators of physiological responses throughout plant life span. Therefore, a proper homeostasis is maintained by regulation of their active levels. Besides degradation, CKs are deactivated by uridine diphosphate glycosyltransferases (UGTs). Physiologically, CKs active levels decline in senescing organs, providing a signal to nutrients that a shift to reproductive tissues has begun. In this work, we show CK glucosides distribution in Arabidopsis leaves during major developmental transition phases. Besides continuous accumulation of N-glucosides we detected sharp maximum of the glucosides in senescence. This is caused prevalently by N7-glucosides followed by N9-glucosides and specifically also by trans-zeatin-O-glucoside (tZOG). Interestingly, we observed a similar trend in response to exogenously applied CK. In Arabidopsis, only three UGTs deactivate CKs in vivo: UGT76C1, UGT76C2 and UGT85A1. We thereby show that UGT85A1 is specifically expressed in senescent leaves whereas UGT76C2 is activated rapidly in response to exogenously applied CK. To shed more light on the UGTs physiological roles, we performed a comparative study on UGTs loss-of-function mutants, characterizing a true ugt85a1-1 loss-of-function mutant for the first time. Although no altered phenotype was detected under standard condition we observed reduced chlorophyll degradation with increased anthocyanin accumulation in our experiment on detached leaves accompanied by senescence and stress related genes modulated expression. Among the mutants, ugt76c2 possessed extremely diminished CK N-glucosides levels whereas ugt76c1 showed some specificity toward cis-zeatin (cZ). Besides tZOG, a broader range of CK glucosides was decreased in ugt85a1-1. Performing CK metabolism gene expression profiling, we revealed that activation of CK degradation pathway serves as a general regulatory mechanism of disturbed CK homeostasis followed by decreased CK signaling in all UGT mutants. In contrast, a specific regulation of CKX7, CKX1 and CKX2 was observed for each individual UGT mutant isoform after exogenous CK uptake. Employing an in silico prediction we proposed cytosolic localization of UGT76C1 and UGT76C2, that we further confirmed by GFP tagging of UGT76C2. Integrating all the results, we therefore hypothesize that UGTs possess different physiological roles in Arabidopsis and serve as a fine-tuning mechanism of active CK levels in cytosol.

• Keywords
• Arabidopsis, GFP subcellular localization, cytokinin, glycosyltransferase, senescence,
• Publication type
• Journal Article MeSH