The forms and sources of cytokinins in developing white lupine seeds and fruits.
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A comprehensive range of cytokinins (CK) was identified and quantified by gas chromatography-mass spectrometry in tissues of and in xylem and phloem serving developing white lupine (Lupinus albus) fruits. Analyses were initiated at anthesis and included stages of podset, embryogenesis, and seed filling up to physiological maturation 77 d post anthesis (DPA). In the first 10 DPA, fertilized ovaries destined to set pods accumulated CK. The proportion of cis-CK:trans-CK isomers was initially 10:1 but declined to less than 1:1. In ovaries destined to abort, the ratio of cis-isomers to trans-isomers remained high. During early podset, accumulation of CK (30-40 pmol ovary(-1)) was accounted for by xylem and phloem translocation, both containing more than 90% cis-isomers. During embryogenesis and early seed filling (40-46 DPA), translocation accounted for 1% to 14% of the increases of CK in endosperm (20 nmol fruit(-1)) and seed coat (15 nmol fruit(-1)), indicating synthesis in situ. High CK concentrations in seeds (0.6 micromol g(-1) fresh weight) were transient, declining rapidly to less than 1% of maximum levels by physiological maturity. These data pose new questions about the localization and timing of CK synthesis, the significance of translocation, and the role(s) of CK forms in reproductive development. (+info)
Genomic organization and transcriptional regulation of maize ZmRR1 and ZmRR2 encoding cytokinin-inducible response regulators.
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Maize genomic clones encoding cytokinin-inducible response regulators, ZmRR1 and ZmRR2, have been isolated. In comparison with the corresponding cDNAs, ZmRR2 was found to be interrupted in the translated region by an intron whereas ZmRR1 was not. The 5'-flanking regions of the two genes shared conserved regions and putative cis-elements, which had been identified in maize or other plant species. The run-on transcription assay and the analysis of stable maize transformants of ZmRR1 promoter-beta-glucuronidase fusion gene revealed that the accumulation of the transcripts in response to cytokinins is, at least in parts, attributed by transcriptional activation. (+info)
Characterisation of a cysteine protease cDNA from Lolium multiflorum leaves and its expression during senescence and cytokinin treatment.
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A cysteine protease cDNA clone (See1) highly homologous to barley aleurain was isolated from Lolium multiflorum leaves. During leaf senescence, expression of the See1 mRNA and protein was strongly enhanced. In dark-incubated leaf segments, cytokinin delayed senescence and reduced expression of both See1 mRNA and protein. (+info)
Effects of elevated [CO(2)] and nitrogen nutrition on cytokinins in the xylem sap and leaves of cotton.
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We measured the level of xylem-derived cytokinins (CKs) entering a cotton leaf, and the CK levels in the same leaf, thus enabling xylem sap and foliar CKs to be compared concurrently. Although zeatin was the dominant CK in xylem sap, zeatin, dihydrozeatin, and N(6)-(2-isopentenyl) adenine were present in approximately equimolar levels in leaves. Elevated [CO(2)] (EC) has an effect on the levels of cytokinins in sap and leaf tissues. This effect was modulated by the two levels of root nitrogen nutrition (2 and 12 mM nitrate). Growth enhancement (70%) in EC over plants in ambient [CO(2)] (AC) was observed for both nitrogen nutrition treatments. Low-nitrogen leaves growing in EC exhibited photosynthetic acclimation, whereas there was no sign of photosynthetic acclimation in high-nitrogen grown leaves. Under these prevailing conditions, xylem sap and leaf tissues were obtained for CK analysis. Higher nitrogen nutrition increased the delivery per unit leaf area of CKs to the leaf at AC. EC caused a greater increase in CK delivery to the leaf at low nitrogen conditions (106%) than at high nitrogen conditions (17%). EC induced a significant increase in CK content in low-nitrogen leaves, whereas CK content in leaf tissues was similar for high-nitrogen leaves growing in AC and EC. (+info)
Differential effects of methyl jasmonate on the expression of the early light-inducible proteins and other light-regulated genes in barley.
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The effects of methyl jasmonate (JA-Me) on early light-inducible protein (ELIP) expression in barley (Hordeum vulgare L. cv Apex) have been studied. Treatment of leaf segments with JA-Me induces the same symptoms as those exhibited by norflurazon bleaching, including a loss of pigments and enhanced light stress that results in increased ELIP expression under both high- and low-light conditions. The expression of both low- and high-molecular-mass ELIP families is considerably down-regulated by JA-Me at the transcript and protein levels. This repression occurs despite increased photoinhibition measurable as a massive degradation of D1 protein and a delayed recovery of photosystem II activity. In JA-Me-treated leaf segments, the decrease of the photochemical efficiency of photosystem II under high light is substantially more pronounced as compared to controls in water. The repression of ELIP expression by JA-Me is superimposed on the effect of the increased light stress that leads to enhanced ELIP expression. The fact that the reduction of ELIP transcript levels is less pronounced than those of light-harvesting complex II and small subunit of Rubisco transcripts indicates that light stress is still affecting gene expression in the presence of JA-Me. The jasmonate-induced protein transcript levels that are induced by JA-Me decline under light stress conditions. (+info)
Convergent pathways for lipochitooligosaccharide and auxin signaling in tobacco cells.
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Lipochitooligosaccharides (LCOs) are a novel class of plant growth regulators that activate in tobacco protoplasts the expression of AXI1, a gene implicated in auxin signaling. Transient assays with a chimeric P(AXI)-GUS expression plasmid revealed that the N-octadecenoylated monosaccharide GlcN has all structural requirements for a biological active glycolipid, whereas the inactive N-acylated GalN epimer inhibits LCO action. Specific inhibition of LCO and auxin action shows that both signals are transduced within the tobacco cell via separate pathways that converge at or before AXI1 transcription. Cytokinin is suggested to be a common effector of LCO and auxin signaling. We also show that activation of AXI1 correlates with growth factor-induced cell division. (+info)
An alternative cytokinin biosynthesis pathway.
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Studies of de novo cytokinin biosynthesis in isopentenyltransferase (ipt)-transformed Arabidopsis thaliana, involving in vivo deuterium labeling and mass spectrometry, showed that the biosynthetic rate of zeatinriboside-5'-monophosphate was around 66-fold higher than that of isopentenyladenosine-5'-monophosphate (iPMP), the proposed primary product of the Agrobacterium ipt. Double tracer analysis, using [(2)H(6)] isopentenyladenosine and deuterium oxide, provided evidence for an alternative, iPMP-independent, biosynthetic pathway for zeatin-type cytokinins, present in both ipt-expressing and wild-type Arabidopsis thaliana. Reduction of the biosynthetic flux in the alternative pathway by use of mevastatin, an inhibitor for 3-hydroxy-3-methylglutaryl CoA reductase, indicated a terpenoid origin for the side-chain precursor of the iPMP independent pathway. (+info)
Characterization of the response of the Arabidopsis response regulator gene family to cytokinin.
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We examined the expression of a family of Arabidopsis response regulators (ARR) and found that the steady-state levels of RNA for most are elevated very rapidly by cytokinin. Using nuclear run-on assays we demonstrated that this increase in ARR transcript levels in response to cytokinin is due, at least in part, to increased transcription. The start site of transcription for the ARR5 gene was identified using primer extension analysis. A DNA fragment comprised of 1.6 kb upstream of the ARR5 transcript start site conferred cytokinin-inducible gene expression when fused to a beta-glucuronidase reporter, confirming that the transcription rate of ARR5 is elevated by cytokinin. This reporter construct was also used to examine the spatial pattern of ARR5 expression. The highest levels of expression were observed in the root and shoot apical meristems, at the junction of the pedicle and the silique, and in the central portion of mature roots. The expression of ARR5 in the apical meristems was confirmed by whole mount in situ analysis of seedlings and is consistent with a role for cytokinin in regulating cell division in vivo. (+info)