Showing papers in "Physiologia Plantarum in 2008"

Potassium transport and plant salt tolerance.

Abstract: Salinity is a major abiotic stress affecting approximately 7% of the world's total land area resulting in billion dollar losses in crop production around the globe. Recent progress in molecular genetics and plant electrophysiology suggests that the ability of a plant to maintain a high cytosolic K+/Na+ ratio appears to be critical to plant salt tolerance. So far, the major efforts of plant breeders have been aimed at improving this ratio by minimizing Na+ uptake and transport to shoot. In this paper, we discuss an alternative approach, reviewing the molecular and ionic mechanisms contributing to potassium homeostasis in salinized plant tissues and discussing prospects for breeding for salt tolerance by targeting this trait. Major K+ transporters and their functional expression under saline conditions are reviewed and the multiple modes of their control are evaluated, including ameliorative effects of compatible solutes, polyamines and supplemental calcium. Subsequently, the genetic aspects of inheritance of K+ transport 'markers' are discussed in the general context of salt tolerance as a polygenic trait. The molecular identity of 'salt tolerance' genes is analysed, and prospects for future research and breeding are examined.

Reactive oxygen signaling and abiotic stress.

Abstract: Reactive oxygen species (ROS) play a dual role in plant biology acting on the one hand as important signal transduction molecules and on the other as toxic by-products of aerobic metabolism that accumulate in cells during different stress conditions. Because of their toxicity as well as their important signaling role, the level of ROS in cells is tightly controlled by a vast network of genes termed the 'ROS gene network'. Using mutants deficient in key ROS-scavenging enzymes, we have defined a signaling pathway that is activated in cells in response to ROS accumulation. Interestingly, many of the key players in this pathway, including different zinc finger proteins and WRKY transcription factors, are also central regulators of abiotic stress responses involved in temperature, salinity and osmotic stresses. Here, we describe our recent findings and discuss how ROS integrate different signals originating from different cellular compartments during abiotic stress.

Using chlorophyll fluorescence to assess the fraction of absorbed light allocated to thermal dissipation of excess excitation

Abstract: In the present study we explored the possibility of assessing the allocation of photons absorbed by photosystem II (PSII) antennae to thermal energy dissipation and photosynthetic electron transport in leaves of several plant species under field conditions. Changes in chlorophyll fluorescence parameters were determined in situ over the course of an entire day in the field in sun-exposed leaves of two species with different maximal rates of photosynthesis, Helianthus annuus (sunflower) and Vinca major. Leaves of Vinca minor (periwinkle) growing in a deeply shaded location were also monitored. We propose using diurnal changes in the efficiency of open PSII centers (F′v/F′m) in these sun and shade leaves to (a) assess diurnal changes in the allocation of absorbed light to photochemistry and thermal energy dissipation and, furthermore, (b) make an estimate of changes in the rate of thermal energy dissipation, an analogous expression to the rate of photochemistry. The fraction of light absorbed in PSII antennae that is dissipated thermally (D) is proposed to be estimated from D = 1-F′v/F′m, in analogy to the widely used estimation of the fraction of light absorbed in PSII antennae (P) that is utilized in PSII photochemistry from P = F′v/F′m× qP (where qP is the coefficient for photochemical quenching; Genty, B., Briantais, J.-M. & Baker, N. R. 1989. Biochim. Biophys. Acta 990: 87-92). The rate of thermal dissipation is consequently given by D × PFD (photon flux density), again in analogy to the rate of photochemistry P × PFD, both assuming a matching behavior of photosystems I and II. Characterization of energy dissipation from the efficiency of open PSII centers allows an assessment from a single set of measurements at any time of day; this is particularly useful under field conditions where the fully relaxed reference values of variable or maximal fluorescence needed for the computation of nonphotochemical quenching may not be available. The usefulness of the assessment described above is compared with other currently used parameters to quantify nonphotochemical and photochemical chlorophyll fluorescence quenching.

Potassium influences on yield and quality production for maize, wheat, soybean and cotton.

Abstract: Potassium is one of the principle plant nutrients underpinning crop yield production and quality determination. While involved in many physiological processes, potassium's impact on water relations, photosynthesis, assimilate transport and enzyme activation can have direct consequences on crop productivity. Potassium deficiency can lead to a reduction in both the number of leaves produced and the size of individual leaves. Coupling this reduced amount of photosynthetic source material with a reduction in the photosynthetic rate per unit leaf area, and the result is an overall reduction in the amount of photosynthetic assimilates available for growth. The production of less photosynthetic assimilates and reduced assimilate transport out of the leaves to the developing fruit greatly contributes to the negative consequences that deficiencies of potassium have on yield and quality production. Goals aimed toward increasing crop productivity and improved quality dictate either increased potassium supply or more efficient use of potassium. Developing plants that more efficiently use potassium might be a worthwhile goal for geneticists.

Metabolomics for plant stress response

Abstract: Stress in plants could be defined as any change in growth condition(s) that disrupts metabolic homeostasis and requires an adjustment of metabolic pathways in a process that is usually referred to as acclimation. Metabolomics could contribute significantly to the study of stress biology in plants and other organisms by identifying different compounds, such as by-products of stress metabolism, stress signal transduction molecules or molecules that are part of the acclimation response of plants. These could be further tested by direct measurements, correlated with changes in transcriptome and proteome expression and confirmed by mutant analysis. In this review, we will discuss recent application of metabolomics and system biology to the area of plant stress response. We will describe approaches such as metabolic profiling and metabolic fingerprinting as well as combination of different 'omics' platforms to achieve a holistic view of the plant response stress and conduct detailed pathway analysis.

Size exclusion limits and lateral heterogeneity of the stomatal foliar uptake pathway for aqueous solutes and water-suspended nanoparticles

Abstract: Penetration rates of foliar-applied polar solutes are highly variable and the underlying mechanisms are not yet fully understood. The contribution of stomata especially, is still a matter of debate. Thus, the size exclusion limits of the stomatal foliar uptake pathway, its variability and its transport capacity have been investigated. The size exclusion limits were analyzed by studying the penetration of water-suspended hydrophilic particles of two different sizes (43 nm or 1.1 microm diameter) into leaves of Vicia faba (L.). To avoid agglutination of the particles, plants were kept in water-saturated atmosphere. Penetration of the larger particles was never detected, whereas after 2 to 9 days, the smaller particles occasionally penetrated the leaf interior through stomatal pores. Permeability of stomata to Na(2)-fluorescein along the leaf blade of Allium porrum (L.) was highly variable and not correlated with the position on the leaf. When evaporated residues of the foliar-applied solutions were rewetted repeatedly, approximately 60% of the previously penetrated stomata were penetrated again. The average rate constant of penetration of an individual stoma was in the same order of magnitude as typical rate constants reported for the cuticular pathway. The observed sparseness of stomatal penetration together with its high lateral variability but local and temporal persistency was taken as evidence that stomata contributing to uptake differ from non-penetrated ones in the wettability of their guard cell cuticle. These results show that the stomatal pathway is highly capacitive because of its large size exclusion limit above 10 nm and its high transport velocity, but at the same time the high variability renders this pathway largely unpredictable.

The effect of potassium nutrition on pest and disease resistance in plants.

Abstract: Providing a fast growing world population with sufficient food while preserving ecological and energy resources of our planet is one of the biggest challenges in this century. Optimized management of chemical fertilizers and pesticides will be essential for achieving sustainability of intensive farming and requires both empirical data from field trials and advanced fundamental understanding of the molecular processes controlling plant growth. Genes involved in plant responses to nutrient deficiency and pathogen/herbivore attack have been identified, but we are lacking information about the cross-talk between signalling pathways when plants are exposed to a combination of abiotic and biotic stress factors. The focus of this review is on the relationship between the potassium status of plants and their susceptibility to pathogens and herbivorous insects. We combine field evidence on potassium-disease interaction with existing knowledge on metabolic and physiological factors that could explain such interaction, and present new data on metabolite profiles and hormonal pathways from the model plant Arabidopsis thaliana. The latter provides evidence that facilitated entry and development of pathogens or insects in(to) potassium-deficient plants as a result of physical and metabolic changes is counteracted by an increased defence. A genetic approach should now be applied to establish a causal relationship between disease susceptibility on the one hand and individual enzymatic and signal components on the other. Once identified, these can be used to design agricultural strategies that support the nutritional status of the crops while exploiting their inherent potential for defence.

Crops and genotypes differ in efficiency of potassium uptake and use.

Abstract: Cultivars with increased efficiency of uptake and utilization of soil nutrients are likely to have positive environmental effects through reduced usage of chemicals in agriculture. This review assesses the available literature on differential uptake and utilization efficiency of K in farming systems. Large areas of agricultural land in the world are deficient in K (e.g. 3/4 of paddy soils in China, 2/3 of the wheatbelt in Southern Australia), with export in agricultural produce (especially hay) and leaching (especially in sandy soils) contributing to lowering of K content in the soil. The capacity of a genotype to grow and yield well in soils low in available K is K efficiency. Genotypic differences in efficiency of K uptake and utilization have been reported for all major economically important plants. The K-efficient phenotype is a complex one comprising a mixture of uptake and utilization efficiency mechanisms. Differential exudation of organic compounds to facilitate release of non-exchangeable K is one of the mechanisms of differential K uptake efficiency. Genotypes efficient in K uptake may have a larger surface area of contact between roots and soil and increased uptake at the root-soil interface to maintain a larger diffusive gradient towards roots. Better translocation of K into different organs, greater capacity to maintain cytosolic K+ concentration within optimal ranges and increased capacity to substitute Na+ for K + are the main mechanisms underlying K utilization efficiency. Further breeding for increased K efficiency will be dependent on identification of suitable markers and compounding of efficiency mechanisms into locally adapted germplasm.

Role of magnesium in carbon partitioning and alleviating photooxidative damage

Abstract: Magnesium (Mg) deficiency exerts a major influence on the partitioning of dry matter and carbohydrates between shoots and roots. One of the very early reactions of plants to Mg deficiency stress is the marked increase in the shoot-to-root dry weight ratio, which is associated with a massive accumulation of carbohydrates in source leaves, especially of sucrose and starch. These higher concentrations of carbohydrates in Mg-deficient leaves together with the accompanying increase in shoot-to-root dry weight ratio are indicative of a severe impairment in phloem export of photoassimilates from source leaves. Studies with common bean and sugar beet plants have shown that Mg plays a fundamental role in phloem loading of sucrose. At a very early stage of Mg deficiency, phloem export of sucrose is severely impaired, an effect that occurs before any noticeable changes in shoot growth, Chl concentration or photosynthetic activity. These findings suggest that accumulation of carbohydrates in Mg-deficient leaves is caused directly by Mg deficiency stress and not as a consequence of reduced sink activity. The role of Mg in the phloem-loading process seems to be specific; resupplying Mg for 12 or 24 h to Mg-deficient plants resulted in a very rapid recovery of sucrose export. It appears that the massive accumulation of carbohydrates and related impairment in photosynthetic CO2 fixation in Mg-deficient leaves cause an over-reduction in the photosynthetic electron transport chain that potentiates the generation of highly reactive O2 species (ROS). Plants respond to Mg deficiency stress by marked increases in antioxidative capacity of leaves, especially under high light intensity, suggesting that ROS generation is stimulated by Mg deficiency in chloroplasts. Accordingly, it has been found that Mg-deficient plants are very susceptible to high light intensity. Exposure of Mg-deficient plants to high light intensity rapidly induced leaf chlorosis and necrosis, an outcome that was effectively delayed by partial shading of the leaf blade, although the Mg concentrations in different parts of the leaf blade were unaffected by shading. The results indicate that photooxidative damage contributes to development of leaf chlorosis under Mg deficiency, suggesting that plants under high-light conditions have a higher physiological requirement for Mg. Maintenance of a high Mg nutritional status of plants is, thus, essential in the avoidance of ROS generation, which occurs at the expense of inhibited phloem export of sugars and impairment of CO2 fixation, particularly under high-light conditions.

Beneficial effects of potassium on human health

Abstract: Until recently, humans consumed a diet high in potassium. However, with the increasing consumption of processed food, which has potassium removed, combined with a reduction in the consumption of fruits and vegetables, there has been a large decrease in potassium intake which now, in most developed countries, averages around 70 mmol day-1, i.e. only one third of our evolutionary intake. Much evidence shows that increasing potassium intake has beneficial effects on human health. Epidemiological and clinical studies show that a high-potassium diet lowers blood pressure in individuals with both raised blood pressure and average population blood pressure. Prospective cohort studies and outcome trials show that increasing potassium intake reduces cardiovascular disease mortality. This is mainly attributable to the blood pressure-lowering effect and may also be partially because of the direct effects of potassium on the cardiovascular system. A high-potassium diet may also prevent or at least slow the progression of renal disease. An increased potassium intake lowers urinary calcium excretion and plays an important role in the management of hypercalciuria and kidney stones and is likely to decrease the risk of osteoporosis. Low serum potassium is strongly related to glucose intolerance, and increasing potassium intake may prevent the development of diabetes that occurs with prolonged treatment with thiazide diuretics. Reduced serum potassium increases the risk of lethal ventricular arrhythmias in patients with ischaemic heart disease, heart failure and left ventricular hypertrophy, and increasing potassium intake may prevent this. The best way to increase potassium intake is to increase the consumption of fruits and vegetables.

Antioxidant enzymatic activity is linked to waterlogging stress tolerance in citrus

Abstract: Soil flooding constitutes a seasonal factor that negatively affects plant performance and crop yields. In this work, the relationship between oxidative damage and flooding sensitivity was addressed in three citrus genotypes with different abilities to tolerate waterlogging. We examined leaf visible damage, oxidative damage in terms of malondialdehyde (MDA) concentration, leaf proline concentration, leaf and root ascorbate and glutathione contents and the antioxidant enzyme activities superoxide dismutase (EC 1.15.1.1), ascorbate peroxidase (EC 1.11.1.11), catalase (EC 1.11.1.6) and glutathione reductase (EC 1.8.1.7). No differences in the extent of oxidative damage relative to controls were found among genotypes. However, a different ability to delay the apparition of oxidative damage was associated to a higher tolerance to waterlogging. This ability was linked to an enhanced activated oxygen species' scavenging capacity in terms of an increased antioxidant enzyme activity and higher content in polar antioxidant compounds. Therefore, the existence of a direct relationship between stress sensitivity and the early accumulation of MDA is proposed. In addition, data indicate that the protective role of proline has to be considered minimal as its accumulation was inversely correlated with tolerance to the stress. The positive antioxidant response in Carrizo citrange (Poncirus trifoliata L. Raf. x Citrus sinensis L. Osb.) and Citrumelo CPB 4475 (Poncirus trifoliata L. Raf. x Citrus paradisi L. Macf.) might be responsible for a higher tolerance to flooding stress, whereas in Cleopatra mandarin (Citrus reshni Hort. Ex Tan.), the early accumulation of MDA seems to be associated to an impaired ability for H2O2 scavenging.

Equivalent pore radii of hydrophilic foliar uptake routes in stomatous and astomatous leaf surfaces--further evidence for a stomatal pathway.

Abstract: Foliar uptake pathways for hydrophilic solutes were studied by the analysis of co-uptake of 15N-labelled urea, NH4+ or NO3- and 13C-labelled sucrose across leaf surfaces of various plant species. Uptake of N (y) and sucrose (x) were strongly correlated. Curvilinear regression revealed significantly positive intercepts with the y-axis indicating the involvement of a sucrose-excluding pathway consisting of small pores with radii <0.5 nm. Depending on plant species, N source, leaf side and aperture of stomata, these small pores accounted for 6-62% of total N uptake. Regression analysis revealed that in stomatous leaf surfaces of Vicia faba L., Coffea arabica L. and Prunus cerasus L., the remaining N uptake occurred via another pathway with an estimated average pore radius (r(P)) greater than 20 nm. This is two orders of magnitude greater than previous estimations of cuticular r(P), indicating that this pathway, which was only found in stomatous leaf surfaces, was probably not located in the cuticle but at the surfaces of the stomatal pores. In astomatous leaf surfaces of C. arabica and Populus x canadensis Moench, average r(P) was 2.0 and 2.4 nm, respectively, which is four to eight times larger than previous estimations of cuticular r(P). These results indicate that for polar solutes, the size exclusion limits of plant surfaces can be considerably larger than previously estimated. The far-reaching implications of these findings are discussed.

Physiological and cytological mechanisms of silicon-induced resistance in rice against blast disease.

Abstract: Rice (Oryza sativa L.) blast disease caused by Magnaporthe grisea is one of the most destructive diseases in the rice-growing areas of the world. Silicon is an important nutritional element especially for rice. Two near-isogenic lines of rice with different resistance to blast disease, i.e. CO39 (susceptible) and C101LAC (Pi-1) (resistant), were selected to determine the effects of Si amendment on the severity and incidence of rice blast disease. The physiological and cytological mechanisms involved in the induced disease resistance by silicon were investigated. Exogenous Si application at a concentration of 2 mM reduced the disease index by 45% for CO39 and 56% for C101LAC (Pi-1). Si application alone did not change lignin content and the activities of defense-related enzymes including peroxidase (POD), polyphenol oxidase (PPO) and phenylalanine ammonia-lyase (PAL) in rice leaves of both isogenic lines. However, after inoculation with M. grisea, Si-treated rice plants significantly increased the activities of POD, PPO and PAL in leaves of both isogenic lines. Si and lignin content were also significantly increased in Si-treated inoculated seedlings. Environmental scanning electron microscope observations revealed that Si amendment resulted in higher Si deposit on dumbbell bodies in the rice leaves and silicon papilla accumulation on the guard cell of stoma. These results suggest that silicon-induced defense response and cell silicification of rice leaves altogether contribute to the silicon-induced rice resistance to blast disease.

Adaptations of higher plant cell walls to water loss: drought vs desiccation.

Abstract: Water-deficit stress poses unique challenges to plant cells dependent on a hydrostatic skeleton and a polysaccharide-rich cell wall for growth and development. How the plant cell wall is adapted to loss of water is of interest in developing a general understanding of water stress tolerance in plants and of relevance in strategies related to crop improvement. Drought tolerance involves adaptations to growth under reduced water potential and the concomitant restructuring of the cell wall that allow growth processes to occur at lower water contents. Desiccation tolerance, by contrast, is the evolution of cell walls that are capable of losing the majority of cellular water without suffering permanent and irreversible damage to cell wall structure and polymer organization. This minireview highlights common features and differences between these two water-deficit responses observed in plants, emphasizing the role of the cell wall, while suggesting future research avenues that could benefit fundamental understanding in this area.

Meta‐topolin, an alternative to benzyladenine in tissue culture?

Abstract: The metabolism and the in vitro effects of the cytokinin N6-(3-hydroxybenzyl)adenine (meta-topolin, mT) were compared with those of N6-benzyladenine (BA), N6-benzyl-9-(2-tetrahydropyranyl)adenine (BPA) and N6-benzyladenine-9-riboside ([9R]BA) in micropropagated Spathiphyllum floribundum Schott cv. Petite. In vitro, BA, BPA and [9R]BA were mainly converted into the stable derivative N6-benzyladenine-9-glucoside ([9G]BA), located at the basal part of the plant. N6-(3-O-β-D-glucopyranosyl)benzyladenine-9-riboside {(OG)[9R]mT} was the main derivative of mT. This new cytokinin-O-glucoside, which was present in all plant parts, was metabolized much faster than [9G]BA during acclimatization. The effect of BA and mT on in vitro shoot and root production and post vitro rooting was compared. Only mT combined a good shoot production with in vitro root formation. The plants that developed on medium with 10 µM or more mT rooted better during acclimatization than those developed on medium with equimolar concentrations of BA.

Sensing environmental temperature change through imbalances between energy supply and energy consumption: Redox state of photosystem II

Abstract: A basic requirement of all photosynthetic organisms is a balance between overall energy supply through temperature-independent photochemical reactions and energy consumption through the temperature-dependent biochemical reactions of photosynthetic electron transport and contiguous metabolic pathways. Since the turnover of photosystem II (PSII) reaction centers is a limiting step in the conversion of light energy into ATP and NADPH, any energy imbalance may be sensed through modulation of the redox state of PSII. This can be estimated in vivo by chlorophyll a fluorescence as changes in the redox state of PSII, or photosystem II excitation pressure, which reflects changes in the redox poise of intersystem electron transport carriers. Through comparisons of photosynthetic adjustment, we show that growth at low temperature mimics growth at high light. We conclude that terrestrial plants, green algae and cyanobacteria do not respond to changes in growth temperature or growth irradiance per se, but rather, respond to changes in the redox state of intersystem electron transport as reflected by changes in PSII excitation pressure, We suggest that this chloroplastic redox sensing mechanism may be an important component for sensing abiotic stresses in general. Thus, in addition to its role in energy transduction, the chloroplast may also be considered a primary sensor of environmental change through a redox sensing/signalling mechanism that acts synergistically with other signal transduction pathways to elicit the appropriate molecular and physiological responses.

Low leaf hydraulic conductance associated with drought tolerance in soybean

Abstract: Lack of water is the most serious environmental constraint on agricultural production. More efficient use of water resources is a key solution for increased plant productivity in water-deficit environments. We examined the hydraulic characteristics of a 'slow wilting' phenotype in soybean (Glycine max Merr.), PI 416937, which has been shown to have relatively constant transpiration rates above a threshold atmospheric vapor pressure deficit (VPD). The VPD response of PI 416937 was confirmed. Three experiments are reported to examine the hypothesis that the VPD response was a result of low hydraulic conductance in leaves as compared to two other soybean genotypes. Results are reported from experiments to measure transpiration response to VPD when xylem water potential was maintained at zero, leaf rehydration response and leaf carbon assimilation response to petiole cutting. Major interspecific differences in leaf hydraulic properties were observed. The observed low leaf hydraulic conductance in PI 416937 is consistent with an increased water use efficiency, and an increased water conservation by limiting transpiration rates under high evaporative conditions but allowing normal gas exchange rates under more moderate evaporative conditions.

Cellular mechanisms of potassium transport in plants

Abstract: Potassium (K(+)) is the most abundant ion in the plant cell and is required for a wide array of functions, ranging from the maintenance of electrical potential gradients across cell membranes, to the generation of turgor, to the activation of numerous enzymes. The majority of these functions depend more or less directly upon the activities and regulation of membrane-bound K(+) transport proteins, operating over a wide range of K(+) concentrations. Here, we review the physiological aspects of potassium transport systems in the plasma membrane, re-examining fundamental problems in the field such as the distinctions between high- and low-affinity transport systems, the interactions between K(+) and other ions such as NH(4)(+) and Na(+), the regulation of cellular K(+) pools, the generation of electrical potentials and the problems involved in measurement of unidirectional K(+) fluxes. We place these discussions in the context of recent discoveries in the molecular biology of K(+) acquisition and produce an overview of gene families encoding K(+) transporters.

Effects of long-term chilling on ultrastructure and antioxidant activity in leaves of two cucumber cultivars under low light.

Abstract: Cucumber (Cucumis sativus L.) cv. Xintaimici (a chilling-resistant cultivar) and cv. Jinyan no. 4 (a chilling-sensitive cultivar) were subjected to two temperatures (15/15 and 25/18°C) under low light (100 μmol m−2 s−1) to understand the relationship between ultrastructural changes and the antioxidant abilities caused by low temperature (15/15°C). We also aimed to find indicators for chilling resistance that could be used on a routine basis in breeding programs of greenhouse crops. At the 15/15°C treatment, the membranes of chloroplast, mitochondrion, ER and plasma were not significantly changed in Xintaimici, whereas they were seriously affected in Jinyan no. 4. This result was consistent with the changes of malonaldehyde in chilling-stressed cucumber leaves. The antioxidant activities were changed under low temperature according to cultivar-expected resistance, relating in part to the described ultrastructural changes. The activities of superoxide dismutase (EC 1.15.1.1) and guaiacol peroxidase (EC 1.11.1.7) increased in chilling-stressed leaves of both cultivars, but the two enzymes were not responsible for the difference between cucumber cultivars. At 15/15°C, contents of GSH and activities of glutathione reductase (GR, EC 1.6.4.2) increased more in leaves of Xintaimici than in those of Jinyan no. 4, while catalase (CAT, EC 1.11.1.6) activities decreased less. GSH, GR and CAT were affected by low temperature and cultivars and correlated with the difference in ultrastructure between chilling-stressed cucumber cultivars. We propose that the three antioxidants might be therefore used as biochemical indicators to screen chilling-resistant cucumber cultivars.

Role of phenylalanine ammonia-lyase in heat pretreatment-induced chilling tolerance in banana fruit.

Abstract: Increasing evidence suggests that phenylalanine ammonia-lyase (PAL, EC 4.3.1.5) is associated with low temperature stress in plant tissues. Banana fruit are highly susceptible to chilling injury. However, little is known about the role of PAL (i.e. gene expression, protein level and activity) in fruit chilling. In this work, the involvement of PAL induced by heat treatment (38 degrees C for 3 days) prior to storage (8 degrees C) in chilling tolerance was investigated. The PAL inhibitor 2-aminoindan-2-phosphonic acid (AIP) was also used to further study the role of PAL in the chilling tolerance. The results showed that mRNA transcripts (MaPAL1 and MaPAL2) and PAL protein levels increased during storage at chilling temperature. Heat treatment prior to storage alleviated chilling injury and enhanced PAL activity, protein amount and MaPAL1 and MaPAL2 transcript levels. The increases in parameters of PAL upon heat pretreatment were all inhibited by AIP treatment, which resulted in aggravation of chilling injury. Thus, these findings indicate that the induction of PAL by heat pretreatment was regulated at both the transcriptional and the translational levels and that PAL may play a role in heat pretreatment-induced chilling tolerance of banana fruit.

The role of activated oxygen species in plant disease resistance

Abstract: One of the earliest responses of plant cells to pathogens is the production of activated oxygen species. In several plant species, this oxidative burst appears to be generated by a signalling pathway similar to that employed by mammalian neutrophils during immune responses. Similar to neutrophils, elicitor receptors in the plant plasma membrane likely activate G proteins and phospholipase C leading to increased intracellular Ca2+ and protein phosphorylation and ultimately, formation of an active NADPH oxidase complex in the plasma membrane. The NADPH oxidase produces O2− which can be rapidly converted to H2O2. Work from a number of laboratories supports the concept that H2O2 plays a central role in mediating diverse aspects of disease resistance. Potential functions of H2O2 include its participation in direct killing of pathogen cells as part of the hypersensitive response, oxidative cross-linking of plant cell wall proteins to render the wall less digestible by microbial enzymes, and as an intracellular signal regulating defense-related gene expression.

Regulation of plant cytosolic glyceraldehyde 3-phosphate dehydrogenase isoforms by thiol modifications.

Abstract: Cytosolic NAD-dependent glyceraldehyde 3-P dehydrogenase (GAPDH; GapC; EC 1.2.1.12) catalyzes the oxidation of triose phosphates during glycolysis in all organisms, but additional functions of the protein has been put forward. Because of its reactive cysteine residue in the active site, it is susceptible to protein modification and oxidation. The addition of GSSG, and much more efficiently of S-nitrosoglutathione, was shown to inactivate the enzymes from Arabidopsis thaliana (isoforms GapC1 and 2), spinach, yeast and rabbit muscle. Inactivation was fully or at least partially reversible upon addition of DTT. The incorporation of glutathione upon formation of a mixed disulfide could be shown using biotinylated glutathione ethyl ester. Furthermore, using the biotin-switch assay, nitrosylated thiol groups could be shown to occur after treatment with nitric oxide donors. Using mass spectrometry and mutant proteins with one cysteine lacking, both cysteines (Cys-155 and Cys-159) were found to occur as glutathionylated and as nitrosylated forms. In preliminary experiments, it was shown that both GapC1 and GapC2 can bind to a partial gene sequence of the NADP-dependent malate dehydrogenase (EC 1.2.1.37; At5g58330). Transiently expressed GapC-green fluorescent protein fusion proteins were localized to the nucleus in A. thaliana protoplasts. As nuclear localization and DNA binding of GAPDH had been shown in numerous systems to occur upon stress, we assume that such mechanism might be part of the signaling pathway to induce increased malate-valve capacity and possibly other protective systems upon overreduction and initial formation of reactive oxygen and nitrogen species as well as to decrease and protect metabolism at the same time by modification of essential cysteine residues.

Relative contribution of AtHAK5 and AtAKT1 to K+ uptake in the high‐affinity range of concentrations

Abstract: The relative contribution of the high-affinity K(+) transporter AtHAK5 and the inward rectifier K(+) channel AtAKT1 to K(+) uptake in the high-affinity range of concentrations was studied in Arabidopsis thaliana ecotype Columbia (Col-0). The results obtained with wild-type lines, with T-DNA insertion in both genes and specific uptake inhibitors, show that AtHAK5 and AtAKT1 mediate the NH4+-sensitive and the Ba(2+)-sensitive components of uptake, respectively, and that they are the two major contributors to uptake in the high-affinity range of Rb(+) concentrations. Using Rb(+) as a K(+) analogue, it was shown that AtHAK5 mediates absorption at lower Rb(+) concentrations than AtAKT1 and depletes external Rb(+) to values around 1 muM. Factors such as the presence of K(+) or NH4+ during plant growth determine the relative contribution of each system. The presence of NH4+ in the growth solution inhibits the induction of AtHAK5 by K(+) starvation. In K(+)-starved plants grown without NH4+, both systems are operative, but when NH4+ is present in the growth solution, AtAKT1 is probably the only system mediating Rb(+) absorption, and the capacity of the roots to deplete Rb(+) is reduced.

Redox signal integration: from stimulus to networks and genes

Abstract: Recent research has established redox-dependent thiol modification of proteins as a major regulatory layer superimposed on most cell functional categories in plants. Modern proteomics and forward as well as reverse genetics approaches have enabled the identification of a high number of novel targets of redox regulation. Redox-controlled processes range from metabolism to transport, transcription and translation. Gene activity regulation by transcription factors such as TGA, Athb-9 and RAP2 directly or indirectly is controlled by the redox state. Knowledge on putative redox sensors such as the peroxiredoxins, on redox transmitters including thioredoxins and glutaredoxins and biochemical mechanisms of their linkage to the metabolic redox environment has emerged as the framework of a functional redox regulatory network. Its basic principle is similar in eukaryotic cells and particularly complex in the photosynthesizing chloroplast. Methods and knowledge are now at hand to develop a quantitative understanding of redox signalling and the redox regulatory network in the eukaryotic cell.

Metabolism of salicylic acid in wild-type, ugt74f1 and ugt74f2 glucosyltransferase mutants of Arabidopsis thaliana.

Abstract: Arabidopsis thaliana contains two salicylic acid (SA) glucosyltransferase enzymes designated UGT74F1 and UGT74F2. UGT74F1 forms only SA 2-O-beta-D-glucose (SAG), while UGT74F2 forms both SAG and the SA glucose ester (SGE). In an attempt to determine the in vivo role of each SA glucosyltransferase (SAGT), the metabolism of SA in ugt74f1 and ugt74f2 mutants was examined and compared with that of the wild-type. The three major metabolites formed in wild-type Arabidopsis included SAG, SGE, and 2,5-dihydroxbenzoic acid 2-O-beta-D-glucose (DHB2G). This is the first description of DHB2G as a major metabolite of SA in plants. The major metabolites of SA formed in ugt74f1 mutants were SGE, SAG and 2,5-dihydroxybenzoic acid 5-O-beta-D-glucose (DHB5G). DHB5G was not formed in the wild-type plants. SAG and DHB2G were the main metabolites of SA in ugt74f2 mutants. The ugt74f2 mutant was unable to form SGE. Only SGE could be detected during in vitro SAGT assays of untreated wild-type and ugt74f1 mutants. This activity was because of constitutive UGT74F2 activity. Both SGE and SAG could be formed during in vitro assays of SA-pretreated wild-type and ugt74f1 leaves. Neither SAG nor SGE could be detected during the in vitro SAGT assays of untreated ugt74f2 leaves. Only SAG was formed during the in vitro SAGT assays of SA-pretreated ugt74f2 leaves. The SAG formation was a result of the UGT74F1 activity. This work demonstrates that changes in the activity of either SAGT enzyme can have a dramatic effect on the metabolism of exogenously supplied SA in Arabidopsis.

Inorganic carbon acquisition in potentially toxic and non‐toxic diatoms: the effect of pH‐induced changes in seawater carbonate chemistry

Abstract: The effects of pH-induced changes in seawater carbonate chemistry on inorganic carbon (C(i)) acquisition and domoic acid (DA) production were studied in two potentially toxic diatom species, Pseudo-nitzschia multiseries and Nitzschia navis-varingica, and the non-toxic Stellarima stellaris. In vivo activities of carbonic anhydrase (CA), photosynthetic O(2) evolution and CO(2) and HCO(3)(-) uptake rates were measured by membrane inlet MS in cells acclimated to low (7.9) and high pH (8.4 or 8.9). Species-specific differences in the mode of carbon acquisition were found. While extracellular carbonic anhydrase (eCA) activities increased with pH in P. multiseries and S. stellaris, N. navis-varingica exhibited low eCA activities independent of pH. Half-saturation concentrations (K(1/2)) for photosynthetic O(2) evolution, which were highest in S. stellaris and lowest in P. multiseries, generally decreased with increasing pH. In terms of carbon source, all species took up both CO(2) and HCO(3)(-). K(1/2) values for inorganic carbon uptake decreased with increasing pH in two species, while in N. navis-varingica apparent affinities did not change. While the contribution of HCO(3)(-) to net fixation was more than 85% in S. stellaris, it was about 55% in P. multiseries and only approximately 30% in N. navis-varingica. The intracellular content of DA increased in P. multiseries and N. navis-varingica with increasing pH. Based on our data, we propose a novel role for eCA acting as C(i)-recycling mechanism. With regard to pH-dependence of growth, the 'HCO(3)(-) user' S. stellaris was as sensitive as the 'CO(2) user' N. navis-varingica. The suggested relationship between DA and carbon acquisition/C(i) limitation could not be confirmed.

Sulfur starvation in rice: the effect on photosynthesis, carbohydrate metabolism, and oxidative stress protective pathways.

Abstract: Sulfur-deficient plants generate a lower yield and have a reduced nutritional value. The process of sulfur acquisition and assimilation play an integral role in plant metabolism, and response to sulfur deficiency involves a large number of plant constituents. Rice (Oryza sativa) is the second most consumed cereal grain, and the effects of sulfur deprivation in rice were analyzed by measuring changes in photosynthesis, carbohydrate metabolism, and antioxidants. The photosynthetic apparatus was severely affected under sulfur deficiency. The Chl content was reduced by 49% because of a general reduction of PSI I and PSI and the associated light-harvesting antenna. The PSI I efficiency was 31% lower at growth light, and the ability of PSI to photoreduce NADP + was decreased by 61%. The Rubisco content was also significantly reduced in the sulfur-deprived plants. The imbalances between PSII and PSI, and between photosynthesis and carbon fixation led to a general over-reduction of the photosynthetic electron carriers (higher 1 - qp). Chromatographic analysis showed that the level of monosaccharides was lower and starch content higher in the sulfur-deprived plants. In contrast, no changes in metabolite levels were found in the tricarboxylic acid or Calvin cycle. The level ofthethiol-containingantioxidant, GSH, was 70% lower and the redox state was significantly more oxidized. These changes in GSH status led to an upregulation of the cytosolic isoforms of GSH reductase and monodehydroascorbate reductase. In addition, alternative antioxidants like flavonoids and anthocyanins were increased in the sulfur-deprived plants.

Adjustments of water use efficiency by stomatal regulation during drought and recovery in the drought-adapted Vitis hybrid Richter-110 (V. berlandieri x V. rupestris).

Abstract: The hybrid Richter-110 (Vitis berlandieri x Vitis rupestris) (R-110) has the reputation of being a genotype strongly adapted to drought. A study was performed with plants of R-110 subjected to water withholding followed by re-watering. The goal was to analyze how stomatal conductance (g(s)) is regulated with respect to different physiological variables under water stress and recovery, as well as how water stress affects adjustments of water use efficiency (WUE) at the leaf level. Water stress induced a substantial stomatal closure and an increase in WUE, which persisted many days after re-watering. The g(s) during water stress was mainly related to the content of ABA in the xylem and partly related to plant hydraulic conductivity but not to leaf water potential. By contrast, low g(s) during re-watering did not correlate with ABA contents and was only related to a sustained decreased hydraulic conductivity. In addition to a complex physiological regulation of stomatal closure, g(s) and rate of transpiration (E) were strongly affected by leaf-to-air vapor pressure deficit (VPD) in a way dependent of the treatment. Interestingly, E increased with increasing VPD in control plants, but decreased with increasing VPD in severely stressed plants. All together, the fine stomatal regulation in R-110 resulted in very high WUE at the leaf level. This genotype is revealed to be very interesting for further studies on the physiological mechanisms leading to regulation of stomatal responsiveness and WUE in response to drought.

Impact of climate change on crop nutrient and water use efficiencies.

Abstract: Implicit in discussions of plant nutrition and climate change is the assumption that we know what to do relative to nutrient management here and now but that these strategies might not apply in a changed climate. We review existing knowledge on interactive influences of atmospheric carbon dioxide concentration, temperature and soil moisture on plant growth, development and yield as well as on plant water use efficiency (WUE) and physiological and uptake efficiencies of soil-immobile nutrients. Elevated atmospheric CO 2 will increase leaf and canopy photosynthesis, especially in C3 plants, with minor changes in dark respiration. Additional CO 2 will increase biomass without marked alteration in dry matter partitioning, reduce transpiration of most plants and improve WUE. However, spatiotemporal variation in these attributes will impact agronomic performance and crop water use in a site-specific manner. Nutrient acquisition is closely associated with overall biomass and strongly influenced by root surface area. When climate change alters soil factors to restrict root growth, nutrient stress will occur. Plant size may also change but nutrient concentration will remain relatively unchanged; therefore, nutrient removal will scalewith growth. Changes in regional nutrient requirements will be most remarkable where we alter cropping systems to accommodate shifts in ecozones or alter farming systems to capture new uses from existing systems. For regions and systems where we currently do an adequate job managing nutrients, we stand a good chance of continued optimization under a changed climate. If we can and should do better, climate change will not help us.

How microspores transform into haploid embryos: changes associated with embryogenesis induction and microspore-derived embryogenesis.

Abstract: Microspore embryogenesis is the most powerful androgenic pathway to produce haploid and doubled haploid plants. To deviate a microspore toward embryogenesis, a number of factors, different for each species, must concur at the same time and place. Once induced, the microspore undergoes numerous changes at different levels, from overall morphology to gene expression. Induction of microspore embryogenesis not only implies the expression of an embryogenic program, but also a stress-related cellular response and a repression of the gametophytic program to revert the microspore to a totipotent status. In this review, we compile the most recent advances in the understanding of the changes undergone by the induced microspore to readapt to the new developmental scenario. We devote special attention to the efforts made to uncover changes in the transcriptome of the induced microspore and microspore-derived embryo (MDE). Finally, we discuss the influence that an in vitro environment exerts over the MDE, as compared with its zygotic counterpart.