Chloroplasts regulate leaf senescence: delayed senescence in transgenic ndhF-defective tobacco

Mitochondrial involvement has not been identified in the programmed cell death (PCD) of leaf senescence which suggests that processes such as those involving reactive oxygen species (ROS) are controlled by chloroplasts. We report that transgenic tobacco (DeltandhF), with the plastid ndhF gene knocked-out, shows low levels of the plastid Ndh complex, homologous to mitochondrial complex I, and more than a 30-day-delay in leaf senescence with respect to wt. The comparison of activities and protein levels and analyses of genetic and phenotypic traits of wtxDeltandhF crosses indicate that regulatory roles of mitochondria in animal PCD are assumed by chloroplasts in leaf senescence. The Ndh complex would increase the reduction level of electron transporters and the generation of ROS. Chloroplastic control of leaf senescence provides a nonclassical model of PCD and reveals an unexpected role of the plastid ndh genes that are present in most higher plants.     

Signal transduction in leaf senescence

Leaf senescence is a complex developmental phase that involves both degenerative and nutrient recycling processes. It is characterized by loss of chlorophyll and the degradation of proteins, nucleic acids, lipids, and nutrient remobilization. The onset and progression of leaf senescence are controlled by an array of environmental cues (such as drought, darkness, extreme temperatures, and pathogen attack) and endogenous factors (including age, ethylene, jasmonic acid, salicylic acid, abscisic acid, and cytokinin). This review discusses the major breakthroughs in signal transduction during the onset of leaf senescence, in dark- and drought-mediated leaf senescence, and in various hormones regulating leaf senescence achieved in the past several years. Various signals show different mechanisms of controlling leaf senescence, and cross-talks between different signaling pathways make it more complex. Key senescence regulatory networks still need to be elucidated, including cross-talks and the interaction mechanisms of various environmental signals and internal factors.     

Molecular regulation of leaf senescence

Leaf senescence is a process of programmed cell death, which is induced in an age-dependent manner and by various environmental cues. The mechanisms that regulate the induction and progression of leaf senescence remain unclear because of their complexity. However, recent genetic and reverse-genetic approaches have identified key components of the regulation of leaf senescence and have revealed glimpses of the underlying molecular mechanisms.     

Cytokinin inhibition of leaf senescence

The senescence delaying effect of cytokinin is well known, however, the details behind how this process occurs remain unclear. Efforts to improve understanding of this phenomenon have led to the identification in Arabidopsis of specific cytokinin signaling components through which senescence signal responses are regulated. These include the cytokinin receptor (AHK3), the type-B response regulator (ARR2) and the recently identified cytokinin response factor (CRF6). At the mechanistic end of this process, it was found that increased cell-wall invertase activity which occurs in response to cytokinin is both necessary and sufficient for the inhibition of senescence. Yet, a direct link between the signaling and mechanistic steps of a cytokinin regulated senescence process has yet to be demonstrated. This may be in part because the relationship between senescence and primary metabolism implied by the key role of cell-wall invertase is the subject of two apparently opposing bodies of evidence. Here we briefly summarize and propose a model in which cytokinin mediated changes in sink/source relationships leads to delayed senescence which is consistent with existing evidence both for and against sugars as a trigger for developmental senescence.     

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     

Protease activity during rice leaf senescence

A protease activity was detected in rice (Oryza sativa L. cv. Ratna) leaves that hydrolysed hemoglobin more efficiently than bovine serum albumin. The activity was high when the enzyme was extracted and assayed with tris-maleate buffer [tris (hydroxymethyl) methyl amino-maleate] pH 7.0 rather than with water or with citrate-phosphate buffer pH 7.0. The enzyme had a strong dependence on sulfhydryl groups for its activity without which it was inaotive. The pH optimum was 7.0 and the temperature optimum was 40 °C. Protease activity expressed per unit leaf fresh weight (absolute activity) increased only little during senescence of detached rice leaves while the same activity expressed per unit soluble protein content (specific activity) increased by a greater factor (about 5 times) than absolute activity. Total and soluble protein content decreased during the senescence of detached leaves. Benzimidazole (10-3M) and kinetin (0.5xl0-5M) treatment arrested the increase of the protease activity and the deorease in the protein content during detached leaf senescence. It was indicative that protease protein was more stable than the bulk of other proteins in senescing leaves.     

Towards understanding abscisic acid-mediated leaf senescence

<正>Senescence is a necessary part of most complex organisms that controlled by many complicated genetic programs.However,unlike animals and humans,leaf senescence has a great impact on nutrient recycling from source to sink to promote reproductive success,thus has strong adaptive advantages in plants[1,2].Therefore,from the agricultural     

Cytokinin biochemistry in relation to leaf senescence. VIII. Translocation, metabolism and biosynthesis of cytokinins in relation to sequential leaf senescence of tobacco

Cytokinin bases (zeatin and dihydrozeatin) and ribosides (zeatin riboside and dihydrozeatin riboside) were identified as major cytokinins in tobacco xylem sap by radioimmunoassay. When ³H-labelled zeatin riboside or dihydrozeatin riboside were supplied to tobacco plants via the xylem, leaves of differing maturity did not differ appreciably in level of radioactivity or in metabolism of the cytokinin. The major metabolites of zeatin riboside in leaves were adenine, adenosine and adenine nucleotides, whereas that of dihydrozeatin riboside was dihydrozeatin 7-glucoside. Incorporation of [¹⁴C]adenine into zeatin was evident in upper green leaves. indicating that young leaves have the capacity to synthesize cytokinins in situ. In contrast, fully expanded green leaves and senescing tobacco leaves exhibited little or no incorporation of [¹⁴C]adenine into cytokinins. This difference in cytokinin biosynthetic capacity may contribute to the differing cytokinin levels in leaves of different matirity, and may participate in control of sequential leaf senescence in tobacco.     

Hormonal regulation of leaf senescence in Lilium

In addition to floral senescence and longevity, the control of leaf senescence is a major factor determining the quality of several cut flowers, including Lilium, in the commercial market. To better understand the physiological process underlying leaf senescence in this species, we evaluated: (i) endogenous variation in the levels of phytohormones during leaf senescence, (ii) the effects of leaf darkening in senescence and associated changes in phytohormones, and (iii) the effects of spray applications of abscisic acid (ABA) and pyrabactin on leaf senescence. Results showed that while gibberellin 4 (GA(4)) and salicylic acid (SA) contents decreased, that of ABA increased during the progression of leaf senescence. However, dark-induced senescence increased ABA levels, but did not affect GA(4) and SA levels, which appeared to correlate more with changes in air temperature and/or photoperiod than with the induction of leaf senescence. Furthermore, spray applications of pyrabactin delayed the progression of leaf senescence in cut flowers. Thus, we conclude that (i) ABA plays a major role in the regulation of leaf senescence in Lilium, (ii) darkness promotes leaf senescence and increases ABA levels, and (iii) exogenous applications of pyrabactin inhibit leaf senescence in Lilium, therefore suggesting that it acts as an antagonist of ABA in senescing leaves of cut lily flowers.     

Altered membrane lipase expression delays leaf senescence

A cDNA clone encoding a lipase that is up-regulated in senescing leaves and flower petals has been isolated by screening an expression library. The abundance of the lipase mRNA increases as flowers and leaves begin to senesce, and expression of the gene is also induced by treatment with ethylene. Transgenic Arabidopsis plants in which levels of the senescence-induced lipase protein have been reduced show delayed leaf senescence.     

Cytokinin biochemistry in relation to leaf senescence. VII. Endogenous cytokinin levels and exogenous applications of cytokinins in relation to sequential leaf senescence of tobacco

The cytokinin complex in tobacco leaves of various maturities was characterized by radioimmunoassay and mass spectrometry. Zeatin was the major base, whereas zeatin riboside was identified as the main riboside. in leaves of all maturities studied. Relative to upper younger leaves, the basal yellow leaves had reduced levels of both cytokinin bases and ribosides. Exogenous applications of dihydrozeatin and zeatin to detached tobacco leaves in amounts sufficient to delay senescence, elevated cytokinin base and riboside levels 2–5 fold. Presenescent and senescent leaves of intact plants showed quantitatively similar changes in cytokinin content. which therefore appear to be of significance in control of senescence. When supplied exogenously, the principal cytokinin bases found to occur in tobacco leaves (zeatin and dihydrozeatin) were markedly more effective than auxins and gibberellic acid in retarding senescence. Localised application of cytokinins to leaf blades of detopped plants was much less effective than application to intact plants. The cytokinin induced senescence retardation in tobacco leaves was independent of effects on directed metabolite transport. Evidence that endogenous levels of active cytokinins in intact tobacco leaves are involved in control of sequential leaf senescence is discussed.     

Sink removal and leaf senescence in soybean : cultivar effects

Three cultivars of soybean (Glycine max [L.] Merr. cvs Harper, McCall, and Maple Amber) were grown in the field and kept continuously deflowered throughout the normal seedfill period. For all cultivars, deflowering led to delayed leaf abscission and a slower rate of chlorophyll loss. Compared to control plants, photosynthesis and ribulose 1,5-bis-phosphate carboxylase/oxygenase (Rubisco) level declined slightly faster for deflowered Harper, but for both McCall and Maple Amber, leaves of deflowered plants maintained approximately 20% of maximum photosynthesis and Rubisco level 1 month after control plants had senesced. Deflowering led to decreased leaf N remobilization and increased starch accumulation for all cultivars, but cultivars differed in that for McCall and Maple Amber, N and starch concentrations slowly but steadily declined over time whereas for Harper, N and starch concentrations remained essentially constant over time. SDS-PAGE of leaf proteins indicated that for all cultivars, deflowering led to accumulation of four polypeptides (80, 54, 29, and 27 kilodaltons). Western analysis using antisera prepared against the 29 and 27 kilodalton polypeptides verified that these polypeptides were the glycoproteins previously reported to accumulate in vacuoles of paraveinal mesophyll cells of depodded soybean plants. The results indicated that depending on the cultivar, sink removal can lead to either slightly faster or markedly slower loss of photosynthesis and Rubisco. This difference, however, was not associated with the ability to synthesize leaf storage proteins. For any particular cultivar, declines in chlorophyll, photosynthesis, and Rubisco were initiated at approximately the same time for control and deflowered plants. Thus, even though cultivars differed in rate of decay of photosynthetic rate and Rubisco level in response to sink removal, the initiation of leaf senescence was not influenced by presence or absence of developing fruits.     

Relative rates of delivery of xylem solute to shoot tissues: Possible relationship to sequential leaf senescence

The rates of delivery of regulatory solutes such as cytokinins and mineral ions from the roots to competing shoot tissues can influence rates of metabolism and development. A 15 min pulse of a synthetic xylem mobile and phloem-immobile solute, acid fuchsin, was used to quantify relative rates of solute delivery to competing organs on excised transpiring bean shoots (Phaseolus vulgaris L. cv. Contender) at different stages of development. Stem, flower and fruit tissues received comparatively low rates of solute delivery. The relative rate of solute delivery to newly opened leaves was initially low, but increased during rapid leaf expansion and then declined progressively as the leaves exceeded 70% of their final area. The relative rate of solute delivery to tissues of the basal primary leaves declined progressively from 2 weeks onwards. This decline appeared to be caused by progressive internally regulated increases in both stomatal resistances and hydraulic resistances to xylem flow up to and into the leaf blade. Thus combined abaxial and adaxial stomatal resistance values in the primary leaves (Rs) increased from 3 to ≥ 7 s cm^?1 between 2 and 5 weeks. Similarly, mean values for the connection resistances (Rc) to hydraulic flow into the primary leaves rose from 7 to 13 TPa · s · m^?1 between 2 and 4 weeks. In the same period pathway resistance from stem to primary leaf petioles (Rp), as determined by direct pressure flow assay, increased from 7 to 15 TPa · s · m^?1. Senescence-associated declines in protein and chlorophyll levels in the primary leaves were initiated in parallel with, or after, declines in relative rates of solute delivery. The provision of extra illumination at the basal leaf level between 2 and 5 weeks did not prevent declines in chlorophyll and soluble protein or increases in stomatal resistance. We suggest that internally programmed changes in the hydraulic architecture of the plant progressively divert xylem-transported root supplies of nutrients and cytokinins from basal to more apical leaves and thus regulate the progressive senescence of leaves from the base upwards.     

Strigolactone signaling regulates rice leaf senescence in response to a phosphate deficiency

Strigolactones (SLs) act as plant hormones that inhibit shoot branching and stimulate secondary growth of the stem, primary root growth, and root hair elongation. In the moss Physcomitrella patens, SLs regulate branching of chloronemata and colony extension. In addition, SL-deficient and SL-insensitive mutants show delayed leaf senescence. To explore the effects of SLs on leaf senescence in rice (Oryza sativa L.), we treated leaf segments of rice dwarf mutants with a synthetic SL analogue, GR24, and evaluated their chlorophyll contents, ion leakage, and expression levels of senescence-associated genes. Exogenously applied GR24 restored normal leaf senescence in SL-deficient mutants, but not in SL-insensitive mutants. Most plants highly produce endogenous SLs in response to phosphate deficiency. Thus, we evaluated effects of GR24 under phosphate deficiency. Chlorophyll levels did not differ of in the wild-type between the sufficient and deficient phosphate conditions, but increased in the SL-deficient mutants under phosphate deficiency, leading in the strong promotion of leaf senescence by GR24 treatment. These results indicate that the mutants exhibited increased responsiveness to GR24 under phosphate deficiency. In addition, GR24 accelerated leaf senescence in the intact SL-deficient mutants under phosphate deficiency as well as dark-induced leaf senescence. The effects of GR24 were stronger in d10 compared to d17. Based on these results, we suggest that SLs regulate leaf senescence in response to phosphate deficiency.