Accelerated Leaf Senescence Research Articles

Knockdown of OsHox33, a member of the class III homeodomain-leucine zipper gene family, accelerates leaf senescence in rice

The class III homeodomain-leucine zipper (HD-Zip III) gene family plays important roles in plant growth and development, including regulation of apical embryo patterning, embryonic shoot meristem formation, leaf polarity, vascular development, and meristem function, with a particularly crucial function in leaf development. Although HD-Zip III members are highly conserved in land plants, previous studies, such as genetic analyses based on multiple mutants in Arabidopsis and other plants, suggest that various HD-Zip III family genes have evolved with distinct functions and pleiotropic effects on plant growth and development. In this study, we analyzed a HD-Zip III member, OsHox33, and demonstrated that it plays an important role in age-dependent leaf senescence in rice. We constructed two specific RNAi vectors derived from the 5'-end region and 3'-UTR of OsHox33 to knockdown its expression. Transgenic plants harboring either RNAi construct displayed similar phenotypes of precocious leaf senescence symptoms, suggesting that knockdown of OsHox33 accelerates leaf senescence in rice. pOsHox33::GUS fusion expression and RT-PCR revealed that OsHox33 is highly expressed in young organs, especially in young meristems such as shoot apical meristems, intercalary meristems, and young callus. In addition, real-time PCR indicated that OsHox33 was more highly expressed in young leaves than in old leaves. To further investigate OsHox33 function, we analyzed chloroplast ultrastructure in different-aged leaves of RNAi plants, and found that OsHox33 knockdown accelerated chloroplast degradation, which is consistent with RNAi phenotypes. Finally, real-time PCR studies showed that OsHox33 can regulate the expression of GS1 and GS2, two senescence-associated genes. Taken together, the work presented here provides new insights into the function of HD-Zip III members in plants.

Senescence and yield responses to plant density in stay green and earlier-senescing maize hybrids from Argentina

Increases in maize (Zea mays L.) yield over the past few decades have been associated with breeding for tolerance to progressively higher plant densities. Since high plant density exacerbates interplant competition, it has been suggested that improved resource capture through delayed senescence might be advantageous in such situations. The main objectives of this work were to determine (1) the time-course of canopy senescence, (2) post-silking C and N accumulation and (3) yield responses of contemporary maize hybrids with different expression of the stay green (SG) character grown in a range of plant densities from moderate to intense crowding stress. Three experiments consisting of a combination of different plant densities (from 6 to 10plm−2) and commercial hybrids with different timing of senescence were carried out. High density accelerated leaf senescence at the lower canopy layer. The SG hybrids delayed senescence and retained green leaves at physiological maturity at all tested densities. One of these hybrids (NK880), with a strong SG character, retained green leaves at all canopy layers, even at the lower layer exposed to limiting irradiance. Lower canopy leaves maintained high respiratory rates in NK880, while leaves of the NSG hybrid (DK682) senesced and their respiration became not detectable. At the highest tested density, the NSG DK682 achieved greater grain yields than the SG NK880. Increased density reduced kernel weight (KW), and this decrease was more pronounced for the SG NK880 (6–18% comparing 10 vs. 8plm−2). In spite of delayed senescence in NK880, no hybrid differences were found for post-silking dry matter accumulation and plant dry matter at physiological maturity. Unexpectedly, plant nitrogen content (Nc) at harvest was similar (Exp. I) or even lower (P<0.05, Exp. II) in the SG NK880. This was the result of lower net N accumulation during the post-silking period (Exp. I) or lower Nc achieved at silking (Exp. II) in the SG NK880. A strong positive relation was found between KW and N concentration in kernels, with %N in kernels being below the critical N concentration to achieve potential KW (around 1.4%) in the SG hybrid. This suggests that yield in NK880 was limited by N. In the SG genotype, N remobilization from vegetative organs did not seem to compensate for the N deficit for optimum grain growth. In summary, at high densities the NK880 hybrid displayed a strong, constitutive SG character, even if it accumulated less N, and senescence delay was not reflected in higher grain yield.

ETHYLENE-INSENSITIVE3 Is a Senescence-Associated Gene That Accelerates Age-Dependent Leaf Senescence by Directly Repressing miR164 Transcription in Arabidopsis

Numerous endogenous and environmental signals regulate the intricate and highly orchestrated process of plant senescence. Ethylene is a well-known inducer of senescence, including fruit ripening and flower and leaf senescence. However, the underlying molecular mechanism of ethylene-induced leaf senescence remains to be elucidated. Here, we examine ethylene-insensitive3 (EIN3), a key transcription factor in ethylene signaling, and find that EIN3 is a functional senescence-associated gene. Constitutive overexpression or temporary activation of EIN3 is sufficient to accelerate leaf senescence symptoms. Conversely, loss of EIN3 and EIN3-Like1 (its close homolog) function leads to a delay in age-dependent and ethylene-, jasmonic acid-, or dark-induced leaf senescence. We further found that EIN3 acts downstream of ORESARA2 (ORE2)/ORE3/EIN2 to repress miR164 transcription and upregulate the transcript levels of ORE1/NAC2, a target gene of miR164. EIN3 directly binds to the promoters of microRNA164 (miR164), and this binding activity progressively increases during leaf ageing. Genetic analysis revealed that overexpression of miR164 or knockout of ORE1/NAC2 represses EIN3-induced early-senescence phenotypes. Collectively, our study defines a continuation of the signaling pathway involving EIN2-EIN3-miR164-NAC2 in regulating leaf senescence and provides a mechanistic insight into how ethylene promotes the progression of leaf senescence in Arabidopsis thaliana.

Leaf cDNA-AFLP analysis of two citrus species differing in manganese tolerance in response to long-term manganese-toxicity

BackgroundVery little is known about manganese (Mn)-toxicity-responsive genes in citrus plants. Seedlings of ‘Xuegan’ (Citrus sinensis) and ‘Sour pummelo’ (Citrus grandis) were irrigated for 17 weeks with nutrient solution containing 2 μM (control) or 600 μM (Mn-toxicity) MnSO4. The objectives of this study were to understand the mechanisms of citrus Mn-tolerance and to identify differentially expressed genes, which might be involved in Mn-tolerance.ResultsUnder Mn-toxicity, the majority of Mn in seedlings was retained in the roots; C. sinensis seedlings accumulated more Mn in roots and less Mn in shoots (leaves) than C. grandis ones and Mn concentration was lower in Mn-toxicity C. sinensis leaves compared to Mn-toxicity C. grandis ones. Mn-toxicity affected C. grandis seedling growth, leaf CO2 assimilation, total soluble concentration, phosphorus (P) and magenisum (Mg) more than C. sinensis. Using cDNA-AFLP, we isolated 42 up-regulated and 80 down-regulated genes in Mn-toxicity C. grandis leaves. They were grouped into the following functional categories: biological regulation and signal transduction, carbohydrate and energy metabolism, nucleic acid metabolism, protein metabolism, lipid metabolism, cell wall metabolism, stress responses and cell transport. However, only 7 up-regulated and 8 down-regulated genes were identified in Mn-toxicity C. sinensis ones. The responses of C. grandis leaves to Mn-toxicity might include following several aspects: (1) accelerating leaf senescence; (2) activating the metabolic pathway related to ATPase synthesis and reducing power production; (3) decreasing cell transport; (4) inhibiting protein and nucleic acid metabolisms; (5) impairing the formation of cell wall; and (6) triggering multiple signal transduction pathways. We also identified many new Mn-toxicity-responsive genes involved in biological and signal transduction, carbohydrate and protein metabolisms, stress responses and cell transport.ConclusionsOur results demonstrated that C. sinensis was more tolerant to Mn-toxicity than C. grandis, and that Mn-toxicity affected gene expression far less in C. sinensis leaves. This might be associated with more Mn accumulation in roots and less Mn accumulation in leaves of Mn-toxicity C. sinensis seedlings than those of C. grandis seedlings. Our findings increase our understanding of the molecular mechanisms involved in the responses of plants to Mn-toxicity.

Identification and functional characterization of a rice NAC gene involved in the regulation of leaf senescence

BackgroundAs the final stage of leaf development, leaf senescence may cause the decline of photosynthesis and gradual reduction of carbon assimilation, which makes it a possible limiting factor for crop yield. NACs are plant-specific transcription factors and some NACs have been confirmed to play important roles in regulating leaf senescence.ResultsIn this study, we reported a member of the NAC transcription factor family named OsNAP whose expression is associated with leaf senescence, and investigated its preliminary function during the process of leaf senescence. The results of qRT-PCR showed that the OsNAP transcripts were accumulated gradually in response to leaf senescence and treatment with methyl jasmonic acid (MeJA). A subcellular localization assay indicated that OsNAP is a nuclear-localized protein. Yeast one-hybrid experiments indicated that OsNAP can bind the NAC recognition site (NACRS)-like sequence. OsNAP-overexpressing transgenic plants displayed an accelerated leaf senescence phenotype at the grain-filling stage, which might be caused by the elevated JA levels and the increased expression of the JA biosynthesis-related genes LOX2 and AOC1, and showed enhanced tolerance ability to MeJA treatment at the seedling stage. Nevertheless, the leaf senescence process was delayed in OsNAP RNAi transgenic plants with a dramatic drop in JA levels and with decreased expression levels of the JA biosynthesis-related genes AOS2, AOC1 and OPR7.ConclusionsThese results suggest that OsNAP acts as a positive regulator of leaf senescence and this regulation may occur via the JA pathway.

ASR1 Mediates Glucose-Hormone Cross Talk by Affecting Sugar Trafficking in Tobacco Plants

Asr (for ABA, stress, ripening) genes are exclusively found in the genomes of higher plants, and the encoded proteins have been found localized both to the nucleus and cytoplasm. However, before the mechanisms underlying the activity of ASR proteins can be determined, the role of these proteins in planta should be deciphered. Results from this study suggest that ASR is positioned within the signaling cascade of interactions among glucose, abscisic acid, and gibberellins. Tobacco (Nicotiana tabacum) transgenic lines with reduced levels of ASR protein showed impaired glucose metabolism and altered abscisic acid and gibberellin levels. These changes were associated with dwarfism, reduced carbon dioxide assimilation, and accelerated leaf senescence as a consequence of a fine regulation exerted by ASR to the glucose metabolism. This regulation resulted in an impact on glucose signaling mediated by Hexokinase1 and Snf1-related kinase, which would subsequently have been responsible for photosynthesis, leaf senescence, and hormone level alterations. It thus can be postulated that ASR is not only involved in the control of hexose uptake in heterotrophic organs, as we have previously reported, but also in the control of carbon fixation by the leaves mediated by a similar mechanism.

The Effect of Priming and Salinity on Physiological and Chemical Characteristics of Wheat (Triticum aestivum L.)

In order to study of the effect of priming and salinity on physiological and chemical characteristics of wheat(Triticum aestivum L.), an experiment was carried out at the Experimental Farm of Shiraz University. Results showedthat primed plants significantly reduced its gas exchanges by accelerating senescence under a series of salt stress,which became more serious along with the increasing of salt concentrations, especially at 21 d after anthesis. Undereach level of salt stress, dry matter accumulation of primed plants was always higher than the non-primed plants.Primed plants had higher potassium selectivity against sodium than non-primed plants with the former could maintainrelatively stable balance of ions, potassium/sodium was found not to be the limited factor for salt tolerant plants, butit was in salt-sensitive plants. Net photosynthesis (Pn) significantly positively correlated with leaf potassium/sodium(K+/Na+), relative water content (RWC), and leaf area duration (P < 0.01). So those four parameters might be idealcriterions of salt tolerance in wheat. In conclusion, salt stresses caused significant declines in growth period of wheatby accelerating leaf senescence at reproductive stage. Primed plants of wheat successfully preserved normal growthby maintaining Pn, K+/Na+, leaf area duration (LAD) and dry matter accumulation (DMA), while non-primed plantsdecreased considerably in those parameters. The improvement of photosynthesis and related traits in reproductivestage was a key to the growth of wheat under saline conditions.

Sweet potato calmodulin SPCAM is involved in salt stress-mediated leaf senescence, H2O2 elevation and senescence-associated gene expression

The sweet potato calmodulin gene, SPCAM, was previously cloned and shown to participate in ethephon-mediated leaf senescence, H2O2 elevation and senescence-associated gene expression. In this report, an association of SPCAM with NaCl stress is reported. Expression of SPCAM was significantly enhanced by NaCl on days 1 and 2 after salt treatment in a dose-dependent manner and drastically decreased again on the third day. Starting on day 6, salt stress also remarkably promoted leaf senescence, H2O2 elevation and senescence-associated gene expression in a dose-dependent manner. These salt stress-mediated effects were strongly inhibited by chlorpromazine, a calmodulin inhibitor, and the chlorpromazine-induced repression could be reversed by exogenous application of purified calmodulin fusion protein. These data suggest an involvement of calmodulin in salt stress-mediated leaf senescence, H2O2 elevation and senescence-associated gene expression in sweet potato. Exogenous application of SPCAM fusion protein alone, however, did not significantly accelerate leaf senescence and senescence-associated gene expression, but only showed a slight effect 12 days after treatment. These data suggest that additional components are involved in salt stress-mediated leaf senescence in sweet potato, possibly induced by and coordinated with SPCAM. In conclusion, the sweet potato calmodulin gene is NaCl-inducible and participates in salt stress-mediated leaf senescence, H2O2 elevation and senescence-associated gene expression.

Interactions Between Temperature and Sugars in the Regulation of Leaf Senescence in the Perennial Herb Arabis alpina L.F

Annual plants usually flower and set seed once before senescence results in the death of the whole plant (monocarpic senescence). Leaf senescence also occurs in polycarpic perennials; even in "evergreen" species individual leaves senesce. In the annual model Arabidopsis thaliana sugars accumulate in the senescent leaves and senescence is accelerated by high sugar availability. Similar to A. thaliana, sugar contents increased with leaf age in the perennial Arabis alpina grown under warm conditions (22 °C day/18 night). At 5 °C, sugar contents in non-senescent leaves were higher than at a warm temperature, but dependent on the accession, either sugars did not accumulate or their contents decreased in old leaves. In A. alpina plants grown in their natural habitat in the Alps, sugar contents declined with leaf age. Growth at a cold temperature slightly delayed senescence in A. alpina. In both warm and cold conditions, an external glucose supply accelerated senescence, but natural variation was found in this response. In conclusion, sugar accumulation under warm conditions could accelerate leaf senescence in A. alpina plants, but genotype-specific responses and interactions with growth temperature are likely to influence senescence under natural conditions.

Growth under elevated atmospheric CO2 concentration accelerates leaf senescence in sunflower (Helianthus annuus L.) plants

Some morphogenetic and metabolic processes were sensitive to a high atmospheric CO2 concentration during sunflower primary leaf ontogeny. Young leaves of sunflower plants growing under elevated CO2 concentration exhibited increased growth, as reflected by the high specific leaf mass referred to as dry weight in young leaves (16days). The content of photosynthetic pigments decreased with leaf development, especially in plants grown under elevated CO2 concentrations, suggesting that high CO2 accelerates chlorophyll degradation, and also possibly leaf senescence. Elevated CO2 concentration increased the oxidative stress in sunflower plants by increasing H2O2 levels and decreasing activity of antioxidant enzymes such as catalase and ascorbate peroxidase. The loss of plant defenses probably increases the concentration of reactive oxygen species in the chloroplast, decreasing the photosynthetic pigment content as a result. Elevated CO2 concentration was found to boost photosynthetic CO2 fixation, especially in young leaves. High CO2 also increased the starch and soluble sugar contents (glucose and fructose) and the C/N ratio during sunflower primary leaf development. At the beginning of senescence, we observed a strong increase in the hexoses to sucrose ratio that was especially marked at high CO2 concentration. These results indicate that elevated CO2 concentration could promote leaf senescence in sunflower plants by affecting the soluble sugar levels, the C/N ratio and the oxidative status during leaf ontogeny. It is likely that systemic signals produced in plants grown with elevated CO2, lead to early senescence and a higher oxidation state of the cells of these plant leaves.

Nitrogen availability impacts oilseed rape (Brassica napus L.) plant water status and proline production efficiency under water-limited conditions.

Large amounts of nitrogen (N) fertilizers are used in the production of oilseed rape. However, as low-input methods of crop management are introduced crops will need to withstand temporary N deficiency. In temperate areas, oilseed rape will also be affected by frequent drought periods. Here we evaluated the physiological and metabolic impact of nitrate limitation on the oilseed rape response to water deprivation. Different amounts of N fertilizer were applied to plants at the vegetative stage, which were then deprived of water and rehydrated. Both water and N depletion accelerated leaf senescence and reduced leaf development. N-deprived plants exhibited less pronounced symptoms of wilting during drought, probably because leaves were smaller and stomata were partially closed. Efficiency of proline production, a major stress-induced diversion of nitrogen metabolism, was assessed at different positions along the whole plant axis and related to leaf developmental stage and water status indices. Proline accumulation, preferentially in younger leaves, accounted for 25–85 % of the free amino acid pool. This was mainly due to a better capacity for proline synthesis in fully N-supplied plants whether they were subjected to drought or not, as deduced from the expression patterns of the proline metabolism BnP5CS and BnPDH genes. Although less proline accumulated in the oldest leaves, a significant amount was transported from senescing to emerging leaves. Moreover, during rehydration proline was readily recycled. Our results therefore suggest that proline plays a significant role in leaf N remobilization and in N use efficiency in oilseed rape.Electronic supplementary materialThe online version of this article (doi:10.1007/s00425-012-1636-8) contains supplementary material, which is available to authorized users.

Interactive effects of elevated ozone and temperature on carbon allocation of silver birch (Betula pendula) genotypes in an open-air field exposure

In the present experiment, the single and combined effects of elevated temperature and ozone (O(3)) on four silver birch genotypes (gt12, gt14, gt15 and gt25) were studied in an open-air field exposure design. Above- and below-ground biomass accumulation, stem growth and soil respiration were measured in 2008. In addition, a (13)C-labelling experiment was conducted with gt15 trees. After the second exposure season, elevated temperature increased silver birch above- and below-ground growth and soil respiration rates. However, some of these variables showed that the temperature effect was modified by tree genotype and prevailing O(3) level. For instance, in gt14 soil respiration was increased in elevated temperature alone (T) and in elevated O(3) and elevated temperature in combination (O(3) + T) treatments, but in other genotypes O(3) either partly (gt12) or totally nullified (gt25) temperature effects on soil respiration, or acted synergistically with temperature (gt15). Before leaf abscission, all genotypes had the largest leaf biomass in T and O(3) + T treatments, whereas at the end of the season temperature effects on leaf biomass depended on the prevailing O(3) level. Temperature increase thus delayed and O(3) accelerated leaf senescence, and in combination treatment O(3) reduced the temperature effect. Photosynthetic : non-photosynthetic tissue ratios (P : nP ratios) showed that elevated temperature increased foliage biomass relative to woody mass, particularly in gt14 and gt12, whereas O(3) and O(3) + T decreased it most clearly in gt25. O(3)-caused stem growth reductions were clearest in the fastest-growing gt14 and gt25, whereas mycorrhizal root growth and sporocarp production increased under O(3) in all genotypes. A labelling experiment showed that temperature increased tree total biomass and hence (13)C fixation in the foliage and roots and also label return was highest under elevated temperature. Ozone seemed to change tree (13)C allocation, as it decreased foliar (13)C excess amount, simultaneously increasing (13)C excess obtained from the soil. The present results suggest that warming has potential to increase silver birch growth and hence carbon (C) accumulation in tree biomass, but the final magnitude of this C sink strength is partly counteracted by temperature-induced increase in soil respiration rates and simultaneous O(3) stress. Silver birch populations' response to climate change will also largely depend on their genotype composition.

A Novel Protein RLS1 with NB–ARM Domains Is Involved in Chloroplast Degradation during Leaf Senescence in Rice

Leaf senescence, a type of programmed cell death (PCD) characterized by chlorophyll degradation, is important to plant growth and crop productivity. It emerges that autophagy is involved in chloroplast degradation during leaf senescence. However, the molecular mechanism(s) involved in the process is not well understood. In this study, the genetic and physiological characteristics of the rice rls1 (rapid leaf senescence 1) mutant were identified. The rls1 mutant developed small, yellow-brown lesions resembling disease scattered over the whole surfaces of leaves that displayed earlier senescence than those of wild-type plants. The rapid loss of chlorophyll content during senescence was the main cause of accelerated leaf senescence in rls1. Microscopic observation indicated that PCD was misregulated, probably resulting in the accelerated degradation of chloroplasts in rls1 leaves. Map-based cloning of the RLS1 gene revealed that it encodes a previously uncharacterized NB (nucleotide-binding site)-containing protein with an ARM (armadillo) domain at the carboxyl terminus. Consistent with its involvement in leaf senescence, RLS1 was up-regulated during dark-induced leaf senescence and down-regulated by cytokinin. Intriguingly, constitutive expression of RLS1 also slightly accelerated leaf senescence with decreased chlorophyll content in transgenic rice plants. Our study identified a previously uncharacterized NB-ARM protein involved in PCD during plant growth and development, providing a unique tool for dissecting possible autophagy-mediated PCD during senescence in plants.

Assimilate Supply as a Yield Determination Factor in Spring Wheat under High Temperature Conditions in the Mediterranean Zone of South-East Turkey

Wheat in the Mediterranean zone often encounters high temperatures during the terminal growth stage. This study investigated whether assimilate supply by source-function plays a dominant role in determining grain production in spring wheat exposed to high terminaltemperatures in the Mediterranean zone of south-east Turkey. The spring wheat cultivar Adana99 was grown under irrigated conditions sowing according to the current schedule (CS plants) and late-sowing (LS plants) for two years. Grain yield of CS plants, which were subjected to relatively lower temperatures during the terminal growth stage, was higher than that of LS plants, which were subjected to relatively higher temperatures. A high temperature in the post-anthesis period accelerated leaf senescence and reduced radiation-use efficiency. Halving the plant density at anthesis, which increases assimilate supply to grains, significantly increased the grain dry weight (GDW) and whole plant dry weight (WPDW) in the post-anthesis period. Grain filling percentage (F%, observed/final GDW) in thinned plants, which was regarded as potential F%, showed a single logistic equation based on cumulative temperature after anthesis, regardless of post-anthesis temperatures. In the LS plants, the daily increase in WPDM ( Δwpdw) decreased, but the peak of an increase rate of potential GDW estimated from F% was shifted to earlier-filling stage, resulting in reduced GDW at harvest. The final GDW in both years closely correlated with the estimated Δwpdw in the grain-filling period. These results suggest that source-function for assimilate supply in the post-anthesis period is one of the important yield-determining processes in spring wheat subjected to high temperatures.

Water Use Efficiency and Physiological Response of Rice Cultivars under Alternate Wetting and Drying Conditions

One of the technology options that can help farmers cope with water scarcity at the field level is alternate wetting and drying (AWD). Limited information is available on the varietal responses to nitrogen, AWD, and their interactions. Field experiments were conducted at the International Rice Research Institute (IRRI) farm in 2009 dry season (DS), 2009 wet season (WS), and 2010 DS to determine genotypic responses and water use efficiency of rice under two N rates and two water management treatments. Grain yield was not significantly different between AWD and continuous flooding (CF) across the three seasons. Interactive effects among variety, water management, and N rate were not significant. The high yield was attributed to the significantly higher grain weight, which in turn was due to slower grain filling and high leaf N at the later stage of grain filling of CF. AWD treatments accelerated the grain filling rate, shortened grain filling period, and enhanced whole plant senescence. Under normal dry-season conditions, such as 2010 DS, AWD reduced water input by 24.5% than CF; however, it decreased grain yield by 6.9% due to accelerated leaf senescence. The study indicates that proper water management greatly contributes to grain yield in the late stage of grain filling, and it is critical for safe AWD technology.

Control of barley (Hordeum vulgare L.) development and senescence by the interaction between a chromosome six grain protein content locus, day length, and vernalization.

Regulatory processes controlling traits such as anthesis timing and whole-plant senescence are of primary importance for reproductive success and for crop quality and yield. It has previously been demonstrated that the presence of alleles associated with high grain protein content (GPC) at a locus on barley chromosome six leads to accelerated leaf senescence, and to strong (>10-fold) up-regulation of several genes which may be involved in senescence control. One of these genes (coding for a glycine-rich RNA-binding protein termed HvGR-RBP1) exhibits a high degree of similarity to Arabidopsis glycine-rich RNA-binding protein 7 (AtGRP7), which has been demonstrated to accelerate flowering under both long-day (LD) and short-day (SD) conditions, but not after vernalization. Development of near-isogenic barley lines, differing in the allelic state of the GPC locus, was compared from the seedling stage to maturity under both SD and LD and after vernalization under LD. Intriguingly, pre-anthesis plant development [measured by leaf emergence timing and pre-anthesis (sequential) leaf senescence] was enhanced in high-GPC germplasm. Differences were more pronounced under SD than under LD, but were eliminated by vernalization, associating observed effects with floral induction pathways. By contrast, differences in post-anthesis flag leaf and whole-plant senescence between low- and high-GPC germplasm persisted under all tested conditions, indicating that the GPC locus, possibly through HvGR-RBP1, impacts on both developmental stages. Detailed molecular characterization of this experimental system may allow the dissection of cross-talk between signalling pathways controlling early plant and floral development on one side, and leaf/whole-plant senescence on the other side.

Overexpression of AtCHX24, a member of the cation/H + exchangers, accelerates leaf senescence in Arabidopsis thaliana

Leaf senescence, the final stage of leaf development, occurs in an age-dependent manner but can be finely regulated by other developmental and environmental factors. Despite the discovery of many genes involved in leaf senescence, the molecular genetic mechanisms of leaf senescence are still unclear. In this study, an activation-tagging based suppressor screen was performed to identify Arabidopsis genes that could suppress the delayed leaf senescence phenotypes of oresara9-1 ( ore9-1) when overexpressed. The suppressor1 of ore9 dominant ( sor1-D) was caused by the overexpression of AtCHX24, a putative cation/H + exchanger. The sor1-D mutation suppressed the phenotypes of ore9 in age-dependent and dark-induced senescence. Furthermore, the sor1-D mutation restored the delayed senescence phenotypes of ore1 and ore3. The sor1-D mutant also exhibited increased sensitivity to pH changes during dark-induced leaf senescence. Collectively, overexpression of AtCHX24 results in accelerated leaf senescence and these results suggest that AtCHX24 plays an important role in regulating leaf senescence.

Photosynthetic alterations of pea leaves infected systemically by pea enation mosaic virus: A coordinated decrease in efficiencies of CO(2) assimilation and photosystem II photochemistry.

We have investigated photosynthetic changes of fully expanded pea leaves infected systemically by pea enation mosaic virus (PEMV) that often attacks legumes particularly in northern temperate regions. Atypical compatible virus-host interaction was monitored during 40 post-inoculation days (dpi). Aninitial PEMV-induced decrease in photosynthetic CO(2) assimilation was detected at 15dpi, when the virus appeared in the measured leaves. This decrease was not induced by stomata closure and corresponded with a decrease in the efficiency of photosystem II photochemistry (Φ(PSII)). Despite of a slight impairment of oxygen evolution at this stage, PSII function was not primarily responsible for the decrease in Φ(PSII). Chlorophyll fluorescence imaging revealed that Φ(PSII) started to decrease from the leaf tip to the base. More pronounced symptoms of PEMV disease appeared at later stages, when a typical mosaic and enations appeared in the infected leaves and oxidative damage of cell membranes was detected. From 30dpi, a degradation of photosynthetic pigments accelerated, stomata were closing and corresponding pronounced decline in CO(2) assimilation was observed. A concomitant photoprotective responses, i.e. an increase in non-photochemical quenching and accumulation of de-epoxidized xanthophylls, were also detected. Interestingly, alternative electron sinks in chloroplasts were not stimulated by PEMV infection, which is in contradiction to earlier reports dealing with virus-induced plant stresses. The presented results show that the PEMV-induced alterations in mature pea leaves accelerated leaf senescence during which a decrease in Φ(PSII) took place in coordinated manner with an inhibition of CO(2) assimilation.

Effects of Soil Salinity and Plant Density on Yield and Leaf Senescence of Field‐Grown Cotton

AbstractThe response of cotton to constant salinity has been well documented under controlled conditions, but its response to changing salinity under field conditions is poorly understood. Using a split‐plot design, we conducted a 2‐year field experiment to determine the effects of soil salinity and plant density on plant biomass, boll load, harvest index and leaf senescence in relation to cotton yield in three fields with similar fertility but varying salinity. The main plots were assigned to weak (electrical conductivity of soil saturated paste extract, ECe = 5.5 dS m−1), moderate (ECe = 10.1 dS m−1) and strong (ECe = 15.0 dS m−1) soil salinity levels, while plant density (3.0, 4.5 and 7.5 plants m−2) was assigned to the subplots. Soil salinity had a negative effect on seedcotton yield, but the negative effect was compensated for by increased plant density under strong‐salinity conditions. Seedcotton yield under weak salinity changed little with varying plant density, but the medium plant density yielded better than the low or high plant density under moderate salinity. Plants accumulated 49 and 112 % more Na+ in leaves under moderate and strong salinity than under weak salinity. Strong salinity also led to higher boll load and early leaf senescence. Plant density had no effect on Na+ accumulation in leaves, but greatly reduced boll load and delayed leaf senescence. Plant biomass, maximum leaf area index and harvest index were greatly affected by salinity, plant density and their interaction. Accelerated leaf senescence under strong salinity was attributed to the high boll load and increased accumulation of toxic ions like Na+ in leaves, while delayed leaf senescence with increased plant density was attributed to the reduced boll load. Optimal yield can only be obtained with proper coordination of total biomass and harvest index by modification of plant density based on salinity levels.

Accelerated leaf senescence takes part in enhanced resistance in cucumber mosaic virus inoculated pepper leaves

Altered photosynthetic reactions in cucumber mosaic virus (CMV) inoculated leaves of virus resistant lines L113 and L57 and susceptible pepper (Capsicum annuum L.) plants cv. Albena grown in controlled environment and in the field were investigated. The CMV inoculated leaves of virus resistant lines developed different symptoms—necrotic local lesions on L113 and chlorotic spots on L57 while the same leaves of susceptible cv. Albena were symptomless. The changes in Photosystem II (PSII) and PSI electron transport were evaluated by chlorophyll fluorescence, and far-red (FR) light induced leaf absorbance A 810–860. CMV infection caused a decrease in maximal PSII quantum yield, F v/F m, in susceptible leaves. Increased non-photochemical fluorescence quenching in CMV-inoculated leaves of both resistant lines were observed. In CMV-inoculated leaves of all tested plants FR light induced P700 oxidation was decreased. In the present study, the viral-infected pepper plants grown in controlled environment to avoid the effects of abiotic factors were used as model system that allow us to investigate the differences in leaf senescence in CMV-inoculated leaves of susceptible and resistant pepper lines expressing different symptoms. Earlier leaf falls of inoculated leaves as a result of accelerated leaf senescence is important for building successful secondary virus resistance strategy following fast responses such as hypersensitive reaction.