Effects of cytokinin and senescence-inducing factors on expression of P ARR5 -GUS gene construct during leaf senescence in transgenic Arabidopsis thaliana plants

ARR5-gene expression was studied in the course of natural leaf senescence and detached leaf senescence in the dark using Arabidopsis thaliana plants transformed with the P _ARR5 -GUS gene construct. GUS-activity was measured as a marker of ARR5-gene expression. Chlorophyll and total protein amounts were also estimated to evaluate leaf senescence. Natural leaf senescence was accompanied by the progressive decline in the GUS-activity in leaves of the 2nd and 3rd nodes studied, and this shift of GUS-activity was more pronounced than the loss of chlorophyll content. The ability of the ARR5-gene promoter to respond to cytokinin was not eliminated during natural leaf senescence, as was demonstrated by a cytokinin-induced increase in GUS activity in leaves after their detachment and incubation on benzyladenine (BA, 5 × 10⁻⁶ M) in the dark. Leaf senescence in the dark was associated with the further decrease in the GUS-activity. The ARR5-gene promoter response to cytokinin was enhanced with the increase of the age of plants, taken as a source of leaves for cytokinin treatments. Hence, although the expression of the ARR5 gene reduces during natural and dark/detached leaf senescence, the ARR5-gene sensitivity to cytokinin was maintained in both cases and even increased with the leaf age. This data suggest that the ARR5 gene, which belongs to the type-A negative regulators of plant response to cytokinin, could be a feedback regulator able to prevent retardation by cytokinin of leaf senescence when it is important for plant life. Growth regulators either reduced ARR5 gene response to cytokinin during senescence of mature detached leaves in the dark (SA, meJA, ABA, SP) or increased it (IAA), thus modifying the resulting rate of its expression.     

The role of hexokinase in plant sugar signal transduction and growth and development

Previous studies have revealed a central role of Arabidopsis thaliana hexokinases (AtHXK1 and AtHXK2) in the glucose repression of photosynthetic genes and early seedling development. However, it remains unclear whether HXK can modulate the expression of diverse sugar-regulated genes. On the basis of the results of analyses of gene expression in HXK transgenic plants, we suggest that three distinct glucose signal transduction pathways exist in plants. The first is an AtHXK1-dependent pathway in which gene expression is correlated with the AtHXK1-mediated signaling function. The second is a glycolysis-dependent pathway that is influenced by the catalytic activity of both AtHXK1 and the heterologous yeast Hxk2. The last is an AtHXK1-independent pathway in which gene expression is independent of AtHXK1. Further investigation of HXK transgenic Arabidopsis discloses a role of HXK in glucose-dependent growth and senescence. In the absence of exogenous glucose, plant growth is limited to the seedling stage with restricted true leaf development even after a 3-week culture on MS medium. In the presence of glucose, however, over-expressing Arabidopsis or yeast HXK in plants results in the repression of growth and true leaf development, and early senescence, while under-expressing AtHXK1 delays the senescence process. These studies reveal multiple glucose signal transduction pathways that control diverse genes and processes that are intimately linked to developmental stages and environmental conditions.     

Overexpression of OsRab7B3, a Small GTP-Binding Protein Gene,Enhances Leaf Senescence in Transgenic Rice

Rab family proteins are small GTP-binding proteins involved in intracellular trafficking. They play critical roles in several plant development processes. Different expression patterns of 46 Rabs in the rice genome were examined in various rice tissues and in leaves treated with plant growth regulators and under senescence conditions. One of the OsRab genes, OsRab7B3, closely associated with senescence in expression pattern, was chosen for functional analysis. Expression of sGFP under the control of the OsRab7B3 promoter increased in leaves when ABA and NaCl were applied or when kept in dark. In transgenic rice overexpressing OsRab7B3, the senescence-related genes were upregulated and leaf senescence was significantly enhanced under dark conditions. Moreover, leaf yellowing occurred earlier in the transgenic plants than in the wild type at the ripening stage. Hence it is suggested that OsRab7B3 act as a stress–inducible gene that plays an important role in the leaf senescence process.     

Interrelationship between ethylene and growth regulators in the senescence of lettuce leaf discs

The interrelationship between ethylene and growth regulators in the senescence of romaine lettuce (Lactuca sativa L.) leaves was studied. Gibberellic acid (GA₃), kinetin, and 3-indoleacetic acid (IAA) retarded chlorophyll loss from leaf discs which were floated on hormone solutions. Abscisic acid (ABA) and ethephon enhanced chlorophyll loss and antagonized the senescence-retarding effect of GA₃ and kinetin. A high concentration of IAA (10^–4 M) caused accelerated chlorophyll loss, whereas a similar concentration of kinetin neither retarded nor promoted chlorophyll loss. The ineffectiveness of IAA and kinetin at their supraoptimal concentrations in retarding leaf senescence was related to increased production of ethylene induced in the treated leaf discs. GA₃ was the most effective in retarding chlorophyll loss and did not stimulate ethylene production at all. The senescence-enhancing effect of ABA was not mediated by ethylene. However, the moderately increased production of ethylene, induced by relatively high concentrations of ABA, could act synergistically with the latter to accelerate chlorophyll loss. It is proposed that the effectiveness of exogenously applied hormones, both in enhancing and retarding senescence, is greatly affected by the endogenous ethylene concentration of the treated plant tissue.Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel, No. 2571-E, 1988 series.     

激素处理及非生物胁迫下枳PtPPF-1基因的表达特性

PtPPF-1是从枳叶片中分离出的与豌豆中的PPF-1具有很高同源性的基因。PPF-1基因能通过控制叶绿体发育而延缓叶片衰老,同时PPF-1能被外源GA3诱导,是与植物营养生长密切相关的基因。本研究以实生苗枳[Poncirus trifoliata(L.)Raf.]为材料,采用半定量RT-PCR对不同激素处理及非生物胁迫(低温、干旱、盐)下枳叶中PtPPF-1的表达情况进行了分析,以期明确PtPPF-1的功能。结果表明,GA3I、AA、ABA促进了PtPPF-1的表达,KT对PtPPF-1的表达有抑制作用;低温、干旱、盐胁迫能诱导PtPPF-1的表达。推测GA3、IAA、ABA信号通路可能与KT信号通路作用于PtPPF-1的表达存在不同的机制。     

大豆microRNA基因GmMIR160A负调控植物叶片衰老进程

:叶片衰老是受内外多种因子影响的遗传发育进程.生长素、细胞分裂素和乙烯等多种植物激素是调 控叶片衰老的重要内部因子,它们通过长或短距离运输形成叶片组织内特定的区域分布和浓度梯度,从而直 接或间接参与植物叶片衰老过程.分子遗传学表明,细胞分裂素和乙烯分别是叶片衰老的抑制子和正调节 子,而生长素如何参与叶片衰老的分子机制目前还不清晰.植物体内成熟小分子RNA 由小RNA 基因转录 并通过特定酶加工形成的２１~２３bp的双链RNA分子.这些小分子通过不完全配对方式抑制其靶基因转录 和/或表达,参与植物生长发育多个过程,然而这类小RNA 分子如何调控植物叶片衰老发育过程目前则还鲜 有报告.大豆是重要的油料作物,具有典型的单次结实性衰老特征.研究大豆叶片衰老具有重要的科学意义 和深远的应用价值.该文采用实时荧光定量PCR(qPCR)技术分析大豆(Glycinemax)microRNA基因Gm- MIR１６０A 的表达模式,发现大豆第一复叶中GmMIR１６０A 表达受外源生长素和黑暗处理的诱导,暗示该基 因是生长素快速响应的叶片衰老相关基因.为进一步探究GmMIR１６０A 在大豆叶片发育中的功能,构建了 肾上腺皮质激素(Glucocorticoid,GR)类似物地塞米松(Dexamethasone,DEX)诱导表达GmMIR１６０A 双元表 达载体并通过农杆菌介导的子叶节方法转化野生型大豆.通过抗性筛选和基因组PCR 鉴定并结合表型分 析,共获得了４株诱导表达的稳定遗传转基因植株(株系OXＧ３、OXＧ５、OXＧ７和OXＧ８).GmMIR１６０A 过表达 植株根、茎、叶、花和果实在形态学上与野生型相比无显著差异,但叶片的叶绿素含量增加、最大光量子效率 (Fv/Fm)增强.进一步分子分析发现,转基因大豆叶片中GmARFs 和衰老标记基因(GmCYSP１)表达明显下 降,表明大豆Gma-miR１６０通过抑制靶基因GmARFs 的表达来负调控植物叶片的衰老进程.该文揭示了生 长素通过小分子RNA调控叶片发育一条新途径,为研究植物激素调控植物叶片衰老提供了新的思路.     

Ethylene regulates the timing of leaf senescence in Arabidopsis

The plant hormone ethylene influences many aspects of plant growth and development, including some specialized forms of programmed senescence such as fruit ripening and flower petal senescence. To study the relationship between ethylene and leaf senescence, etr1-1, an ethylene-insensitive mutant in Arabidopsis, was used. Comparative analysis of rosette leaf senescence between etr1-1 and wild-type plants revealed that etr1-1 leaves live approximately 30% longer than the wild-type leaves. Delayed leaf senescence in etr1-1 coincided with delayed induction of senescence-associated genes (SAGs) and higher expression levels of photosynthesis-associated genes (PAGs). In wild-type plants, exogenous ethylene was able to further accelerate induction of SAGs and decrease expression of PAGs. The extended period of leaf longevity in etr1-1 was associated with low levels of photosynthetic activity. Therefore, the leaves in etr1-1 functionally senesced even though the apparent life span of the leaf was prolonged.     

Photo- and antioxidant protection and salicylic acid accumulation during post-anthesis leaf senescence in Salvia lanigera grown under Mediterranean climate

Post-anthesis leaf senescence is a key developmental process in the life of plants as it is the time during which material built up by the plant during its growth phase is mobilized into reproductive tissues. Here we aimed to study the extent of photo- and antioxidant protection and salicylic acid (SA) accumulation during post-anthesis leaf senescence in a perennial plant, Salvia lanigera Poir. grown under Mediterranean field conditions. SA levels increased sharply (up to 2.7-fold) during early stages of leaf senescence until fruit and seed formation occurred (i.e. 4 weeks after anthesis). Later on, SA levels kept at constant high levels until leaf abscission occurred (i.e. 7 weeks after anthesis). Reductions in chlorophyll and carotenoid (lutein, violaxanthin and β-carotene) levels occurred progressively during leaf senescence. In contrast, xanthophyll cycle de-epoxidation increased during early stages of leaf senescence and remained constant later, similar to SA accumulation. Indeed, xanthophyll cycle de-epoxidation strongly positively correlated with SA levels (r² = 0.92). The maximum efficiency of PSII (F_v/F_m ratio) kept around 0.80 throughout the experiment, except during the latest stage of leaf senescence (i.e. after fruit and seed formation), when this ratio decreased to 0.72, thus indicating damage to PSII. It is concluded that endogenous SA levels increase sharply during early stages of post-anthesis leaf senescence and concomitantly with activation of photoprotection mechanisms, such as xanthophyll cycle-dependent excess energy dissipation, thus avoiding damage to PSII until fruit and seed formation have been accomplished.     

MdARF8: An Auxin Response Factor Involved in Jasmonate Signaling Pathway in Malus domestica

Auxin response factor (ARF) is a key component of auxin signal. The study of MdARF8 gene in apple shows that it is involved in the process of jasmonate regulating plant growth and development. Methyl jasmonate (MeJA) treatment inhibited the growth process of apple calli, and ARF8 played a negative regulatory role in this pathway. The results of ectopic expression in Arabidopsis showed that MdARF8 could reduce the sensitivity of Arabidopsis to MeJA and alleviate the phenotype of promoting leaf senescence and inhibiting taproot elongation. Further results showed that the dysplastic phenotype of transgenic Arabidopsis root hair could be partially recovered by MeJA treatment. This study provided valuable clues for functional characterization of ARF8 and signal crosstalk between jasmonate and auxin in apple.

     

The Chlorophyll <Emphasis Type="Italic">a</Emphasis> Fluorescence Characteristic in Different Types of Leaf Senescence in <Emphasis Type="Italic">Alhagi sparsifolia</Emphasis>

Senescence is both a highly controlled and a strictly regulated process that is gene dependent. To study the PSII reaction in different types of leaf senescence processes, stem girdling was performed on Alhagi sparsifolia to investigate the leaf status in the control, natural senescence, and girdling-induced senescence leaves. The results showed that during senescence, leaf soluble sugar content, starch content, and the energy absorbed by the unit reaction center (ABS/RC) increased; whereas leaf photosynthetic rate, photosynthetic pigment content, maximum photochemical efficiency (φ _Po), and energy used by the acceptor site in electron transfer (ET_o/RC) decreased. The result of the present research implied that stem girdling significantly accelerated leaf senescence, which was due to the accumulation of carbohydrate. Natural senescence is a highly controlled process, which is an ordered process played by genes, whereas girdling-induced senescence is a disordered one. In addition, natural senescence slightly inhibits the acceptor site of PSII but did not damage the donor site of PSII. Conversely, girdling-induced senescence not only damaged the donor site of PSII (for example, oxygen-evolving complex), but also significantly inhibited the acceptor site of PSII. Moreover, both types of senescence led to an increase in the energy absorbed by the unit reaction center (ABS/RC), which subsequently resulted in an increasing excitation pressure in the reaction center (DI_o/RC), as well as additional saved carotenoid for absorbing residual light energy and quenching reactive oxygen species during senescence.     

Overexpression of CpSIZ1, a SIZ/PIAS-Type SUMO E3 Ligase from Wintersweet (Chimonanthus praecox), Delays Flowering,Accelerates Leaf Senescence and Enhances Cold Tolerance in Arabidopsis

Wintersweet (Chimonanthus praecox L.) is a traditional winter-flowering plant in China and a popular cut flower in winter. Its unique flowering characteristics under cold stress may involve the regulation of a large number of proteins. Protein post-translational modification is an important regulating type for the gene function. However, little is known about the post-translational modification in wintersweet. SUMOylation is an important post-translational modification in eukaryotes. Small ubiquitin-like modifier (SUMO) E3 ligases perform multiple functional regulatory activities in plants via SUMOylation. Here, we cloned and identified a SIZ/PIAS-type SUMO E3 ligase, CpSIZ1, from wintersweet. CpSIZ1 shared high similarity with other SIZ1 proteins. Quantitative real-time PCR (qRT-PCR) indicated that CpSIZ1 was expressed in all tissues tested, with the highest expression in flower wither period of stage 6, and followed by mature leaves except for different flower development stages. The ectopic expression of CpSIZ1 in Arabidopsis, including the CpSIZ1 overexpression in siz1-2 mutant (HB line) and CpSIZ1 overexpression in WT (OE line), not only promoted vegetative growth, delayed flowering and accelerated leaf senescence, but also improve the cold tolerance in Arabidopsis. Therefore, our studies have enrich the understanding of function of SIZ1 gene in woody plant, and provide a good foundation for further research on the post-translational modification regulation mechanism in this winter-flowering plant.

     

Carbon-11 Reveals Opposing Roles of Auxin and Salicylic Acid in Regulating Leaf Physiology,Leaf Metabolism,and Resource Allocation Patterns that Impact Root Growth in Zea mays

Auxin (IAA) is an important regulator of plant development and root differentiation. Although recent studies indicate that salicylic acid (SA) may also be important in this context by interfering with IAA signaling, comparatively little is known about its impact on the plant’s physiology, metabolism, and growth characteristics. Using carbon-11, a short-lived radioisotope (t _1/2 = 20.4 min) administered as ¹¹CO₂ to maize plants (B73), we measured changes in these functions using SA and IAA treatments. IAA application decreased total root biomass, though it increased lateral root growth at the expense of primary root elongation. IAA-mediated inhibition of root growth was correlated with decreased ¹¹CO₂ fixation, photosystem II (PSII) efficiency, and total leaf carbon export of ¹¹C-photoassimilates and their allocation belowground. Furthermore, IAA application increased leaf starch content. On the other hand, SA application increased total root biomass, ¹¹CO₂ fixation, PSII efficiency, and leaf carbon export of ¹¹C-photoassimilates, but it decreased leaf starch content. IAA and SA induction patterns were also examined after root-herbivore attack by Diabrotica virgifera to place possible hormone crosstalk into a realistic environmental context. We found that 4 days after infestation, IAA was induced in the midzone and root tip, whereas SA was induced only in the upper proximal zone of damaged roots. We conclude that antagonistic crosstalk exists between IAA and SA which can affect the development of maize plants, particularly through alteration of the root system’s architecture, and we propose that the integration of both signals may shape the plant’s response to environmental stress.     

Effect of Indol-3-yl Acetic Acid on Photosynthetic Characteristics of Wheat Flag Leaf During Grain Filling

Area and fresh and dry masses of flag leaf show two phases of development during grain filling in Triticum aestivum. The initial large increase in leaf size is mainly due to water intake. Contents of chlorophylls and carotenoids, reducing sugars, and sucrose, Hill reaction rate, and photosynthetic activity increased during leaf growth, but a noticeable decline in these parameters followed throughout leaf senescence. The maximum accumulation of polysaccharides and proteins occurred at the beginning of grain set, but a continuous decline in their absolute values was manifested during grain filling. Grain priming with indol-3-yl acetic acid (IAA) at 25 mg kg^-1 stimulated the flag leaf growth, namely its fresh and dry masses and its area. Furthermore, the stimulatory effect was mainly due to the increase in the pigment formation that in turn increased the photosynthetic activity of flag leaf during grain filling. On the other hand, the highest dose of IAA (50 mg kg^-1) attenuated the growth and physiological activity of flag leaf through its inhibitory action on leaf fresh and dry masses, leaf area, pigments, saccharides and protein formation, as well as its effect on ¹⁴CO₂ assimilation.     

Effects of varied water regimes on root length,dry matter partitioning and endogenous plant growth regulators in sunflower (Helianthus annuus L.)

Abstract

A field experiment was conducted to quantify the effect of varied water regimes on root length, partitioning of dry matter and plant growth regulators by using sunflower genotypes differing in maturity and drought tolerance. Significant depressing effect of drought stress was evident on traits (i.e., reproductive dry matter, leaf area index and cytokinin concentrations in leaves). However, root/shoot, reproductive/vegetative ratios and Abscisic acid (ABA) concentration were found to increase under drought stress. Drought stress also changed the dry matter accumulation pattern of genotypes. In most cases it reduced the days to reach the maximum peak showing early senescence.

ABA was identified as a multi-functional plant growth regulator under drought stress, causing early senescence of plants and translocation of assimilates to the roots and reproductive part while root growth under drought stress was explained by the indole-acetic acid (IAA) concentrations. Maintaining higher cytokinin contents were involved in accumulation of higher reproductive dry matter under drought stress. Although ABA and IAA were both involved in the development of defense responses during the adaptation and survival to drought stress but higher productivity under drought stress was only realized through maintaining higher cytokinin contents.     

Photosynthesis and chlorophyllafluorescence during flag leaf senescence of field-grown wheat plants

The characteristics of photosynthetic gas exchange, chlorophyll a fluorescence, and xanthophyll cycle pigments during flag leaf senescence of field-grown wheat plants were investigated. With senescence progressing, the light-saturated net CO₂ assimilation rate expressed either on a basis of leaf area or chlorophyll decreased significantly. The apparent quantum yield of net photosynthesis decreased when expressed on a leaf area basis but increased when expressed on a chlorophyll basis. The maximal efficiency of PSII photochemistry decreased very little while actual PSII efficiency, photochemical quenching, and the efficiency of excitation capture by open PSII centers decreased considerably. At the same time, non-photochemical quenching increased significantly. A substantial decrease in the contents of violaxanthin and zeaxanthin, but a slight decrease in the content of antheraxanthin were observed. However, the de-epoxidation status of the xanthophyll cycle was positively correlated with progressive senescence. This increase was due mainly to a smaller decrease in zeaxanthin than in violaxanthin. Our results suggest that PSII apparatus remained functional, but a down-regulation of PSII occurred under the steady state of photosynthesis in senescent flag leaves. Such a down-regulation was associated with the closure of PSII centers and an enhanced xanthophyll cycle-related thermal dissipation in the PSII antennae.