Integrated Signaling in Flower Senescence: An Overview

Flower senescence is the terminal phase of developmental processes that lead to the death of flower, which include, flower wilting, shedding of flower parts and fading of blossoms. Since it is a rapid process as compared to the senescence of other parts of the plant it therefore provides excellent model system for the study of senescence. During flower senescence, developmental and environmental stimuli enhance the upregulation of catabolic processes causing breakdown and remobilization of cellular constituents. Ethylene is well known to play regulatory role in ethylene-sensitive flowers while in ethylene-insensitive flowers abscisic acid (ABA) is thought to be primary regulator. Subsequent to perception of flower senescence signal, death of petals is accompanied by the loss of membrane permeability, increase in oxidative and decreased level of protective enzymes. The last stages of senescence involve the loss of of nucleic acids (DNA and RNA), proteins and organelles, which is achieved by activation of several nucleases, proteases and wall modifiers. Environmental stimuli such as pollination, drought and other stresses also affect senescence by hormonal imbalance. In this article we have covered the following: perception mechanism and specificity of flower senescence, flower senescence-associated events, like degradation of cell membranes, proteins and nucleic acids, environmental/external factors affecting senescence, like pollination and abiotic stress, hormonal and non-hormonal regulation of flower/petal senescence and finally the senescence associated genes (SAGs) have also been described.Key Words: environmental factors, ethylene, flowers, petals, plant hormones, pollination, programmed cell death, senescence, senescence-associated genes     

Possible involvement of abscisic acid in senescence of daylily petals

Daylily flowers (Hemerocallis hybrid, cv. Stella d'Oro) senesce and die autonomously over a 24 h period after opening. Investigations were performed to determine some of the mechanisms that lead to death of the petals. The flowers are insensitive to ethylene, but exogenous ABA prematurely upregulates events that occur during natural senescence, such as loss or differential membrane permeability, increases in lipid peroxidation and the induction of proteinase and RNase activities. Furthermore, the same patterns of proteinase and RNase activities appearing on activity gels during natural senescence are induced prematurely by ABA. The mRNA profile from ABA-treated, prematurely senescing petals visualized by differential display shows a high degree of similarity to the mRNA profile of naturally senescing petals 18 h later. In addition, endogenous ABA increases before flower opening and continues to increase during petal senescence. An osmotic stress by sorbitol increases endogenous levels of ABA and upregulates the same parameters of senescence as those occurring during natural senescence and after application of ABA. The mRNA profile from sorbitol-treated, prematurely senescing petals, but somewhat less similarity to mRNA from ABA-treated petals. The possibility is discussed that ABA is a constituent of the signal transduction chain leading to programmed cell death of daylily petals.     

Role of ethylene in the senescence of isolated hibiscus petals

Senescence of petals isolated from flowers of Hibiscus rosa-sinensis L. (cv Pink Versicolor) was associated with increased ethylene production. Exposure to ethylene (10 microliters per liter) accelerated the onset of senescence, as indicated by petal in-rolling, and stimulated ethylene production. Senescence was also hastened by basal application of 1-aminocyclopropane-1-carboxylic acid (ACC). Aminooxyacetic acid, an inhibitor of ethylene biosynthesis, effectively inhibited ethylene production by petals and delayed petal in-rolling. In marked contrast to these results with mature petals, immature petals isolated from flowers the day before flower opening did not respond to ethylene in terms of an increase in ethylene production or petal in-rolling. Furthermore, treatment with silver thiosulfate the day before flower opening effectively prevented petal senescence, while silver thiosulfate treatment on the morning of flower opening was ineffective. Application of ACC to both immature and mature petals greatly stimulated ethylene production indicating the presence of an active ethylene-forming enzyme in both tissues. Immature petals contained less free ACC than mature, presenescent petals and appeared to possess a more active system for converting ACC into its conjugated form. Thus, while the nature of the lack of responsiveness of immature petals to ethylene is unknown, ethylene production in hibiscus petals appears to be regulated by the control over ACC availability.     

Programmed Cell Death in Floral Organs: How and Why do Flowers Die?

BACKGROUND: Flowers have a species-specific, limited life span with an irreversible programme of senescence, which is largely independent of environmental factors, unlike leaf senescence, which is much more closely linked with external stimuli. TIMING: Life span of the whole flower is regulated for ecological and energetic reasons, but the death of individual tissues and cells within the flower is co-ordinated at many levels to ensure correct timing. Some floral cells die selectively during organ development, whereas others are retained until the whole organ dies. TRIGGERS: Pollination is an important floral cell death trigger in many species, and its effects are mediated by the plant growth regulator (PGR) ethylene. In some species ethylene is a major regulator of floral senescence, but in others it plays a very minor role and the co-ordinating signals involved remain elusive. Other PGRs such as cytokinin and brassinosteroids are also important but their role is understood only in some specific systems. MECHANISMS: In two floral cell types (the tapetum and the pollen-tube) there is strong evidence for apoptotic-type cell death, similar to that in animal cells. However, in petals there is stronger evidence for an autophagous type of cell death involving endoplasmic reticulum-derived vesicles and the vacuole. Proteases are important, and homologues to animal caspases, key regulators of animal cell death, exist in plants. However, their role is not yet clear. COMPARISON WITH OTHER ORGANS: There are similarities to cell death in other plant organs, and many of the same genes are up-regulated in both leaf and petal senescence; however, there are also important differences for example in the role of PGRs. CONCLUSIONS: Understanding gene regulation may help to understand cell death in floral organs better, but alone it cannot provide all the answers.     

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.     

Intracellular energy depletion triggers programmed cell death during petal senescence in tulip

Programmed cell death (PCD) in petals provides a model system to study the molecular aspects of organ senescence. In this study, the very early triggering signal for PCD during the senescence process from young green buds to 14-d-old petals of Tulipa gesneriana was determined. The opening and closing movement of petals of intact plants increased for the first 3 d and then gradually decreased. DNA degradation and cytochrome c (Cyt c) release were clearly observed in 6-d-old flowers. Oxidative stress or ethylene production can be excluded as the early signal for petal PCD. In contrast, ATP was dramatically depleted after the first day of flower opening. Sucrose supplementation to cut flowers maintained their ATP levels and the movement ability for a longer time than in those kept in water. The onset of DNA degradation, Cyt c release, and petal senescence was also delayed by sucrose supplementation to cut flowers. These results suggest that intracellular energy depletion, rather than oxidative stress or ethylene production, may be the very early signal to trigger PCD in tulip petals.     

Translocation of 14C, carbohydrate content and activity of the enzymes of sucrose metabolism in rose petals temperatures

Both export of ¹⁴C from the source leaves of roses (Rosa × hybrida cv. Golden Times) and import of ¹⁴C to the petals were reduced by plant exposure to low night temperature. However, the import was affected to a greater extent than the export. During all stages of flower bud development the concentration of reducing sugars in petals of roses grown at reduced night temperature was lower than in petals of plants grown at higher night temperature. There was no significant difference in starch content in response to the night temperature, and the content of starch decreased toward complete flower bud opening. The concentration of sucrose in flowers at the low night temperature remained low during all stages of flower bud development, while at the high night temperature the concentration of sucrose increased during flower bud development, reaching a peak at the stage when petals start to unfold. At both temperatures the concentration of sucrose declined at complete flower opening. The activity of sucrose synthase (EC 2.4.1.14) was inhibited by low temperature in young rose shoots more than in the petals, while the activity of acid invertase (EC 3.2.1.26) was affected similarly in both tissues by the temperature treatments.     

Fructan Hydrolysis Drives Petal Expansion in the Ephemeral Daylily Flower

Dry weight, water content, soluble carbohydrate content, and carbohydrate composition of daylily (Hemerocallis hybrid cv Cradle Song) flower petals were monitored in the 3 d leading up to full opening and in the first day of senescence. Timing of events was related to the time (hour 0) when flower expansion was 60% complete. Petal dry weight increased linearly from hour -62 (tight bud) to hour 10 (fully developed flower), then fell rapidly to hour 34 as senescence advanced. Increase in water content was proportional to dry weight increase from hour -62 to hour -14, but was more rapid as the bud cracked and the flower opened, giving an increase in fresh weight/dry weight ratio. Soluble carbohydrate was 50% of petal dry weight up to hour 10, then decreased during senescence to reach 4% by hour 34. Up until hour -14, fructan accounted for 80% of the soluble carbohydrate in the petals, whereas hexose accounted for only 2%. Fructan hydrolysis started just prior to bud crack at hour -14, reaching completion by hour 10 when no detectable fructan remained, and fructose plus glucose accounted for more than 80% of the total soluble carbohydrate. The proportion of sucrose remained constant throughout development. Osmolality of petal cell sap increased significantly during fructan hydrolysis, from 0.300 to 0.340 osmolal. Cycloheximide applied to excised buds between hour -38 and hour -14 halted both fructan hydrolysis and flower expansion. The findings suggest that onset of fructan hydrolysis, with the concomitant large increase in osmoticum, is an important event driving flower expansion in daylily.     

Is a cysteine proteinase inhibitor involved in the regulation of petal wilting in senescing carnation (Dianthus caryophyllus L.) flowers?

Senescence of carnation petals is accompanied by autocatalytic ethylene production and wilting of the petals; the former is caused by the expression of 1-aminocyclopropane-1-carboxylate (ACC) synthase and ACC oxidase genes and the latter is related to the expression of a cysteine proteinase (CPase) gene. CPase is probably responsible for the degradation of proteins, leading to the decomposition of cell components and resultant cell death during the senescence of petals. The carnation plant also has a gene for the CPase inhibitor (DC-CPIn) that is expressed abundantly in petals at the full opening stage of flowers. In the present study, DC-CPIn cDNA was cloned and expressed in E. coli. The recombinant DC-CPIn protein completely inhibited the activities of a proteinase (CPase) extracted from carnation petals and papain. Northern blot analysis showed that the mRNA for CPase (DC-CP1) accumulated in large amounts, whereas that for DC-CPIn disappeared, corresponding to the onset of petal wilting in flowers undergoing natural senescence and exogenous ethylene-induced senescence. Based on these findings, a role of DC-CPIn in the regulation of petal wilting is suggested; DC-CPIn acts as a suppressor of petal wilting, which probably functions to fine-tune petal wilting in contrast to coarse tuning, the up-regulation of CPase activity by gene expression.     

Gibberellin Is Involved in the Regulation of Cell Death-mediated Apical Senescence in G2 Pea

Senescence is the process of programmed degradation. The G2 line of pea exhibits apical senescence-delaying phenotype under short-day （SD） conditions, but the mechanism regulating the apical senescence is still largely unknown. Gibberellin （GA） was proved to be able to delay this apical senescence phenotype in G2 pea grown under long-day （LD） conditions. Here we show that the initiation of cell death signals in the terminal floral meristem was involved in the regulation of apical senescence in pea plants. SD signals prevented the formation of the cell death region in the apical mersitem. Moreover, GA3 treatment could effectively inhibit the occurrence of cell death-mediated apical senescence in LD-grown apical buds. Therefore, our data suggest that the prevention of apical senescence in SD-grown G2 pea through GA3 treatment may be largely responsible for the regulation of occurrence of the DNA fragmentation in apical meristem.     

Spatial and temporal distribution of mineral nutrients and sugars throughout the lifespan of <Emphasis Type="Italic">Hibiscus rosa-sinensis</Emphasis> L. flower

Although the physiological and molecular mechanisms of flower development and senescence have been extensively investigated, a whole-flower partitioning study of mineral concentrations has not been carried out. In this work, the distribution of sucrose, total reducing sugars, dry and fresh weight and macro and micronutrients were analysed in Hibiscus rosa-sinensis L. petals, stylestigma including stamens and ovary at different developmental stages (bud, open and senescent flowers). Total reducing sugars showed the highest value in petals of bud flowers, then fell during the later stages of flower development whereas sucrose showed the highest value in petals of senescent flowers. In petals, nitrogen and phosphorus content increased during flower opening, then nitrogen level decreased in senescent flowers. The calcium, phosphorus and boron concentrations were highest in ovary tissues whatever the developmental stage. Overall, the data presented suggests that the high level of total reducing sugars prior the onset of flower opening contributes to support petal cells expansion, while the high amount of sucrose at the time of petal wilting may be viewed as a result of senescence. Furthermore, this study discusses how the accumulation of particular mineral nutrients can be considered in a tissue specific manner for the activation of processes directly connected with reproduction.     

Characterization of a novel lipoxygenase-independent senescence mechanism in Alstroemeria peruviana floral tissue

The role of lipoxygenase (lox) in senescence of Alstroemeria peruviana flowers was investigated using a combination of in vitro assays and chemical profiling of the lipid oxidation products generated. Phospholipids and galactolipids were extensively degraded during senescence in both sepals and petals and the ratio of saturated/unsaturated fatty acids increased. Lox protein levels and enzymatic activity declined markedly after flower opening. Stereochemical analysis of lox products showed that 13-lox was the major activity present in both floral tissues and high levels of 13-keto fatty acids were also synthesized. Lipid hydroperoxides accumulated in sepals, but not in petals, and sepals also had a higher chlorophyll to carotenoid ratio that favors photooxidation of lipids. Loss of membrane semipermeability was coincident for both tissue types and was chronologically separated from lox activity that had declined by over 80% at the onset of electrolyte leakage. Thus, loss of membrane function was not related to lox activity or accumulation of lipid hydroperoxides per se and differs in these respects from other ethylene-insensitive floral tissues representing a novel pattern of flower senescence.     

Effects of Ethylene and Sucrose on Translocation of Dry Matter and 14C-Sucrose in the Cut Flower of the Glasshouse Carnation (Dianthus caryophyllus) During Senescence

The translocation and distribution of dry matter were studiedin the floral and vegetative parts of the cut carnation duringsenescence. The change in dry weights of the tissues and theamount of radioactivity recovered from them after feeding with¹⁴C-sucrose were measured. Treatments with ethylene and sucrosewere used to alter the rate of senescence of the flowers. Sucrosemoved through the stem relatively unchanged but was rapidlyinverted and metabolized in the petals. During natural ageing,¹⁴C moved from the stem to the flower and the movement was enhancedby exogenous sucrose, which also reduced the loss of dry matterin the petals and promoted their growth. Treatment with ethylenecaused petals to wilt and lose dry weight, and ovaries to enlargeand increase in dry weight. The distribution of radioactivityin flowers fed with 14C-sucrose before and after ethylene treatmentsupported the observation that dry matter was translocated betweenthe flower parts. The results indicate that a change in thesource-ink relationships of the flower parts contributes tothe factors that determine the rate of flower senescence.     

Characterization of proteolytic activity during senescence in daylilies

From 12 to 24 h after the opening of daylily flowers ( Hemerocallis hybrid cv. Stella d'Oro), the petals begin to degrade and the protein levels of soluble, microsomal‐ and plastid‐enriched fractions decrease by 50%, on a per petal basis. To help determine some of the components for the cell death program in daylily petals, we studied the mechanisms that regulate this loss of protein. Enzyme activities capable of digesting native daylily protein, gelatin, and azocasein markedly increase after flower opening, and their appearance is inhibited by the translation inhibitor, cycloheximide. Protein hydrolysis in vitro is prevented by inhibitors of cysteine, serine and metalloproteinases. Immunoblots using antibodies to ubiquitin pathway enzymes indicate that the ubiquitin system is not senescence specific. However, ion leakage is delayed by two inhibitors of the 26S proteasome. We propose that programmed cell death in daylily petals may involve the increase in activity of at least three classes of proteinases, and discuss the possibility that these proteinases may operate in concert with the ubiquitin pathway.     

Activity of Ageing Carnation Flower Parts and the Effects of 1-(Malonylamino)cyclopropane-1-Carboxylic Acid-Induced Ethylene

Peak levels of 1-aminocyclopropane-l-carboxylic acid (ACC) in flower parts of ageing carnations (Dianthus caryophyllus L. cv Scanea 3C) were detected 6 to 9 days after flower opening. The ethylene climacteric and the first visible sign of wilting was observed 7 days after opening. The concentration of conjugated ACC in these same tissues peaked at day three with reduction of 70% by day 4. From day 5 to day 9 all parts followed a diurnal pattern of increasing in conjugate levels 1 day and decreasing the next. Concentrations of conjugated ACC were significantly higher than those of ACC in all ageing parts. Preclimacteric petals treated with ACC or 1-(malonylamino)-cycloprane-1-carboxylic acid (MACC), started to senesce 30 to 36 hours after treatment. When petals were treated with MACC plus by 0.1 millimolar aminoethyoxyvinylglycine, premature senescence was induced, while ethylene production was suppressed relative to MACC-treated petals. Petals treated with MACC and silver complex produced ethylene, but did not senesce. The MACC-induced ethylene was inhibited by the addition of 1.0 millimolar CoC1₂. These results demonstrate MACC-induced senescence in preclimacteric petals. The patterns of ACC and MACC detected in the flower parts support the view that an individual part probably does not export an ethylene precursor to the remainder of the flower inducing senescence.