The Arabidopsis sex1 mutant is defective in the R1 protein, a general regulator of starch degradation in plants, and not in the chloroplast hexose transporter

Starch is the major storage carbohydrate in higher plants and of considerable importance for the human diet and for numerous technical applications. In addition, starch can be accumulated transiently in chloroplasts as a temporary deposit of carbohydrates during ongoing photosynthesis. This transitory starch has to be mobilized during the subsequent dark period. Mutants defective in starch mobilization are characterized by high starch contents in leaves after prolonged periods of darkness and therefore are termed starch excess (sex) mutants. Here we describe the molecular characterization of the Arabidopsis sex1 mutant that has been proposed to be defective in the export of glucose resulting from hydrolytic starch breakdown. The mutated gene in sex1 was cloned using a map-based cloning approach. By complementation of the mutant, immunological analysis, and analysis of starch phosphorylation, we show that sex1 is defective in the Arabidopsis homolog of the R1 protein and not in the hexose transporter. We propose that the SEX1 protein (R1) functions as an overall regulator of starch mobilization by controlling the phosphate content of starch.     

Modulation of Reserve Mobilization by Sucrose,Glutamine, and Abscisic Acid During Seedling Establishment in Sunflower

We carried out in vitro feeding experiments using sunflower as a model to differentiate the modulatory effects of metabolites (sucrose and glutamine) and hormones (gibberellic acid and abscisic acid) on reserve mobilization, metabolite partitioning, and key enzyme activities. Exogenous sucrose negatively not only modulated the mobilization of carbon reserves (oils and starch), but it also delayed the degradation of nitrogen reserves (storage proteins) in the cotyledons. Similarly, exogenous glutamine negatively not only modulated storage protein hydrolysis, but it also retarded oil and starch degradation. Different from the metabolites, exogenous abscisic acid affected only the mobilization of oils and storage proteins. Sucrose and glutamine caused non-reducing sugar accumulation in the cotyledons and axis, but abscisic acid did not change the content of these compounds in both seedling parts. Curiously, glutamine failed to cause amino acid accumulation in the cotyledons and abscisic acid increased the amino acid content in both cotyledons and axis. Gibberellic acid did not stimulate reserve mobilization and metabolite consumption. Although the mobilization of oils, storage proteins, and starch has been delayed by sucrose and glutamine, these metabolites augmented the activity of isocitrate lyase, acid proteases, and amylases. Only abscisic acid reduced amylase activity and increased glutamine synthetase activity. Accordingly, sucrose and glutamine exert a “crossed effect” on reserve mobilization, that is, sucrose delays storage protein hydrolysis and glutamine retards oil and starch degradation. These effects may be mediated by non-reducing sugars and they are, at least in part, different from those exerted by abscisic acid.     

Oxylipins and plant abiotic stress resistance

Oxylipins are signaling molecules formed enzymatically or spontaneously from unsaturated fatty acids in all aerobic organisms. Oxylipins regulate growth, development, and responses to environmental stimuli of organisms. The oxylipin biosynthesis pathway in plants includes a few parallel branches named after first enzyme of the corresponding branch as allene oxide synthase, hydroperoxide lyase, divinyl ether synthase, peroxygenase, epoxy alcohol synthase, and others in which various biologically active metabolites are produced. Oxylipins can be formed non-enzymatically as a result of oxygenation of fatty acids by free radicals and reactive oxygen species. Spontaneously formed oxylipins are called phytoprostanes. The role of oxylipins in biotic stress responses has been described in many published works. The role of oxylipins in plant adaptation to abiotic stress conditions is less studied; there is also obvious lack of available data compilation and analysis in this area of research. In this work we analyze data on oxylipins functions in plant adaptation to abiotic stress conditions, such as wounding, suboptimal light and temperature, dehydration and osmotic stress, and effects of ozone and heavy metals. Modern research articles elucidating the molecular mechanisms of oxylipins action by the methods of biochemistry, molecular biology, and genetics are reviewed here. Data on the role of oxylipins in stress signal transduction, stress-inducible gene expression regulation, and interaction of these metabolites with other signal transduction pathways in cells are described. In this review the general oxylipin-mediated mechanisms that help plants to adjust to a broad spectrum of stress factors are considered, followed by analysis of more specific responses regulated by oxylipins only under certain stress conditions. New approaches to improvement of plant resistance to abiotic stresses based on the induction of oxylipin-mediated processes are discussed.     

植物响应非生物胁迫的磷酸化修饰组学研究进展

植物具有固着生活的特点,高温、低温、干旱和盐等生境中常见的非生物胁迫会严重影响植物的生长发育。蛋白质磷酸化是植物应对非生物胁迫的重要机制,主要通过蛋白质的磷酸化和去磷酸化修饰来调控植物细胞对外界胁迫的应激反应,在植物细胞快速传递胁迫信号并激活对胁迫环境的形态、生理和分子水平适应机制的过程中起重要作用。该文主要介绍了植物磷酸化蛋白质的富集、检测和鉴定技术,并对近年来国内外有关植物响应高温、低温、干旱、淹水、盐、养分亏缺和元素毒害等非生物胁迫的磷酸化修饰蛋白组学研究进展进行综述。     

Source-sink regulation of cotyledonary reserve mobilization during cashew (Anacardium occidentale) seedling establishment under NaCl salinity

Seedling establishment is a critical process to crop productivity, especially under saline conditions. This work was carried out to investigate the hypothesis that reserve mobilization is coordinated with salt-induced inhibition of seedling growth due to changes in source-sink relations. To test this hypothesis, cashew nuts (Anacardium occidentale) were sown in vermiculite irrigated daily with distilled water (control) or 50mM NaCl and they were evaluated at discrete developmental stages from the seed germination until the whole seedling establishment. The salt treatment coordinately delayed the seedling growth and the cotyledonary reserve mobilization. However, these effects were more pronounced at late seedling establishment than in earlier stages. The storage protein mobilization was affected by salt stress before the lipid and starch breakdown. The globulin fraction represented the most important storage proteins of cashew cotyledons, and its mobilization was markedly delayed by NaCl along the seedling establishment. Free amino acids were mostly retained in the cotyledons of salt-treated seedlings when the mobilization of storage proteins, lipids and starch was strongly delayed. Proline was not considerably accumulated in the cotyledons of cashew seedlings as a response to NaCl salinity. According to these results it is noteworthy that the salt-induced inhibition of seedling growth is narrowly coordinated with the delay of reserve mobilization and the accumulation of hydrolysis products in cotyledons. Also, it was evidenced that free amino acids, especially those related to nitrogen transport, are potential signals involved in the regulation of storage protein hydrolysis during cashew seedling establishment under NaCl salinity.     

The effects of season and water availability on chemical composition,secondary metabolites and biological activity in plants

Plants react towards changes in their environment, which can be a result of biotic or abiotic activities. Numerous studies have investigated the effects of abiotic stress on plants, and how it affects the primary as well as secondary metabolism. Generally it is accepted that plants react to environmental stress by increasing secondary metabolites. This is however a very broad and simplified explanation and often inaccurate. Various examples are provided where plants react positively, and often negatively towards seasonal variation and water availability, resulting in a lowering of certain secondary metabolites concentration, while others are increased. Furthermore species differences, cultivars and interaction of other environmental factors such as temperature complicates a simple conclusion from the effect of stress on plants. The differential expression of genes in different species and in different metabolic pathways ensures a complex and very specific reaction of a plant to environmental stress. Overall the paper provides support for a complex and intricate response system which differs for each plant species, and could be explained by understanding and studying the different metabolic pathways responsible for secondary metabolite production.     

Effects of NaCl, Na2SO4, and mannitol on storage lipid mobilization in the cotyledons and roots of purple alfalfa seedlings

The effects of NaCl, Na₂SO₄, and mannitol on mobilization of storage lipids in the cotyledon mesophyll and root meristem cells during the germination of purple alfalfa (Medicago sativa L.) seeds were studied using the transmission electron microscopy technique. The ultrastructural analysis showed that the number and shape of specific organelles, oleosomes, depended on the osmotic effects of a salt used. In control seedlings grown on distilled water, lipid mobilization in roots terminated by the 4th day, and in cotyledons, only by the 8th day. The disruptions in the storage lipid mobilization in the treated seedlings were specific and depended on the stress agent used, the osmotic pressure induced by this agent, as well as on the tissue type. The presence of a great number of oleosomes in the cytoplasm of cells subjected to osmotic stress, in contrast to the control ones, demonstrated the inhibition of mobilization processes. All concentrations of Na₂SO₄ and mannitol suppressed lipid mobilization in the cotyledons and roots. At the same time, NaCl markedly suppressed this process in the root cells only at a higher osmotic pressure of the solution. The results obtained suggest that the cytological analysis of cotyledon storage-lipid mobilization can tentatively assess the resistance of dicotyledonous plants to abiotic stresses.     

Wheat LEA genes, PMA80 and PMA1959, enhance dehydration tolerance of transgenic rice (Oryza sativa L.)

Drought and salt stresses are two major factors that lower plant productivity. Transgenic approaches offer powerful means to better understand and then minimize loss of yield due to these abiotic stresses. In this study, we have generated transgenic rice plants expressing a wheat LEA group 2 protein (PMA80) gene, and separately the wheat LEA group 1 protein (PMA1959) gene. Molecular analysis of the transgenic plants revealed the stable integration of the transgenes. Immunoblot analysis showed the presence of the LEA group 2 protein (39 kDa) and the LEA group 1 protein (25 kDa) in most of the plant lines. Second-generation transgenic plants were subjected to dehydration or salt stress. The results showed that accumulation of either PMA80 or PMA1959 correlates with increased tolerance of transgenic rice plants to these stresses.     

The polyamine spermine protects against high salt stress in Arabidopsis thaliana

It is well known that changes in abiotic conditions such as the concentration of ions, temperature and humidity lead to modulation of polyamine contents in plants. However, little is known about the relevant parts these polyamines play in abiotic stress responses. Here we addressed a specific role of spermine during high salt stress using an Arabidopsis double knockout-mutant plant (acl5/spms) which cannot produce spermine. The mutant showed higher sensitivity to high salt than wild type plants. This phenotype was cured by exogenous spermine but not by the other polyamines putrescine and spermidine, suggesting a strong link between spermine-deficiency and NaCl-hypersensitivity. The mutant was also hypersensitive to high levels of KCl but not to MgCl₂ or to high osmoticum. NaCl-hypersensitivity of the mutant was compromised by treatment with Ca²⁺ channel blockers. Moreover, the mutant showed poor growth on Ca²⁺-depleted Murashige–Skoog agar media. The data suggest that the absence of spermine causes an imbalance in Ca²⁺ homeostasis in the mutant plant. Based on the data obtained, we propose a model for a role of spermine in high salt stress responses.     

A cytosolic glucosyltransferase is required for conversion of starch to sucrose in Arabidopsis leaves at night

Maltose is exported from the Arabidopsis chloroplast as the main product of starch degradation at night. To investigate its fate in the cytosol, we characterised plants with mutations in a gene encoding a putative glucanotransferase (disproportionating enzyme; DPE2), a protein similar to the maltase Q (MalQ) gene product involved in maltose metabolism in bacteria. Use of a DPE2 antiserum revealed that the DPE2 protein is cytosolic. Four independent mutant lines lacked this protein and displayed a decreased capacity for both starch synthesis and starch degradation in leaves. They contained exceptionally high levels of maltose, and elevated levels of glucose, fructose and other malto-oligosaccharides. Sucrose levels were lower than those in wild-type plants, especially at the start of the dark period. A glucosyltransferase activity, capable of transferring one of the glucosyl units of maltose to glycogen or amylopectin and releasing the other, was identified in leaves of wild-type plants. Its activity was sufficient to account for the rate of starch degradation. This activity was absent from dpe2 mutant plants. Based on these results, we suggest that DPE2 is an essential component of the pathway from starch to sucrose and cellular metabolism in leaves at night. Its role is probably to metabolise maltose exported from the chloroplast. We propose a pathway for the conversion of starch to sucrose in an Arabidopsis leaf.