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Senescing Leaves (senescing + leaf)

Distribution by Scientific Domains

Life Sciences	100%

Selected Abstracts

Antioxidant and Pigment Composition during Autumnal Leaf Senescence in Woody Deciduous Species Differing in their Ecological Traits

PLANT BIOLOGY, Issue 5 2003
J. I. García-Plazaola
Abstract: Photoprotection mechanisms have been studied during autumnal senescence in sun and shade leaves of woody plants with different ecological characteristics and senescence patterns. Three of them belonging to the same family, Betulaceae: the shade-intolerant and early successional species (Betula alba L.), the shade-tolerant and late successional species (Corylus avellana L.), and an N-fixing tree with low N resorption efficiency (Alnus glutinosa L.). The other two species: a shade-intolerant (Populus tremula L.) and a shade-tolerant (Cornus sanguinea L.), were chosen because of their ability to accumulate anthocyanins during autumnal leaf senescence. The study of plants with different ecological strategies allowed us to establish general trends in photoprotection mechanisms during autumnal senescence, when nutrient remobilisation occurs, but also during whole leaf ontogeny. We have not found a clear relationship between shade tolerance and the level of photoprotection; the main difference between both groups of species being the presence of ,-carotene in shade leaves of shade-tolerant species. Preceding autumn, nitrogen resorption started in mid-summer and occurred in parallel with a slight and continuous ascorbate, chlorophyll and carotenoid degradation. However, the ascorbate pool remained highly reduced and lipid oxidation did not increase at this time. Contrasting with ascorbate, ,-tocopherol accumulated progressively in all species. Only during the last stages of senescence was chlorophyll preferentially degraded with respect to carotenoids, leading to the yellowing of leaves, except in A. glutinosa in which a large retention of chlorophyll and N took place. Senescing leaves were characterised, except in C. sanguinea, by a relative increase in the proportion of de-epoxidised xanthophylls: zeaxanthin, antheraxanthin and lutein. The light-induced accumulation of anthocyanins in C. sanguinea could play an additional protective role, compensating for the low retention of de-epoxidised xanthophylls. These different strategies among deciduous species are consistent with a role for photoprotective compounds in enhancing nitrogen remobilization and storage for the next growing season. [source]

Inactivation of the cytochrome P450 gene CYP82E2 by degenerative mutations was a key event in the evolution of the alkaloid profile of modern tobacco

NEW PHYTOLOGIST, Issue 3 2007
Manohar Chakrabarti
Summary ,,The alkaloid profile of cultivated tobacco (Nicotiana tabacum) is different from that of its two progenitors, Nicotiana sylvestris and Nicotiana tomentosiformis, in that tobacco accumulates nicotine as the most abundant alkaloid, while its ancestors convert nicotine to nornicotine in the senescing leaf. The nicotine-retaining phenotype of tobacco is thought to have evolved through the inactivation of the conversion loci inherited from its two progenitors. Here, the genetic changes associated with the inactivation of the conversion locus derived from N. sylvestris were investigated. ,,Candidate genes were isolated from a N. sylvestris senescing leaf cDNA library and characterized by heterologous gene expression in yeast, site-directed mutagenesis and quantitative real-time polymerase chain reaction. ,,A cytochrome P450 gene, designated NsylCYP82E2, was isolated from N. sylvestris. Located on the chromosomal fragment defined by the N. sylvestris conversion locus, NsylCYP82E2 confers high nicotine N -demethylase (NND) activity in the senescing leaves of N. sylvestris, but the gene is inactivated by two degenerative mutations in tobacco. ,,Collectively with previously published data, these results show that inactivation of NND genes by degenerative mutations and/or transcriptional suppression played a key role in the evolution of the alkaloid profile of modern tobacco. [source]

The different fates of mitochondria and chloroplasts during dark-induced senescence in Arabidopsis leaves

PLANT CELL & ENVIRONMENT, Issue 12 2007
OLIVIER KEECH
ABSTRACT Senescence is an active process allowing the reallocation of valuable nutrients from the senescing organ towards storage and/or growing tissues. Using Arabidopsis thaliana leaves from both whole darkened plants (DPs) and individually darkened leaves (IDLs), we investigated the fate of mitochondria and chloroplasts during dark-induced leaf senescence. Combining in vivo visualization of fates of the two organelles by three-dimensional reconstructions of abaxial parts of leaves with functional measurements of photosynthesis and respiration, we showed that the two experimental systems displayed major differences during 6 d of dark treatment. In whole DPs, organelles were largely retained in both epidermal and mesophyll cells. However, while the photosynthetic capacity was maintained, the capacity of mitochondrial respiration decreased. In contrast, IDLs showed a rapid decline in photosynthetic capacity while maintaining a high capacity for mitochondrial respiration throughout the treatment. In addition, we noticed an unequal degradation of organelles in the different cell types of the senescing leaf. From these data, we suggest that metabolism in leaves of the whole DPs enters a ,stand-by mode' to preserve the photosynthetic machinery for as long as possible. However, in IDLs, mitochondria actively provide energy and carbon skeletons for the degradation of cell constituents, facilitating the retrieval of nutrients. Finally, the heterogeneity of the degradation processes involved during senescence is discussed with regard to the fate of mitochondria and chloroplasts in the different cell types. [source]

Inactivation of the cytochrome P450 gene CYP82E2 by degenerative mutations was a key event in the evolution of the alkaloid profile of modern tobacco

Catching a red herring: autumn colours and aphids

OIKOS, Issue 11 2009
T. C. R. White
The purpose of this note is not to support any particular hypothesis explaining the evolution of red coloured autumn leaves, but to present evidence that shows existing knowledge does not support one such hypothesis , that red coloured leaves evolved as a signal to protect trees from aphids feeding and laying eggs on them in autumn. An alternative hypothesis is that autumn-feeding aphids are senescence-feeders, evolved to feed only on senescing leaves. These aphids are programmed to detect and feed on such leaves when they are still green and yellow and actively exporting their nutrients. Aphids reject or ignore red leaves because they are no longer good food, not because they are protecting the trees from the aphids. [source]

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

PLANT BIOTECHNOLOGY JOURNAL, Issue 2 2004
Paul R. H. Robson
Summary We have genetically modified maize plants to delay leaf senescence. A senescence-enhanced promoter from maize (PSEE1) was used to drive expression of the Agrobacterium cytokinin biosynthesis gene IPT in senescing leaf tissue. Three maize lines expressing IPT from PSEE1, 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 PSEE1XbaIPTNOS 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. [source]