RESUMO
Exposure to abiotic stresses accelerates leaf senescence in most crop plant species, thereby reducing photosynthesis and other assimilatory processes. In some cases, genotypes with delayed leaf senescence (i.e. 'stay-green') show stress resistance, particularly in cases of water deficit, and this has led to the proposal that senescence delay improves crop performance under some abiotic stresses. In this review, we summarize the evidence for increased resistance to abiotic stress, mostly water deficit, in genotypes with delayed senescence, and specifically focus on the physiological mechanisms and agronomic conditions under which the stay-green trait may ameliorate grain yield under stress.
Assuntos
Produtos Agrícolas , Senescência Vegetal , Estresse Fisiológico , Produtos Agrícolas/fisiologia , Produtos Agrícolas/crescimento & desenvolvimento , Produtos Agrícolas/genética , Senescência Vegetal/fisiologia , Folhas de Planta/fisiologiaRESUMO
In C3 cereals such as wheat and barley, grain filling was traditionally explained as being sustained by assimilates from concurrent leaf photosynthesis and remobilization from the stem. In recent decades, a role for ear photosynthesis as a contributor to grain filling has emerged. This review analyzes several aspects of this topic: (i) methodological approaches for estimation of ear photosynthetic contribution to grain filling; (ii) the existence of genetic variability in the contribution of the ear, and evidence of genetic gains in the past; (iii) the controversy of the existence of C4 metabolism in the ear; (iv) the response of ear photosynthesis to water deficit; and (v) morphological and physiological traits possibly related to ear temperature and thermal balance of the ear. The main conclusions are: (i) there are a number of methodologies to quantify ear photosynthetic activity (e.g. gas exchange and chlorophyll fluorescence) and the contribution of the ear to grain filling (individual ear shading, ear emergence in shaded canopies, and isotope composition); (ii) the contribution of ear photosynthesis seems to have increased in modern wheat germplasm; (iii) the contribution of the ear to grain filling increases under resource-limitation (water deficit, defoliation, or pathogen infection); (iv) there is genetic variability in the contribution of the ear in wheat, opening up the possibility to use this trait to ameliorate grain yield; (v) current evidence supports the existence of C3 metabolism rather than C4 metabolism; (vi) the ear is a 'dehydration avoider organ' under drought; and (vii) thermal balance in the ear is a relevant issue to explore, and more research is needed to clarify the underlying morphological and physiological traits.
Assuntos
Grão Comestível , Hordeum , Fotossíntese , Folhas de Planta , Triticum/genéticaRESUMO
Leaf senescence is characterized by massive degradation of chloroplast proteins, yet the protease(s) involved is(are) not completely known. Increased expression and/or activities of serine, cysteine, aspartic, and metalloproteases were detected in senescing leaves, but these studies have not provided information on the identities of the proteases responsible for chloroplast protein breakdown. Silencing some senescence-associated proteases has delayed progression of senescence symptoms, yet it is still unclear if these proteases are directly involved in chloroplast protein breakdown. At least four cellular pathways involved in the traffic of chloroplast proteins for degradation outside the chloroplast have been described (i.e., "Rubisco-containing bodies," "senescence-associated vacuoles," "ATI1-plastid associated bodies," and "CV-containing vesicles"), which differ in their dependence on the autophagic machinery, and the identity of the proteins transported and/or degraded. Finding out the proteases involved in, for example, the degradation of Rubisco, may require piling up mutations in several senescence-associated proteases. Alternatively, targeting a proteinaceous protein inhibitor to chloroplasts may allow the inhibitor to reach "Rubisco-containing bodies," "senescence-associated vacuoles," "ATI1-plastid associated bodies," and "CV-containing vesicles" in essentially the way as chloroplast-targeted fluorescent proteins re-localize to these vesicular structures. This might help to reduce proteolytic activity, thereby reducing or slowing down plastid protein degradation during senescence.
RESUMO
Chlorophyll (Chl) loss is the main visible symptom of senescence in leaves. The initial steps of Chl degradation operate within the chloroplast, but the observation that 'senescence-associated vacuoles' (SAVs) contain Chl raises the question of whether SAVs might also contribute to Chl breakdown. Previous confocal microscope observations (Martínez et al., 2008) showed many SAVs containing Chl. Isolated SAVs contained Chl a and b (with a Chl a/b ratio close to 5) and lower levels of chlorophyllide a. Pheophytin a and pheophorbide a were formed after the incubation of SAVs at 30°C in darkness, suggesting the presence of Chl-degrading activities in SAVs. Chl in SAVs was bound to a number of 'green bands'. In the most abundant green band of SAVs, Western blot analysis showed the presence of photosystem I (PSI) Chl-binding proteins, including the PsaA protein of the PSI reaction center and the apoproteins of the light-harvesting complexes (Lhca 1-4). This was confirmed by: (i) measurements of 77-K fluorescence emission spectra showing a single emission peak at around 730 nm in SAVs; (ii) mass spectrometry of the most prominent green band with the slowest electrophoretic mobility; and (iii) immunofluorescence detection of PsaA in SAVs observed through confocal microscopy. Incubation of SAVs at 30°C in darkness caused a steady decrease in PsaA levels. Overall, these results indicate that SAVs may be involved in the degradation of PSI proteins and their associated chlorophylls during the senescence of leaves.
Assuntos
Clorofila/metabolismo , Cloroplastos/metabolismo , Nicotiana/metabolismo , Complexo de Proteína do Fotossistema I/metabolismo , Folhas de Planta/metabolismo , Proteínas de Plantas/metabolismo , Vacúolos/metabolismo , Envelhecimento , Senescência Celular , Escuridão , Plastídeos/metabolismo , ProteóliseRESUMO
The apoplast, i.e. the cellular compartment external to the plasma membrane, undergoes important changes during senescence. Apoplastic fluid volume increases quite significantly in senescing leaves, thereby diluting its contents. Its pH elevates by about 0.8 units, similar to the apoplast alkalization in response to abiotic stresses. The levels of 159 proteins decrease, whereas 24 proteins increase in relative abundance in the apoplast of senescing leaves. Around half of the apoplastic proteins of non-senescent leaves contain a N-terminal signal peptide for secretion, while all the identified senescence-associated apoplastic proteins contain the signal peptide. Several of the apoplastic proteins that accumulate during senescence also accumulate in stress responses, suggesting that the apoplast may constitute a compartment where developmental and stress-related programs overlap. Other senescence-related apoplastic proteins are involved in cell wall modifications, proteolysis, carbohydrate, ROS and amino acid metabolism, signaling, lipid transport, etc. The most abundant senescence-associated apoplastic proteins, PR2 and PR5 (e.g. pathogenesis related proteins PR2 and PR5) are related to leaf aging rather than to the chloroplast degradation program, as their levels increase only in leaves undergoing developmental senescence, but not in dark-induced senescent leaves. Changes in the apoplastic space may be relevant for signaling and molecular trafficking underlying senescence.
RESUMO
Rust is one of the most important biotic stress factors that affect poplars. The aims of this work were: (i) to analyze the changes in growth and nitrogen (N) accumulation in Populus deltoides W. Bartram ex Marshall plants infected with rust (Melampsora medusae Thümen.) and to determine how internal N stores are affected by the disease, in plants growing under two N availabilities in the soil; and (ii) to evaluate the impact of rust in the early sprout in the following growing season and the cumulative effect of the disease after repeated infections. Two clones with different susceptibility to rust were analyzed. At leaf level, rust reduced gas exchange capacity, water conductance in liquid phase and photosynthetic rate in both clones. At plant level, rust reduced plant growth, accelerated leaf senescence and abscission occurred with a higher concentration of leaf N. Even though N concentration in stems and roots were not significantly reduced by rust, total N accumulation in perennial tissues was reduced in infected plants. The vigor of the early sprout of plants infected by rust in the previous season was lower than that of non-infected plants. Therefore, rust affects plant growth by reducing the photosynthetic capacity and leaf area duration, and by decreasing internal nutrient recycling. As nutrient reserves in perennial tissues are lower, rust infection reduces not only the growth of the current season, but also has a cumulative effect on the following years. The reduction of growth was similar in both clones. High availability of N in the soil had no effect on leaf physiology but increased plant growth, delayed leaf senescence and abscission, and increased total N accumulation. If fertilization increases plant growth (stem and root dry mass) it can mitigate the negative effect of the pathogen in the reduction of nutrient storages and future growth.
Assuntos
Basidiomycota , Nitrogênio/metabolismo , Doenças das Plantas , Populus/metabolismo , Árvores/metabolismo , Doenças das Plantas/microbiologia , Populus/crescimento & desenvolvimento , Populus/microbiologia , Estações do Ano , Solo/química , Árvores/crescimento & desenvolvimento , Árvores/microbiologiaRESUMO
Leaf senescence is a concerted physiological process involving controlled degradation of cellular structures and reallocation of breakdown products to other plant organs. It is accompanied by increased production of reactive oxygen species (ROS) that are proposed to signal cell death, although both the origin and the precise role of ROS in the execution of this developmental program are still poorly understood. To investigate the contribution of chloroplast-associated ROS to natural leaf senescence, we used tobacco plants expressing a plastid-targeted flavodoxin, an electron shuttle flavoprotein present in prokaryotes and algae. When expressed in plants, flavodoxin specifically prevents ROS formation in chloroplasts during stress situations. Senescence symptoms were significantly mitigated in these transformants, with decreased accumulation of chloroplastic ROS and differential preservation of chlorophylls, carotenoids, protein contents, cell and chloroplast structures, membrane integrity and cell viability. Flavodoxin also improved maintenance of chlorophyll-protein complexes, photosynthetic electron flow, CO2 assimilation, central metabolic routes and levels of bioactive cytokinins and auxins in aging leaves. Delayed induction of senescence-associated genes indicates that the entire genetic program of senescence was affected by flavodoxin. The results suggest that ROS generated in chloroplasts are involved in the regulation of natural leaf senescence.
RESUMO
Plant senescence is accompanied by a marked increase in proteolytic activities, and cysteine proteases (Cys-protease) represent the prevailing class among the responsible proteases. Cys-proteases predominantly locate to lytic compartments, i.e., to the central vacuole (CV) and to senescence-associated vacuoles (SAVs), the latter being specific to the photosynthetic cells of senescing leaves. Cellular fractionation of vacuolar compartments may facilitate Cys-proteases purification and their concentration for further analysis. Active Cys-proteases may be analyzed by different, albeit complementary approaches: (1) in vivo examination of proteolytic activity by fluorescence microscopy using specific substrates which become fluorescent upon cleavage by Cys-proteases, (2) protease labeling with specific probes that react irreversibly with the active enzymes, and (3) zymography, whereby protease activities are detected in polyacrylamide gels copolymerized with a substrate for proteases. Here we describe the three methods mentioned above for detection of active Cys-proteases and a cellular fractionation technique to isolate SAVs.
Assuntos
Envelhecimento , Cisteína Proteases/metabolismo , Fenômenos Fisiológicos Vegetais , Vacúolos/enzimologia , Ativação Enzimática , Proteínas de Plantas/metabolismo , Coloração e RotulagemRESUMO
Rust produced by Melampsora sp. is considered one of the most relevant diseases in poplar plantations. Growth reduction in poplar plantations takes place because rust, like other pathogens, alters leaf physiology. There is not a complete evaluation of several of the physiological traits that can be affected by rust at leaf level. Therefore, the aim of this work was to evaluate, in an integrative way and in the same pathosystem, which physiological processes are affected when Populus deltoides Bartr. ex Marsh. leaves are infected by rust (Melampsora medusae Thümen). Leaves of two clones with different susceptibility to rust were analyzed. Field and pot experiments were performed, and several physiological traits were measured in healthy and infected leaves. We conclude that rust affects leaf mesophyll integrity, and so water movement in the leaf in liquid phase is affected. As a consequence, gas exchange is reduced, affecting both carbon fixation and transpiration. However, there is an increase in respiration rate, probably due to plant and fungal respiration. The increase in respiration rate is important in the reduction of net photosynthetic rate, but also some damage in the photosynthetic apparatus limits leaf capacity to fix carbon. The decrease in chlorophyll content would start later and seems not to explain the reduction in net photosynthetic rate. Both clones, although they have different susceptibility to rust, are affected in the same physiological mechanisms.
Assuntos
Basidiomycota/fisiologia , Doenças das Plantas/microbiologia , Folhas de Planta/fisiologia , Populus/fisiologia , Interações Hospedeiro-Patógeno , FotossínteseRESUMO
Senescence involves increased expression of proteases, which may participate in nitrogen recycling or cellular signalling. 2D zymograms detected two protein species with increased proteolytic activity in senescing leaves of Arabidopsis thaliana. A proteomic analysis revealed that both protein species correspond to a subtilisin protease encoded by At3g14067, termed Senescence-Associated Subtilisin Protease (SASP). SASP mRNA levels and enzyme activity increase during leaf senescence in leaves senescing during both the vegetative or the reproductive phase of the plant life cycle, but this increase is more pronounced in reproductive plants. SASP is expressed in all above-ground organs, but not in roots. Putative AtSASP orthologues were identified in dicot and monocot crop species. A phylogenetic analysis shows AtSASP and its putative orthologues clustering in one discrete group of subtilisin proteases in which no other Arabidospsis subtilisin protease is present. Phenotypic analysis of two knockout lines for SASP showed that mutant plants develop more inflorescence branches during reproductive development. Both AtSASP and its putative rice orthologue (OsSASP) were constitutively expressed in sasp-1 to complement the mutant phenotype. At maturity, sasp-1 plants produced 25% more inflorescence branches and siliques than either the wild-type or the rescued lines. These differences were mostly due to an increased number of second and third order branches. The increased number of siliques was compensated for by a small decrease (5.0%) in seed size. SASP downregulates branching and silique production during monocarpic senescence, and its function is at least partially conserved between Arabidopsis and rice.
Assuntos
Proteínas de Arabidopsis/genética , Proteínas de Arabidopsis/metabolismo , Arabidopsis/enzimologia , Arabidopsis/crescimento & desenvolvimento , Regulação da Expressão Gênica de Plantas , Sementes/crescimento & desenvolvimento , Sementes/genética , Subtilisinas/genética , Subtilisinas/metabolismo , Sequência de Aminoácidos , Arabidopsis/química , Arabidopsis/genética , Proteínas de Arabidopsis/química , Regulação da Expressão Gênica no Desenvolvimento , Filogenia , Proteômica , Sementes/enzimologia , Alinhamento de Sequência , Subtilisinas/químicaRESUMO
Degradation of chloroplasts and chloroplast components is a distinctive feature of leaf senescence. In spite of its importance in the nutrient economy of plants, knowledge about the mechanism(s) involved in the breakdown of chloroplast proteins is incomplete. A novel class of vacuoles, "senescence-associated vacuoles" (SAVs), characterized by intense proteolytic activity appear during senescence in chloroplast-containing cells of leaves. Since SAVs contain some chloroplast proteins, they are candidate organelles to participate in chloroplast breakdown. In this review we discuss the characteristics of SAVs, and their possible involvement in the degradation of Rubisco, the most abundant chloroplast protein. Finally, SAVs are compared with other extra-plastidial protein degradation pathways operating in senescing leaves.
RESUMO
Breakdown of leaf proteins, particularly chloroplast proteins, is a massive process in senescing leaves. In spite of its importance in internal N recycling, the mechanism(s) and the enzymes involved are largely unknown. Senescence-associated vacuoles (SAVs) are small, acidic vacuoles with high cysteine peptidase activity. Chloroplast-targeted proteins re-localize to SAVs during senescence, suggesting that SAVs might be involved in chloroplast protein degradation. SAVs were undetectable in mature, non-senescent tobacco leaves. Their abundance, visualized either with the acidotropic marker Lysotracker Red or by green fluorescent protein (GFP) fluorescence in a line expressing the senescence-associated cysteine protease SAG12 fused to GFP, increased during senescence induction in darkness, and peaked after 2-4 d, when chloroplast dismantling was most intense. Increased abundance of SAVs correlated with higher levels of SAG12 mRNA. Activity labelling with a biotinylated derivative of the cysteine protease inhibitor E-64 was used to detect active cysteine proteases. The two apparently most abundant cysteine proteases of senescing leaves, of 40kDa and 33kDa were detected in isolated SAVs. Rubisco degradation in isolated SAVs was completely blocked by E-64. Treatment of leaf disks with E-64 in vivo substantially reduced degradation of Rubisco and leaf proteins. Overall, these results indicate that SAVs contain most of the cysteine protease activity of senescing cells, and that SAV cysteine proteases are at least partly responsible for the degradation of stromal proteins of the chloroplast.
Assuntos
Senescência Celular , Cloroplastos/enzimologia , Cisteína Proteases/metabolismo , Nicotiana/enzimologia , Folhas de Planta/enzimologia , Proteínas de Plantas/metabolismo , Vacúolos/enzimologia , Senescência Celular/efeitos dos fármacos , Senescência Celular/efeitos da radiação , Cloroplastos/efeitos dos fármacos , Cloroplastos/genética , Cloroplastos/efeitos da radiação , Cisteína Proteases/genética , Inibidores de Cisteína Proteinase/farmacologia , Escuridão , Regulação para Baixo/efeitos dos fármacos , Regulação para Baixo/efeitos da radiação , Folhas de Planta/efeitos dos fármacos , Folhas de Planta/genética , Folhas de Planta/efeitos da radiação , Proteínas de Plantas/antagonistas & inibidores , Proteínas de Plantas/genética , Proteólise/efeitos dos fármacos , Proteólise/efeitos da radiação , Nicotiana/efeitos dos fármacos , Nicotiana/genética , Nicotiana/efeitos da radiação , Vacúolos/efeitos dos fármacos , Vacúolos/genética , Vacúolos/efeitos da radiaçãoRESUMO
The subcellular localization of NO generation in soybean cotyledons, and the relationship between NO synthesis and in vivo chloroplast performance were studied. Employing the NO probe 4-aminomethyl-2',7'-difluorofluorescein diacetate (DAF-FM DA) and fluorescence microscopy, a strongly punctuated fluorescence was detected in mesophyll cells. The co-localization of DAF-FM and chlorophyll fluorescence, in confocal laser microscopy images, indicated the presence of NO in the chloroplasts. NO visualization was dependent on light, seedling age, and chloroplast function throughout cotyledons lifespan. The addition of herbicides with action in chloroplasts (DCMU and paraquat) dramatically reduced the quantum yield of photosystem II (φ(PSII)), and lead to images with absence of punctuated green fluorescence. Moreover, electron paramagnetic resonance signals corresponding to NO-spin trap adduct observed in cotyledon homogenates decreased significantly by the treatment with herbicides, as compared to controls. Neither chloroplast function nor NO content were significantly different in cotyledons from plants growing in the presence of ammonium or nitrate as the nitrogen source. These findings suggest that chloroplasts are organelles that contribute to NO synthesis in vivo, and that their proper functionality is essential for maintaining NO levels in soybean cotyledons.
Assuntos
Cloroplastos/metabolismo , Cotilédone/metabolismo , Glycine max/metabolismo , Óxido Nítrico/metabolismo , Clorofila/metabolismo , Cloroplastos/efeitos dos fármacos , Cotilédone/efeitos dos fármacos , Diurona/farmacologia , Transporte de Elétrons/efeitos dos fármacos , Fluoresceínas/metabolismo , Herbicidas/farmacologia , Luz , Células do Mesofilo/metabolismo , Paraquat/farmacologia , Complexo de Proteína do Fotossistema II/metabolismo , Glycine max/efeitos dos fármacosRESUMO
The main symptoms of leaf senescence are the degradation of chlorophyll and proteins (which may be accompanied by ammonium accumulation), and an increase of electrolyte leakage (EL), which has been traditionally attributed to disruption of cell membranes. The aim of this study was to determine if ammonium efflux contributes to the increase EL in senescing barley leaves. During senescence of detached leaves the increase of EL correlated with ammonium leakage (r(2) = 0.82) and ammonium content in tissues (r(2) = 0.73), but not with K(1+) leakage (r(2) = 0.23). Although lower amounts of ammonium accumulated in senescing attached leaves, again changes in EL paralleled ammonium accumulation. EL increased early during senescence even though ion leakage was selective (leaves leaked proportionally more ammonium than K(1+)), and membranes appeared intact as judged from staining with the cell impermeant stain propidium iodide. Detached leaves maintained their capacity to regreen after 3 days of senescence-acceleration in darkness, i.e., membrane integrity was not severely compromised. During the early stages of senescence, EL increases due to ammonium accumulation (possibly resulting from protein degradation) even if there is no massive disruption of cell membranes. Therefore, increased EL in senescing leaves is not an unequivocal symptom of cell membrane damage.
Assuntos
Membrana Celular/metabolismo , Eletrólitos/metabolismo , Hordeum/metabolismo , Folhas de Planta/metabolismo , Compostos de Amônio Quaternário/metabolismo , Morte Celular , Membrana Celular/fisiologia , Clorofila/análise , Clorofila/metabolismo , Escuridão , Hordeum/fisiologia , Microscopia de Fluorescência , Permeabilidade , Folhas de Planta/fisiologia , Proteínas de Plantas/análise , Proteínas de Plantas/metabolismo , Potássio/análise , Potássio/metabolismo , Propídio/metabolismo , Compostos de Amônio Quaternário/análise , Fatores de TempoRESUMO
Cellular proteins are extensively degraded during leaf senescence, and this correlates with an up-regulation of protease gene expression, particularly cysteine proteases. The objectives of this work were (i) to detect cysteine proteases associated with senescence of wheat leaves under different conditions and (ii) to find out their subcellular location. Activity labelling of cysteine proteases with the biotinylated inhibitor DCG-04 detected five bands at 27, 36, 39, 42, and 46 kDa in leaves of wheat senescing under continuous darkness. In-gel activity assays showed that these proteases are only active in an acid milieu (pH 4), and their activity increased several-fold in senescing leaves. Fractionation experiments showed that the senescence-associated cysteine proteases of 36, 39, 42, and 46 kDa localize to a vacuolar-enriched fraction. The vacuolar cysteine proteases of 36, 39, and 42 kDa increased in activity in attached flag leaves senescing naturally during post-anthesis, and in attached leaves of plants subjected to a period of water deficit. Thus, the activity of these vacuolar cysteine proteases is associated with developmental (post-anthesis) senescence and with senescence induced by stress factors (i.e. protracted darkness or drought). This suggests that vacuoles are involved in senescence-associated cellular degradation, and that different senescence-inducing factors may converge on a single degradation pathway.
Assuntos
Cisteína Endopeptidases/metabolismo , Folhas de Planta/enzimologia , Triticum/enzimologia , Vacúolos/enzimologia , Perfilação da Expressão Gênica , Regulação da Expressão Gênica de Plantas , Concentração de Íons de Hidrogênio , Reprodução/fisiologia , Fatores de Tempo , Água/metabolismoRESUMO
The natural variation in leaf and plant longevity in Arabidopsis thaliana was analysed in a set of 45 ecotypes and 155 recombinant inbred lines derived from a Cape Verde Islands (Cvi) x Landsberg erecta (Ler) cross. Post-bolting longevity was inversely related to time to flowering and rosette leaf number in the set of 45 ecotypes, with Cvi having the longest and Ler the shortest post-bolting longevity. The recombinant inbred line population was tested under low or high soil nutrient levels (LN or HN, respectively). Three quantitative trait loci (QTL), one in chromosome 3 and two in chromosomes 1 and 5, were associated with longevity of the 6th rosette leaf under LN and HN, respectively. Four QTL for post-bolting longevity were found in chromosomes 1, 3, 4, and 5, and two in chromosomes 1 and 5 under LN and HN, respectively. An epistatic interaction affecting post-bolting longevity under LN, but not HN, was detected. Ler and Cvi carry a mix of increasing and decreasing alleles for the QTL affecting longevity of the 6th leaf and post-bolting longevity. Longevity of the 6th rosette leaf was associated with different QTL than post-bolting longevity, and it was affected by different QTL depending on nutrient availability. By contrast, the major QTL affecting post-bolting longevity exerted significant effects irrespective of soil nutrient availability.
Assuntos
Arabidopsis/genética , Locos de Características Quantitativas , Arabidopsis/crescimento & desenvolvimento , Meio Ambiente , Epistasia Genética , Flores , Genes de Plantas , Variação Genética , Folhas de Planta/crescimento & desenvolvimento , Plantas Geneticamente Modificadas , Fatores de TempoRESUMO
Vacuolar compartments associated with leaf senescence and the subcellular localization of the senescence-specific cysteine-protease SAG12 (senescence-associated gene 12) were studied using specific fluorescent markers, the expression of reporter genes, and the analysis of high-pressure frozen/freeze-substituted samples. Senescence-associated vacuoles (SAVs) with intense proteolytic activity develop in the peripheral cytoplasm of mesophyll and guard cells in Arabidopsis and soybean. The vacuolar identity of these compartments was confirmed by immunolabeling with specific antibody markers. SAVs and the central vacuole differ in their acidity and tonoplast composition: SAVs are more acidic than the central vacuole and, whereas the tonoplast of central vacuoles is highly enriched in gamma-TIP (tonoplast intrinsic protein), the tonoplast of SAVs lacks this aquaporin. The expression of a SAG12-GFP fusion protein in transgenic Arabidopsis plants shows that SAG12 localizes to SAVs. The analysis of Pro(SAG12):GUS transgenic plants indicates that SAG12 expression in senescing leaves is restricted to SAV-containing cells, for example, mesophyll and guard cells. A homozygous sag12 Arabidopsis mutant develops SAVs and does not show any visually detectable phenotypical alteration during senescence, indicating that SAG12 is not required either for SAV formation or for progression of visual symptoms of senescence. The presence of two types of vacuoles in senescing leaves could provide different lytic compartments for the dismantling of specific cellular components. The possible origin and functions of SAVs during leaf senescence are discussed.
Assuntos
Arabidopsis/enzimologia , Arabidopsis/ultraestrutura , Glycine max/enzimologia , Glycine max/ultraestrutura , Vacúolos/enzimologia , Arabidopsis/genética , Proteínas de Arabidopsis/fisiologia , Cloroplastos , Cisteína Endopeptidases/fisiologia , Concentração de Íons de Hidrogênio , Mutação , Folhas de Planta/enzimologia , Folhas de Planta/ultraestrutura , Plantas Geneticamente Modificadas , Glycine max/genética , Fatores de Tempo , Vacúolos/químicaRESUMO
The aim of this study was to explore the role of the mitochondrial alternative oxidase (AOX) in the protection of photosynthesis during drought in wheat leaves. The relative water contents of water-replete and drought-exposed wheat plants were 97.2+/-0.3 and 75+/-2, respectively. Drought increased the amount of leaf AOX protein and also enhanced the rate of AOX-dependent O(2) uptake by the respiratory electron transport chain. The amount of the reduced, active form of the AOX protein was specifically increased by drought. The AOX inhibitor salicylhydroxamic acid (1 mM; SHAM) inhibited 70% of AOX activity in vivo in both water-replete and drought-exposed plants. Plants treated with SHAM were then exposed to low (100), high (350), or excess light (800 mumol photons m(-2) s(-1)) for 90 min. SHAM did not modify chlorophyll a fluorescence quenching parameters in water-replete controls after any of these treatments. However, while the maximal quantum yield of photosystem II (PSII) electron transport (F(v)/F(m)) was not affected by SHAM, the immediate quantum yield of PSII electron transport (Phi(PSII)) and photochemical quenching (qP) were gradually reduced by increasing irradiance in SHAM-treated drought-exposed plants, the decrease being most pronounced at the highest irradiance. Non-photochemical quenching (NPQ) reached near maximum levels in plants subjected to drought at high irradiance. However, a combination of drought and low light caused an intermediate increase in NPQ, which attained higher values when AOX was inhibited. Taken together, these results show that up-regulation of the respiratory AOX pathway protects the photosynthetic electron transport chain from the harmful effects of excess light.
Assuntos
Desastres , Mitocôndrias/fisiologia , Oxirredutases/metabolismo , Fotossíntese/fisiologia , Triticum/fisiologia , Clorofila/metabolismo , Cloroplastos/enzimologia , Cloroplastos/fisiologia , Transporte de Elétrons , Proteínas Mitocondriais , Consumo de Oxigênio , Folhas de Planta/enzimologia , Folhas de Planta/fisiologia , Proteínas de Plantas/genética , Triticum/enzimologiaRESUMO
Photosynthesis, respiration, and other processes produce reactive oxygen species (ROS) that can cause oxidative modifications to proteins, lipids, and DNA. The production of ROS increases under stress conditions, causing oxidative damage and impairment of normal metabolism. In this work, oxidative damage to various subcellular compartments (i.e. chloroplasts, mitochondria, and peroxisomes) was studied in two cultivars of wheat differing in ascorbic acid content, and growing under good irrigation or drought. In well-watered plants, mitochondria contained 9-28-fold higher concentrations of oxidatively modified proteins than chloroplasts or peroxisomes. In general, oxidative damage to proteins was more intense in the cultivar with the lower content of ascorbic acid, particularly in the chloroplast stroma. Water stress caused a marked increase in oxidative damage to proteins, particularly in mitochondria and peroxisomes. These results indicate that mitochondria are the main target for oxidative damage to proteins under well-irrigated and drought conditions.
Assuntos
Mitocôndrias/metabolismo , Estresse Oxidativo , Folhas de Planta/metabolismo , Triticum/metabolismo , Ácido Ascórbico/análise , Cloroplastos/metabolismo , Desidratação , Oxirredução , Consumo de Oxigênio , Peroxissomos/metabolismo , Espécies Reativas de Oxigênio/metabolismoRESUMO
In chloroplasts, stromal and thylakoid-bound ascorbate peroxidases (tAPX) play a major role in the removal of H(2)O(2) produced during photosynthesis. Here, we report that hexaploid wheat (Triticum aestivum) expresses three homeologous tAPX genes (TaAPX-6A, TaAPX-6B, and TaAPX-6D) mapping on group-6 chromosomes. The tAPX activity of a mutant line lacking TaAPX-6B was 40% lower than that of the wild type. When grown at high-light intensity photosystem II electron transfer, photosynthetic activity and biomass accumulation were significantly reduced in this mutant, suggesting that tAPX activity is essential for photosynthesis. Despite the reduced tAPX activity, mutant plants did not exhibit oxidative damage probably due to the reduced photochemical activity. This might be the result of a compensating mechanism to prevent oxidative damage having as a consequence a decrease in growth of the tAPX mutant plants.