Senescing Leaf Tissue Research Articles

Leaf senescence is delayed in maize expressing the Agrobacterium IPT gene under the control of a novel maize senescence‐enhanced promoter

We have genetically modified maize plants to delay leaf senescence. A senescence-enhanced promoter from maize (P(SEE1)) was used to drive expression of the Agrobacterium cytokinin biosynthesis gene IPT in senescing leaf tissue. Three maize lines expressing IPT from P(SEE1), Sg1, Sg2 and Sg3, were analysed in detail, representing mild, intermediate and extreme expression, respectively, of the delayed-senescence phenotype. Backcross populations segregating for the presence or absence of the P(SEE1Xba)IPTNOS transgene also simultaneously segregated for the senescence phenotype. At the time of ear leaf emergence, individuals of lines Sg1 and Sg2 segregating for the presence of the transgene carried about three fewer senescing leaves than control (transgene-minus) segregants, and IPT transcript levels were higher in leaves at incipient senescence than in young leaves. Leaves of transgenic Sg3 plants were significantly greener than controls and progressed directly from fully green to bleached and dead without an intervening yellowing phase. IPT transcript abundance in this line was not related to the initiation of senescence. Extended greenness was accompanied by a delay in the loss of photosynthetic capacity with leaf age. The delayed-senescence trait was associated with relatively minor changes in morphology and development. The phenotype was particularly emphasized in plants grown in low soil nitrogen. The reduced ability of the extreme transgenic line Sg3 to recycle internal nitrogen from senescing lower leaves accounted for significant chlorosis in emerging younger leaves when plants were grown in low nutrient conditions. This study demonstrates that the agronomically important delayed-senescence ('stay-green') trait can be engineered into a monocot crop, and is the first example outside Arabidopsis of senescence modification using a homologous senescence-enhanced promoter.

Identification of genes associated with perianth senescence in Daffodil ( Narcissus pseudonarcissus L. ‘Dutch Master’)

We used a PCR-based subtractive procedure to isolate genes that increased in abundance in the perianth of incipiently senescent daffodil ( Narcissus pseudonarcissus L. ‘Dutch Master’) flowers. Using RNA gel blot analysis we confirmed that 18 of the transcripts were senescence-associated. A number of these have homologs in previously-studied senescing floral and leaf tissue systems (cysteine proteases, serine proteases, endonuclease, vacuole processing enzyme). Some showed homology to genes encoding proteins with transport functions (auxin efflux carrier protein, low affinity nitrate transporter), others to enzymes that may have a role in cellular signaling (RSH-like protein). A surprising number of the isolated genes either showed no homology with previously-reported sequences or were sequences with no known function. A strong association of some of the transcripts with senescence was also indicated by their enhanced accumulation in the perianth of detached daffodil flowers, which senesce earlier than attached flowers.

Delivering copper within plant cells

Two genes recently identified in Arabidopsis thaliana may be involved in sequestering free copper ions in the cytoplasm and delivering copper to post-Golgi vesicles. The genes, COPPER CHAPERONE and RESPONSIVE TO ANTAGONIST1 are homologous to copper-trafficking genes from yeast and humans. This plant copper-delivery pathway is required to create functional ethylene receptors. The pathway may also facilitate the transport of copper from senescing leaf tissue. In addition, several other genes have been identified recently that may have a role in copper salvage during senescence.

Relationship of forest productivity to nutrient and carbon supply-a modeling analysis.

A simple model of photosynthetic and nutritional controls over foliar dynamics is analyzed to compare the magnitude of the growth response of forest stands to increased rates of photosynthesis and nutrient supply. According to the model, productivity achieved at canopy closure is sensitive to nutrient supply, except where nutrient availability exceeds the plants' uptake capacity. Plants growing under nutrient-limited conditions can only respond positively to enhanced photosynthetic rates if they simultaneously increase their nutrient uptake, or reduce nutrient concentrations in stem, branch, root or senescing leaf tissue, or shift their carbon allocation in favor of biomass components with low nutrient concentrations. In particular, a response is more likely where considerable internal cycling of nitrogen occurs before leaf senescence, or where foliar allocation decreases with decreasing leaf nutrient concentration.

Leaf senescence in a non-yellowing mutant of Festuca pratensis Huds.

A study was made of linolenic acid-dependent oxidative chlorophyll bleaching (CHLOX) by thylakoid membranes from senescing leaf tissue of a normal cultivar (cv. Rossa) and a non-yellowing mutant genotype (Bf 993) of Festuca pratensis Huds. To overcome the problem of variation in levels of endogenous chlorophyll substrate in membranes from different sources, light-harvesting complex (LHC) was used to supplement thylakoid pigment. It was shown that CHLOX is associated with both Photosystem I and LHC-rich thylakoid subfractions but that purified LHC has negligible associated CHLOX activity and stimulates the rate of bleaching by isolated entire chloroplast membranes. Non-senescent tissue of Bf 993 and Rossa had essentially identical thylakoid CHLOX levels, which subsequently declined during senescence in darkness. The half-life of CHLOX from the mutant was three times greater than that of the normal genotype. In both cultivars, the amount of CHLOX assayed in thylakoids isolated at different times during senescence was more than adequate to support the corresponding in-vivo rate of pigment degradation as calculated from the half-life for chlorophyll. It was concluded that the non-yellowing mutation is not expressed through a lack of CHLOX activity. The role of linolenic acid metabolism in the regulation of thylakoid structure and function during senescence, and as a likely site of the non-yellowing lesion, are discussed.

Leaf senescence in a non-yellowing mutant of Festuca pratensis

Soluble and thylakoid membrane polypeptides from senescing leaf tissue of Rossa, a normal yellowing Festuca pratensis genotype, were fractionated by sodium dodecyl sulphate polyacrylamide gel electrophoresis and compared with those of the non-yellowing mutant Bf 993. Subunits of ribulose-1,5-bisphosphate carboxylase were the major soluble polypeptides and declined to low levels in senescing leaves of both genotypes. The major thylakoid polypeptides were those associated with the chlorophyllprotein complexes CPI and CPII. The levels of all thylakoid polypeptide species fell during senescence of Rossa leaf tissue but Bf993 lamellae retained CPI, CPII and a number of other hydrophobic low molecular weight polypeptides. The increasing hydrophobicity and decreasing protein complement of Bf 993 thylakoids were reflected in a fall in membrane density from 1.16 to 1.13 g cm(-3) over 8 d of senescence and a decline in the extractability of chlorophyll-containing membranes in the same period. In Bf993 the molar ratio of chlorophyll to hydrophobic membrane protein increased from 92 at day 0 to 296 at day 8. In the same time the ratio for Rossa increased from 88 to 722 and 8 d-senesced Rossa tissue yielded less than 2% of the solvent-soluble protein it contained at day 0 as compared with 24% for the protein of Bf993. These results are discussed in relation to the nature of the non-yellowing lesion.

Oleosomes in flag leaves of wheat; their distribution, composition and fate during senesence and rust-infection

Oleosomes, up to 14μm in diameter, were found in mesophyll and bundle sheath cells of the flag and lower leaves of wheat cv Professeur Marchal. They develop in flag leaves at least 10 d before anthesis, possibly from fatty acids secreted by the plastids, and persist in mature and senescing leaf tissue. Oleosomes are bordered with an osmiophilic layer rather than a unit membrane. The major lipids of oleosomes, isolated 20 d after anthesis, are triacylglycerols (50%) and sterol or wax exter (34%). The dominant fatty acids of both lipid classes are plamitic (16:0) and stearic (18:0) acids which accounts for the low osmiophilia of the oleosomes. The function of the oleosomes is unknown but they may act as short-term energy reserves. Oleosomes persist in leaves infected with brown rust, even in cells penetrated by haustoria. Yellowish-brown oleosomes found in senescing and rust-infected leaves may be formed by the release and coalescence of pigmented plastoglobuli.

Activity profiles of enzymes involved in glutamine and glutamate metabolism in the apple during autumnal senescence

AbstractSeasonal changes in glutamine synthetase (EC 6.3.1.2), glutamate synthase (EC 2.6.1.53), and glutamate dehydrogenase (EC 1.4.1.3) were measured in both senescing leaf and bark tissues of ‘Golden Delicious’ apple trees (Malus domestica Borkh.). From the measured enzyme activities we attempted to estimate the in vivo catalytic potentials of the enzymes with special reference to nitrogen mobilization and conservation of senescing apple trees. The cumulative glutamine synthetase activity of leaf tissue was about three times higher than that of bark. The estimated catalytic potential of leaf glutamine synthetase was 800‐fold higher than the actual protein nitrogen loss of senescing leaves. The cumulative glutamate synthase activity of bark was about six times higher than that of leaf. The estimated catalytic potential of bark glutamate synthase was 160‐times higher than the actual protein nitrogen gain in that tissue. The cumulative glutamate dehydrogenase activities in leaf and bark tissue were approximately the same. However, the catalytic potential of leaf glutamate dehydrogenase was twice that of leaf glutamate synthase. It is thus concluded that the physiological role of glutamine synthetase in senescing leaf tissue is to furnish the amide(s) prior to mobilization of nitrogen to storage tissue. The higher activity of glutamate synthase in bark tissue could provide a mechanism to transform the imported amide nitrogen to amino nitrogen of glutamate for storage protein synthesis. The possible regulatory factors upon the activity of these enzymes in the tissues of senescing apple trees are discussed.

Relationship of senescence of corn ( Zea mays L.) stalk pith, cob parenchyma and first developed leaf tissues to RNA content and synthesis

In stalk parenchyma tissue of WF9 × 38-11 single cross corn ( Zea mays L.). there was a per cell increase in RNA synthesis and a slight increase in total RNA as cells became older. In cob parenchyma tissue of WF9 × 38-11 single cross corn, oth total RNA content and RNA synthesis per cell decreased with the age of cells. In first developed leaf tissue of FR43 × FR14A single cross corn, RNA synthesis increased steadily but only slightly over its life span. The pattern of RNA synthesis and destruction in senescing leaf tissue of seedlings and two sources of parenchyma tissue of maturing plants appeared to differ.

Uptake of 2-[14C]abscisic acid by senescing leaf tissue of radish, Raphanus sativus L.

The uptake of [(14)C]abscisic acid by radish leaf discs rises one to two days after excision and then declines to six days. This pattern of uptake is not identical to the uptake of [(14)C]sucrose. The uptake of both [(14)C]abscisic acid and [(14)C]sucrose is substantially reduced by anaerobic conditions.

Lipid Transformations in Greening and Senescing Leaf Tissue

Analyses were made of chlorophyll a and b and fatty acids (18:3, 18:2, 18:1, 18:0, 16:2, 16:1, and 16:0) of greening and senescing leaf tissue. Those dark-grown tissues given a prior treatment of red, far red, or red followed by far red light showed similar increases in chlorophylls and linolenate (18:3) when exposed to continuous white light. In contrast, green barley (Hordeum vulgare L.) leaves placed in the dark lost chlorophylls and fatty acids, especially 18:3. Senescing cocklebur (Xanthium strumarium L.) leaf tissue showed a decline in chlorophyll and fatty acids, especially again 18:3. Abscisic acid, but not sucrose, accelerated these senescent changes. Radioactive acetate incorporation into the galacto-lipids and phospholipids of senescing cocklebur leaf tissue increased and then the radioactivity of the lipids decreased in senescent tissues.

The effects of abscisic acid, kinetin and 5-fluorouracil on ribonucleic acid and protein synthesis in senescing radish leaf disks

The effects of abscisic acid and kinetin on RNA synthesis in senescing radish leaf disks were investigated using the improved resolution afforded by polyacrylamide gel electrophoresis. Kinetin stimulated and abscisic acid inhibited incorporation of radioactivity into cytoplasmic ribosomal RNA and soluble RNA. Chloroplast ribosomal RNA synthesis appeared to be confined to the period of leaf expansion and was not detected in fully mature leaves. The effects of kinetin in retarding and of abscisic acid in accelerating leaf senescence were not altered by the inhibition of cytoplasmic ribosomal RNA synthesis with 5-fluorouracil. Following inhibition of cytoplasmic ribosomal RNA synthesis with 5-fluorouracil, kinetin stimulated and abscisic acid inhibited incorporation of radioactivity into polydisperse RNA. These results are discussed in relation to the possible mode of action of kinetin and abscisic acid in senescing leaf tissue.

The Control of RNA, Protein and Chlorophyll Synthesis in Senescing Leaf Tissue of Kale by Coumarin and Growth Retardants

The Control of RNA, Protein and Chlorophyll Synthesis in Senescing Leaf Tissue of Kale by Coumarin and Growth Retardants

Coumarin, Phosfon D and CCC - the Inhibitors of Chlorophyll and Protein Degradation in Senescing Leaf Tissue of Kale

Coumarin, Phosfon D and CCC - the Inhibitors of Chlorophyll and Protein Degradation in Senescing Leaf Tissue of Kale