Plant organ senescence – regulation by manifold pathways

Senescence is the final stage of plant ontogeny before death. Senescence may occur naturally because of age or may be induced by various endogenous and exogenous factors. Despite its destructive character, senescence is a precisely controlled process that follows a well‐defined order. It is often inseparable from programmed cell death (PCD), and a correlation between these processes has been confirmed during the senescence of leaves and petals. Despite suggestions that senescence and PCD are two separate processes, with PCD occurring after senescence, cell death responsible for senescence is accompanied by numerous changes at the cytological, physiological and molecular levels, similar to other types of PCD. Independent of the plant organ analysed, these changes are focused on initiating the processes of cellular structural degradation via fluctuations in phytohormone levels and the activation of specific genes. Cellular structural degradation is genetically programmed and dependent on autophagy. Phytohormones/plant regulators are heavily involved in regulating the senescence of plant organs and can either promote [ethylene, abscisic acid (ABA), jasmonic acid (JA), and polyamines (PAs)] or inhibit [cytokinins (CKs)] this process. Auxins and carbohydrates have been assigned a dual role in the regulation of senescence, and can both inhibit and stimulate the senescence process. In this review, we introduce the basic pathways that regulate senescence in plants and identify mechanisms involved in controlling senescence in ephemeral plant organs. Moreover, we demonstrate a universal nature of this process in different plant organs; despite this process occurring in organs that have completely different functions, it is very similar. Progress in this area is providing opportunities to revisit how, when and which way senescence is coordinated or decoupled by plant regulators in different organs and will provide a powerful tool for plant physiology research.     

Senescence and programmed cell death: substance or semantics?

The terms senescence and programmed cell death (PCD) have led to some confusion. Senescence as visibly observed in, for example, leaf yellowing and petal wilting, has often been taken to be synonymous with the programmed death of the constituent cells. PCD also obviously refers to cells, which show a programme leading to their death. Some scientists noted that leaf yellowing, if it has not gone too far, can be reversed. They suggested calling leaf yellowing, before the point of no return, 'senescence' and the process after it 'PCD'. However, this runs into several problems. It is counter to the historical definitions of senescence, both in animal and plant science, which stipulate that senescence is programmed and directly ends in death. It would also mean that only leaves and shoots show senescence, whereas several other plant parts, where reversal has not (yet) been shown, have no senescence, but only PCD. This conflicts with ordinary usage (as in root and flower senescence). Moreover, a programme can be reversible and therefore it is not counter to logic to regard the cell death programme as potentially reversible. In green leaf cells a decision to die, in a programmed way, has been taken, in principle, before the cells start to remobilize their contents (that is, before visible yellowing) and only rarely is this decision reversed. According to the arguments developed here there are no good reasons to separate a senescence phase and a subsequent PCD phase. Rather, it is asserted, senescence in cells is the same as PCD and the two are fully synchronous.     

Senescence Is Induced in Individually Darkened Arabidopsis Leaves, but Inhibited in Whole Darkened Plants

It has long been known that leaf senescence can be induced in many plant species by detaching leaves and placing them in the darkness. It recently has been shown that entire Arabidopsis plants placed in the darkness are not induced to senesce, as judged by visible yellowing and certain molecular markers. Here, we show that when individual Arabidopsis leaves are darkened, but not when entire plants are darkened, senescence is induced in the covered leaves. This induction of senescence is highly localized. The phenomenon is leaf age dependent in that it occurs more rapidly and strongly in older leaves than in younger ones, as is the case with many forms of induced senescence. Whole adult plants placed in darkness, in contrast, show delayed senescence, although seedlings lacking primary leaves do not. These observations imply that the light status of the entire plant affects the senescence of individual leaves. A model summarizing the results is presented.     

Correlative controls of senescence and plant death in Arabidopsis thaliana (Brassicaceae)

Like most monocarpic plants, longevity of Arabidopsis thaliana plants is controlled by the reproductive structures; however, they appear to work differently from most dicots studied. Neither male- and female-sterility mutations (ms1-1 and bell1, respectively) nor surgical removal of the stems with inflorescences (bolts) at various stages significantly increased the longevity of individual rosette leaves, yet the mutants and treated plants lived 20-50 d longer, measured by the death of the last rosette and/or the last cauline leaf. A series of growth mutations (clv2-4, clv3-2, det3, vam1 enh, and dark green) also increased plant longevity by 20-30 d but did not delay the overall development of the plants. The mutations prolonged plant life through the production of new leaves and stems with inflorescences (bolts) rather than by extending leaf longevity. In growing stems, the newly-formed leaves may induce senescence in the older leaves; however, removal of the younger leaves did not significantly increase the life of the older leaves on the compressed stems of Arabidopsis. Since plants that produce more bolts also live longer, the reproductive load (dry weight) of the bolts did not seem to drive leaf or whole plant senescence here. The developing reproductive structures caused the death of the plant by preventing regeneration of leaves and bolts, which are green and presumably photosynthetic. They also exerted a correlative control (repression) on the development of additional reproductive structures.     

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.     

水稻叶片早衰成因及分子机理研究进展

植物叶片衰老是叶片发育的最终阶段,也是植物在长期进化过程中形成的适应性机制。水稻(Oryza sativa)叶片的衰老对其产量和品质影响极大,相关研究主要集中在早衰。该文综述了水稻早衰及其调控基因的研究进展,尤其对水稻叶片早衰的形成原因、发生过程、生理变化及防治措施进行了阐述,以期为深入解析水稻早衰的分子机制奠定理论基础,同时为水稻育种提供参考。     

Arabidopsis onset of leaf death mutants identify a regulatory pathway controlling leaf senescence

The onset of leaf senescence is controlled by leaf age and ethylene can promote leaf senescence within a specific age window. We exploited the interaction between leaf age and ethylene and isolated mutants with altered leaf senescence that are named as onset of leaf death (old) mutants. Early leaf senescence mutants representing three genetic loci were selected and their senescence syndromes were characterised using phenotypical, physiological and molecular markers. old1 is represented by three recessive alleles and displayed earlier senescence both in air and upon ethylene exposure. The etiolated old1 seedlings exhibited a hypersensitive triple response. old2 is a dominant trait and the mutant plants were indistinguishable from the wild-type when grown in air but showed an earlier senescence syndrome upon ethylene treatment. old3 is a semi-dominant trait and its earlier onset of senescence is independent of ethylene treatment. Analyses of the chlorophyll degradation, ion leakage and SAG expression showed that leaf senescence was advanced in ethylene-treated old2 plants and in both air-grown and ethylene-treated old1 and old3 plants. Epistatic analysis indicated that OLD1 might act downstream of OLD2 and upstream of OLD3 and mediate the interaction between leaf age and ethylene. A genetic model was proposed that links the three OLD genes and ethylene into a regulatory pathway controlling the onset of leaf senescence.     

Developmental and age-related processes that influence the longevity and senescence of photosynthetic tissues in arabidopsis.

Factors that influence the longevity and senescence of photosynthetic tissues of Arabidopsis were investigated. To determine the influence of reproductive development on the timing of somatic tissue senescence, the longevity of rosette leaves of the Landsberg erecta strain and of isogenic mutant lines in which flowering is delayed (co-2) or sterile flowers are produced (ms1-1) were compared. No difference in the timing of senescence of individual leaves was observed between these lines, indicating that somatic tissue longevity is not governed by reproductive development in this species. To examine the role of differential gene expression in the process of leaf senescence, cDNA clones representing genes that are differentially expressed in senescing tissues were isolated. Sequence analysis of one such clone indicated homology to previously cloned cysteine proteinases, which is consistent with a role for the product of this gene in nitrogen salvage. RNA gel blot analysis revealed that increased expression of senescence-associated genes is preceded by declines in photosynthesis and in the expression of photosynthesis-associated genes. A model is presented in which it is postulated that leaf senescence is triggered by age-related declines in photosynthetic processes.     

拟南芥莲座叶片发育过程中P1基因的表达特性

该实验对CDF1类似蛋白基因(P1)在拟南芥叶片发育不同阶段的定量PCR结果显示,P1基因在拟南芥叶片发育的所有时期均可表达,但在茎生叶和衰老叶中的表达水平明显高于成熟叶和幼叶。GUS报告基因表达的组织化学染色结果显示,P1启动子在拟南芥叶片中有较高的驱动活性;在营养生长阶段的幼苗和植株(4~5周)的所有叶片中均能检测到GUS表达,但在植株转入生殖生长阶段后(6周及以后),GUS表达主要集中在逐渐衰老的叶中,并随着叶片衰老程度加剧GUS染色程度也越深,这一结果与GUS荧光定量检测结果一致。通过分析P1基因启动子上可能存在的顺式调控元件,发现茉莉酸甲酯、热压、干旱和水杨酸等均能够引起叶片衰老调控元件的响应,证实P1的表达受到这些因素的调控。研究表明,P1在拟南芥莲座叶片中很可能参与了对上游衰老信号的响应,该研究结果为进一步探究P1在叶片衰老过程中的分子功能验证奠定了基础。     

Mechanisms of the light-dependent induction of cell death in tobacco plants with delayed senescence

The relationship between leaf senescence and cell death was investigated using tobacco with delayed senescence due to auto-regulated production of cytokinin (SAG12-IPT). Although leaf senescence ultimately results in cell death, the results show that senescence and cell death can be uncoupled: in nutrient-deficient, but not in fertilized SAG12-IPT plants, necrotic lesions were detected in old, but otherwise green leaves. By contrast, wild-type leaves of the same age were yellow, but not necrotic. Chlorophyll fluorescence analysis revealed an over-reduction of the electron transport chain in old SAG12-IPT leaves, in combination with characteristic spatial patterns of minimum fluorescence (F0) quantum efficiency of open photosystem II centres (F(v)/F(m)) and non-photochemical quenching (NPQ), as determined by fluorescence imaging. The same patterns of F0, F(v)/F(m), and NPQ were induced by incubation of leaf discs from nutrient-deficient SAG12-IPT plants under illumination, but not in the dark, indicating that light-dependent reactions were responsible for the cell death. RT-PCR analysis showed that the pathogenesis-related (PR) genes PR-1b and PR-Q were strongly induced in old SAG12-IPT tobacco leaves with necrotic lesions. In addition, the ethylene-synthesis gene ACO was induced before lesions became visible in SAG12-IPT. It is proposed that over-reduction of the electron transport chain in combination with decreased electron consumption due to nutrient-deficiency led to oxidative stress, which, mediated by ethylene formation, can induce PR gene expression and hypersensitive cell death. Probably as a consequence of inefficient nutrient mobilization, flower development was prematurely aborted and reproduction thereby impaired in nutrient-deficient SAG12-IPT plants.     

The molecular biology of leaf senescence

Senescence is a complex, highly regulated, developmental phasein the life of a leaf that results in the co-ordinated degradationof macromolecules and the subsequent mobilization of componentsto other parts of the plant. The application of molecular biologytechniques to the study of leaf senescence has, in the lastfew years, enabled the isolation and characterization of a largerange of cDNA clones representing genes that show increasedexpression in senescing leaves. The analysis of these genesand identification of the function of the encoded proteins willallow a picture of the complex processes that take place duringsenescence to be assembled. To date, genes encoding degradativeenzymes such as proteases and nucleases, enzymes involved inlipid and carbohydrate metabolism and enzymes involved in nitrogenmobilization have all been identified as senescence-enhancedgenes. A variety of other genes of no obvious senescence-relatedfunction have also been identified; their role in senescencemay be less predictable and, possibly, more interesting. The combined action of several internal and external signalsmay be involved in the induction of senescence. Analysis ofthe regulatory mechanisms controlling the expression of senescence-inducedgenes will allow the signalling pathways that are involved inthe regulation of senescence to be elucidated. Experiments withtransgenic plants and mutants are already shedding light onthe role played by cytokinins and ethylene in regulating senescencein leaves. Key words: Senescence, cDNA clones, gene expression, signals     

Leaf senescence is delayed in tobacco plants expressing the maize homeobox gene knotted1 under the control of a senescence-activated promoter.

Leaf senescence is an active process involving remobilization of nutrients from senescing leaves to other parts of the plant. Whereas senescence is accompanied by a decline in leaf cytokinin content, supplemental cytokinin delays senescence. Plants that overexpress isopentenyl transferase (ipt), a cytokinin-producing gene, or knotted1 (kn1), a homeobox gene, have many phenotypes in common. Many of these phenotypes are characteristic of altered cytokinin physiology. The effect of kn1 on leaf senescence was tested by driving its expression using the promoter of the senescence-associated gene SAG12. SAG:kn1 tobacco plants showed a marked delay in leaf senescence but otherwise developed normally. The delay in senescence was revealed by an increase in chlorophyll content in SAG:kn1 leaves relative to leaves of the control plants and by a decrease in the number of dead leaves. Senescence was also delayed in detached leaves of SAG:kn1 plants. Delayed senescence was accompanied by increased leaf cytokinin content in older leaves expressing kn1. These experiments extend the current understanding of kn1 function and suggest that in addition to mediating meristem maintenance, kn1 is capable of regulating the onset of senescence in leaves.     

Molecular analysis of programmed cell death during senescence in Arabidopsis thaliana and Brassica oleracea: cloning broccoli LSD1, Bax inhibitor and serine palmitoyltransferase homologues

This study was undertaken to characterize the programmed cell death (PCD) processes that occur during detached and natural on-plant senescence and correlate them with the expression of putative regulatory genes that may be involved in the process. DNA fragmentation and TUNEL analysis of broccoli florets showed that DNA was processed into fragments of approximately 180 bp after 48 h of harvest-induced tissue senescence. Characteristic laddering patterns were also visible in Arabidopsis leaves undergoing natural on-plant senescence and during detached senescence. Several recently isolated plant proteins have been assigned a PCD role, for example, the zinc finger containing protein, LSD1 (lesion simulating disease); Bax inhibitor (BI); and serine palmitoyltransferase (SPT), an enzyme in the sphingolipid signalling pathway. Two cDNAs encoding each of these proteins were isolated from broccoli (BoBI-1, BoBI-2, BoLSD1, BoLSD2, BoSPT1, BoSPT2), and the mRNAs increased during harvest-induced senescence in floret tissue. Expression of the Arabidopsis homologues (AtBI-1, AtLSD1, AtSPT1) were also characterized during detached leaf senescence in Arabidopsis leaves. AtBI-1 expression was constitutively expressed during detached senescence, AtLSD1 expression remained constitutively low, and AtSPT1 expression increased during detached senescence.     

Rice tillering dwarf mutant dwarf3 has increased leaf longevity during darkness-induced senescence or hydrogen peroxide-induced cell death

Senescence or cell death in plant leaves is known to be inducible by darkness or H(2)O(2). When the Arabidopsis gene MAX2/ORE9 is disrupted, leaf senescence or cell death in response to the above stimuli is delayed. Because the rice (Oryza sativa L.) gene DWARF3 (D3) is orthologous to MAX2/ORE9, we wished to know whether disruption of D3 also results in increased longevity in leaves. We found that darkness-induced senescence or H(2)O(2)-induced cell death in the third leaf [as measured by chlorophyll degradation, membrane ion leakage and expression of senescence-associated genes (SAGs)] in a d3 rice mutant was delayed by 1-3 d compared to that in its reference line Shiokari. Moreover, the mRNA levels of D3, HTD1 and D10, which are orthologs of Arabidopsis MAX2/ORE9, MAX3 and MAX4, respectively, increased during cell death. These results suggest that D3 protein in rice, like MAX2/ORE9 in Arabidopsis, is involved in leaf senescence or cell death.     

Expression of the <Emphasis Type="Italic">Apx</Emphasis> gene family during leaf senescence of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

Oxygen-free radicals are thought to play an essential role in senescence. Therefore, the expression patterns of the small gene family encoding the H₂O₂ scavenging enzymes ascorbate peroxidase (APX; EC 1.11.1.11) were analyzed during senescence of Arabidopsis thaliana (L.) Heinh. Applying real-time RT-PCR, the mRNA levels were quantified for three cytosolic (APX1, APX2, APX6), two chloroplastic types (stromal sAPX, thylakoid tAPX), and three microsomal (APX3, APX4, APX5) isoforms identified in the genome of Arabidopsis. The genes of chloroplastic thylakoid-bound tAPX and the microsomal APX4 exhibit a strong age-related decrease of mRNA level in leaves derived from one rosette as well as in leaves derived from plants of different ages. In contrast to the tAPX, the mRNA of sAPX was only reduced in old leaves of old plants. The microsomal APX3 and APX5, and the cytosolic APX1, APX2, and APX6 did not show remarkable age-related changes in mRNA levels. The data show that expression of the individual APX genes is differentially regulated during senescence indicating possible functional specialization of respective isoenzymes. The hydrogen peroxide levels seem to be controlled very precisely in different cell compartments during plant development.