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Subject: search.filters.subject.Cytokinins

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    The LONELY GUY gene family: from mosses to wheat, the key to the formation of active cytokinins in plants
    (John Wiley and Sons Ltd on behalf of the Society for Experimental Biology and The Association of Applied Biologists, 2022-04-07) Chen L; Jameson GB; Guo Y; Song J; Jameson PE
    LONELY GUY (LOG) was first identified in a screen of rice mutants with defects in meristem maintenance. In plants, LOG codes for cytokinin riboside 5'-monophosphate phosphoribohydrolase, which converts inactive cytokinin nucleotides directly to the active free bases. Many enzymes with the PGGxGTxxE motif have been misannotated as lysine decarboxylases; conversely not all enzymes containing this motif are cytokinin-specific LOGs. As LOG mutants clearly impact yield in rice, we investigated the LOG gene family in bread wheat. By interrogating the wheat (Triticum aestivum) genome database, we show that wheat has multiple LOGs. The close alignment of TaLOG1, TaLOG2 and TaLOG6 with the X-ray structures of two functional Arabidopsis thaliana LOGs allows us to infer that the wheat LOGs 1-11 are functional LOGs. Using RNA-seq data sets, we assessed TaLOG expression across 70 tissue types, their responses to various stressors, the pattern of cis-regulatory elements (CREs) and intron/exon patterns. TaLOG gene family members are expressed variously across tissue types. When the TaLOG CREs are compared with those of the cytokinin dehydrogenases (CKX) and glucosyltransferases (CGT), there is close alignment of CREs between TaLOGs and TaCKXs reflecting the key role of CKX in maintaining cytokinin homeostasis. However, we suggest that the main homeostatic mechanism controlling cytokinin levels in response to biotic and abiotic challenge resides in the CGTs, rather than LOG or CKX. However, LOG transgenics and identified mutants in rice variously impact yield, providing interesting avenues for investigation in wheat.
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    Cytokinin, jasmonates and postharvest physiology of Asparagus officinalis L. : a thesis submitted for the degree of Master of Science in Plant Biology, Massey University,
    (Massey University, 1998) Gapper, Nigel Esteven
    The asparagus spear is a rapidly growing shoot, dependent on the crown and storage roots for substrate . Once harvested, spears have a very short shelf-life. Investigations to date point to a physiological cause of this deterioration rather than a pathogenic one. Since loss of membrane integrity is a notable feature of the postharvest deterioration, spears were treated immediately following harvest with cytokinin (which promotes membrane integrity), and jasmonic acid (produced by deteriorating membranes). Treated plant material was collected and monitored for physiological and compositional changes. Results show a reduction in postharvest elongation of spears treated with cytokinin, and a reduction of shelf-life of spears treated with jasmonic acid, when compared with control spears treated with water. Also an extension of shelf-life was observed for spears treated with cytokinin. We quantified jasmonates using ELISA in spears after harvest, and also in naturally senescing cladophylls. Jasmonate concentration increased in spears rapidly after harvest, which is most likely to be in response to wounding. Results also showed that jasmonates may be involved in desiccation stress and cessation of elongation in asparagus spears. Jasmonate production and metabolism appears to be more ordered during natural foliar senescence than during harvest induced senescence of the spear. Jasmonic acid and dihydrojasmonic acid are metabolised to cucurbic acid during the later stages of natural foliar senescence. The presence of jasmonates in asparagus spears was confirmed by electrospray ionisation mass spectrometry. This analysis also enabled to identify a novel jasmonate, tryptophan-dihydrojasmonic acid amino acid conjugate.
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    Cytokinins and phase change in Pinus radiata : morphological, physiological and molecular studies : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Biology at Massey University, Palmerston North, New Zealand
    (Massey University, 1999) Zhang, Huaibi
    Phase change in higher plants is a developmental process during which changes occur at morphological, physiological and molecular levels. In Pinus radiata, buds of juvenile trees produce photosynthetically functional primary needles while buds from mature trees do not produce such primary needles. Cytokinin, however, causes production of primary needles from mature buds in vitro (Horgan, 1987). Pursuing this observation, morphological and anatomical examinations of the buds were carried out using light microscopy. The results showed that the cytokinin-induced transition from mature to juvenile bud morphology may be through resetting the fate of fascicle meristems and/or foliar primordia. To determine if a correlation existed between the endogenous cytokinin content and the maturation status of the buds, buds from the juvenile and mature P. radiata were analysed using a range of modern techniques, including column complex purification, immunoaffinity purification, normal and reverse HPLC, radioimmunoassay and electrospray tandem mass spectrometry. A wide spectrum of endogenous cytokinins were detected in the bud tissues, including five novel forms discovered in this work. Quantitative analyses revealed a general trend with seedling buds > juvenile (J4) buds > mature (M4) buds > mature (M8) buds for the combined concentration of free base and riboside cytokinins. High concentrations of phosphorylated cytokinins were found in the mature buds but not the juvenile buds. Novel cytokinin glucosides were the most abundant forms in the buds, with zeatin-9-(glucopyranosyl-1,3-ribosyl) and dihydrozeatin-9-(glucopyranosyl-1,3-ribosyl) being higher in the mature buds and isopentenyladenine-9-(glucopyranosyl-l,3-ribosyl) being higher in the juvenile buds. Overall, particular patterns of cytokinins in the field buds reflected the maturation status of the buds. Extensive metabolism of 6-benzylaminopurine occurred, including the production of the novel forms, 6-benzylaminopurine-9-(glucopyranosyl-1,3-ribosyl) and phosphorylated 6-benzylaminopurine-9-(glucopyranosyl-1,3-ribosyl), during the in vitro 'rejuvenation' of mature buds to the juvenile phenotype. Among the metabolites, the abundance of 6-benzylaminopurine, 6-benzylaminopurine riboside and 6-benzylaminopurine-9-(glucopyranosyl-1,3-ribosyl) was high while phosphorylated forms were very low over the duration of the experiment. The patterns of metabolites reflected the patterns of endogenous cytokinins observed in juvenile buds. The results also indicated that 6-benzylaminopurine did not regulate phase-specific traits by increasing endogenous cytokinins. Molecular tools were used to clone cytokinin-responsive genes which may also be involved in the regulation of phase change. A cDNA sequence (Prcr5) was cloned using a modified mRNA differential display technique. Northern analyses showed that cytokinin promoted and maintained the expression of Prcr5 at a high level during rejuvenation of the mature buds in vitro. The deduced PrCR5 protein sequence displays homology to Ginseng RNases and PR-10. A possible function of the Prcr5 gene in the regulation of phase change is discussed. A cDNA sequence (Prcab) coding for a chlorophyll a/b binding protein was also cloned. Although expression of the cab gene has been reported to be associated with phase change in other species, no such change was observed in P. radiata.
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    Role of cytokinin and ethylene during senescence in broccoli (Brassica oleracea var. Italica) : a thesis submitted for the degree of Doctor of Philosophy at Massey University
    (Massey University, 2003) Gapper, Nigel Esteven
    Broccoli (Brassica oleracea var. italica) deteriorates rapidly following harvest. The two plant hormones ethylene and cytokinin are known to act antagonistically on harvest-induced senescence in broccoli: ethylene acts by accelerating the process, whereas additional cytokinin delays it. The overall aim of this thesis was to gain a better understanding of how these two hormones control postharvest senescence. The effects of exogenous cytokinin (6-benzyl aminopurine, 6-BAP), 1-aminocyclopropane-1-carboxylic acid (ACC) and sucrose on senescence-associated gene expression were measured in both wild-type plants and transgenic plants harbouring an antisense tomato ACC oxidase gene (pTOM13). Exogenous cytokinin caused both a reduction (BoACO) and an increase (BoACS) in ethylene biosynthetic gene expression as well as reduced expression of genes encoding sucrose transporters and carbohydrate metabolising enzymes, indicating a significant role for cytokinin in the delay of senescence. Transgenic broccoli was produced using Agrobacterium tumefaciens-mediated transformation. Ethylene biosynthesis was targeted via an antisense BoACO2 gene fused to the harvest-induced asparagine synthetase (AS) promoter from asparagus. In addition, broccoli was transformed with constructs harbouring the Agrobacterium tumefaciens isopentenyl transferase (ipt) gene using the senescence-associated SAG12 and floral-associated MYB305 gene promoters to enhance the levels of cytokinin either during senescence or in floral tissue, respectively. The presence of the antisense AS-ACO construct was associated with an increased rate of transformation when compared to control constructs. Physiological analyses of mature plants showed that the antisense AS-ACO gene construct caused delayed senescence in both detached leaves and detached heads. Gene expression analyses of harvested floret tissue from AS-ACO lines showed decreases in transcript levels of senescence marker genes compared to wild-type and transgenic control lines, as well as a reduction in expression of sucrose transporter and carbohydrate metabolising genes, confirming the key role of ethylene in the promotion of senescence. In addition, genes involved with cytokinin biosynthesis and metabolism were isolated by PCR using primers based on Arabidopsis clones. The four broccoli ipt sequences aligned closely to four of the Arabidopsis sequences and were subsequently named BoIPT4, BoIPT5, BoIPT6 and BoIPT7. A cytokinin oxidase clone (BoCKX) was also isolated from broccoli. The four BoIPT genes were expressed in a number of different tissues, suggesting that the different genes may be involved in different biological processes in the plant. BoIPT4 was expressed early and BoCKX expressed late in florets during senescence. A model depicting the regulation of senescence in broccoli through the expression of cytokinin biosynthesis and metabolism genes, and their interaction with ethylene and carbohydrate metabolism is presented and discussed.