Showing papers on "Plant physiology published in 2009"

Plant drought stress: effects, mechanisms and management

Abstract: Scarcity of water is a severe environmental constraint to plant productivity. Drought-induced loss in crop yield probably exceeds losses from all other causes, since both the severity and duration of the stress are critical. Here, we have reviewed the effects of drought stress on the growth, phenology, water and nutrient relations, photosynthesis, assimilate partitioning, and respiration in plants. This article also describes the mechanism of drought resistance in plants on a morphological, physiological and molecular basis. Various management strategies have been proposed to cope with drought stress. Drought stress reduces leaf size, stem extension and root proliferation, disturbs plant water relations and reduces water-use efficiency. Plants display a variety of physiological and biochemical responses at cellular and whole-organism levels towards prevailing drought stress, thus making it a complex phenomenon. CO2 assimilation by leaves is reduced mainly by stomatal closure, membrane damage and disturbed activity of various enzymes, especially those of CO2 fixation and adenosine triphosphate synthesis. Enhanced metabolite flux through the photorespiratory pathway increases the oxidative load on the tissues as both processes generate reactive oxygen species. Injury caused by reactive oxygen species to biological macromolecules under drought stress is among the major deterrents to growth. Plants display a range of mechanisms to withstand drought stress. The major mechanisms include curtailed water loss by increased diffusive resistance, enhanced water uptake with prolific and deep root systems and its efficient use, and smaller and succulent leaves to reduce the transpirational loss. Among the nutrients, potassium ions help in osmotic adjustment; silicon increases root endodermal silicification and improves the cell water balance. Low-molecular-weight osmolytes, including glycinebetaine, proline and other amino acids, organic acids, and polyols, are crucial to sustain cellular functions under drought. Plant growth substances such as salicylic acid, auxins, gibberrellins, cytokinin and abscisic acid modulate the plant responses towards drought. Polyamines, citrulline and several enzymes act as antioxidants and reduce the adverse effects of water deficit. At molecular levels several drought-responsive genes and transcription factors have been identified, such as the dehydration-responsive element-binding gene, aquaporin, late embryogenesis abundant proteins and dehydrins. Plant drought tolerance can be managed by adopting strategies such as mass screening and breeding, marker-assisted selection and exogenous application of hormones and osmoprotectants to seed or growing plants, as well as engineering for drought resistance.

Drought stress in plants: A review on morphological characteristics and pigments composition

Abstract: Plant growth and productivity is adversely affected by nature's wrath in the form of various biotic and abiotic stress factors. Water deficit is one of the major abiotic stresses, which adversely affects crop growth and yield. These changes are mainly related to altered metabolic functions, one of those is either loss of or reduced synthesis of photosynthetic pigments. This result in declined light harvesting and generation of reducing powers, which are a source of energy for dark reactions of photosynthesis. These changes in the amounts of photosynthetic pigments are closely associated to plant biomass yield. This review describes some aspects of drought induced changes in morphological, physiological and pigments composition in higher plants.

Green light drives leaf photosynthesis more efficiently than red light in strong white light: revisiting the enigmatic question of why leaves are green.

Abstract: The literature and our present examinations indicate that the intra-leaf light absorption profile is in most cases steeper than the photosynthetic capacity profile. In strong white light, therefore, the quantum yield of photosynthesis would be lower in the upper chloroplasts, located near the illuminated surface, than that in the lower chloroplasts. Because green light can penetrate further into the leaf than red or blue light, in strong white light, any additional green light absorbed by the lower chloroplasts would increase leaf photosynthesis to a greater extent than would additional red or blue light. Based on the assessment of effects of the additional monochromatic light on leaf photosynthesis, we developed the differential quantum yield method that quantifies efficiency of any monochromatic light in white light. Application of this method to sunflower leaves clearly showed that, in moderate to strong white light, green light drove photosynthesis more effectively than red light. The green leaf should have a considerable volume of chloroplasts to accommodate the inefficient carboxylation enzyme, Rubisco, and deliver appropriate light to all the chloroplasts. By using chlorophylls that absorb green light weakly, modifying mesophyll structure and adjusting the Rubisco/chlorophyll ratio, the leaf appears to satisfy two somewhat conflicting requirements: to increase the absorptance of photosynthetically active radiation, and to drive photosynthesis efficiently in all the chloroplasts. We also discuss some serious problems that are caused by neglecting these intra-leaf profiles when estimating whole leaf electron transport rates and assessing photoinhibition by fluorescence techniques.

Evidence for a Role of Gibberellins in Salicylic Acid-Modulated Early Plant Responses to Abiotic Stress in Arabidopsis Seeds

Abstract: Salicylic acid (SA) is a plant hormone mainly associated with the induction of defense mechanism in plants, although in the last years there is increasing evidence on the role of SA in plant responses to abiotic stress. We recently reported that an increase in endogenous SA levels are able to counteract the inhibitory effects of several abiotic stress conditions during germination and seedling establishment of Arabidopsis thaliana and that this effect is modulated by gibberellins (GAs) probably through a member of the GASA (Giberellic Acid Stimulated in Arabidopsis) gene family, clearly showing the existence of a cross talk between these two plant hormones in Arabidopsis.

Cytokinin-Dependent Photorespiration and the Protection of Photosynthesis during Water Deficit

Abstract: We investigated the effects of P(SARK)IPT (for Senescence-Associated Receptor KinaseIsopentenyltransferase) expression and cytokinin production on several aspects of photosynthesis in transgenic tobacco (Nicotiana tabacum cv SR1) plants grown under optimal or restricted (30% of optimal) watering regimes. There were no significant differences in stomatal conductance between leaves from wild-type and transgenic P(SARK)-IPT plants grown under optimal or restricted watering. On the other hand, there was a significant reduction in the maximum rate of electron transport as well as the use of triose-phosphates only in wild-type plants during growth under restricted watering, indicating a biochemical control of photosynthesis during growth under water deficit. During water deficit conditions, the transgenic plants displayed an increase in catalase inside peroxisomes, maintained a physical association among chloroplasts, peroxisomes, and mitochondria, and increased the CO(2) compensation point, indicating the cytokinin-mediated occurrence of photorespiration in the transgenic plants. The contribution of photorespiration to the tolerance of transgenic plants to water deficit was also supported by the increase in transcripts coding for enzymes involved in the conversion of glycolate to ribulose-1,5-bisphosphate. Moreover, the increase in transcripts indicated a cytokinin-induced elevation in photorespiration, suggesting the contribution of photorespiration in the protection of photosynthetic processes and its beneficial role during water stress.

Differential Response of Gray Poplar Leaves and Roots Underpins Stress Adaptation during Hypoxia

Abstract: The molecular and physiological responses of gray poplar (Populus × canescens) following root hypoxia were studied in roots and leaves using transcript and metabolite profiling. The results indicate that there were changes in metabolite levels in both organs, but changes in transcript abundance were restricted to the roots. In roots, starch and sucrose degradation were altered under hypoxia, and concurrently, the availability of carbohydrates was enhanced, concomitant with depletion of sucrose from leaves and elevation of sucrose in the phloem. Consistent with the above, glycolytic flux and ethanolic fermentation were stimulated in roots but not in leaves. Various messenger RNAs encoding components of biosynthetic pathways such as secondary cell wall formation (i.e. cellulose and lignin biosynthesis) and other energy-demanding processes such as transport of nutrients were significantly down-regulated in roots but not in leaves. The reduction of biosynthesis was unexpected, as shoot growth was not affected by root hypoxia, suggesting that the up-regulation of glycolysis yields sufficient energy to maintain growth. Besides carbon metabolism, nitrogen metabolism was severely affected in roots, as seen from numerous changes in the transcriptome and the metabolome related to nitrogen uptake, nitrogen assimilation, and amino acid metabolism. The coordinated physiological and molecular responses in leaves and roots, coupled with the transport of metabolites, reveal important stress adaptations to ensure survival during long periods of root hypoxia.

Isoprene synthesis protects transgenic tobacco plants from oxidative stress.

Abstract: Isoprene emission represents a significant loss of carbon to those plant species that synthesize this highly volatile and reactive compound. As a tool for studying the role of isoprene in plant physiology and biochemistry, we developed transgenic tobacco plants capable of emitting isoprene in a similar manner to and at rates comparable to a naturally emitting species. Thermotolerance of photosynthesis against transient high-temperature episodes could only be observed in lines emitting high levels of isoprene; the effect was very mild and could only be identified over repetitive stress events. However, isoprene-emitting plants were highly resistant to ozone-induced oxidative damage compared with their non-emitting azygous controls. In ozone-treated plants, accumulation of toxic reactive oxygen species (ROS) was inhibited, and antioxidant levels were higher. Isoprene-emitting plants showed remarkably decreased foliar damage and higher rates of photosynthesis compared to non-emitting plants immediately following oxidative stress events. An inhibition of hydrogen peroxide accumulation in isoprene-emitting plants may stall the programmed cell death response which would otherwise lead to foliar necrosis. These results demonstrate that endogenously produced isoprene provides protection from oxidative damage.

The Role of Tobacco Aquaporin1 in Improving Water Use Efficiency, Hydraulic Conductivity, and Yield Production Under Salt Stress

Abstract: Tobacco (Nicotiana tabacum; C3) plants increase their water use efficiency (WUE) under abiotic stress and are suggested to show characteristics of C4 photosynthesis in stems, petioles, and transmitting tract cells. The tobacco stress-induced Aquaporin1 (NtAQP1) functions as both water and CO(2) channel. In tobacco plants, overexpression of NtAQP1 increases leaf net photosynthesis (A(N)), mesophyll CO(2) conductance, and stomatal conductance, whereas its silencing reduces root hydraulic conductivity (L(p)). Nevertheless, interaction between NtAQP1 leaf and root activities and its impact on plant WUE and productivity under normal and stress conditions have never been suggested. Thus, the aim of this study was to suggest a role for NtAQP1 in plant WUE, stress resistance, and productivity. Expressing NtAQP1 in tomato (Solanum lycopersicum) plants (TOM-NtAQP1) resulted in higher stomatal conductance, whole-plant transpiration, and A(N) under all conditions tested. In contrast to controls, where, under salt stress, L(p) decreased more than 3-fold, TOM-NtAQP1 plants, similar to maize (Zea mays; C4) plants, did not reduce L(p) dramatically (only by approximately 40%). Reciprocal grafting provided novel evidence for NtAQP1's role in preventing hydraulic failure and maintaining the whole-plant transpiration rate. Our results revealed independent, albeit closely related, NtAQP1 activities in roots and leaves. This dual activity, which increases the plant's water use and A(N) under optimal and stress conditions, resulted in improved WUE. Consequently, it contributed to the plant's stress resistance in terms of yield production under all tested conditions, as demonstrated in both tomato and Arabidopsis (Arabidopsis thaliana) plants constitutively expressing NtAQP1. The putative involvement of NtAQP1 in tobacco's C4-like photosynthesis characteristics is discussed.

SAUR39, a Small Auxin-Up RNA Gene, Acts as a Negative Regulator of Auxin Synthesis and Transport in Rice

Abstract: The phytohormone auxin plays a critical role for plant growth by regulating the expression of a set of genes. One large auxin-responsive gene family of this type is the small auxin-up RNA (SAUR) genes, although their function is largely unknown. The expression of the rice (Oryza sativa) SAUR39 gene showed rapid induction by transient change in different environmental factors, including auxin, nitrogen, salinity, cytokinin, and anoxia. Transgenic rice plants overexpressing the SAUR39 gene resulted in lower shoot and root growth, altered shoot morphology, smaller vascular tissue, and lower yield compared with wild-type plants. The SAUR39 gene was expressed at higher levels in older leaves, unlike auxin biosynthesis, which occurs largely in the meristematic region. The transgenic plants had a lower auxin level and a reduced polar auxin transport as well as the down-regulation of some putative auxin biosynthesis and transporter genes. Biochemical analysis also revealed that transgenic plants had lower chlorophyll content, higher levels of anthocyanin, abscisic acid, sugar, and starch, and faster leaf senescence compared with wild-type plants at the vegetative stage. Most of these phenomena have been shown to be negatively correlated with auxin level and transport. Transcript profiling revealed that metabolic perturbations in overexpresser plants were largely due to transcriptional changes of genes involved in photosynthesis, senescence, chlorophyll production, anthocyanin accumulation, sugar synthesis, and transport. The lower growth and yield of overexpresser plants was largely recovered by exogenous auxin application. Taken together, the results suggest that SAUR39 acts as a negative regulator for auxin synthesis and transport.

Comparative effects of salt-stress and alkali-stress on the growth, photosynthesis, solute accumulation, and ion balance of barley plants

Abstract: We compared the effects of salt-stresses (SS, 1: 1 molar ratio of NaCl to Na2SO4) and alkali-stresses (AS, 1: 1 molar ratio of NaHCO3 to Na2CO3) on the growth, photosynthesis, solute accumulation, and ion balance of barley seedlings, to elucidate the mechanism of AS (high-pH) damage to plants and the physiological adaptive mechanism of plants to AS. The effects of SS on the water content, root system activity, membrane permeability, and the content of photosynthetic pigments were much less than those of AS. However, AS damaged root function, photosynthetic pigments, and the membrane system, led to the severe reductions in water content, root system activity, content of photosynthetic pigments, and net photosynthetic rate, and a sharp increase in electrolyte leakage rate. Moreover, with salinity higher than 60 mM, Na+ content increased slowly under SS and sharply under AS. This indicates that high-pH caused by AS might interfere with control of Na+ uptake in roots and increase intracellular Na+ to a toxic level, which may be the main cause of some damage emerging under higher AS. Under SS, barley accumulated organic acids, Cl−, SO42−, and NO3− to balance the massive influx of cations, the contribution of inorganic ions to ion balance was greater than that of organic acids. However, AS might inhibit absorptions of NO3− and Cl−, enhance organic acid synthesis, and SO42− absorption to maintain intracellular ion balance and stable pH.

Plant-Derived Sucrose Is a Key Element in the Symbiotic Association between Trichoderma virens and Maize Plants

Abstract: Fungal species belonging to the genus Trichoderma colonize the rhizosphere of many plants, resulting in beneficial effects such as increased resistance to pathogens and greater yield and productivity. However, the molecular mechanisms that govern the recognition and association between Trichoderma and their hosts are still largely unknown. In this report, we demonstrate that plant-derived sucrose (Suc) is an important resource provided to Trichoderma cells and is also associated with the control of root colonization. We describe the identification and characterization of an intracellular invertase from Trichoderma virens (TvInv) important for the mechanisms that control the symbiotic association and fungal growth in the presence of Suc. Gene expression studies revealed that the hydrolysis of plant-derived Suc in T. virens is necessary for the up-regulation of Sm1, the Trichoderma-secreted elicitor that systemically activates the defense mechanisms in leaves. We determined that as a result of colonization of maize (Zea mays) roots by T. virens, photosynthetic rate increases in leaves and the functional expression of tvinv is crucial for such effect. In agreement, the steady-state levels of mRNA for Rubisco small subunit and the oxygen-evolving enhancer 3-1 were increased in leaves of plants colonized by wild-type T. virens. We conclude that during the symbiosis, the sucrolytic activity in the fungal cells affects the sink activity of roots, directing carbon partitioning toward roots and increasing the rate of photosynthesis in leaves. A discussion of the role of Suc in controlling the fungal proliferation on roots and its pivotal role in the coordination of plant-microbe associations is provided.

Enhancement of Carotenoid Biosynthesis in Transplastomic Tomatoes by Induced Lycopene-to-Provitamin A Conversion

Abstract: Carotenoids are essential pigments of the photosynthetic apparatus and an indispensable component of the human diet. In addition to being potent antioxidants, they also provide the vitamin A precursor β-carotene. In tomato (Solanum lycopersicum) fruits, carotenoids accumulate in specialized plastids, the chromoplasts. How the carotenoid biosynthetic pathway is regulated and what limits total carotenoid accumulation in fruit chromoplasts is not well understood. Here, we have introduced the lycopene β-cyclase genes from the eubacterium Erwinia herbicola and the higher plant daffodil (Narcissus pseudonarcissus) into the tomato plastid genome. While expression of the bacterial enzyme did not strongly alter carotenoid composition, expression of the plant enzyme efficiently converted lycopene, the major storage carotenoid of the tomato fruit, into provitamin A (β-carotene). In green leaves of the transplastomic tomato plants, more lycopene was channeled into the β-branch of carotenoid biosynthesis, resulting in increased accumulation of xanthophyll cycle pigments and correspondingly reduced accumulation of the α-branch xanthophyll lutein. In fruits, most of the lycopene was converted into β-carotene with provitamin A levels reaching 1 mg per g dry weight. Unexpectedly, transplastomic tomatoes also showed a >50% increase in total carotenoid accumulation, indicating that lycopene β-cyclase expression enhanced the flux through the pathway in chromoplasts. Our results provide new insights into the regulation of carotenoid biosynthesis and demonstrate the potential of plastids genome engineering for the nutritional enhancement of food crops.

Dynamic acclimation of photosynthesis increases plant fitness in changing environments.

Abstract: Plants growing in different environments develop with different photosynthetic capacities—developmental acclimation of photosynthesis. It is also possible for fully developed leaves to change their photosynthetic capacity—dynamic acclimation. The importance of acclimation has not previously been demonstrated. Here, we show that developmental and dynamic acclimation are distinct processes. Furthermore, we demonstrate that dynamic acclimation plays an important role in increasing the fitness of plants in natural environments. Plants of Arabidopsis (Arabidopsis thaliana) were grown at low light and then transferred to high light for up to 9 d. This resulted in an increase in photosynthetic capacity of approximately 40%. A microarray analysis showed that transfer to high light resulted in a substantial but transient increase in expression of a gene, At1g61800, encoding a glucose-6-phosphate/phosphate translocator GPT2. Plants where this gene was disrupted were unable to undergo dynamic acclimation. They were, however, still able to acclimate developmentally. When grown under controlled conditions, fitness, measured as seed output and germination, was identical, regardless of GPT2 expression. Under naturally variable conditions, however, fitness was substantially reduced in plants lacking the ability to acclimate. Seed production was halved in gpt2− plants, relative to wild type, and germination of the seed produced substantially less. Dynamic acclimation of photosynthesis is thus shown to play a crucial and previously unrecognized role in determining the fitness of plants growing in changing environments.

Rice Aldehyde Dehydrogenase7 Is Needed for Seed Maturation and Viability

Abstract: Aldehyde dehydrogenases (ALDHs) catalyze the irreversible oxidation of a wide range of reactive aldehydes to their corresponding carboxylic acids. Although the proteins have been studied from various organisms and at different growth stages, their roles in seed development have not been well elucidated. We obtained T-DNA insertional mutants in OsALDH7, which is remarkably inducible by oxidative and abiotic stresses. Interestingly, endosperms from the osaldh7 null mutants accumulated brown pigments during desiccation and storage. Extracts from the mutant seeds showed a maximum absorbance peak at 360 nm, the wavelength that melanoidin absorbs. Under UV light, those extracts also exhibited much stronger fluorescence than the wild type, suggesting that the pigments are melanoidin. These pigments started to accumulate in the late seed developmental stage, the time when OsALDH7 expression began to increase significantly. Purified OsALDH7 protein showed enzyme activities to malondialdehyde, acetaldehyde, and glyceraldehyde. These results suggest that OsALDH7 is involved in removing various aldehydes formed by oxidative stress during seed desiccation. The mutant seeds were more sensitive to our accelerated aging treatment and accumulated more malondialdehyde than the wild type. These data imply that OsALDH7 plays an important role in maintaining seed viability by detoxifying the aldehydes generated by lipid peroxidation.

Effect of 28-homobrassinolide on the drought stress-induced changes in photosynthesis and antioxidant system of Brassica juncea L.

Abstract: To investigate the effect of exogenously applied 28-homobrassinolide (HBL) on drought-stressed plants, photosynthesis and antioxidant systems were examined in Indian mustard (Brassica juncea L.). Seedlings of Indian mustard were subjected to drought stress for 7 days at the 8–14 (DS1)/15–21 (DS2) days’ stage of growth and then returned to normal conditions of growth. These seedlings were sprayed with HBL (0.01 μM) at the 30-day stage and were sampled at 60 days to assess the changes in growth, photosynthesis and antioxidant enzymes. Plants exposed to stress at either of the stages of growth exhibited a significant decrease in growth and photosynthesis. The exposure of plants to stress at an earlier stage (DS1) was more inhibitory than that at a later stage (DS2). However, the follow-up treatment with HBL significantly improved the values of these parameters and also overcame the inhibitory effect of water stress. The activity of antioxidant enzymes [catalase (E.C. 1.11.1.6), peroxidase (E.C. 1.11.1.7) and superoxide dismutase (E.C. 1.15.1.1)] and proline content in leaves exhibited an increase in response to both the treatment factors, where their interaction had an additive effect. It was, therefore, concluded that the elevated antioxidant system, at least in part, was responsible for amelioration of the drought stress.

Role of silicon in plant resistance to water stress.

Abstract: Agricultural productivity is strongly affected by different abiotic stresses, among which water stress is the major environmental constraint limiting plants growth. The primary reason for water stress is drought or high salt concentration in soil (salinity). Because both of these stress factors lead to numerous physiological and biochemical changes in plants and result in serious loss in yields, there is a pressing need for finding the effective ways for increasing crops’ resistance to stress factors. One of the alternative methods involving alleviation of negative stress effects might be application of silicon as a fertiliser (root or foliar supply). Many plants, particularly monocotyledonous species, contain large amounts of Si (up to 10% of dry mass). In spite of the high Si accumulation in plants (its amount may equal concentration of macronutrients), until now it has not been considered as an essential element for higher plants. Many reports have shown that silicon may play a very important role in increasing plant resistance to noxious environmental factors. Hence, Si is recognised as a beneficial element for plants growing under biotic and abiotic stresses. The main form of Si which is available and easily taken up by plants is monosilicic acid (H 4 SiO 4 ). Plants take up Si from soil solution both passively and actively. Some dicotyledonous plants such as legumes tend to exclude Si from tissues – rejective uptake. These plants are unable to accumulate Si and they do not benefit from silicon. Under water stress conditions, silicon might enhance plants’ resistance to stress and ameliorate growth of plants. These beneficial effects may result from better and more efficient osomoregulation, improved plant water status, reduction in water loss by transpiration, maintenance of adequate supply of essential nutrients, restriction in toxic ions uptake and efficient functioning of antioxidative mechanisms. Based on the current knowledge and presented data, it can be concluded that the

Cold-tolerant crop species have greater temperature homeostasis of leaf respiration and photosynthesis than cold-sensitive species.

Abstract: Some plant species show constant rates of respiration and photosynthesis measured at their respective growth temperatures (temperature homeostasis), whereas others do not. However, it is unclear what species show such temperature homeostasis and what factors affect the temperature homeostasis. To analyze the inherent ability of plants to acclimate respiration and photosynthesis to different growth temperatures, we examined 11 herbace-ous crops with different cold tolerance. Leaf respiration (R(area)) and photosynthetic rate (P(area)) under high light at 360 microl l(-1) CO(2) concentrations were measured in plants grown at 15 and 30 degrees C. Cold-tolerant species showed a greater extent of temperature homeostasis of both R(area) and P(area) than cold-sensitive species. The underlying mechanisms which caused differences in the extent of temperature homeostasis were examined. The extent of temperature homeostasis of P(area) was not determined by differences in leaf mass and nitrogen content per leaf area, but by differences in photosynthetic nitrogen use efficiency (PNUE). Moreover, differences in PNUE were due to differences in the maximum catalytic rate of Rubisco, Rubisco contents and amounts of nitrogen invested in Rubisco. These findings indicated that the temperature homeostasis of photosynthesis was regulated by various parameters. On the other hand, the extent of temperature homeostasis of R(area) was unrelated to the maximum activity of the respiratory enzyme (NAD-malic enzyme). The R(area)/P(area) ratio was maintained irrespective of the growth temperatures in all the species, suggesting that the extent of temperature homeostasis of R(area) interacted with the photosynthetic rate and/or the homeostasis of photosynthesis.

Wheat cryptochromes: subcellular localization and involvement in photomorphogenesis and osmotic stress responses.

Abstract: Cryptochromes (CRYs) are blue light receptors important for plant growth and development. Comprehensive information on monocot CRYs is currently only available for rice (Oryza sativa). We report here the molecular and functional characterization of two CRY genes, TaCRY1a and TaCRY2, from the monocot wheat (Triticum aestivum). The expression of TaCRY1a was most abundant in seedling leaves and barely detected in roots and germinating embryos under normal growth conditions. The expression of TaCRY2 in germinating embryos was equivalent to that in leaves and much higher than the TaCRY1a counterpart. Transition from dark to light slightly affected the expression of TaCRY1a and TaCRY2 in leaves, and red light produced a stronger induction of TaCRY1a. Treatment of seedlings with high salt, polyethylene glycol, and abscisic acid (ABA) up-regulated TaCRY2 in roots and germinating embryos. TaCRY1a displays a light-responsive nucleocytoplasmic shuttling pattern similar to that of Arabidopsis (Arabidopsis thaliana) CRY1, contains nuclear localization domains in both the N and C termini, and includes information for nuclear export in its N-terminal domain. TaCRY2 was localized to the nucleus in the dark. Expression of TaCRY1a-green fluorescent protein or TaCRY2-green fluorescent protein in Arabidopsis conferred a shorter hypocotyl phenotype under blue light. These transgenic Arabidopsis plants showed higher sensitivity to high-salt, osmotic stress, and ABA treatment during germination and postgermination development, and they displayed altered expression of stress/ABA-responsive genes. The primary root growth in transgenic seedlings was less tolerant of ABA. These observations indicate that TaCRY1 and TaCRY2 might be involved in the ABA signaling pathway in addition to their role in primary blue light signal transduction.

Proline antioxidant role in the common ice plant subjected to salinity and paraquat treatment inducing oxidative stress

Abstract: Leaves of 4-week-old (juvenile) and 9-week-old (adult) plants of the halophyte Mesembryanthemum crystallinum L. (the common ice plant), cultured under controlled conditions in the phytotron, were treated with paraquat (0.1 μM), which produces superoxide radical, and (or) paraquat combined with introduction of NaCl (100 mM) or proline (5 mM) into nutrient medium. After a 20-h dark period (23°C), plants were transferred into light (4 h at 54.1 W/m2 of photosynthetically active radiation) for stimulation of O° 2 − formation in plastids. Activities of antioxidant enzymes, the contents of MDA, H2O2, chlorophyll, and free proline were measured in leaves. Plant responses in two age groups, which differed in the type of photosynthesis (juvenile plants had C3 type of photosynthesis, whereas adult plants were at the transition stage to Crassulacean Acid Metabolism (CAM) photosynthesis), differed in the levels of constitutive proline and proline, induced by NaCl and paraquat, as well as in activities of superoxide dismutase (SOD) and catalase. Changes in SOD activity and proline accumulation in response to paraquat treatment combined with NaCl revealed opposite dependence to accumulation of proline: the more proline accumulated in leaves, the lower activity of the enzyme. In response to paraquat treatment, the content of chlorophylls a and b most drastically declined in juvenile plants. Negative effect of salinity on the content of chlorophylls was lower than that of paraquat and was almost the same in plants of both age groups. Protective effect of exogenous proline was most profound in the case of paraquat treatment. Exogenous proline decreased the rate of lipid peroxidation, the content of superoxide radical and, consequently, SOD activity (almost fivefold), and increased the content of chlorophylls (a and b) in leaves of adult plants. The obtained data suggest that stress-induced accumulation of proline in the common ice plant has both osmoprotectory and antioxidant functions.

Sugar accumulation, photosynthesis and growth of two indica rice varieties in response to salt stress

Abstract: Sugar, a final product of photosynthesis, is reported to be involved in the defense mechanisms of plants against abiotic stresses such as salinity, water deficiency, extreme temperature and mineral toxicity. Elements involved in photosynthesis, sugar content, water oxidation, net photosynthetic rate, activity of enzyme and gene expression have therefore been studied in Homjan (HJ), salt-tolerant, and Pathumthani 1 (PT1), salt-sensitive, varieties of rice. Fructose-1,6-biphosphatase (FBP) and fructokinase (FK) genes were rapidly expressed in HJ rice when exposed to salt stress for 1–6 h and to a greater degree than in PT1 rice. An increase in FBP enzyme activity was found in both roots and leaves of the salt-tolerant variety after exposure to salt stress. A high level of sugar and a delay in chlorophyll degradation were found in salt-tolerant rice. The total sugar content in leaf and root tissues of salt-tolerant rice was 2.47 and 2.85 times higher, respectively, than in the salt-sensitive variety. Meanwhile, less chlorophyll degradation was detected. Salt stress may promote sugar accumulation, thus preventing the degradation of chlorophyll. Water oxidation by the light reaction of photosynthesis in the salt-tolerant variety was greater than that in the salt-sensitive variety, indicated by a high maximum quantum yield of PSII (Fv/Fm) and quantum efficiency of PSII (ΦPSII) with low nonphotochemical quenching (NPQ), leading to a high net photosynthetic rate. In addition, the overall growth performances in the salt-tolerant variety were higher than those in the salt-sensitive variety. The FBP gene expression and enzyme activity, sugar accumulation, pigment stabilization, water oxidation and net photosynthetic rate parameters in HJ rice should be further investigated as multivariate salt-tolerant indices for the classification of salt tolerance in rice breeding programs.

Effects of salt stress on photosynthesis, PSII photochemistry and thermal energy dissipation in leaves of two corn (Zea mays L.) varieties.

Abstract: The effect of four different NaCl concentrations (from 0 to 102 mM NaCl) on seedlings leaves of two corn (Zea mays L.) varieties (Aristo and Arper) was investigated through chlorophyll (Chl) a fluorescence parameters, photosynthesis, stomatal conductance, photosynthetic pigments concentration, tissue hydration and ionic accumulation. Salinity treatments showed a decrease in maximal efficiency of PSII photochemistry (Fv/Fm) in dark-adapted leaves. Moreover, the actual PSII efficiency (ϕPSII), photochemical quenching coefficient (qp), proportion of PSII centers effectively reoxidized, and the fraction of light used in PSII photochemistry (%P) were also dropped with increasing salinity in light-adapted leaves. Reductions in these parameters were greater in Aristo than in Arper. The tissue hydration decreased in salt-treated leaves as did the photosynthesis, stomatal conductance (gs) and photosynthetic pigments concentration essentially at 68 and 102 mM NaCl. In both varieties the reduction of photosynthesis was mainly due to stomatal closure and partially to PSII photoinhibition. The differences between the two varieties indicate that Aristo was more susceptible to salt-stress damage than Arper which revealed a moderate regulation of the leaf ionic accumulation.

Role of cytokinin and salicylic acid in plant growth at low temperatures

Abstract: Low temperature restrains plant growth by inhibiting the cell cycle, and phytohormones play important roles in this case; however, the molecular mechanisms whereby phytohormones affect growth at low temperature are largely unknown. When grown at 23, 16, 10, and 4°C, we found that Arabidopsis thaliana could develop with normal morphology, but needed a prolonged period of cultivation. By screening mutants, we could implicate cytokinin and salicylic acid. At 4°C, both amp1 plants, which have an increased level of cytokinin, and wild-type plants treated with exogenous cytokinin, displayed relative growth rates greater than control by increasing total cell number. Additionally, transgenic NahG plants, which have lower salicylic acid content, grew faster than wild-type accompanied by larger cells. Expression of C-repeat binding transcription factors (CBFs), that mediate cold acclimation by stimulation of the expression of cold-inducible genes, was similar in all tested genotypes. Thus CBF expression did not correlate with the observed enhanced growth in mutants. The improved growth coincided with elevated expression of CYCD3;1, especially in NahG plants. At 4°C, enhanced endoreduplication took responsibility for larger cells in NahG plants, while enhanced cell division was observed in amp1 plants.

Effect of 24-epibrassinolide on the photosynthetic activity of radish plants under cadmium stress

Abstract: The present study was conducted to study the effect of 24-epibrassinolide (EBL) on changes of plant growth, net photosynthetic rate, carbonic anhydrase (E.C. 4.2.1.1) and nitrate reductase (E.C.1.6.6.1) activities in the leaves of Raphanus sativus L. under the influence of cadmium (Cd) stress. Cd reduced plant growth, photosynthetic pigment levels, net photosynthetic rate and the activities of carbonic anhydrase and nitrate reductase. However seed application of EBL reduced the toxic effect of Cd on plant growth, pigment content, photosynthesis and enzyme activities. The studies clearly demonstrated the ameliorating effect of 24-epibrassinolide in mitigating the toxicity of Cd in plants.

Promotion of 5-aminolevulinic acid treatment on leaf photosynthesis is related with increase of antioxidant enzyme activity in watermelon seedlings grown under shade condition

Abstract: Watermelon [Citrullus lanatus (Thunb.) Mansfeld] is a photophilic plant, whose net photosynthetic rate was significantly decreased when seedlings were grown under low light condition. However, treatment with 100 mg kg−1 5-aminolevulinic acid (ALA) could significantly restore the photosynthetic ability under the environmental stress. The parameters of leaf gas exchange, chlorophyll modulated fluorescence and fast induction fluorescence of the ALA-treated plants were higher than that of the control. Additionally, ALA treatment increased the activities of antioxidant enzymes, including superoxide dismutase (SOD), peroxidase (POD) and ascorbate peroxidase (APX). Nevertheless, the treatment of diethyldithiocarbamate (DDC), an inhibitor of SOD activity, dramatically depressed photosynthesis of watermelon leaves, while ALA could reverse the inhibition of DDC. Therefore, it can be deduced that ALA promotion on photosynthesis of watermelon leaves under low light stress is attributed to its promotion on antioxidant enzyme activities, and the increased activities of the enzymes, which are mainly located near the reaction centers of PSI, can scavenge superoxide anions, leading to an increase of apparent electron transport rate and an alleviation of photosynthetic photoinhibition under the stressed environment.

Plastic and adaptive responses of plant respiration to changes in atmospheric CO2 concentration

Abstract: The concentration of atmospheric CO 2 has increased from below 200 μl l -1 during last glacial maximum in the late Pleistocene to near 280 μl l -1 at the beginning of the Holocene and has continuously increased since the onset of the industrial revolution. Most responses of plants to increasing atmospheric CO 2 levels result in increases in photosynthesis, water use efficiency and biomass. Less known is the role that respiration may play during adaptive responses of plants to changes in atmospheric CO 2 . Although plant respiration does not increase proportionally with CO 2 -enhanced photosynthesis or growth rates, a reduction in respiratory costs in plants grown at subambient CO 2 can aid in maintaining a positive plant C-balance (i.e. enhancing the photosynthesis-to-respiration ratio). The understanding of plant respiration is further complicated by the presence of the alternative pathway that consumes photosynthate without producing chemical energy [adenosine triphosphate (ATP)] as effectively as respiration through the normal cytochrome pathway. Here, we present the respiratory responses of Arabidopsis thaliana plants selected at Pleistocene (200 μl l -1 ), current Holocene (370μl l -1 ), and elevated (700 μl l -1 ) concentrations of CO 2 and grown at current CO 2 levels. We found that respiration rates were lower in Pleistocene-adapted plants when compared with Holocene ones, and that a substantial reduction in respiration was because of reduced activity of the alternative pathway. In a survey of the literature, we found that changes in respiration across plant growth forms and CO 2 levels can be explained in part by differences in the respiratory energy demand for maintenance of biomass. This trend was substantiated in the Arabidopsis experiment in which Pleistocene-adapted plants exhibited decreases in respiration without concurrent reductions in tissue N content. Interestingly, N-based respiration rates of plants adapted to elevated CO 2 also decreased. As a result, ATP yields per unit of N increased in Pleistocene-adapted plants compared with current CO 2 adapted ones. Our results suggest that mitochondrial energy coupling and alternative pathway-mediated responses of respiration to changes in atmospheric CO 2 may enhance survival of plants at low CO 2 levels to help overcome a low carbon balance. Therefore, increases in the basal activity of the alternative pathway are not necessarily associated to metabolic plant stress in all cases.

Proteomic analysis of β-aminobutyric acid priming and abscisic acid – induction of drought resistance in crabapple (Malus pumila): effect on general metabolism, the phenylpropanoid pathway and cell wall enzymes

Abstract: In a variety of herbaceous, model and crop plants, DL-β-aminobutyric acid (BABA), has been shown to enhance both biotic and abiotic stress resistance by potentiating rather than inducing resistance responses but studies in woody plants are lacking. In the present study, two-dimensional difference in-gel electrophoresis (DIGE) was used to quantify differences in protein abundance in leaf tissue from BABA-treated, abscisic acid (ABA)-treated and untreated (control) plants during a 10 d drought stress. ABA-treated seedlings were most resistant to water loss followed by BABA-treated seedlings and then untreated seedlings. Although some similarity was observed in the proteome of ABA- and BABA-treated seedlings, a dramatic shift in the proteome occurred earlier in ABA-treated seedlings. Some proteins showed almost identical patterns of increase or decrease in abundance in both BABA- and ABA-treated seedlings, supporting the concept that BABA-induced abiotic stress resistance in plants is achieved by potentiating an ABA-regulated pathway. Some proteins, however, were induced or suppressed only in BABA-primed plants, indicating that BABA may also mediate resistance via some ABA-independent pathways. Based on the putative function of the identified proteins, we propose that changes in cell wall enzymes and a suppression of lignin biosynthesis may play a specific role in BABA-primed drought resistance.