Senescence-Associated Changes during Long-day-induced Leaf Senescence and the Nature of the Graft-transmissible Senescence Substance in Kleinia articulata

Schwabe, W. W. and Kulkarni, V. J. 1987. Senescence-associatedchanges during long-day-induced leaf senescence and the natureof the graft-transmissible senescence substance in Kleinia articulata.— J. exp. Bot. 38: 1741–1755. The long-day-induced senescence in Kleinia articulata leaveswas characterized by a loss in fresh and dry weight, in therate of leaf expansion and progressive loss of chlorophyll inthe detached rooted leaves. Ultrastructural examination of mesophyllcells of leaves from plants grown in continuous light showedthat osmiophilic globules accumulating in the chloroplasts werethe first visible sign of senescence in the organdies. Thesefirst signs of senescence could be detected in very young leavesof plants in continuous light, even before the leaves had expanded.Attempts were made to study the cause of this photoperiodicsenescence which, from previous work, appeared to involve agraft-transmissible substance. Leaves in continuous light showed reduced stomatal opening andextracts from them had very much higher activity in the Commelinastomatal closure assay (ABA-like activity ?) compared with non-senescingleaves grown in short days (8 h). However, even if all the activitywere due to ABA, this on its own does not appear to be the senescencesubstance because a much longer exposure to continuous lightwas required to induce irreversible senescence than to reachmaximum stomatal closure promoting activity in the bioassay.Moreover, severe water stress (high ABA?) did not lead to senescenceunless combined with continuous light or ethylene treatment.It is postulated that while ABA may play an important role inKleinia leaf senescence its lethal effect may not be realizedunless ethylene-induced membrane changes may synergisticallyassist. Key words: Leaf senescence, ABA, Daylength, stomatal movement, Kleinia     

Failure of the Tomato Trans-Acting Short Interfering RNA Program to Regulate AUXIN RESPONSE FACTOR3 and ARF4 Underlies the Wiry Leaf Syndrome

Interfering with small RNA production is a common strategy of plant viruses. A unique class of small RNAs that require microRNA and short interfering (siRNA) biogenesis for their production is termed trans-acting short interfering RNAs (ta-siRNAs). Tomato (Solanum lycopersicum) wiry mutants represent a class of phenotype that mimics viral infection symptoms, including shoestring leaves that lack leaf blade expansion. Here, we show that four WIRY genes are involved in siRNA biogenesis, and in their corresponding mutants, levels of ta-siRNAs that regulate AUXIN RESPONSE FACTOR3 (ARF3) and ARF4 are reduced, while levels of their target ARFs are elevated. Reducing activity of both ARF3 and ARF4 can rescue the wiry leaf lamina, and increased activity of either can phenocopy wiry leaves. Thus, a failure to negatively regulate these ARFs underlies tomato shoestring leaves. Overexpression of these ARFs in Arabidopsis thaliana, tobacco (Nicotiana tabacum), and potato (Solanum tuberosum) failed to produce wiry leaves, suggesting that the dramatic response in tomato is exceptional. As negative regulation of orthologs of these ARFs by ta-siRNA is common to land plants, we propose that ta-siRNA levels serve as universal sensors for interference with small RNA biogenesis, and changes in their levels direct species-specific responses.     

A Role for Glutamine Synthetase in the Remobilization of Leaf Nitrogen during Natural Senescence in Rice Leaves

Changes in the levels of cytosolic glutamine synthetase (GS1) and chloroplastic glutamine synthetase (GS2) polypeptides and of corresponding mRNAs were determined in leaves of hydroponically grown rice (Oryza sativa) plants during natural senescence. The plants were grown in the greenhouse for 105 days at which time the thirteenth leaf was fully expanded. This was counted as zero time for senescence of the twelfth leaf. The twelfth leaf blade on the main stem was analyzed over a time period of −7 days (98 days after germination) to +42 days (147 days after germination). Total GS activity declined to less than a quarter of its initial level during the senescence for 35 days and this decline was mainly caused by a decrease in the amount of GS2 polypeptide. Immunoblotting analyses showed that contents of other chloroplastic enzymes, such as ribulose-1,5-bisphosphate carboxylase/oxygenase and Fd-glutamate synthase, declined in parallel with GS2. In contrast, the GS1 polypeptide remained constant throughout the senescence period. Translatable mRNA for GS1 increased about fourfold during the senescence for 35 days. During senescence, there was a marked decrease in content of glutamate (to about one-sixth of the zero time value); glutamate is the major form of free amino acid in rice leaves. Glutamine, the major transported amino acid, increased about threefold compared to the early phase of the harvest in the senescing rice leaf blades. These observations suggest that GS1 in senescing leaf blades is responsible for the synthesis of glutamine, which is then transferred to the growing tissues in rice plants.     

An Interaction between the Effects of Kinetin and Gibberellin in Retarding Leaf Senescence

The relation hetween the effects of kinetin and gibberellin on retardation of leaf senescence was studied. Leaf discs were incubated for five days in several hormone concentrations, the chlorophyll was extracted and its amount estimated spectrophotometrically. Investigation of leaves from actively growing plants of Taraxacum megalorrhizon and Tropaeolum majus showed that an interaction existed between the effect of both hormones. Leaf age, light intensity and day length had a marked effect on the degree of response to the hormonal treatments, hut the change in response effected by these conditions remained similar for both hormones. Possible interpretations of the interaction observed between the effects of kinetin and gibberellin are discussed.     

AUXIN RESPONSE FACTOR 2 Intersects Hormonal Signals in the Regulation of Tomato Fruit Ripening

The involvement of ethylene in fruit ripening is well documented, though knowledge regarding the crosstalk between ethylene and other hormones in ripening is lacking. We discovered that AUXIN RESPONSE FACTOR 2A (ARF2A), a recognized auxin signaling component, functions in the control of ripening. ARF2A expression is ripening regulated and reduced in the rin, nor and nr ripening mutants. It is also responsive to exogenous application of ethylene, auxin and abscisic acid (ABA). Over-expressing ARF2A in tomato resulted in blotchy ripening in which certain fruit regions turn red and possess accelerated ripening. ARF2A over-expressing fruit displayed early ethylene emission and ethylene signaling inhibition delayed their ripening phenotype, suggesting ethylene dependency. Both green and red fruit regions showed the induction of ethylene signaling components and master regulators of ripening. Comprehensive hormone profiling revealed that altered ARF2A expression in fruit significantly modified abscisates, cytokinins and salicylic acid while gibberellic acid and auxin metabolites were unaffected. Silencing of ARF2A further validated these observations as reducing ARF2A expression let to retarded fruit ripening, parthenocarpy and a disturbed hormonal profile. Finally, we show that ARF2A both homodimerizes and interacts with the ABA STRESS RIPENING (ASR1) protein, suggesting that ASR1 might be linking ABA and ethylene-dependent ripening. These results revealed that ARF2A interconnects signals of ethylene and additional hormones to co-ordinate the capacity of fruit tissue to initiate the complex ripening process.     

The Metabolism of Oat Leaves during Senescence: II. Senescence in Leaves Attached to the Plant

The course of senescence in the first leaves of light-grown Avena seedlings when attached to the plant has been compared with that previously studied in detached leaves and leaf segments. Proteolysis in the leaf, whether attached or detached, is accompanied by markedly polar basipetal transport of amino acids. This polar transport can be superimposed on the known transport of amino acids towards a locally applied cytokinin. In the intact plant, it results in a strong movement into the roots. The reducing sugars, which are set free in senescence, do not participate appreciably in this polar transport phenomenon.     

Involvement of the Phospholipid Sterol Acyltransferase1 in Plant Sterol Homeostasis and Leaf Senescence

Genes encoding sterol ester-forming enzymes were recently identified in the Arabidopsis (Arabidopsis thaliana) genome. One belongs to a family of six members presenting homologies with the mammalian Lecithin Cholesterol Acyltransferases. The other one belongs to the superfamily of Membrane-Bound O-Acyltransferases. The physiological functions of these genes, Phospholipid Sterol Acyltransferase1 (PSAT1) and Acyl-CoA Sterol Acyltransferase1 (ASAT1), respectively, were investigated using Arabidopsis mutants. Sterol ester content decreased in leaves of all mutants and was strongly reduced in seeds from plants carrying a PSAT1-deficient mutation. The amount of sterol esters in flowers was very close to that of the wild type for all lines studied. This indicated further functional redundancy of sterol acylation in Arabidopsis. We performed feeding experiments in which we supplied sterol precursors to psat1-1, psat1-2, and asat1-1 mutants. This triggered the accumulation of sterol esters (stored in cytosolic lipid droplets) in the wild type and the asat1-1 lines but not in the psat1-1 and psat1-2 lines, indicating a major contribution of the PSAT1 in maintaining free sterol homeostasis in plant cell membranes. A clear biological effect associated with the lack of sterol ester formation in the psat1-1 and psat1-2 mutants was an early leaf senescence phenotype. Double mutants lacking PSAT1 and ASAT1 had identical phenotypes to psat1 mutants. The results presented here suggest that PSAT1 plays a role in lipid catabolism as part of the intracellular processes at play in the maintenance of leaf viability during developmental aging.Sterols are components of most eukaryotic membranes; as such, they are important regulators of membrane fluidity and thus influence membrane properties, functions, and structure (Demel and De Kruyff, 1976; Bloch, 1983; Schuler et al., 1991; Roche et al., 2008). Unlike animals, in which cholesterol is most often the unique end product of sterol biosynthesis, each plant species has its own distribution of sterols, with the three most common phytosterols being sitosterol, stigmasterol, and campesterol (Benveniste, 2004). In addition to their free sterol form, phytosterols are also found as conjugates, particularly fatty acyl sterol esters (SE). Since SE are hardly integrated into the bilayer of the membranes (Hamilton and Small, 1982), the biochemical process of sterol acylation is believed to play a crucial role in maintaining free sterol concentration in the cell membranes (Lewis et al., 1987; Dyas and Goad, 1993; Chang et al., 1997; Sturley, 1997; Schaller, 2004). In other words, SE are generally thought to constitute a storage pool of sterols when those are present in amounts greater than immediately required for the cells. For instance, in plants, accumulation of SE has been described during seed maturation and senescence or when plant cell cultures reach stationary phase (Dyas and Goad, 1993, and refs. therein) as well as in mutant lines overproducing sterols (Gondet et al., 1994; Schaller et al., 1995).In mammals and yeast, the genes involved in sterol esterification have been known for a long time. These genes encode two types of enzymes responsible for the formation of SE in animals, the Acyl-Coenzyme A:Cholesterol Acyltransferase (ACAT) and the Lecithin:Cholesterol Acyltransferase (LCAT). ACAT, which catalyzes an acyl-CoA-dependent acylation, is a membrane-bound enzyme acting inside the cells (Chang et al., 1997). LCAT, which is evolutionarily unrelated to ACAT, catalyzes transacylation of acyl groups from phospholipids to sterols. It is a soluble enzyme present in the blood stream, where it is an important regulator of circulating cholesterol (Jonas, 2000). The budding yeast Saccharomyces cerevisiae has two enzymes of the ACAT type for the synthesis of SE (Yang et al., 1996).In plants, genes encoding enzymes responsible for SE formation have long been unknown. Based on biochemical studies, it was suggested that phospholipids and/or neutral lipids could serve as acyl donors (Garcia and Mudd, 1978a, 1978b; Zimowski and Wojciechowski, 1981a, 1981b). The identification in the Arabidopsis (Arabidopsis thaliana) genome of two genes involved in sterol esterification was based on homology searches. First, the phospholipid:sterol acyltransferase gene AtPSAT1 (At1g04010) was found to display consistent identity with the mammalian LCAT and then was biochemically characterized by expression in Arabidopsis (Noiriel, 2004; Banas et al., 2005). The encoded protein was shown to be associated with microsomal membranes and to catalyze the transfer of unsaturated fatty acyl groups from position sn-2 of phosphatidylethanolamine (and phosphatidylcholine to a lesser extent) to sterols. The preferred acceptor molecules of PSAT1 were cholesterol, a minor biosynthetic end product in Arabidopsis, then campesterol and sitosterol, the two main end products. Sterol coincubation studies performed with this microsomal enzymatic assay showed that sterol precursors such as cycloartenol or obtusifoliol, which were poor substrates when incubated alone, were preferentially acylated in the presence of sitosterol, suggesting an activation of the enzyme by sitosterol (Banas et al., 2005). Another sterol acyltransferase gene, AtASAT1 (At3g51970), was identified in a survey of members of the Arabidopsis superfamily of membrane-bound O-acyltransferases with a yeast ACAT mutant functional complementation approach (Chen et al., 2007). AtASAT1 encodes a protein structurally related to the animal and yeast ACATs. This enzyme was shown to prefer saturated fatty acyl-CoAs as acyl donors and cycloartenol as the acyl acceptor. Overexpression of AtASAT1 in seeds of Arabidopsis resulted in a strong accumulation of cycloartenol fatty acyl esters accompanied by an increase of the whole SE content of these seeds and, in spite of a slight decrease of the free sterol pool, an increase of the total sterol content of the transgenic seeds by up to 60% compared with that of the wild type (Chen et al., 2007). We took advantage of the availability of Arabidopsis T-DNA insertion mutants of these two genes to investigate their respective physiological roles. Here, we report on the involvement of AtPSAT1 in leaf senescence, its major contribution to SE formation in leaves and seeds, and also its essential role in free sterol homeostasis in these organs.     

Cytokinin Biochemistry in Relation to Leaf Senescence : II. The Metabolism of 6-Benzylaminopurine in Soybean Leaves and the Inhibition of Its Conjugation

The metabolism of [³H]6-benzylamino purine was studied in presenescent and early senescent soybean (Glycine max [L.] Merr.) leaves. In both types of leaves, the metabolism was essentially the same. The principal metabolite was identified as β-(6-benzylaminopurin-9-yl)alanine by mass spectral studies, which included discharge ionization-secondary ion mass spectrometry and pulsed positive ion-negative ion-chemical ionization mass spectrometry. Conversion to this alanine conjugate was found to be inhibited 2,4-dichlorophenoxyacetic acid and 5,7-dichloroindoleacetic acid.