Salicylic acid induced physiological and biochemical changes in wheat seedlings under water stress

Salicylic acid (SA) is an important signal molecule modulating plantresponses to stress. It is recently reported to induce multiple stresstolerancein plants including drought. An experiment was, therefore, conducted toascertain the effect of salicylic acid on the growth and metabolic profile ofwheat seedlings under water stress. Irrespective of the SA concentration(1–3 mM) and water stress, SA treated plants showed, ingeneral, a higher moisture content, dry mass, carboxylase activity of Rubisco,superoxide dismutase (SOD) activity and total chlorophyll compared to those ofuntreated seedlings. SA treatment, under water stress, protected nitratereductase (NR) activity and maintained, especially at 3 mM SAconcentration, the protein and nitrogen content of leaves compared to watersufficient seedlings. Results signify the role of SA in regulating the droughtresponse of plants and suggest that SA could be used as a potential growthregulator, for improving plant growth under water stress.     

Role of salicylic acid on physiological and biochemical mechanism of salinity stress tolerance in plants

Salinity stress is one of the major abiotic stresses affecting plant growth and productivity globally. In order to improve the yields of plants growing under salt stress bear remarkable importance to supply sustainable agriculture. Acclimation of plants to salinized condition depends upon activation of cascade of molecular network involved in stress sensing/perception, signal transduction, and the expression of specific stress-related genes and metabolites. Isolation of salt overly sensitive (SOS) genes by sos mutants shed us light on the relationship between ion homeostasis and salinity tolerance. Regulation of antioxidative system to maintain a balance between the overproduction of reactive oxygen species and their scavenging to keep them at signaling level for reinstating metabolic activity has been elucidated. However, osmotic adaptation and metabolic homeostasis under abiotic stress environment is required. Recently, role of phytohormones like Abscisic acid, Jasmonic acid, and Salicylic acid in the regulation of metabolic network under osmotic stress condition has emerged through crosstalk between chemical signaling pathways. Thus, abiotic stress signaling and metabolic balance is an important area with respect to increase crop yield under suboptimal conditions. This review focuses on recent developments on improvement in salinity tolerance aiming to contribute sustainable plant yield under saline conditions in the face of climate change.     

Ectopic expression of C 4 photosynthetic pathway genes improves carbon assimilation and alleviate stress tolerance for future climate change

Alteration in atmospheric carbon dioxide concentration and other environmental factors are the significant cues of global climate change. Environmental factors affect the most fundamental biological process including photosynthesis and different metabolic pathways. The feeding of the rapidly growing world population is another challenge which imposes pressure to improve productivity and quality of the existing crops. C4 plants are considered the most productive, containing lower photorespiration, and higher water-use & N-assimilation efficiencies, compared to C3 plants. Besides, the C4-photosynthetic genes not only play an important role in carbon assimilation but also modulate abiotic stresses. In this review, fundamental three metabolic processes (C4, C3, and CAM) of carbon dioxide assimilation, the evolution of C4-photosynthetic genes, effect of elevated CO2 on photosynthesis, and overexpression of C4-photosynthetic genes for higher photosynthesis were discussed. Kranz-anatomy is considered an essential prerequisite for the terrestrial C4 carbon assimilation, but single-celled C4 plant species changed this well-established paradigm. C4 plants are insensitive to an elevated CO2 stress condition but performed better under stress conditions. Overexpression of essential C4-photosynthetic genes such as PEPC, PPDK, and NADP-ME in C3 plants like Arabidopsis, tobacco, rice, wheat, and potato not only improved photosynthesis but also provided tolerance to various environmental stresses, especially drought. The review provides useful information for sustainable productivity and yield under elevated CO2 environment, which to be explored further for CO2 assimilation and also abiotic stress tolerance. Additionally, it provides a better understanding to explore C4-photosynthetic gene(s) to cope with global warming and prospective adverse climatic changes.     

Oxidative stress induced expression of monodehydroascorbate reductase gene in Eleusine coracana

Abiotic stresses constitute a serious threats to the world food security as they cause significant economic losses in terms of reduction in crop productivity and also greatly limit the geographical locations where crops can be grown. Exposure to abiotic stress causes over-production of reactive oxygen species, leading to oxidative stress in plants. Induction of oxidative stress is primarily responsible for a variety of detrimental changes in the cellular physiology. However, plants have evolved intricate anti-oxidative defence machinery, for their survival under stress. Plant defence strategies for stress tolerance rely on the expression of anti-oxidative genes required for scavenging the toxic reactive oxygen species. Monodehydroascorbate reductase is one of the key anti-oxidant enzyme responsible for scavenging reactive oxygen species. In the present study, efforts have been made to understand the role of monodehydroascorbate reductase in finger millet under different abiotic stresses (drought, salt and UV radiation). The study establishes a differential link between mdar gene expression and enzyme activity under oxidative stress that is validated under different types of imposed stresses. Alteration in correlation between gene expression and enzyme activities under varying magnitude of oxidative stress is elucidated.     

Plant abiotic stress: deciphering remedial strategies for emerging problem

Growing in their natural environment, plants often encounter unfavorable environmental conditions that interrupt normal plant growth and productivity. Drought, high/low temperature and saline soils are the most common abiotic stresses that plants encounter in their natural environments. Molecular and genomic analyses have facilitated gene discovery and enabled genetic engineering using several functional or regulatory genes that are known to be involved in stress response and preliminary tolerance, to activate specific or broad pathways related to abiotic stress tolerance in plants. Through the use of transgenic technology, goals such as production of plants with desired traits that were unattainable with traditional selection programs are achieved. This review deals with recent advancement in understanding the role of various stress responsive genes and their critical importance for explaining the control mechanism of abiotic stress tolerance and engineering stress tolerant crops based on the expression of specific stress related genes.     

Emerging trends in the functional genomics of the abiotic stress response in crop plants

Plants are exposed to different abiotic stresses, such as water deficit, high temperature, salinity, cold, heavy metals and mechanical wounding, under field conditions. It is estimated that such stress conditions can potentially reduce the yield of crop plants by more than 50%. Investigations of the physiological, biochemical and molecular aspects of stress tolerance have been conducted to unravel the intrinsic mechanisms developed during evolution to mitigate against stress by plants. Before the advent of the genomics era, researchers primarily used a gene-by-gene approach to decipher the function of the genes involved in the abiotic stress response. However, abiotic stress tolerance is a complex trait and, although large numbers of genes have been identified to be involved in the abiotic stress response, there remain large gaps in our understanding of the trait. The availability of the genome sequences of certain important plant species has enabled the use of strategies, such as genome-wide expression profiling, to identify the genes associated with the stress response, followed by the verification of gene function by the analysis of mutants and transgenics. Certain components of both abscisic acid-dependent and -independent cascades involved in the stress response have already been identified. Information originating from the genome-wide analysis of abiotic stress tolerance will help to provide an insight into the stress-responsive network(s), and may allow the modification of this network to reduce the loss caused by stress and to increase agricultural productivity.     

The carboxylase activity of Rubisco and the photosynthetic performance in aquatic plants

Summary Activated carboxylase activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), as well as photosynthetic rates were measured for 42 species of freshwater and marine macrophytes. While the carboxylase activity varied greatly among the species investigated (0.2–12.5 mol CO₂ mg^–1 chlorophyll min^–1), the submersed freshwater plants showed significantly lower activities than emergent, floating leaved or secondary submersed forms. The variability in photosynthetic rates correlated with the carboxylase activity only for the marine macroalgae, and their photosynthesis to carboxylase activity ratios were close to 1. These plants also had a consistently high inorganic carbon transport capability, and it is suggested that ribulose-1,5-bisphosphate carboxylase/oxygenase activity is an important internal factor regulating the photosynthetic capacity within this plant group where, apparently, the internal CO₂ concentration is high and photorespiration is suppressed. Among the freshwater forms, it appears that their much lower inorganic carbon transport ability, rather than their carboxylase activity, limits the photosynthetic process.     

Trehalose and its applications in plant biotechnology

Trehalose, a nonreducing disaccharide of glucose, is one of the most effective osmoprotectants. Several strategies leading to its accumulation have been envisaged in both model and crop plants using genes of bacterial, yeast and, more recently, plant origin. Significant levels of trehalose accumulation have been shown to cause abiotic stress tolerance in transgenic plants. In this review, we describe the most biologically relevant features of trehalose: chemical and biological properties; occurrence and metabolism in organisms with special reference to plants; protective role in stabilizing molecules; physiological role in plants with special reference to carbohydrate metabolism. The emphasis of this review, however, will be on manipulation of trehalose metabolism to improve abiotic stress tolerance in plants.     

Altered night-time CO2 concentration affects the growth, physiology and biochemistry of soybean

Soybean plants (Glycine max (L.) Merr. c.v. Williams) were grown in CO₂ controlled, natural-light growth chambers under one of four atmospheric CO₂ concentrations ([CO₂]): (1) 250 μmol mol^–1 24 h d^–1[250/250]; (2) 1000 μmol mol^–1 24 h d^–1[1000/1000]; (3) 250 μmol mol^–1 during daylight hours and 1000 μmol mol^–1 during night-time hours [250/1000] or (4) 1000 μmol mol^–1 during daylight hours and 250 μmol mol^–1 during night-time hours [1000/250]. During the vegetative growth phase few physiological differences were observed between plants exposed to a constant 24 h [CO₂] (250/250 and 1000/1000) and those that were switched to a higher or lower [CO₂] at night (250/1000 and 1000/250), suggesting that the primary physiological responses of plants to growth in elevated [CO₂] is apparently a response to daytime [CO₂] only. However, by the end of the reproductive growth phase, major differences were observed. Plants grown in the 1000/250 regime, when compared with those in the 1000/1000 regime, had significantly more leaf area and leaf mass, 27% more total plant dry mass, but only 18% of the fruit mass. After 12 weeks of growth these plants also had 19% higher respiration rates and 32% lower photosynthetic rates than the 1000/1000 plants. As a result the ratio of carbon gain to carbon loss was reduced significantly in the plants exposed to the reduced night-time [CO₂]. Plants grown in the opposite switching environment, 250/1000 versus 250/250, showed no major differences in biomass accumulation or allocation with the exception of a significant increase in the amount of leaf mass per unit area. Physiologically, those plants exposed to elevated night-time [CO₂] had 21% lower respiration rates, 14% lower photosynthetic rates and a significant increase in the ratio of carbon gain to carbon loss, again when compared with the 250/250 plants. Biochemical differences also were found. Ribulose-1,5-bisphosphate carboxylase/ oxygenase concentrations decreased in the 250/ 1000 treatment compared with the 250/250 plants, and phosphoenolpyruvate carboxylase activity decreased in the 1000/250 compared with the 1000/1000 plants. Glucose, fructose and to a lesser extent sucrose concentrations also were reduced in the 1000/250 treatment compared with the 1000/1000 plants. These results indicate that experimental protocols that do not maintain elevated CO₂ levels 24 h d^–1 can have significant effects on plant biomass, carbon allocation and physiology, at least for fast-growing annual crop plants. Furthermore, the results suggest some plant processes other than photosynthesis are sensitive to [CO₂] and under ecologically relevant conditions, such as high night-time [CO₂], whole plant carbon balance can be affected.     

Enhanced tolerance to oxidative stress in transgenic tobacco plants expressing three antioxidant enzymes in chloroplasts

The effect of simultaneous expression of genes encoding three antioxidant enzymes, copper zinc superoxide dismutase (CuZnSOD, EC 1.15.1.1), ascorbate peroxidase (APX, EC 1.11.1.11), and dehydroascorbate (DHA) reductase (DHAR, EC 1.8.5.1), in the chloroplasts of tobacco plants was investigated under oxidative stress conditions. In previous studies, transgenic tobacco plants expressing both CuZnSOD and APX in chloroplast (CA plants), or DHAR in chloroplast showed enhanced tolerance to oxidative stresses, such as paraquat and salt. In this study, in order to develop transgenic plants that were more resistant to oxidative stress, we introduced the gene encoding DHAR into CA transgenic plants. Mature leaves of transgenic plants expressing all three antioxidant genes (CAD plants) had approximately 1.6–2.1 times higher DHAR activity, and higher ratios of reduced ascorbate (AsA) to DHA, and oxidized glutathione (GSSG) to reduced glutathione (GSH) compared to CA plants. CAD plants were more resistant to paraquat-induced stress, exhibiting only 18.1% reduction in membrane damage relative to CA plants. In addition, seedlings of CAD plants had enhanced tolerance to NaCI (100 mM) compared to CA plants. These results indicate that the simultaneous expression of multiple antioxidant enzymes, such as CuZnSOD, APX, and DHAR, in chloroplasts is more effective than single or double expression for developing transgenic plants with enhanced tolerance to multiple environmental stresses.     

Rate-limiting factors in leaf photosynthesis. I. Carbon fluxes in the calvin cycle

Rates of photosynthesis of spinach leaves were varied by varying light intensity and CO₂ concentration. Metabolism of the leaves was then arrested by freezing them in liquid nitrogen. Chloroplasts were isolated by a nonaqueous procedure. In the chloroplast fractions, levels of intermediates of the carbon reduction cycle were determined and considered in relation to the photosynthetic flux situation of the leaves at the time before freezing. During induction of photosynthesis, ribulose 1,5-bisphosphate levels increased in parallel with CO₂ fixation. In the steady state, a similar relation between ribulose 1,5-bisphosphate levels and CO₂ uptake was observed at light intensities between 0 and 50 W·m⁻². A further increase in light intensity increased CO₂ fixation rates but not ribulose 1,5-bisphosphate levels. Increasing the CO₂ concentration resulted in increased CO₂ uptake, whereas ribulose 1,5-bisphosphate levels decreased. Even under CO₂ saturation, ribulose 1,5-bisphosphate levels were about 100 nmol/mg chlorophyll corresponding to about 3.5 mM ribulose 1,5-bisphosphate in the chloroplast stroma. This suggests that even under CO₂ saturation, ribulose-1,5-bisphosphate carboxylase limits photosynhetic CO₂ uptake. Mass action ratios calculated from measured metabolite levels demonstrated that the thermodynamic gradient required for the regeneration of ribulose 1,5-bisphosphate from hexosephosphate and triosephosphate increased considerably as photosynthetic flux increased. Similar calculations revealed that the enzymatic apparatus responsible for the reduction of 3-phosphoglycerate to dihydroxyacetone phosphate is not displaced much from equilibrium even under maximum rates of photosynthesis at saturating CO₂. The same is true for aldolase. Fructose-1,6-bisphosphatase also did not limit Calvin cycle turnover. Only at very low light intensities and during the first minutes of the induction period was the ratio of fructose 1,6-bisphosphate to fructose 6-phosphate high. This observation was more readily explained in terms of fructose 1,6-bisphosphate binding to ribulose-1,5-bisphosphate carboxylase than by a rate limitation imposed by insufficient activation of fructose-1,6-bisphosphatase.     

Sequence similarity based identification of abiotic stress responsive genes in chickpea

Chickpea (Cicer arietinum L.) is an important food legume crop, particularly for the arid regions including Indian subcontinent. Considering the detrimental effect of drought, temperature and salt stress on crop yield, efforts have been initiated in the direction of developing improved varieties and designing alternate strategies to sustain chickpea production in adverse environmental conditions. Identification of genes that confer abiotic stress tolerance in plants remains a challenge in contemporary plant breeding. The present study focused on the identification of abiotic stress responsive genes in chickpea based on sequence similarity approach exploiting known abiotic stress responsive genes from model crops or other plant species. Ten abiotic stress responsive genes identified in other plants were partially amplified from eight chickpea genotypes and their presence in chickpea was confirmed after sequencing the PCR products. These genes have been functionally validated and reported to play significant role in stress response in model plants like Arabidopsis, rice and other legume crops. Chickpea EST sequences available at NCBI EST database were used for the identification of abiotic stress responsive genes. A total of 8,536 unique coding long sequences were used for identification of chickpea homologues of these abiotic stress responsive genes by sequence similarity search (BLASTN and BLASTX). These genes can be further explored towards achieving the goal of developing superior chickpea varieties providing improved yields under stress conditions using modern molecular breeding approaches.     

Impact of feed processing and mixed ruminal culture on the fate of recombinant EPSP synthase and endogenous canola plant DNA

The impact of feed processing and in vitro ruminal cultures on the persistence of recombinant and canola-specific endogenous DNA was studied using various canola substrates (whole seed, cracked seed, meal and diet). For both, parental and genetically modified substrates, ribulose-1,5-bisphosphate carboxylase/oxygenase gene was amplifiable up to varying time points. Persistence of recombinant DNA, encoding 5-enolpyruvylshikimate-3-phosphate synthase (1,363 bp) was detected up to 8 h for meal and 4 h for mixed diet. Upon processing of canola, DNA large enough to contain intact plant genes remains. In an in vitro environment, plant DNA was rapidly degraded upon its release into rumen fluid.     

Induction of abiotic stress tolerance in plants by endophytic microbes

Endophytes are micro‐organisms including bacteria and fungi that survive within healthy plant tissues and promote plant growth under stress. This review focuses on the potential of endophytic microbes that induce abiotic stress tolerance in plants. How endophytes promote plant growth under stressful conditions, like drought and heat, high salinity and poor nutrient availability will be discussed. The molecular mechanisms for increasing stress tolerance in plants by endophytes include induction of plant stress genes as well as biomolecules like reactive oxygen species scavengers. This review may help in the development of biotechnological applications of endophytic microbes in plant growth promotion and crop improvement under abiotic stress conditions.

Significance and Impact of the Study

Increasing human populations demand more crop yield for food security while crop production is adversely affected by abiotic stresses like drought, salinity and high temperature. Development of stress tolerance in plants is a strategy to cope with the negative effects of adverse environmental conditions. Endophytes are well recognized for plant growth promotion and production of natural compounds. The property of endophytes to induce stress tolerance in plants can be applied to increase crop yields. With this review, we intend to promote application of endophytes in biotechnology and genetic engineering for the development of stress‐tolerant plants.