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

Stress-induced senescence is a global agro-economic problem. Cytokinins are considered to be key plant anti-senescence hormones, but despite this practical function their use in agriculture is limited because cytokinins also inhibit root growth and development. We explored new cytokinin analogs by synthesizing a series of 1,2,3-thiadiazol-5-yl urea derivatives. The most potent compound, 1-(2-methoxy-ethyl)-3-1,2,3-thiadiazol-5-yl urea (ASES - Anti-Senescence Substance), strongly inhibited dark-induced senescence in leaves of wheat (Triticum aestivum L.) and Arabidopsis thaliana. The inhibitory effect of ASES on wheat leaf senescence was, to the best of our knowledge, the strongest of any known natural or synthetic compound. In vivo, ASES also improved the salt tolerance of young wheat plants. Interestingly, ASES did not affect root development of wheat and Arabidopsis, and molecular and classical cytokinin bioassays demonstrated that ASES exhibits very low cytokinin activity. A proteomic analysis of the ASES-treated leaves further revealed that the senescence-induced degradation of photosystem II had been very effectively blocked. Taken together, our results including data from cytokinin content analysis demonstrate that ASES delays leaf senescence by mechanism(s) different from those of cytokinins and, more effectively. No such substance has yet been described in the literature, which makes ASES an interesting tool for research of photosynthesis regulation. Its simple synthesis and high efficiency predetermine ASES to become also a potent plant stress protectant in biotechnology and agricultural industries.

• Keywords
• ASES, cytokinin, photosystem II, senescence, stress, thidiazuron, wheat,
• Publication type
• Journal Article MeSH

A comparative analysis of various parameters that characterize plant morphology, growth, water status, photosynthesis, cell damage, and antioxidative and osmoprotective systems together with an iTRAQ analysis of the leaf proteome was performed in two inbred lines of maize (Zea mays L.) differing in drought susceptibility and their reciprocal F1 hybrids. The aim of this study was to dissect the parent-hybrid relationships to better understand the mechanisms of the heterotic effect and its potential association with the stress response. The results clearly showed that the four examined genotypes have completely different strategies for coping with limited water availability and that the inherent properties of the F1 hybrids, i.e. positive heterosis in morphological parameters (or, more generally, a larger plant body) becomes a distinct disadvantage when the water supply is limited. However, although a greater loss of photosynthetic efficiency was an inherent disadvantage, the precise causes and consequences of the original predisposition towards faster growth and biomass accumulation differed even between reciprocal hybrids. Both maternal and paternal parents could be imitated by their progeny in some aspects of the drought response (e.g., the absence of general protein down-regulation, changes in the levels of some carbon fixation or other photosynthetic proteins). Nevertheless, other features (e.g., dehydrin or light-harvesting protein contents, reduced chloroplast proteosynthesis) were quite unique to a particular hybrid. Our study also confirmed that the strategy for leaving stomata open even when the water supply is limited (coupled to a smaller body size and some other physiological properties), observed in one of our inbred lines, is associated with drought-resistance not only during mild drought (as we showed previously) but also during more severe drought conditions.

• MeSH
• Acclimatization MeSH
• Chimera genetics   physiology   MeSH
• Photosynthesis MeSH
• Stress, Physiological MeSH
• Hybrid Vigor * MeSH
• Zea mays anatomy & histology   genetics   physiology   MeSH
• Plant Leaves anatomy & histology   genetics   physiology   MeSH
• Droughts MeSH
• Proteome analysis   metabolism   MeSH
• Plant Proteins analysis   metabolism   MeSH
• Water metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Proteome MeSH
• Plant Proteins MeSH
• Water MeSH

Annexins are a family of calcium- and membrane-binding proteins that are important for plant tolerance to adverse environmental conditions. Annexins function to counteract oxidative stress, maintain cell redox homeostasis, and enhance drought tolerance. In the present study, an endogenous annexin, STANN1, was overexpressed to determine whether crop yields could be improved in potato (Solanum tuberosum L.) during drought. Nine potential potato annexins were identified and their expression characterized in response to drought treatment. STANN1 mRNA was constitutively expressed at a high level and drought treatment strongly increased transcription levels. Therefore, STANN1 was selected for overexpression analysis. Under drought conditions, transgenic potato plants ectopically expressing STANN1 were more tolerant to water deficit in the root zone, preserved more water in green tissues, maintained chloroplast functions, and had higher accumulation of chlorophyll b and xanthophylls (especially zeaxanthin) than wild type (WT). Drought-induced reductions in the maximum efficiency and the electron transport rate of photosystem II (PSII), as well as the quantum yield of photosynthesis, were less pronounced in transgenic plants overexpressing STANN1 than in the WT. This conferred more efficient non-photochemical energy dissipation in the outer antennae of PSII and probably more efficient protection of reaction centers against photooxidative damage in transgenic plants under drought conditions. Consequently, these plants were able to maintain effective photosynthesis during drought, which resulted in greater productivity than WT plants despite water scarcity. Although the mechanisms underlying this stress protection are not yet clear, annexin-mediated photoprotection is probably linked to protection against light-induced oxidative stress.

• MeSH
• Annexins genetics   metabolism   MeSH
• Chlorophyll metabolism   MeSH
• Photosynthesis MeSH
• Photosystem II Protein Complex metabolism   MeSH
• Stress, Physiological MeSH
• Plants, Genetically Modified MeSH
• Droughts MeSH
• Oxidative Stress MeSH
• Gene Expression Regulation, Plant MeSH
• Recombinant Proteins genetics   metabolism   MeSH
• Genes, Plant MeSH
• Plant Proteins genetics   metabolism   MeSH
• Solanum tuberosum genetics   growth & development   metabolism   MeSH
• Light MeSH
• Xanthophylls metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Annexins MeSH
• Chlorophyll MeSH
• Photosystem II Protein Complex MeSH
• Recombinant Proteins MeSH
• Plant Proteins MeSH
• Xanthophylls MeSH

Arabidopsis (Arabidopsis thaliana) leaf development relies on subsequent phases of cell proliferation and cell expansion. During the proliferation phase, chloroplasts need to divide extensively, and during the transition from cell proliferation to expansion, they differentiate into photosynthetically active chloroplasts, providing the plant with energy. The transcription factor GROWTH REGULATING FACTOR5 (GRF5) promotes the duration of the cell proliferation period during leaf development. Here, it is shown that GRF5 also stimulates chloroplast division, resulting in a higher chloroplast number per cell with a concomitant increase in chlorophyll levels in 35S:GRF5 leaves, which can sustain higher rates of photosynthesis. Moreover, 35S:GRF5 plants show delayed leaf senescence and are more tolerant for growth on nitrogen-depleted medium. Cytokinins also stimulate leaf growth in part by extending the cell proliferation phase, simultaneously delaying the onset of the cell expansion phase. In addition, cytokinins are known to be involved in chloroplast development, nitrogen signaling, and senescence. Evidence is provided that GRF5 and cytokinins synergistically enhance cell division and chlorophyll retention after dark-induced senescence, which suggests that they also cooperate to stimulate chloroplast division and nitrogen assimilation. Taken together with the increased leaf size, ectopic expression of GRF5 has great potential to improve plant productivity.

• MeSH
• Arabidopsis drug effects   genetics   physiology   MeSH
• Cell Division drug effects   MeSH
• Chlorophyll metabolism   MeSH
• Chloroplasts drug effects   metabolism   ultrastructure   MeSH
• Cytokinins pharmacology   MeSH
• Nitrogen deficiency   MeSH
• Photosynthesis * drug effects   MeSH
• Plants, Genetically Modified MeSH
• Plant Leaves drug effects   growth & development   physiology   ultrastructure   MeSH
• 14-3-3 Proteins genetics   metabolism   MeSH
• Gene Expression Regulation, Plant drug effects   MeSH
• Genes, Plant MeSH
• Trans-Activators genetics   metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Chlorophyll MeSH
• Cytokinins MeSH
• Nitrogen MeSH
• GRF5 protein, Arabidopsis MeSH Browser
• 14-3-3 Proteins MeSH
• Trans-Activators MeSH