Photosynthesis in Salt-Adapted Heterotrophic Tobacco Cells and Regenerated Plants

The physiological response of different tobacco varieties to chilling stress during the vigorous growing period

Tobacco is be sensitively affected by chilling injury in the vigorous growth period, which can easily lead to tobacco leaf browning during flue-curing and quality loss, however, the physiological response of tobacco in the prosperous period under low temperature stress is unclear. The physiological response parameters of two tobacco varieties to low temperature stress were determined. The main results were as follows: ① For tobacco in the vigorous growing period subjected to low-temperature stress at 4-16 °C, the tissue structure of chloroplast changed and photosynthetic pigments significantly decreased compared with each control with the increase of intensity of low-temperature stress. ② For tobacco in the vigorous growing period at 10-16 °C, antioxidant capacity of the protective enzyme system, osmotic adjustment capacity of the osmotic adjusting system and polyphenol metabolism in plants gradually increased due to induction of low temperature with the increase of intensity of low-temperature stress. ③ Under low-temperature stress at 4 °C, the protective enzyme system, osmotic adjusting system and polyphenol metabolism of the plants played an insignificant role in stress tolerance, which cannot be constantly enhanced based on low-temperature resistance at 10 °C. This study confirmed that under the temperature stress of 10-16 °C, the self-regulation ability of tobacco will be enhanced with the deepening of low temperature stress, but there is a critical temperature between 4 and 10 °C. The self-regulation ability of plants under low temperature stress will be inhibited.

Effects of Salinity Stress on Chloroplast Structure and Function

Salinity is a growing problem affecting soils and agriculture in many parts of the world. The presence of salt in plant cells disrupts many basic metabolic processes, contributing to severe negative effects on plant development and growth. This review focuses on the effects of salinity on chloroplasts, including the structures and function of these organelles. Chloroplasts house various important biochemical reactions, including photosynthesis, most of which are considered essential for plant survival. Salinity can affect these reactions in a number of ways, for example, by changing the chloroplast size, number, lamellar organization, lipid and starch accumulation, and interfering with cross-membrane transportation. Research has shown that maintenance of the normal chloroplast physiology is necessary for the survival of the entire plant. Many plant species have evolved different mechanisms to withstand the harmful effects of salt-induced toxicity on their chloroplasts and its machinery. The differences depend on the plant species and growth stage and can be quite different between salt-sensitive (glycophyte) and salt-tolerant (halophyte) plants. Salt stress tolerance is a complex trait, and many aspects of salt tolerance in plants are not entirely clear yet. In this review, we discuss the different mechanisms of salt stress tolerance in plants with a special focus on chloroplast structure and its functions, including the underlying differences between glycophytes and halophytes.

Detecting Salt Tolerance in Doubled Haploid Wheat Lines

Improving salt tolerance of genotypes requires a source of genetic variation and multiple accurate selection criteria for discriminating their salt tolerance. A combination of morpho-physiological and biochemical parameters and multivariate analysis was used to detect salt tolerance variation in 15 wheat lines developed by doubled haploid (DHL) technique. They were then compared with the salt-tolerant check cultivar Sakha 93. Salinity stress was investigated at three salinity levels (0, 100, and 200 mM NaCl) for 25 days. Considerable genetic variation was observed for all traits, as was high heritability (>60%) and genetic gain (>20%). Principal component analysis indicated the ability of nine traits (root number, root length, root dry weight, shoot length, shoot dry weight, specific root length, relative water content, membrane stability index, and catalase) to identify differences in salinity tolerance among lines. Three traits (shoot length, shoot dry weight, and catalase) were indicative of salt-tolerance, indicating their importance in improving and evaluating salt tolerant genotypes for breeding programs. The salinity tolerance membership index based on these three traits classified one new line (DHL21) and the check cultivar (Sakha 93) as highly salt-tolerant, DHL25, DHL26, DHL2, DHL11, and DHL5 as tolerant, and DHL23 and DHL12 as intermediate. Discriminant function analysis and MANOVA suggested differences among the five groups of tolerance. Among the donor genotypes, Sakha 93 remained the donor of choice for improving salinity tolerance during the seedling stage. The tolerated lines (DHL21, DHL25, DHL26, DHL2, DHL11, and DHL5) could be also recommended as useful and novel genetic resources for improving salinity tolerance of wheat in breeding programs.

Metabolic Adjustment of Arabidopsis Root Suspension Cells During Adaptation to Salt Stress and Mitotic Stress Memory

Sessile plants reprogram their metabolic and developmental processes during adaptation to prolonged environmental stresses. To understand the molecular mechanisms underlying adaptation of plant cells to saline stress, we established callus suspension cell cultures from Arabidopsis roots adapted to high salt for an extended period of time. Adapted cells exhibit enhanced salt tolerance compared with control cells. Moreover, acquired salt tolerance is maintained even after the stress is relieved, indicating the existence of a memory of acquired salt tolerance during mitotic cell divisions, known as mitotic stress memory. Metabolite profiling using 1H-nuclear magnetic resonance (NMR) spectroscopy revealed metabolic discrimination between control, salt-adapted and stress-memory cells. Compared with control cells, salt-adapted cells accumulated higher levels of sugars, amino acids and intermediary metabolites in the shikimate pathway, such as coniferin. Moreover, adapted cells acquired thicker cell walls with higher lignin contents, suggesting the importance of adjustments of physical properties during adaptation to elevated saline conditions. When stress-memory cells were reverted to normal growth conditions, the levels of metabolites again readjusted. Whereas most of the metabolic changes reverted to levels intermediate between salt-adapted and control cells, the amounts of sugars, alanine, γ-aminobutyric acid and acetate further increased in stress-memory cells, supporting a view of their roles in mitotic stress memory. Our results provide insights into the metabolic adjustment of plant root cells during adaptation to saline conditions as well as pointing to the function of mitotic memory in acquired salt tolerance.

Three-dimensional ultrastructure of chloroplast pockets formed under salinity stress

We investigated the invagination structure of a chloroplast that surrounds organelles such as mitochondria and peroxisomes within a thin layer of chloroplast stroma, which is called a chloroplast pocket. In this study, chloroplast pockets were observed in rice plants subjected to salinity stress but not under moderate growth condition. They included cytosol, transparent structure, lipid bodies, mitochondria, and peroxisomes. We constructed the three-dimensional architecture of chloroplast pockets by using serial images obtained by transmission electron microscopy and focused ion beam-scanning electron microscopy. Three types of chloroplast pockets were observed by transmission electron microscopy: Organelles were completely enclosed in a chloroplast pocket (enclosed type), a chloroplast pocket with a small gap in the middle part (gap type), and a chloroplast pocket with one side open (open type). Of the 70 pockets observed by serial imaging, 35 were enclosed type, and 21 and 14 were gap and open types, respectively. Mitochondria and peroxisomes were often in contact with the chloroplast pockets. Focused ion beam-scanning electron microscopy revealed chloroplasts with a sheet structure partially surrounding peroxisomes. This fact suggests that chloroplasts might construct large sheet structures that would be related to the formation of chloroplast pockets.

Effects of catalase on chloroplast arrangement in Opuntia streptacantha chlorenchyma cells under salt stress

In arid and semiarid regions, low precipitation rates lead to soil salinity problems, which may limit plant establishment, growth, and survival. Herein, we investigated the NaCl stress effect on chlorophyll fluorescence, photosynthetic-pigments, movement and chloroplasts ultrastructure in chlorenchyma cells of Opuntia streptacantha cladodes. Cladodes segments were exposed to salt stress at 0, 100, 200, and 300 mM NaCl for 8, 16, and 24 h. The results showed that salt stress reduced chlorophyll content, F _v /F _m , ΦPSII, and qP values. Under the highest salt stress treatments, the chloroplasts were densely clumped toward the cell center and thylakoid membranes were notably affected. We analyzed the effect of exogenous catalase in salt-stressed cladode segments during 8, 16, and 24 h. The catalase application to salt-stressed cladodes counteracted the NaCl adverse effects, increasing the chlorophyll fluorescence parameters, photosynthetic-pigments, and avoided chloroplast clustering. Our results indicate that salt stress triggered the chloroplast clumping and affected the photosynthesis in O. streptacantha chlorenchyma cells. The exogenous catalase reverted the H₂O₂ accumulation and clustering of chloroplast, which led to an improvement of the photosynthetic efficiency. These data suggest that H₂O₂ detoxification by catalase is important to protect the chloroplast, thus conserving the photosynthetic activity in O. streptacantha under stress.

Understanding abiotic stress tolerance mechanisms in soybean: a comparative evaluation of soybean response to drought and flooding stress

Many sources of drought and flooding tolerance have been identified in soybean, however underlying molecular and physiological mechanisms are poorly understood. Therefore, it is important to illuminate different plant responses to these abiotic stresses and understand the mechanisms that confer tolerance. Towards this goal we used four contrasting soybean (Glycine max) genotypes (PI 567690--drought tolerant, Pana--drought susceptible, PI 408105A--flooding tolerant, S99-2281--flooding susceptible) grown under greenhouse conditions and compared genotypic responses to drought and flooding at the physiological, biochemical, and cellular level. We also quantified these variations and tried to infer their role in drought and flooding tolerance in soybean. Our results revealed that different mechanisms contribute to reduction in net photosynthesis under drought and flooding stress. Under drought stress, ABA and stomatal conductance are responsible for reduced photosynthetic rate; while under flooding stress, accumulation of starch granules played a major role. Drought tolerant genotypes PI 567690 and PI 408105A had higher plastoglobule numbers than the susceptible Pana and S99-2281. Drought stress increased the number and size of plastoglobules in most of the genotypes pointing to a possible role in stress tolerance. Interestingly, there were seven fibrillin proteins localized within the plastoglobules that were up-regulated in the drought and flooding tolerant genotypes PI 567690 and PI 408105A, respectively, but down-regulated in the drought susceptible genotype Pana. These results suggest a potential role of Fibrillin proteins, FBN1a, 1b and 7a in soybean response to drought and flooding stress.

Morphological and physiological responses of cotton (Gossypium hirsutum L.) plants to salinity

Salinization usually plays a primary role in soil degradation, which consequently reduces agricultural productivity. In this study, the effects of salinity on growth parameters, ion, chlorophyll, and proline content, photosynthesis, antioxidant enzyme activities, and lipid peroxidation of two cotton cultivars, [CCRI-79 (salt tolerant) and Simian 3 (salt sensitive)], were evaluated. Salinity was investigated at 0 mM, 80 mM, 160 mM, and 240 mM NaCl for 7 days. Salinity induced morphological and physiological changes, including a reduction in the dry weight of leaves and roots, root length, root volume, average root diameter, chlorophyll and proline contents, net photosynthesis and stomatal conductance. In addition, salinity caused ion imbalance in plants as shown by higher Na+ and Cl- contents and lower K+, Ca2+, and Mg2+ concentrations. Ion imbalance was more pronounced in CCRI-79 than in Simian3. In the leaves and roots of the salt-tolerant cultivar CCRI-79, increasing levels of salinity increased the activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), and glutathione reductase (GR), but reduced catalase (CAT) activity. The activities of SOD, CAT, APX, and GR in the leaves and roots of CCRI-79 were higher than those in Simian 3. CAT and APX showed the greatest H2O2 scavenging activity in both leaves and roots. Moreover, CAT and APX activities in conjunction with SOD seem to play an essential protective role in the scavenging process. These results indicate that CCRI-79 has a more effective protection mechanism and mitigated oxidative stress and lipid peroxidation by maintaining higher antioxidant activities than those in Simian 3. Overall, the chlorophyll a, chlorophyll b, and Chl (a+b) contents, net photosynthetic rate and stomatal conductance, SOD, CAT, APX, and GR activities showed the most significant variation between the two cotton cultivars.

Ultrastructure of Cosmarium strains (Zygnematophyceae, Streptophyta) collected from various geographic locations shows species-specific differences both at optimal and stress temperatures

Plant species collected from various climatic zones and stressed in vitro at various temperatures reveal changes in cellular ultrastructure which are in accordance with the climate at their sampling sites. This observation initiated the investigation to establish if stress at different temperatures may cause diverse extents of changes in the ultrastructure of microalgal strains originating from different geographic zones. The study revealed that the six Cosmarium strains demonstrated ultrastructural characteristics that were consistent with their source location under optimal, low and high temperature conditions, pointing to their preference to specific climatic niches. Interestingly, chloroplasts of all of the Cosmarium strains correspond to a sun-adapted type, which is concomitant with earlier statements that these strains are rendered as high-light adapted algae. The Cosmarium strains developed multiple ultrastructural responses which enabled them to cope with excessive temperatures, occasionally occurring in desmid natural habitats. The appearance of cubic membranes and increased number of plastoglobules may represent the first line in protection against high-temperature stress, which is accompanied by the alteration of protein synthesis and the appearance of stress granules in order to preserve cell homeostasis. However, the prolonged warm- or cold-temperature stress obviously initiated the programmed cell death, as concluded from the appearance of several ultrastructural features observed in all of the Cosmarium strains. The fair acclimation possibilities and the ability to undergo programmed cell death in order to save the population, certainly favor the cosmopolitan distribution of the genus Cosmarium.

A mechanism implicating plastoglobules in thylakoid disassembly during senescence and nitrogen starvation

Plastoglobules are lipid droplets present in all plastid types. In chloroplasts, they are connected to the thylakoid membrane by the outer lipid half-bilayer. The plastoglobule core is composed of neutral lipids most prominently the prenylquinones, triacylglycerols, fatty acid phytyl esters but likely also unknown compounds. During stress and various developmental stages such as senescence, plastoglobule size and number increase due to the accumulation of lipids. However, their role is not limited to lipid storage. Indeed, the characterization of the plastoglobule proteome revealed the presence of enzymes. Importantly it has been demonstrated that these participate in isoprenoid lipid metabolic pathways at the plastoglobule, notably in the metabolism of prenylquinones. Recently, the characterization of two phytyl ester synthases has established a firm metabolic link between PG enzymatic activity and thylakoid disassembly during chloroplast senescence and nitrogen starvation.

When proteomics reveals unsuspected roles: the plastoglobule example

Plastoglobules are globular compartments found in plastids. Before initial proteomic studies were published, these particles were often viewed as passive lipid droplets whose unique role was to store lipids coming from the thylakoid turn-over, or to accumulate carotenoids in the chromoplasts. Yet, two proteomic studies, published concomitantly, suggested for the first time that plastoglobules are more than "junk cupboards" for lipids. Indeed, both studies demonstrated that plastoglobules do not only include structural proteins belonging to the plastoglobulin/fibrillin family, but also contain active enzymes. The specific plastoglobule localization of these enzymes has been confirmed by different approaches such as immunogold localization and GFP protein fusions, thus providing evidence that plastoglobules actively participate in diverse pathways of plastid metabolism. These proteomic studies have been the basis for numerous recent works investigating plastoglobule function. However, a lot still needs to be discovered about the molecular composition and the role of plastoglobules. In this chapter, we will describe how the proteomic approaches have launched new perspectives on plastoglobule functions.

Genotypic variability among soybean genotypes under NaCl stress and proteome analysis of salt-tolerant genotype

The present investigation was conducted to evaluate salt tolerance in ten genotypes of soybean (Glycine max L.). Twelve-day-old seedlings, grown hydroponically, were treated with 0, 25, 50, 75, 100, 125 and 150 mM NaCl for 10 days. Growth, lipid peroxidation and antioxidant enzyme activities were evaluated. Growth, measured in terms of length, fresh weight and dry weight of plants, was drastically reduced in Pusa-24 while there was little effect of NaCl treatment on Pusa-37 genotype of soybean. High level of lipid peroxidation was observed in Pusa-24 as indicated by increased level of malondialdehyde. Activities of superoxide dismutase, catalase, ascorbate peroxidase and glutathione reductase were maximum in Pusa-37 where 9-, 1-, 5- and 6-fold increase over control were observed, respectively. The results suggested that Pusa-24 and Pusa-37 are salt-sensitive and salt-tolerant genotype of soybean, respectively, and antioxidant defence system is involved in conferring the sensitiveness and tolerance in these genotypes. Salt-tolerant genotype Pusa-37, was further analysed by 2-dimensional gel electrophoresis to analyse the differential expression of proteins at high salt stress. In the present study, 173 protein spots were identified. Of these, 40 proteins were responsive to salinity in that they were either up- or downregulated. This study could help us in identifying the possible regulatory switches (gene/s) controlling novel proteins of the salt-tolerant genotype of the crop plants and their possible role in defence mechanism.

Plastid localization of the key carotenoid enzyme phytoene synthase is altered by isozyme, allelic variation, and activity

Plant carotenoids have unique physiological roles related to specific plastid suborganellar locations. Carotenoid metabolic engineering could enhance plant adaptation to climate change and improve food security and nutritional value. However, lack of fundamental knowledge on carotenoid pathway localization limits targeted engineering. Phytoene synthase (PSY), a major rate-controlling carotenoid enzyme, is represented by multiple isozymes residing at unknown plastid sites. In maize (Zea mays), the three isozymes were transiently expressed and found either in plastoglobuli or in stroma and thylakoid membranes. PSY1, with one to two residue modifications of naturally occurring functional variants, exhibited altered localization, associated with distorted plastid shape and formation of a fibril phenotype. Mutating the active site of the enzyme reversed this phenotype. Discovery of differential PSY locations, linked with activity and isozyme type, advances the engineering potential for modifying carotenoid biosynthesis.

Salt-induced chloroplast protrusion is the process of exclusion of ribulose-1,5-bisphosphate carboxylase/oxygenase from chloroplasts into cytoplasm in leaves of rice

Chloroplast protrusions (CPs) are often observed under environmental stresses, but their role has not been elucidated. The formation of CPs was observed in the leaf of rice plants treated with 75 mm NaCl for 14 d. Some CPs were almost separated from the main chloroplast body. In some CPs, inner membrane structures and crystalline inclusions were included. Similar structures surrounded by double membranes were observed in the cytoplasm and vacuole. Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) was detected in CPs and the similar structures in the cytoplasm and vacuole. These results suggest that CP is one of the pathways of Rubisco exclusion from chloroplasts into the cytoplasm under salinity, and the exclusions could be transported to vacuole for their degradation.

Plastid lipid droplets at the crossroads of prenylquinone metabolism

Lipid droplets called plastoglobules (PGs) exist in most plant tissues and plastid types. In chloroplasts, the polar lipid monolayer surrounding these low-density lipoprotein particles is continuous with the outer lipid leaflet of the thylakoid membrane. Often small clusters of two or three PGs, only one of them directly connected to thylakoids, are present. Structural proteins (known as plastid-lipid associated proteins/fibrillins or plastoglobulins) together with lipid metabolic enzymes coat the PGs. The hydrophobic core of PGs contains a range of neutral lipids including the prenylquinones [tocopherols (vitamin E), phylloquinone (vitamin K(1)), and plastoquinone (PQ-9)]. In this review the function of PGs and their associated enzymes in prenylquinone metabolism will be discussed.

Sesuvium portulacastrum maintains adequate gas exchange, pigment composition, and thylakoid proteins under moderate and high salinity

Cuttings of Sesuvium portulacastrum L. (Aizoaceae) were taken from plants cultivated under severe saline conditions. The obtained seedlings were grown on sand and irrigated with nutrient solution over 5 weeks under no (0 mM NaCl), moderate (200 mM NaCl), or high (400 mM NaCl) salinity conditions. A follow-up of gas exchange was performed weekly and pigment levels and patterns of fully expanded leaves were determined after 3 and 5 weeks of treatment. At the end of the 5-week period, immunoblot analysis of the main polypeptides of photosystem I and II was performed with the aim to investigate salt-induced variations in photosystem composition. Net CO2 assimilation rate (Pn) increased under salinity up to 3 weeks of treatment then decreased to reach the value of 0mM-treated plants at the end of the experiment. For stomatal conductance (gs) and intercellular CO2 concentration (Ci), the opposite occurred. These results were concomitant with an increase in practically all pigment levels, mainly under high salinity, with the exception of zeaxanthin. The de-epoxidation index (DEPS index) was much lower under saline than non-saline conditions in the 3rd week, indicating light stress in 0mM-treated plants. At the end of the experiment, this index showed much lower values with no significant differences between treatments, which coincided with no significant differences in gas exchange as well. Protein amounts of D1, CP47, and CP43 did not show noticeable variations with salt treatment, whereas LHCII underwent a slight but significant decrease (-15%) at the highest NaCl concentration. LHCI polypeptides were unaffected by the salt treatments, where conversely, the highest concentration induced a significant decrease in PsaA/B amount (-18%).

Moderate heat stress of Arabidopsis thaliana leaves causes chloroplast swelling and plastoglobule formation

Photosynthesis is inhibited by heat stress. This inhibition is rapidly reversible when heat stress is moderate but irreversible at higher temperature. Absorbance changes can be used to detect a variety of biophysical parameters in intact leaves. We found that moderate heat stress caused a large reduction of the apparent absorbance of green light in light-adapted, intact Arabidopsis thaliana leaves. Three mechanisms that can affect green light absorbance of leaves, namely, zeaxanthin accumulation (absorbance peak at 505 nm), the electrochromic shift (ECS) of carotenoid absorption spectra (peak at 518 nm), and light scattering (peak at 535 nm) were investigated. The change of green light absorbance caused by heat treatment was not caused by changes of zeaxanthin content nor by the ECS. The formation of non-photochemical quenching (NPQ), chloroplast movements, and chloroplast swelling and shrinkage can all affect light scattering inside leaves. The formation of NPQ under high temperature was not well correlated with the heat-induced absorbance change, and light microscopy revealed no appreciable changes of chloroplast location because of heat treatment. Transmission electron microscopy results showed swollen chloroplasts and increased number of plastoglobules in heat-treated leaves, indicating that the structural changes of chloroplasts and thylakoids are significant results of moderate heat stress and may explain the reduced apparent absorbance of green light under moderately high temperature.

Cytological insights into the desiccation biology of a model system: moss protonemata

*Set out here is the first generic account of the cytological effects of dehydration and rehydration and exogenous abscisic acid on moss protonemata. *Protonemal cells were subjected to slow and fast drying regimes, with and without prior exposure to abscisic acid. The cytological changes associated with de- and rehydration were analysed by light, fluorescence and transmission electron microscopy, together with pharmacological studies. *Protonemata survive slow but not fast drying, unless pretreated with abscisic acid. Dehydration elicits profound cytological changes, namely vacuolar fragmentation, reorganization of the endomembrane domains, changes in the thickness of the cell wall and in the morphology of plastids and mitochondria, and the controlled dismantling of the cytoskeleton; these dynamic events are prevented by fast drying. In control cells, abscisic acid elicits changes that partially mimic those associated with slow drying, including controlled disassembly of cytoskeletal elements, thus enabling protonemal cells to survive normally lethal rates of water loss. *Our demonstration that moss protonemata are an ideal system for visualizing and manipulating the cytological events associated with vegetative desiccation tolerance in land plants now opens up the way for genomic dissection of the underlying mechanisms.

Cytosolic and plastoglobule-targeted carotenoid dioxygenases from Crocus sativus are both involved in beta-ionone release

Saffron, the processed stigma of Crocus sativus, is characterized by the presence of several apocarotenoids that contribute to the color, flavor, and aroma of the spice. However, little is known about the synthesis of aroma compounds during the development of the C. sativus stigma. The developing stigma is nearly odorless, but before and at anthesis, the aromatic compound beta-ionone becomes the principal norisoprenoid volatile in the stigma. In this study, four carotenoid cleavage dioxygenase (CCD) genes, CsCCD1a, CsCCD1b, CsCCD4a, and CsCCD4b, were isolated from C. sativus. Expression analysis showed that CsCCD1a was constitutively expressed, CsCCD1b was unique to the stigma tissue, but only CsCCD4a and -b had expression patterns consistent with the highest levels of beta-carotene and emission of beta-ionone derived during the stigma development. The CsCCD4 enzymes were localized in plastids and more specifically were present in the plastoglobules. The enzymatic activities of CsCCD1a, CsCCD1b, and CsCCD4 enzymes were determined by Escherichia coli expression, and subsequent analysis of the volatile products was generated by GC/MS. The four CCDs fell in two phylogenetically divergent dioxygenase classes, but all could cleave beta-carotene at the 9,10(9',10') positions to yield beta-ionone. The data obtained suggest that all four C. sativus CCD enzymes may contribute in different ways to the production of beta-ionone. In addition, the location and precise timing of beta-ionone synthesis, together with its known activity as a fragrance and insect attractant, suggest that this volatile may have a role in Crocus pollination.

Tocotrienols, the unsaturated forms of vitamin E, can function as antioxidants and lipid protectors in tobacco leaves

Vitamin E is a generic term for a group of lipid-soluble antioxidant compounds, the tocopherols and tocotrienols. While tocotrienols are considered as important vitamin E components in humans, with functions in health and disease, the protective functions of tocotrienols have never been investigated in plants, contrary to tocopherols. We took advantage of the strong accumulation of tocotrienols in leaves of double transgenic tobacco (Nicotiana tabacum) plants that coexpressed the yeast (Saccharomyces cerevisiae) prephenate dehydrogenase gene (PDH) and the Arabidopsis (Arabidopsis thaliana) hydroxyphenylpyruvate dioxygenase gene (HPPD) to study the antioxidant function of those compounds in vivo. In young leaves of wild-type and transgenic tobacco plants, the majority of vitamin E was stored in thylakoid membranes, while plastoglobules contained mainly delta-tocopherol, a very minor component of vitamin E in tobacco. However, the vitamin E composition of plastoglobules was observed to change substantially during leaf aging, with alpha-tocopherol becoming the major form. Tocotrienol accumulation in young transgenic HPPD-PDH leaves occurred without any significant perturbation of photosynthetic electron transport. Tocotrienols noticeably reinforced the tolerance of HPPD-PDH leaves to high light stress at chilling temperature, with photosystem II photoinhibition and lipid peroxidation being maintained at low levels relative to wild-type leaves. Very young leaves of wild-type tobacco plants turned yellow during chilling stress, because of the strongly reduced levels of chlorophylls and carotenoids, and this phenomenon was attenuated in transgenic HPPD-PDH plants. While sugars accumulated similarly in young wild-type and HPPD-PDH leaves exposed to chilling stress in high light, a substantial decrease in tocotrienols was observed in the transgenic leaves only, suggesting vitamin E consumption during oxygen radical scavenging. Our results demonstrate that tocotrienols can function in vivo as efficient antioxidants protecting membrane lipids from peroxidation.

Plastoglobule lipid bodies: their functions in chloroplasts and their potential for applications

Plastoglobules are plant lipid bodies localized inside plastids. They have long been considered as mere lipid storage compartments. However, ultrastructural and proteomic data now suggest their involvement in various metabolic pathways, notably the biosynthesis of tocopherols. In this work, the current knowledge on the structure and functions of plastoglobules is reviewed. On the basis of similarities between plastoglobules and seed oleosomes, the potential of plastoglobules for bioengineering applications is discussed.

Tocopherol cyclase (VTE1) localization and vitamin E accumulation in chloroplast plastoglobule lipoprotein particles

Chloroplasts contain lipoprotein particles termed plastoglobules. Plastoglobules are generally believed to have little function beyond lipid storage. Here we report on the identification of plastoglobule proteins using mass spectrometry methods in Arabidopsis thaliana. We demonstrate specific plastoglobule association of members of the plastid lipid-associated proteins/fibrillin family as well as known metabolic enzymes, including the tocopherol cyclase (VTE1), a key enzyme of tocopherol (vitamin E) synthesis. Moreover, comparative analysis of chloroplast membrane fractions shows that plastoglobules are a site of vitamin E accumulation in chloroplasts. Thus, in addition to their lipid storage function, we propose that plastoglobules are metabolically active, taking part in tocopherol synthesis and likely other pathways.

Chlorophyll accumulation is enhanced by osmotic stress in graminaceous chlorophyllic cells

We have developed a new chlorophyllic cell line ('TADH-XO') from the highly water stress tolerant grass Bouteloua gracilis (blue grama). When grown under normal (non-stress) conditions, this new cell line accumulates higher levels of chlorophyll (up to 368.1 microg total chlorophyll g(-1) FW) than a previously obtained cell line ('TIANSJ98'). Both cell lines are capable of developing substantially higher amounts of chlorophyll when subjected to osmotic stress. In order to explain these changes in the chlorophyll kinetics of the chlorophyllic cells, we analyzed the following population variables in cells subjected to polyethylene glycol 8000-induced osmotic stress: growth, viability, chlorophyll (total, 'a' and 'b'), cell size, percentage of green cells and chloroplast (number and size). Although previous studies in some chlorophyllic cells of dicots have already reported that chlorophyll increases under saline stress, in this report we show that, at least in this graminaceous cell line, the increase in chlorophyll is an immediate and proportional response to the osmotic stress applied and not the result of a progressive adaptation process. Consistent with previous studies, the increase in chlorophyll accumulation could be the result of chloroplast development (increased thylakoid number per chloroplast). On the basis of our results, the increases in chlorophyll accumulation previously observed in salt-adapted dicot cells may be the result of the osmotic shock (water deficit), rather than the ionic effect of salt on the physiology of chlorophyllic cells of dicots. Under the cell population experimental approach we followed, our study provides important insights related to the physiological behavior of chlorophyllic cells subjected to osmotic stress.

Efficient leaf ion partitioning, an overriding condition for abscisic acid-controlled stomatal and leaf growth responses to NaCl salinization in two legumes

Two tree medics contrasting in salt tolerance, Medicago arborea and Medicago citrina, were compared to evaluate the relative importance of abscisic acid on leaf growth and stomatal responses to salt stress. Plants were grown for 30 d in solution culture with 1, 50, 100 or 200 mM NaCl. Salinized plants of M. citrina had lower Na+ and Cl- uptake and maintained better leaf growth than M. arborea. In M. citrina, stomatal conductance was only slightly affected by salt and, in consequence, the salt treatment had no significant influence, neither on the CO2 fixation rate nor the transpiration rate in these plants. Moreover, leaf photosynthetic pigments and soluble protein in M. citrina were increased by the presence of NaCl, while a decrease of both parameters with salt was found in M. arborea. However, leaf and xylem ABA increased only in salt-treated M. citrina, while no differences were found among treatments in M. arborea. The role of ion compartmentation, gas exchange parameters and ABA concentrations in relation to salt tolerance in M. arborea and M. citrina is discussed.

Genetic analysis of salt tolerance in arabidopsis. Evidence for a critical role of potassium nutrition

A large genetic screen for sos (for salt overly sensitive) mutants was performed in an attempt to isolate mutations in any gene with an sos phenotype. Our search yielded 28 new alleles of sos1, nine mutant alleles of a newly identified locus, SOS2, and one allele of a third salt tolerance locus, SOS3. The sos2 mutations, which are recessive, were mapped to the lower arm of chromosome V, approximately 2.3 centimorgans away from the marker PHYC. Growth measurements demonstrated that sos2 mutants are specifically hypersensitive to inhibition by Na+ or Li+ and not hypersensitive to general osmotic stresses. Interestingly, the SOS2 locus is also necessary for K+ nutrition because sos2 mutants were unable to grow on a culture medium with a low level of K+. The expression of several salt-inducible genes was superinduced in sos2 plants. The salt tolerance of sos1, sos2, and sos3 mutants correlated with their K+ tissue content but not their Na+ tissue content. Double mutant analysis indicated that the SOS genes function in the same pathway. Based on these results, a genetic model for salt tolerance mechanisms in Arabidopsis is presented in which SOS1, SOS2, and SOS3 are postulated to encode regulatory components controlling plant K+ nutrition that in turn is essential for salt tolerance.

Genetic analysis of salt tolerance in arabidopsis. Evidence for a critical role of potassium nutrition

A large genetic screen for sos (for salt overly sensitive) mutants was performed in an attempt to isolate mutations in any gene with an sos phenotype. Our search yielded 28 new alleles of sos1, nine mutant alleles of a newly identified locus, SOS2, and one allele of a third salt tolerance locus, SOS3. The sos2 mutations, which are recessive, were mapped to the lower arm of chromosome V, approximately 2.3 centimorgans away from the marker PHYC. Growth measurements demonstrated that sos2 mutants are specifically hypersensitive to inhibition by Na+ or Li+ and not hypersensitive to general osmotic stresses. Interestingly, the SOS2 locus is also necessary for K+ nutrition because sos2 mutants were unable to grow on a culture medium with a low level of K+. The expression of several salt-inducible genes was superinduced in sos2 plants. The salt tolerance of sos1, sos2, and sos3 mutants correlated with their K+ tissue content but not their Na+ tissue content. Double mutant analysis indicated that the SOS genes function in the same pathway. Based on these results, a genetic model for salt tolerance mechanisms in Arabidopsis is presented in which SOS1, SOS2, and SOS3 are postulated to encode regulatory components controlling plant K+ nutrition that in turn is essential for salt tolerance.

Photoautotrophic tobacco cells adapted to grow at high salinity

Photoautotrophic tobacco (Nicotiana tabacum var. Wisconsin 38) cell cultures were gradually adapted to grow in media containing the normally inhibitory concentration of 20 g l^-1 NaCl. Both salt-adapted cultures maintained in 20 g l^-1 NaCl (P20) and salt-unadapted (P0) cultures demonstrated similar chloroplast morphology and similar growth characteristics on a dry weight basis, but P20 cells showed reduced growth on a fresh weight basis compared to P0 cells. Compared to P0 cells, intracellular sucrose levels were significantly higher in P20 cells while starch levels in P0 cells were significantly higher than in P20 cells. Levels of intracellular and extracellular reducing sugars, and chlorophyll accumulated to the same degree in P20 and P0 cells, but accumulation was delayed by approximately 13 days in P20 cells. O₂ evolution and¹⁴[CO₂] fixation was more resistant to inhibition by NaCl in P20 cells than in P0 cells. However, significant changes in the abundance of thylakoid membrane proteins could not be demonstrated between P20 and P0 cells although higher levels of Rubisco on a per milligram chlorophyll basis were observed in P0 compared to P20 chloroplasts.