Biotechnological approach of improving plant salt tolerance using antioxidants as markers

Salt stress causes multifarious adverse effects in plants. Of them, production of reactive oxygen species (ROS) is a common phenomenon. These ROS are highly reactive because they can interact with a number of cellular molecules and metabolites thereby leading to a number of destructive processes causing cellular damage. Plants possess to a variable extent antioxidant metabolites, enzymes and non-enzymes, that have the ability to detoxify ROS. In the present review, the emphasis of discussion has been on understanding the role of different antioxidants in plants defense against oxidative stress caused by salt stress. The role of different antioxidants as potential selection criteria for improving plant salt tolerance has been critically discussed. With the advances in molecular biology and availability of advanced genetic tools considerable progress has been made in the past two decades in improving salt-induced oxidative stress tolerance in plants by developing transgenic lines with altered levels of antioxidants of different crops. The potential of this approach in counteracting stress-induced oxidative stress has been discussed at length in this review.     

Genetic engineering of polyamine and carbohydrate metabolism for osmotic stress tolerance in higher plants

Plant growth and productivity are greatly affected by various stress factors. The molecular mechanisms of stress tolerance in plant species have been well established. Metabolic pathways involving the synthesis of metabolites such as polyamines, carbohydrates, proline and glycine betaine have been shown to be associated with stress tolerance. Introduction of the stress-induced genes involved in these pathways from tolerant species to sensitive plants seems to be a promising approach to confer stress tolerance in plants. In cases where single gene is not enough to confer tolerance, metabolic engineering necessitates the introduction of multiple transgenes in plants.     

Developing salt tolerant plants in a new century: a molecular biology approach

Soil salinity is a major abiotic stress in plant agriculture strongly, influencing plant productivity world-wide. Classical breeding for salt tolerance in crop plants has been attempted to improve field performance without success. Therefore, an alternative strategy is to generate salt tolerant plants through genetic engineering. Several species and experimental approaches have been used in order to identify those genes that are important for salt tolerance. Due to high level of salt tolerance, halophytes are good candidates to identify salt tolerance genes. However, other species such as yeast and glycophytes have also been employed. Three approaches are commonly used to identify genes important for salt tolerance. The first approach is to identify genes involved in processes known to be critical for salt tolerance (osmolyte synthesis, ion homeostasis, etc.). The second approach is to identify genes whose expression is regulated by salt stress. This is relatively simply and applicable to any plant species. Genetic amenability of some species allows the third approach, which consists in the identification of salt tolerance determinants based on functionality. At the moment, there is a large number of reports in the literature claiming that plants with increased salt tolerance have been obtained. The main problem is that different plant species, stage of development, organs, promoters and salt conditions used it is difficult to compare the degree of salt tolerance conferred by different genes. In this review, we discuss progress made towards understanding the molecular elements involved in salt stress responses that have been used in transgenic approaches to improve salt tolerance.     

Transgenic approaches to increase dehydration-stress tolerance in plants

Plant productivity is strongly influenced by abiotic stress conditions induced by drought, high salt and low temperature. Plants respond to these conditions with an array of biochemical and physiological adaptations, at least some of which are the result of changes in gene expression. Transgenic approaches offer a powerful means of gaining valuable information to better understand the mechanisms governing stress tolerance. They also offer new opportunities to improve dehydration-stress tolerance in crops by incorporating a gene involved in stress protection into species that lack them. In this review, we discuss progress made towards understanding the molecular elements involved in dehydration-stress responses that have been used to improve salt or drought tolerance following several transgenic approaches. Further, we discuss various strategies being used to produce transgenic plants with increased tolerance to dehydration stress. These include the overproduction of enzymes responsible for biosynthesis of osmolytes, late-embryogenesis-abundant proteins and detoxification enzymes. At this time, there is a need for a careful appraisal of the genes to be selected and promoter elements to be used, because constitutive expression of these genes may not be desirable in all applications. In this context, the advantages and limitations of transgenic approaches currently being used are discussed together with the importance of using stress-inducible promoters and the introduction of multiple genes for the improvement of dehydration-stress tolerance.     

Transgenic plants: An insight into oxidative stress tolerance mechanisms

Environmental stresses considerably limit plant productivity. At the molecular level the negative effect of stress is often mediated by reactive oxygen species-initiated oxidative damage. Hence, it was hypothesised that increased tolerance to several environmental constraints could be achieved through enhanced tolerance to oxidative stress. In recent years much effort has been undertaken to improve oxidative stress tolerance by transforming plants with native or bacterial genes coding either for reactive oxygen species-scavenging enzymes or for enzymes modulating the cellular antioxidant capacity. This review deals with data on transgenic plants with altered antioxidant capacity and focuses on the new insight into the antioxidant defence mechanism given by this type of experimental model.     

植物果聚糖的代谢途径及其在植物抗逆中的功能研究进展

植物果聚糖是一类重要的碳水化合物和渗透调节物质,可以提高植物的抗逆性。目前对植物果聚糖代谢酶基因的研究较多,主要包括相关基因的克隆、表达和利用基因工程技术将果聚糖相关代谢基因转入植物中。该文主要介绍了果聚糖的分布、种类、代谢途径及相关基因的克隆和表达,重点阐述了果聚糖在植物抗逆中的作用及其分子生物学研究进展。     

Abiotic stress response in the moss <Emphasis Type="Italic">Physcomitrella patens</Emphasis>: evidence for an evolutionary alteration in signaling pathways in land plants

The mechanisms plants use to adapt to abiotic stress have been widely studied in a number of seed plants. Major research has been focused on the isolation of stress-responsive genes as a means to understand the molecular events underlying the adaptation process. To study stress-related gene regulation in the moss Physcomitrella patens we have isolated two cDNAs showing homology to highly conserved small hydrophobic proteins from different seed plants. The corresponding genes are up-regulated by dehydration, salt, sorbitol, cold and the hormone abscisic acid, indicating overlapping pathways are involved in the control of these genes. Based on the molecular characterization of the moss homologs we propose that signaling pathways in response to abiotic stress may have been altered during the evolution of land plants.Abbreviation ABA Abscisic acid - EST Expressed sequence tag     

Overexpression of Rice Sphingosine-1-Phoshpate Lyase Gene OsSPL1 in Transgenic Tobacco Reduces Salt and Oxidative Stress Tolerance

Sphingolipids, including sphingosine-1-phosphate (S1P), have been shown to function as signaling mediators to regulate diverse aspects of plant growth, development, and stress response. In this study, we performed functional analysis of a rice (Oryza sativa) S1P lyase gene OsSPL1 in transgenic tobacco plants and explored its possible involvement in abiotic stress response. Overexpression of OsSPL1 in transgenic tobacco resulted in enhanced sensitivity to exogenous abscisic acid (ABA), and decreased tolerance to salt and oxidative stress, when compared with the wild type. Furthermore, the expression levels of some selected stress-related genes in OsSPL1-overexpressing plants were reduced after application of salt or oxidative stress, indicating that the altered responsiveness of stress-related genes may be responsible for the reduced tolerance in OsSPL1-overexpressing tobacco plants under salt and oxidative stress. Our results suggest that rice OsSPL1 plays an important role in abiotic stress responses.     

Generation and analysis of drought stressed subtracted expressed sequence tags from safflower (Carthamus tinctorius L.)

Drought is the most crucial environmental factor that limits productivity of many crop plants. Exploring novel genes and gene combinations is of primary importance in plant drought tolerance research. Stress tolerant genotypes/species are known to express novel stress responsive genes with unique functional significance. Hence, identification and characterization of stress responsive genes from these tolerant species might be a reliable option to engineer the drought tolerance. Safflower has been found to be a relatively drought tolerant crop and thus, it has been the choice of study to characterize the genes expressed under drought stress. In the present study, we have evaluated differential drought tolerance of two cultivars of safflower namely, A1 and Nira using selective physiological marker traits and we have identified cultivar A1 as relatively drought tolerant. To identify the drought responsive genes, we have constructed a stress subtracted cDNA library from cultivar A1 following subtractive hybridization. Analysis of?~1,300 cDNA clones resulted in the identification of 667 unique drought responsive ESTs. Protein homology search revealed that 521 (78?%) out of 667 ESTs showed significant similarity to known sequences in the database and majority of them previously identified as drought stress-related genes and were found to be involved in a variety of cellular functions ranging from stress perception to cellular protection. Remaining 146 (22?%) ESTs were not homologous to known sequences in the database and therefore, they were considered to be unique and novel drought responsive genes of safflower. Since safflower is a stress-adapted oil-seed crop this observation has great relevance. In addition, to validate the differential expression of the identified genes, expression profiles of selected clones were analyzed using dot blot (reverse northern), and northern blot analysis. We showed that these clones were differentially expressed under different abiotic stress conditions. The implications of the analyzed genes in abiotic stress tolerance are discussed in our study.     

Understanding Fe2+ toxicity and P deficiency tolerance in rice for enhancing productivity under acidic soils

Plants experience low phosphorus (P) and high iron (Fe) levels in acidic lowland soils that lead to reduced crop productivity. A better understanding of the relationship between these two stresses at molecular and physiological level will lead to development of suitable strategies to increase crop productivity in such poor soils. Tolerance for most abiotic stresses including P deficiency and Fe toxicity is a quantitative trait in rice. Recent studies in the areas of physiology, genetics, and overall metabolic pathways in response to P deficiency of rice plants have improved our understanding of low P tolerance. Phosphorous uptake and P use efficiency are the two key traits for improving P deficiency tolerance. In the case of Fe toxicity tolerance, QTLs have been reported but the identity and role played by underlying genes is just emerging. Details pertaining to Fe deficiency tolerance in rice are well worked out including genes involved in Fe sensing and uptake. But, how rice copes with Fe toxicity is not clearly understood. This review focuses on the progress made in understanding these key environmental stresses. Finally, an opinion on the key genes which can be targeted for this stress is provided.