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Leaf Senescence (leaf + senescence)
Selected AbstractsAntioxidant and Pigment Composition during Autumnal Leaf Senescence in Woody Deciduous Species Differing in their Ecological TraitsPLANT BIOLOGY, Issue 5 2003J. I. García-Plazaola Abstract: Photoprotection mechanisms have been studied during autumnal senescence in sun and shade leaves of woody plants with different ecological characteristics and senescence patterns. Three of them belonging to the same family, Betulaceae: the shade-intolerant and early successional species (Betula alba L.), the shade-tolerant and late successional species (Corylus avellana L.), and an N-fixing tree with low N resorption efficiency (Alnus glutinosa L.). The other two species: a shade-intolerant (Populus tremula L.) and a shade-tolerant (Cornus sanguinea L.), were chosen because of their ability to accumulate anthocyanins during autumnal leaf senescence. The study of plants with different ecological strategies allowed us to establish general trends in photoprotection mechanisms during autumnal senescence, when nutrient remobilisation occurs, but also during whole leaf ontogeny. We have not found a clear relationship between shade tolerance and the level of photoprotection; the main difference between both groups of species being the presence of ,-carotene in shade leaves of shade-tolerant species. Preceding autumn, nitrogen resorption started in mid-summer and occurred in parallel with a slight and continuous ascorbate, chlorophyll and carotenoid degradation. However, the ascorbate pool remained highly reduced and lipid oxidation did not increase at this time. Contrasting with ascorbate, ,-tocopherol accumulated progressively in all species. Only during the last stages of senescence was chlorophyll preferentially degraded with respect to carotenoids, leading to the yellowing of leaves, except in A. glutinosa in which a large retention of chlorophyll and N took place. Senescing leaves were characterised, except in C. sanguinea, by a relative increase in the proportion of de-epoxidised xanthophylls: zeaxanthin, antheraxanthin and lutein. The light-induced accumulation of anthocyanins in C. sanguinea could play an additional protective role, compensating for the low retention of de-epoxidised xanthophylls. These different strategies among deciduous species are consistent with a role for photoprotective compounds in enhancing nitrogen remobilization and storage for the next growing season. [source] Cloning and Preliminary Characterization of Three Receptor-like Kinase Genes in SoybeanJOURNAL OF INTEGRATIVE PLANT BIOLOGY, Issue 11 2006Yuan-Yuan Ma Abstract Leaf senescence that occurs in the last stage of leaf development is a genetically programmed process. It is very significant to isolate the upstream components in the senescence signaling pathway and to elucidate the molecular mechanisms that control the initiation and progression of leaf senescence. In this study, full-length cDNAs of three receptor-like protein kinase genes, designated rlpk1, rlpk2 and rlpk3, were cloned from artificially-induced senescent soybean (Glycine max L.) primary leaves (GenBank accession AY687390, AY687391, AF338813). The deduced amino acid sequences indicated that they belonged to a receptor-like kinase family. Each of rlpk1 and rlpk2 encodes a leucine-rich repeat (LRR) receptor-like protein kinase. They both comprise a typical signal peptide, several LRR motifs, a single-pass transmem-brane domain, and a cytoplasmic protein kinase domain. No typical extracellular domain of RLPK3 was predicted. Organ-specific expression pattern analysis by reverse-transcription polymerase chain reaction (RT-PCR) revealed higher expression levels of the three genes in cotyledons, roots and flowers. Phylogenetic analysis indicated that RLPK1 and RLPK2 belonged to an independent branch, whereas RLPK3 shared common nodes with several known RLKs responding to abiotic and biotic stresses. The evident alternations of expression profiles of rlpk1 and rlpk2 induced by the artificial senescence-inducing treatment implied involvements of these two RLKs in regulating soybean leaf senescence. (Managing editor: Li-Hui Zhao) [source] Identification and characterization of novel senescence-associated genes from barley (Hordeum vulgare) primary leavesPLANT BIOLOGY, Issue 2008N. Ay Abstract Leaf senescence is the final developmental stage of a leaf. The progression of barley primary leaf senescence was followed by measuring the senescence-specific decrease in chlorophyll content and photosystem II efficiency. In order to isolate novel factors involved in leaf senescence, a differential display approach with mRNA populations from young and senescing primary barley leaves was applied. In this approach, 90 senescence up-regulated cDNAs were identified. Nine of these clones were, after sequence analyses, further characterized. The senescence-associated expression was confirmed by Northern analyses or quantitative RealTime-PCR. In addition, involvement of the phytohormones ethylene and abscisic acid in regulation of these nine novel senescence-induced cDNA fragments was investigated. Two cDNA clones showed homologies to genes with a putative regulatory function. Two clones possessed high homologies to barley retroelements, and five clones may be involved in degradation or transport processes. One of these genes was further analysed. It encodes an ADP ribosylation factor 1-like protein (HvARF1) and includes sequence motifs representing a myristoylation site and four typical and well conserved ARF-like protein domains. The localization of the protein was investigated by confocal laser scanning microscopy of onion epidermal cells after particle bombardment with chimeric HvARF1-GFP constructs. Possible physiological roles of these nine novel SAGs during barley leaf senescence are discussed. [source] Leaf senescence is delayed in maize expressing the Agrobacterium IPT gene under the control of a novel maize senescence-enhanced promoterPLANT BIOTECHNOLOGY JOURNAL, Issue 2 2004Paul R. H. Robson Summary We have genetically modified maize plants to delay leaf senescence. A senescence-enhanced promoter from maize (PSEE1) was used to drive expression of the Agrobacterium cytokinin biosynthesis gene IPT in senescing leaf tissue. Three maize lines expressing IPT from PSEE1, Sg1, Sg2 and Sg3, were analysed in detail, representing mild, intermediate and extreme expression, respectively, of the delayed-senescence phenotype. Backcross populations segregating for the presence or absence of the PSEE1XbaIPTNOS transgene also simultaneously segregated for the senescence phenotype. At the time of ear leaf emergence, individuals of lines Sg1 and Sg2 segregating for the presence of the transgene carried about three fewer senescing leaves than control (transgene-minus) segregants, and IPT transcript levels were higher in leaves at incipient senescence than in young leaves. Leaves of transgenic Sg3 plants were significantly greener than controls and progressed directly from fully green to bleached and dead without an intervening yellowing phase. IPT transcript abundance in this line was not related to the initiation of senescence. Extended greenness was accompanied by a delay in the loss of photosynthetic capacity with leaf age. The delayed-senescence trait was associated with relatively minor changes in morphology and development. The phenotype was particularly emphasized in plants grown in low soil nitrogen. The reduced ability of the extreme transgenic line Sg3 to recycle internal nitrogen from senescing lower leaves accounted for significant chlorosis in emerging younger leaves when plants were grown in low nutrient conditions. This study demonstrates that the agronomically important delayed-senescence (,stay-green') trait can be engineered into a monocot crop, and is the first example outside Arabidopsis of senescence modification using a homologous senescence-enhanced promoter. [source] A delayed leaf senescence mutant is defective in arginyl-tRNA:protein arginyltransferase, a component of the N-end rule pathway in ArabidopsisTHE PLANT JOURNAL, Issue 1 2002Satoko Yoshida Summary We have isolated a delayed-leaf-senescence mutant, designated dls1, from an Arabidopsis T-DNA line. Leaf senescence progresses more slowly in the dls1 mutant than in the wild-type plant in both age-dependent and dark-induced senescence. Genetic analysis revealed that the dls1 is a monogenic recessive mutation that cosegregated with the T-DNA insertion. Isolation of DNA flanking the T-DNA revealed that the T-DNA was inserted into the fourth intron of the AtATE1 gene, which encodes arginyl-tRNA:protein arginyltransferase (EC. 2.3.2.8, R-transferase), a component of the N-end rule proteolytic pathway in yeast and mammals that transfers arginine to the N-terminus of proteins with N-terminal glutamyl or aspartyl residues. AtATE1 transcripts were not detectable in the dls1 mutant by RT-PCR analysis. Introduction of a wild-type AtATE1 gene into the dls1 mutant complemented the dls1 phenotype. We also showed using a transient expression assay system, that the dls1 mutation results in a decreased degradation of proteins with Asp or Glu at their N-termini, and that the introduction of the wild-type AtATE1 gene reverses this deficiency. These results suggest that the normal progression of leaf senescence requires R-transferase activity, and that proteolysis by the N-end rule pathway has an important physiological function in the progress of leaf senescence in plants. [source] Plant oxylipins: role of jasmonic acid during programmed cell death, defence and leaf senescenceFEBS JOURNAL, Issue 17 2009Christiane Reinbothe Plants are continuously challenged by a variety of abiotic and biotic cues. To deter feeding insects, nematodes and fungal and bacterial pathogens, plants have evolved a plethora of defence strategies. A central player in many of these defence responses is jasmonic acid. It is the aim of this minireview to summarize recent findings that highlight the role of jasmonic acid during programmed cell death, plant defence and leaf senescence. [source] The capture and gratuitous disposal of resources by plantsFUNCTIONAL ECOLOGY, Issue 1 2001Thomas H. Summary 1,Every plant will die if light, water or nutrients are withheld for long enough. It is natural to think of plants in general as having evolved a strong drive for resource acquisition as a survival mechanism. All else being equal, an individual that sequesters more material from the environment than its neighbour must be at a competitive advantage. 2,But the resource capture imperative seems at odds with the profligacy of some characteristic developmental and metabolic processes in many plants. Here, using leaf senescence as a vantage point, we consider whether a kind of wilful inefficiency of resource use may not be essential for success as a terrestrial autotroph. [source] Ozone exposure over two growing seasons alters root-to-shoot ratio and chemical composition of birch (Betula pendula Roth)GLOBAL CHANGE BIOLOGY, Issue 10 2003K. Yamaji Abstract Physiological and chemical responses of 17 birch (Betula pendula Roth) clones to 1.5,1.7 × ambient ozone were studied in an open-field experiment over two growing seasons. The saplings were studied for growth, foliar visible injuries, net photosynthesis, stomatal conductance, and chlorophyll, carotenoid, Rubisco, total soluble protein, macronutrient and phenolic concentrations in leaves. Elevated ozone resulted in growth enhancement, changes in shoot-to-root (s/r) ratio, visible foliar injuries, reduced stomatal conductance, lower late-season net photosynthesis, foliar nutrient imbalance, changes in phenolic composition, and reductions in pigment, Rubisco and soluble protein contents indicating accelerated leaf senescence. Majority of clones responded to ozone by changing C allocation towards roots, by stomatal closure (reduced ozone uptake), and by investment in low-cost foliar antioxidants to avoid and tolerate ozone stress. A third of clones, showing increased s/r ratio, relied on inducible efficient high-cost antioxidants, and enhanced leaf production to compensate ozone-caused decline in leaf-level net photosynthesis. However, the best ozone tolerance was found in two s/r ratio-unaffected clones showing a high constitutive amount of total phenolics, investment in low-cost antioxidants and N distribution to leaves, and lower stomatal conductance under ozone stress. The results highlight the importance of phenolic compounds in ozone defence mechanisms in the birch population. Depending on the genotype, ozone detoxification was improved by an increase in either efficient high-cost or less efficient low-cost antioxidative phenolics, with close connections to whole-plant physiology. [source] Global trends in senesced-leaf nitrogen and phosphorusGLOBAL ECOLOGY, Issue 5 2009Zhiyou Yuan ABSTRACT Aim, Senesced-leaf litter plays an important role in the functioning of terrestrial ecosystems. While green-leaf nutrients have been reported to be affected by climatic factors at the global scale, the global patterns of senesced-leaf nutrients are not well understood. Location, Global. Methods, Here, bringing together a global dataset of senesced-leaf N and P spanning 1253 observations and 638 plant species at 365 sites and of associated mean climatic indices, we describe the world-wide trends in senesced-leaf N and P and their stoichiometric ratios. Results, Concentration of senesced-leaf N was highest in tropical forests, intermediate in boreal, temperate, and mediterranean forests and grasslands, and lowest in tundra, whereas P concentration was highest in grasslands, lowest in tropical forests and intermediate in other ecosystems. Tropical forests had the highest N : P and C : P ratios in senesced leaves. When all data were pooled, N concentration significantly increased, but senesced-leaf P concentration decreased with increasing mean annual temperature (MAT) and mean annual precipitation (MAP). The N : P and C : P ratios also increased with MAT and MAP, but C : N ratios decreased. Plant functional type (PFT), i.e. life-form (grass, herb, shrub or tree), phylogeny (angiosperm versus gymnosperm) and leaf habit (deciduous versus evergreen), affected senesced-leaf N, P, N : P, C : N and C : P with a ranking of senesced-leaf N from high to low: forbs , shrubs , trees > grasses, while the ranking of P was forbs , shrubs , trees < grasses. The climatic trends of senesced-leaf N and P and their stoichiometric ratios were similar between PFTs. Main conclusions, Globally, senesced-leaf N and P concentrations differed among ecosystem types, from tropical forest to tundra. Differences were significantly related to global climate variables such as MAT and MAP and also related to plant functional types. These results at the global scale suggest that nutrient feedback to soil through leaf senescence depends on both the climatic conditions and the plant composition of an ecosystem. [source] Changes in the physiology and feed quality of cocksfoot (Dactylis glomerata L.) during regrowthGRASS & FORAGE SCIENCE, Issue 3 2002R. P. Rawnsley Abstract A glasshouse study was undertaken to determine the physiological and morphological changes in cocksfoot (Dactylis glomerata L.) during regrowth after defoliation. Individual plants were arranged in a mini-sward in a randomized complete block design. Treatments involved harvesting each time one new leaf had expanded (one-leaf stage), up to the six-leaf stage, with the plants separated into leaf, stubble (tiller bases) and roots. Stubble and root water-soluble carbohydrate (WSC), stubble and leaf dry matter (DM), tiller number per plant and leaf quality (crude protein (CP), estimated metabolizable energy (ME) and mineral content) were measured to develop optimal defoliation management of cocksfoot-based pastures. WSC concentration in stubble and roots was highest at the five- and six-leaf stages. Mean WSC concentration (g kg,1 DM) was greater in stubble than roots (32·7 ± 5·9 vs. 9·4 ± 1·5 respectively). There was a strong positive linear relationship between plant WSC concentration and leaf DM, root DM and tillers per plant after defoliation (Adj R2 = 0·72, 0·88 and 0·95 respectively). Root DM plant,1 and tiller DM tiller,1 decreased immediately following defoliation and remained low until the three-leaf stage, then increased from the four-leaf stage. Tillers per plant remained stable until the four-leaf stage, after which they increased (from 9·9 ± 0·5 to 15·7 ± 1·0 tillers plant,1). Estimated metabolizable energy concentration (MJ kg,1 DM) was significantly lower at the six-leaf stage (11·01 ± 0·06) than at any previous leaf regrowth stage, whereas CP concentration (g kg,1 DM) decreased with regrowth to the six-leaf stage. Both the levels of ME and CP concentrations were indicative of a high quality forage throughout regrowth (11·37 ± 0·04 and 279 ± 8·0 for ME and CP respectively). Results from this study give a basis for determining appropriate criteria for grazing cocksfoot-based pastures. The optimal defoliation interval for cocksfoot appears to be between the four- and five-leaf stages of regrowth. Delaying defoliation to the four-leaf stage allows time for replenishment of WSC reserves, resumption of root growth and an increase in tillering, and is before herbage is lost and quality falls due to onset of leaf senescence. [source] Setting management limits for the production and utilization of herbage for out-of-season grazingGRASS & FORAGE SCIENCE, Issue 1 2000Laidlaw Three experiments were carried out on perennial ryegrass-dominant swards to provide a basis for recommendations for the limits to (a) building up and timing of utilization of a herbage ,bank' for out-of-season grazing and (b) duration and intensity of early spring grazing in the United Kingdom and Ireland. In experiment 1, the effect of regrowth interval (from 7 September, 20 October, 17 November or 15 December) in autumn on herbage accumulation, leaf turnover and on subsequent spring growth was investigated. Swards regrown from early September reached maximum herbage mass (about 3 t ha,1 DM) and leaf lamina content in mid-November, by which time senescence rate exceeded rate of production of new leaves. New leaf production and senescence rates were greater in swards remaining uncut until December than in those cut in October or November. Time of defoliation up to December had no effect on spring herbage mass in the subsequent spring. Defoliating in March reduced herbage mass in late May by less than 20%. Experiment 2 investigated the progress in herbage growth and senescence in swards regrowing from different times in late summer and autumn to produce herbage for utilization beyond the normal grazing season. Treatments in a randomized block design with three replicates were regrowths from 19 July, 8 August, 30 August and 20 September. Based on a lower ceiling of leaf and total herbage mass being reached with progressively later regrowths, beyond which leaf senescence generally exceeded leaf production and herbage mass declined, it was concluded that currently recommended rotation lengths for this period should extend from 3 weeks in late July to 8 weeks for swards previously grazed in mid-September. In both experiments, leaf senescence commenced earlier (by one leaf-age category) than previously published estimates and so brought forward the time at which senescence rates balanced leaf growth rates. In experiment 3, designed to evaluate the effect of daily grazing period and intensity in early spring on herbage regrowth, dairy cows grazed successive plots (replicates) for 2 or 4 h each day at two intensities (target residual heights of 5 or 7 cm) in March to mid-April. Regrowth rate was similar in all treatments including the ungrazed control, despite soil moisture content being relatively high on occasions. Tiller density was significantly reduced in May by grazing plots in early or mid-April. It is concluded that in autumn there are limits to which rotation lengths should be extended to produce herbage for out-of-season grazing owing to attainment of ceiling yields. Although utilization in early spring may reduce herbage availability in spring, out-of-season utilization need not reduce herbage growth rates in early spring. [source] Grazing history versus current grazing: leaf demography and compensatory growth of three alpine plants in response to a native herbivore (Ochotona collaris)JOURNAL OF ECOLOGY, Issue 2 2002Eliot J. B. McIntire Summary 1 We measured leaf births, leaf deaths and leaf length of three alpine perennial species, Kobresia myosuroides, Erigeron humilis and Oxytropis nigrescens, from sites with different grazing histories (strong or weak) in response to two levels of current season grazing (present or absent) by collared pikas (Ochotona collaris), a small lagomorph, in the south-west Yukon. 2 All three species appeared to tolerate the removal of 58,61% of summer leaf production under natural conditions. Grazing history, which was defined by the location of plants located either < 2 m or > 6 m from boulderfields with a history of occupation by pikas, was the most significant factor determining shifts in leaf births and leaf deaths following herbivory. 3 The only detectable influence of current season herbivory for any measured species was a reduction of leaf length of Kobresia. 4 A comparison of historically grazed with historically ungrazed plants indicated several changes in leaf demography and morphology. Kobresia leaves were generally shorter and had higher rates of production of new leaves. Oxytropis had higher rates of new leaf production. Erigeron had fewer leaf births throughout the summer, but showed a large and highly significant delay in the timing of leaf senescence. 5 These responses can be largely understood as strategies to avoid the predictable intensive late season foraging that is characteristic of pikas. Morphological mechanisms allow these species to tolerate and, more importantly for the herbivore, persist under heavy and chronic grazing. [source] Endopeptidase Isoenzyme Characteristics in Cucumis sativus Leaves During Dark-induced SenescenceJOURNAL OF INTEGRATIVE PLANT BIOLOGY, Issue 4 2007Peng Zhang Abstract The changes and characteristics of endopeptidase (EP) isoenzymes in cucumber (Cucumis sativus L.) leaves during dark-induced senescence were investigated by activity staining after gradient-polyacrylamide gel electrophoresis (G-PAGE) containing co-polymerized gelatin as substrate. The results showed that both the chlorophyll and the protein contents of leaves were decreased, and the protein degradation was correlated with the increase of proteolytic activity during the course of leaf senescence. Meanwhile, nine cucumber endopeptidases isoenzymes (CEP) with 140, 120, 106, 94, 76, 55, 46, 39 and 35 kDa molecular weights were detected. Four of these, CEP2, 3, 4 and CEP9 appeared all the time, but the changes of the activity were different during incubation. Another four CEPs (CEP5, 6, 7 and CEP8) whose activities increased with dark-induced time were only detected in senescent leaves. Furthermore, the biochemical properties of these nine CEP were also characterized. All the CEPs had high activities from 35 °C to 45 °C, and the optimum temperature was found to be 40 °C. However, the activities of CEPs were not detected below 25 °C or over 60 °C. The activity bands appeared at a wide range of pH from 5.0 to 9.0, but the optimum pH was found at 7.0. No CEPs were detected at pH 4 or pH 10. By inhibition analysis we concluded that CEP2, 3, 4 and CEP9 were serine endopeptidases and CEP6 was a kind of cysteine protease. It is suggested that serine endopeptidases might play a major role in cucumber leaf senescence, and for the first time, six senescence-related endopeptidases (CEP1, 5, 6, 7, 8 and 9) were found in cucumber leaves. [source] Cloning and Preliminary Characterization of Three Receptor-like Kinase Genes in SoybeanJOURNAL OF INTEGRATIVE PLANT BIOLOGY, Issue 11 2006Yuan-Yuan Ma Abstract Leaf senescence that occurs in the last stage of leaf development is a genetically programmed process. It is very significant to isolate the upstream components in the senescence signaling pathway and to elucidate the molecular mechanisms that control the initiation and progression of leaf senescence. In this study, full-length cDNAs of three receptor-like protein kinase genes, designated rlpk1, rlpk2 and rlpk3, were cloned from artificially-induced senescent soybean (Glycine max L.) primary leaves (GenBank accession AY687390, AY687391, AF338813). The deduced amino acid sequences indicated that they belonged to a receptor-like kinase family. Each of rlpk1 and rlpk2 encodes a leucine-rich repeat (LRR) receptor-like protein kinase. They both comprise a typical signal peptide, several LRR motifs, a single-pass transmem-brane domain, and a cytoplasmic protein kinase domain. No typical extracellular domain of RLPK3 was predicted. Organ-specific expression pattern analysis by reverse-transcription polymerase chain reaction (RT-PCR) revealed higher expression levels of the three genes in cotyledons, roots and flowers. Phylogenetic analysis indicated that RLPK1 and RLPK2 belonged to an independent branch, whereas RLPK3 shared common nodes with several known RLKs responding to abiotic and biotic stresses. The evident alternations of expression profiles of rlpk1 and rlpk2 induced by the artificial senescence-inducing treatment implied involvements of these two RLKs in regulating soybean leaf senescence. (Managing editor: Li-Hui Zhao) [source] Components of Partial Disease Resistance in Wheat Detected in a Detached Leaf Assay Inoculated with Microdochium majus using First, Second and Third Expanding Seedling LeavesJOURNAL OF PHYTOPATHOLOGY, Issue 4 2006R. A. Browne Abstract The use of first, second and third expanding seedling leaves of wheat (L1, L2 and L3 respectively), inoculated with conidial suspensions of Microdochium majus (syn. Microdochium nivale var. majus) in a detached leaf assay, for detecting components of partial disease resistance (PDR) was investigated across a range of wheat cultivars. Incubation periods (period from inoculation to first appearance of symptoms; a dull grey,green water-soaked lesion) and latent periods (period from inoculation to the first appearance of sporodochia) were longest and lesions smallest on L3. The expression of PDR components on L2 was intermediate to those on L1 and L3. The longer latent periods on L3 typically occurred after leaf senescence contrasting with latent periods on L1 and L2 where sporulation most frequently occurred on relatively green leaf tissue. Cultivar differences in the first appearance of symptoms, incubation period, which occurred before any leaf senescence was observed, correlated significantly across all leaf positions. Similarly cultivar differences in latent period were correlated for L1 and L2. However, latent periods on L3, which were the least consistent between cultivars across experiments, were not correlated with those of L1 or L2 in any experiment. The results indicate that due to the delay in sporulation until after leaf senescence, observations on latent period in L3 are less representative of what occurs in the whole plant where infection of living tissue is of greatest interest. This work indicates that the selection of the first or second expanding leaf of wheat is optimal for the use in the detached leaf assay using M. majus for studying components of PDR. [source] Photosynthetic Responses of a Temperate Liana to Xylella fastidiosa Infection and Water StressJOURNAL OF PHYTOPATHOLOGY, Issue 1 2004A. J. McElrone Abstract Xylella fastidiosa is a xylem-limited bacterial plant pathogen that causes bacterial leaf scorch in its hosts. Our previous work showed that water stress enhances leaf scorch symptom severity and progression along the stem of a liana, Parthenocissus quinquefolia, infected by X. fastidiosa. This paper explores the photosynthetic gas exchange responses of P. quinquefolia, with the aim to elucidate mechanisms behind disease expression and its interaction with water stress. We used a 2 × 2-complete factorial design, repeated over two growing seasons, with high and low soil moisture levels and infected and non-infected plants. In both years, low soil moisture levels reduced leaf water potentials, net photosynthesis and stomatal conductance at all leaf positions, while X. fastidiosa -infection reduced these parameters at basally located leaves only. Intercellular CO2 concentrations were reduced in apical leaves, but increased at the most basal leaf location, implicating a non-stomatal reduction of photosynthesis in leaves showing the greatest disease development. This result was supported by measured reductions in photosynthetic rates of basal leaves at high CO2 concentrations, where stomatal limitation was eliminated. Repeated measurements over the summer of 2000 showed that the effects of water stress and infection were progressive over time, reaching their greatest extent in September. By reducing stomatal conductances at moderate levels of water stress, P. quinquefolia maintained relatively high leaf water potentials and delayed the onset of photosynthetic damage due to pathogen and drought-induced water stress. In addition, chlorophyll fluorescence measurements showed that P. quinquefolia has an efficient means of dissipating excess light energy that protects the photosynthetic machinery of leaves from irreversible photoinhibitory damage that may occur during stress-induced stomatal limitation of photosynthesis. However, severe stress induced by disease and drought eventually led to non-stomatal decreases in photosynthesis associated with leaf senescence. [source] Senescence and hyperspectral reflectance of cotton leaves exposed to ultraviolet-B radiation and carbon dioxidePHYSIOLOGIA PLANTARUM, Issue 2 2004Vijaya Gopal Kakani The objectives of this study were to determine the effects of UV-B radiation and atmospheric carbon dioxide concentrations ([CO2]) on leaf senescence of cotton by measuring leaf photosynthesis and chlorophyll content and to identify changes in leaf hyperspectral reflectance occurring due to senescence and UV-B radiation. Plants were grown in controlled-environment growth chambers at two [CO2] (360 and 720 µmol mol,1) and three levels of UV-B radiation (0, 7.7 and 15.1 kJ m,2 day,1). Photosynthesis, chlorophyll, carotenoids and phenolic compounds along with leaf hyperspectral reflectance were measured on three leaves aged 12, 21 and 30 days in each of the treatments. No interaction was detected between [CO2] and UV-B for any of the measured parameters. Significant interactions were observed between UV-B and leaf age for photosynthesis and stomatal conductance. Elevated [CO2] enhanced leaf photosynthesis by 32%. On exposure to 0, 7.7 and 15.1 kJ of UV-B, the photosynthetic rates of 30-day-old leaves compared with 12-day-old leaves were reduced by 52, 76 and 86%, respectively. Chlorophyll pigments were not affected by leaf age at UV-B radiation of 0 and 7.7 kJ, but UV-B of 15.1 kJ reduced the chlorophylls by 20, 60 and 80% in 12, 21 and 30-day-old leaves, respectively. The hyperspectral reflectance between 726 and 1142 nm showed interaction for UV-B radiation and leaf age. In cotton, leaf photosynthesis can be used as an indicator of leaf senescence, as it is more sensitive than photosynthetic pigments on exposure to UV-B radiation. This study revealed that, cotton leaves senesced early on exposure to UV-B radiation as indicated by leaf photosynthesis, and leaf hyperspectral reflectance can be used to detect changes caused by UV-B and leaf ageing. [source] SAG2 and SAG12 protein expression in senescing Arabidopsis plantsPHYSIOLOGIA PLANTARUM, Issue 2 2003Vojislava Grbi During leaf senescence, nutrients are remobilized from the senescing tissues to the growing parts of the plant. Many senescence-associated genes (SAGs) were identified based on the induction of their transcripts. However, little is known about the protein expression of the corresponding genes. We have raised antibodies against two Arabidopsis SAGs, SAG2 and SAG12, which encode putative cysteine proteases. The SAG2 antibodies recognized a 29-kDa protein that was abundant in senescing leaves, but was also present at low levels in green tissues. SAG12 antibodies labelled a 38-kDa protein present only in senescent leaves. The protein expression of these SAGs parallels their mRNA expression patterns, indicating that control of SAG2 and SAG12 is at the level of transcription or transcript stability. In addition, we found that SAGs are induced during stem senescence with delayed kinetics of their expression relative to leaf expression, suggesting that age-dependent factor(s) regulating the onset of senescence in Arabidopsis may act in tissue-dependent manner. [source] The control of chlorophyll catabolism and the status of yellowing as a biomarker of leaf senescencePLANT BIOLOGY, Issue 2008H. Ougham Abstract The pathway of chlorophyll catabolism during leaf senescence is known in a fair amount of biochemical and cell biological detail. In the last few years, genes encoding a number of the catabolic enzymes have been characterized, including the key ring-opening activities, phaeophorbide a oxygenase (PaO) and red chlorophyll catabolite reductase (RCCR). Recently, a gene that modulates disassembly of chlorophyll,protein complexes and activation of pigment ring-opening has been isolated by comparative mapping in monocot species, positional cloning exploiting rice genomics resources and functional testing in Arabidopsis. The corresponding gene in pea has been identified as Mendel's I locus (green/yellow cotyledons). Mutations in this and other chlorophyll catabolic genes have significant consequences, both for the course of leaf senescence and senescence-like stress responses, notably hypersensitivity to pathogen challenge. Loss of chlorophyll can occur via routes other than the PaO/RCCR pathway, resulting in changes that superficially resemble senescence. Such ,pseudosenescence' responses tend to be pathological rather than physiological and may differ from senescence in fundamental aspects of biochemistry and regulation. [source] A novel upstream regulator of WRKY53 transcription during leaf senescence in Arabidopsis thalianaPLANT BIOLOGY, Issue 2008Y. Miao Abstract Arabidopsis WRKY proteins comprise a family of zinc finger-type transcription factors involved in the regulation of gene expression during pathogen defence, wounding, trichome development and senescence. To better understand the regulatory role of the senescence-related WRKY53 factor, we identified upstream regulatory factors using the yeast one-hybrid system. Among others, we identified a DNA-binding protein with a so far unknown function that contains a transcriptional activation domain and a kinase domain with similarities to HPT kinases. In vitro studies revealed that this activation domain protein (AD protein) can phosphorylate itself and that phosphorylation increases its DNA-binding activity to the WRKY53 promoter region. Using the yeast two-hybrid system, an interaction with proteins that were previously shown to bind to the WRKY53 promoter was tested. The AD protein interacted with MEKK1. The interaction with MEKK1 was confirmed in vivo by bimolecular fluorescence complementation (BiFC); however, the AD protein was not phosphorylated by MEKK1 in vitro and vice versa. This indicates that there may be competition between WRKY53 and AD protein for binding of MEKK1 at the WRKY53 promoter. Overexpression and knockout of the respective gene resulted in changes in transcription levels of WRKY53, indicating that AD protein is a positive regulator of WRKY53 expression. Expression of the AD protein gene can be induced by hydrogen peroxide treatment and reduced by jasmonic acid treatment, as previously shown for WRKY53. [source] Identification and characterization of novel senescence-associated genes from barley (Hordeum vulgare) primary leavesPLANT BIOLOGY, Issue 2008N. Ay Abstract Leaf senescence is the final developmental stage of a leaf. The progression of barley primary leaf senescence was followed by measuring the senescence-specific decrease in chlorophyll content and photosystem II efficiency. In order to isolate novel factors involved in leaf senescence, a differential display approach with mRNA populations from young and senescing primary barley leaves was applied. In this approach, 90 senescence up-regulated cDNAs were identified. Nine of these clones were, after sequence analyses, further characterized. The senescence-associated expression was confirmed by Northern analyses or quantitative RealTime-PCR. In addition, involvement of the phytohormones ethylene and abscisic acid in regulation of these nine novel senescence-induced cDNA fragments was investigated. Two cDNA clones showed homologies to genes with a putative regulatory function. Two clones possessed high homologies to barley retroelements, and five clones may be involved in degradation or transport processes. One of these genes was further analysed. It encodes an ADP ribosylation factor 1-like protein (HvARF1) and includes sequence motifs representing a myristoylation site and four typical and well conserved ARF-like protein domains. The localization of the protein was investigated by confocal laser scanning microscopy of onion epidermal cells after particle bombardment with chimeric HvARF1-GFP constructs. Possible physiological roles of these nine novel SAGs during barley leaf senescence are discussed. [source] Ageing in Plants: Conserved Strategies and Novel PathwaysPLANT BIOLOGY, Issue 5 2003H.-C. Jing Abstract: Ageing increases chaos and entropy and ultimately leads to the death of living organisms. Nevertheless, single gene mutations substantially alter lifespan, revealing that ageing is subject to genetic control. In higher plants, ageing is most obviously manifested by the senescence of leaves, and recent molecular genetic studies, in particular the isolation of Arabidopsis mutants with altered leaf senescence, have greatly advanced our understanding of ageing regulation in plants. This paper provides an overview of the identified genes and their respective molecular pathways. Hormones, metabolic flux, reactive oxygen species and protein degradation are prominent strategies employed by plants to control leaf senescence. Plants predominantly use similar ageing-regulating strategies as yeast and animals but have evolved different molecular pathways. The senescence window concept is proposed to describe the age-dependent actions of the regulatory genes. It is concluded that the similarities and differences in ageing between plants and other organisms are deeply rooted in the evolution of ageing and we hope to stimulate discussion and research in the fascinating field of leaf senescence. [source] Antioxidant and Pigment Composition during Autumnal Leaf Senescence in Woody Deciduous Species Differing in their Ecological TraitsPLANT BIOLOGY, Issue 5 2003J. I. García-Plazaola Abstract: Photoprotection mechanisms have been studied during autumnal senescence in sun and shade leaves of woody plants with different ecological characteristics and senescence patterns. Three of them belonging to the same family, Betulaceae: the shade-intolerant and early successional species (Betula alba L.), the shade-tolerant and late successional species (Corylus avellana L.), and an N-fixing tree with low N resorption efficiency (Alnus glutinosa L.). The other two species: a shade-intolerant (Populus tremula L.) and a shade-tolerant (Cornus sanguinea L.), were chosen because of their ability to accumulate anthocyanins during autumnal leaf senescence. The study of plants with different ecological strategies allowed us to establish general trends in photoprotection mechanisms during autumnal senescence, when nutrient remobilisation occurs, but also during whole leaf ontogeny. We have not found a clear relationship between shade tolerance and the level of photoprotection; the main difference between both groups of species being the presence of ,-carotene in shade leaves of shade-tolerant species. Preceding autumn, nitrogen resorption started in mid-summer and occurred in parallel with a slight and continuous ascorbate, chlorophyll and carotenoid degradation. However, the ascorbate pool remained highly reduced and lipid oxidation did not increase at this time. Contrasting with ascorbate, ,-tocopherol accumulated progressively in all species. Only during the last stages of senescence was chlorophyll preferentially degraded with respect to carotenoids, leading to the yellowing of leaves, except in A. glutinosa in which a large retention of chlorophyll and N took place. Senescing leaves were characterised, except in C. sanguinea, by a relative increase in the proportion of de-epoxidised xanthophylls: zeaxanthin, antheraxanthin and lutein. The light-induced accumulation of anthocyanins in C. sanguinea could play an additional protective role, compensating for the low retention of de-epoxidised xanthophylls. These different strategies among deciduous species are consistent with a role for photoprotective compounds in enhancing nitrogen remobilization and storage for the next growing season. [source] Leaf senescence is delayed in maize expressing the Agrobacterium IPT gene under the control of a novel maize senescence-enhanced promoterPLANT BIOTECHNOLOGY JOURNAL, Issue 2 2004Paul R. H. Robson Summary We have genetically modified maize plants to delay leaf senescence. A senescence-enhanced promoter from maize (PSEE1) was used to drive expression of the Agrobacterium cytokinin biosynthesis gene IPT in senescing leaf tissue. Three maize lines expressing IPT from PSEE1, Sg1, Sg2 and Sg3, were analysed in detail, representing mild, intermediate and extreme expression, respectively, of the delayed-senescence phenotype. Backcross populations segregating for the presence or absence of the PSEE1XbaIPTNOS transgene also simultaneously segregated for the senescence phenotype. At the time of ear leaf emergence, individuals of lines Sg1 and Sg2 segregating for the presence of the transgene carried about three fewer senescing leaves than control (transgene-minus) segregants, and IPT transcript levels were higher in leaves at incipient senescence than in young leaves. Leaves of transgenic Sg3 plants were significantly greener than controls and progressed directly from fully green to bleached and dead without an intervening yellowing phase. IPT transcript abundance in this line was not related to the initiation of senescence. Extended greenness was accompanied by a delay in the loss of photosynthetic capacity with leaf age. The delayed-senescence trait was associated with relatively minor changes in morphology and development. The phenotype was particularly emphasized in plants grown in low soil nitrogen. The reduced ability of the extreme transgenic line Sg3 to recycle internal nitrogen from senescing lower leaves accounted for significant chlorosis in emerging younger leaves when plants were grown in low nutrient conditions. This study demonstrates that the agronomically important delayed-senescence (,stay-green') trait can be engineered into a monocot crop, and is the first example outside Arabidopsis of senescence modification using a homologous senescence-enhanced promoter. [source] The molecular analysis of leaf senescence , a genomics approachPLANT BIOTECHNOLOGY JOURNAL, Issue 1 2003Vicky Buchanan-Wollaston Summary Senescence in green plants is a complex and highly regulated process that occurs as part of plant development or can be prematurely induced by stress. In the last decade, the main focus of research has been on the identification of senescence mutants, as well as on genes that show enhanced expression during senescence. Analysis of these is beginning to expand our understanding of the processes by which senescence functions. Recent rapid advances in genomics resources, especially for the model plant species Arabidopsis, are providing scientists with a dazzling array of tools for the identification and functional analysis of the genes and pathways involved in senescence. In this review, we present the current understanding of the mechanisms by which plants control senescence and the processes that are involved. [source] Stay green trait in grain sorghum: relationship between visual rating and leaf chlorophyll concentrationPLANT BREEDING, Issue 4 2000W. Xu Abstract Post-flowering drought tolerance is referred to as the stay green trait in sorghum. Plants with stay green resist drought-induced premature plant senescence. In breeding programmes, stay green is evaluated under limited irrigation, post-flowering moisture-stress field conditions and visually scored at or soon after physiological grain maturity. The objective of this study was to investigate the relationship between the stay green rating and total leaf chlorophyll content. The parents B35 and Tx7000, and their 98 F, recombinant inbred lines were evaluated in replicated field trials under limited (post-flowering stress) and full-irrigation (non-stress) conditions. After scoring the stay green trait of stressed plants, total leaf chlorophyll contents were measured with a chlorophyll meter (SPAD values) and a spectrophotometer method. The SPAD value had a significant linear relationship with total leaf chlorophyll (R2= 0.91) and with visual stay green rating (with R2= 0.82). Relative water content in top leaves of the stay green lines was about 81%, much higher than non-stay green lines (38%), indicating that the stay green lines kept the stalk transporting system functioning under severe drought conditions, The results indicate that visual stay green ratings were a reliable indicator of leaf senescence an should be useful to sorghum breeders in evaluating progeny when breeding for drought tolerance. [source] Differential gene expression in senescing leaves of two silver birch genotypes in response to elevated CO2 and tropospheric ozonePLANT CELL & ENVIRONMENT, Issue 6 2010SARI KONTUNEN-SOPPELA ABSTRACT Long-term effects of elevated CO2 and O3 concentrations on gene expression in silver birch (Betula pendula Roth) leaves were studied during the end of the growing season. Two birch genotypes, clones 4 and 80, with different ozone growth responses, were exposed to 2× ambient CO2 and/or O3 in open-top chambers (OTCs). Microarray analyses were performed after 2 years of exposure, and the transcriptional profiles were compared to key physiological characteristics during leaf senescence. There were genotypic differences in the responses to CO2 and O3. Clone 80 exhibited greater transcriptional response and capacity to alter metabolism, resulting in better stress tolerance. The gene expression patterns of birch leaves indicated contrasting responses of senescence-related genes to elevated CO2 and O3. Elevated CO2 delayed leaf senescence and reduced associated transcriptional changes, whereas elevated O3 advanced leaf senescence because of increased oxidative stress. The combined treatment demonstrated that elevated CO2 only temporarily alleviated the negative effects of O3. Gene expression data alone were insufficient to explain the O3 response in birch, and additional physiological and biochemical data were required to understand the true O3 sensitivity of these clones. [source] The different fates of mitochondria and chloroplasts during dark-induced senescence in Arabidopsis leavesPLANT CELL & ENVIRONMENT, Issue 12 2007OLIVIER KEECH ABSTRACT Senescence is an active process allowing the reallocation of valuable nutrients from the senescing organ towards storage and/or growing tissues. Using Arabidopsis thaliana leaves from both whole darkened plants (DPs) and individually darkened leaves (IDLs), we investigated the fate of mitochondria and chloroplasts during dark-induced leaf senescence. Combining in vivo visualization of fates of the two organelles by three-dimensional reconstructions of abaxial parts of leaves with functional measurements of photosynthesis and respiration, we showed that the two experimental systems displayed major differences during 6 d of dark treatment. In whole DPs, organelles were largely retained in both epidermal and mesophyll cells. However, while the photosynthetic capacity was maintained, the capacity of mitochondrial respiration decreased. In contrast, IDLs showed a rapid decline in photosynthetic capacity while maintaining a high capacity for mitochondrial respiration throughout the treatment. In addition, we noticed an unequal degradation of organelles in the different cell types of the senescing leaf. From these data, we suggest that metabolism in leaves of the whole DPs enters a ,stand-by mode' to preserve the photosynthetic machinery for as long as possible. However, in IDLs, mitochondria actively provide energy and carbon skeletons for the degradation of cell constituents, facilitating the retrieval of nutrients. Finally, the heterogeneity of the degradation processes involved during senescence is discussed with regard to the fate of mitochondria and chloroplasts in the different cell types. [source] The subcellular distribution of the Arabidopsis histidine phosphotransfer proteins is independent of cytokinin signalingTHE PLANT JOURNAL, Issue 3 2010Jayson A. Punwani Summary Cytokinins are a class of mitogenic plant hormones that play an important role in most aspects of plant development, including shoot and root growth, vascular and photomorphogenic development and leaf senescence. A model for cytokinin perception and signaling has emerged that is similar to bacterial two-component phosphorelays. In this model, binding of cytokinin to the extracellular domain of the Arabidopsis histidine kinase (AHKs) receptors induces autophosphorylation within the intracellular histidine-kinase domain. The phosphoryl group is subsequently transferred to cytosolic Arabidopsis histidine phosphotransfer proteins (AHPs), which have been suggested to translocate to the nucleus in response to cytokinin treatment, where they then transfer the phosphoryl group to nuclear-localized response regulators (Type-A and Type-B ARRs). We examined the effects of cytokinin on AHP subcellular localization in Arabidopsis and, contrary to expectations, the AHPs maintained a constant nuclear/cytosolic distribution following cytokinin treatment. Furthermore, mutation of the conserved phosphoacceptor histidine residue of the AHP, as well as disruption of multiple cytokinin signaling elements, did not affect the subcellular localization of the AHP proteins. Finally, we present data indicating that AHPs maintain a nuclear/cytosolic distribution by balancing active transport into and out of the nucleus. Our findings suggest that the current models indicating relocalization of AHP protein into the nucleus in response to cytokinin are incorrect. Rather, AHPs actively maintain a consistent nuclear/cytosolic distribution regardless of the status of the cytokinin response pathway. [source] Two short-chain dehydrogenase/reductases, NON-YELLOW COLORING 1 and NYC1-LIKE, are required for chlorophyll b and light-harvesting complex II degradation during senescence in riceTHE PLANT JOURNAL, Issue 1 2009Yutaka Sato Summary Yellowing, which is related to the degradation of chlorophyll and chlorophyll,protein complexes, is a notable phenomenon during leaf senescence. NON-YELLOW COLORING1 (NYC1) in rice encodes a membrane-localized short-chain dehydrogenase/reductase (SDR) that is thought to represent a chlorophyll b reductase necessary for catalyzing the first step of chlorophyll b degradation. Analysis of the nyc1 mutant, which shows the stay-green phenotype, revealed that chlorophyll b degradation is required for the degradation of light-harvesting complex II and thylakoid grana in leaf senescence. Phylogenetic analysis further revealed the existence of NYC1-LIKE (NOL) as the most closely related protein to NYC1. In the present paper, the nol mutant in rice was also found to show a stay-green phenotype very similar to that of the nyc1 mutant, i.e. the degradation of chlorophyll b was severely inhibited and light-harvesting complex II was selectively retained during senescence, resulting in the retention of thylakoid grana even at a late stage of senescence. The nyc1 nol double mutant did not show prominent enhancement of inhibition of chlorophyll degradation. NOL was localized on the stromal side of the thylakoid membrane despite the lack of a transmembrane domain. Immunoprecipitation analysis revealed that NOL and NYC1 interact physically in vitro. These observations suggest that NOL and NYC1 are co-localized in the thylakoid membrane and act in the form of a complex as a chlorophyll b reductase in rice. [source] | ||||||||||||||||||||||||||||