Sonali Sengupta

Insight into the salt tolerance factors of a wild halophytic rice, Porteresia coarctata: a physiological and proteomic approach.

Salinity poses a serious threat to yield performance of cultivated rice in South Asian countries. To understand the mechanism of salt-tolerance of the wild halophytic rice, Porteresia coarctata in contrast to the salt-sensitive domesticated rice Oryza sativa, we have compared P. coarctata with the domesticated O. sativa rice varieties under salinity stress with respect to several physiological parameters and changes in leaf protein expression. P. coarctata showed a better growth performance and biomass under salinity stress. Relative water content was conserved in Porteresia during stress and sodium ion accumulation in leaves was comparatively lesser. Scanning electron microscopy revealed presence of two types of salt hairs on two leaf surfaces, each showing a different behaviour under stress. High salt stress for prolonged period also revealed accumulation of extruded NaCl crystals on leaf surface. Changes induced in leaf proteins were studied by two-dimensional gel electrophoresis and subsequent quantitative image analysis. Out of more than 700 protein spots reproducibly detected and analyzed, 60% spots showed significant changes under salinity. Many proteins showed steady patterns of up- or downregulation in response to salinity stress. Twenty protein spots were analyzed by MALDI-TOF, leading to identification of 16 proteins involved in osmolyte synthesis, photosystem functioning, RubisCO activation, cell wall synthesis and chaperone functions. We hypothesize that some of these proteins confer a physiological advantage on Porteresia under salinity, and suggest a pattern of salt tolerance strategies operative in salt-marsh grasses. In addition, such proteins may turn out to be potential targets for recombinant cloning and introgression in salt-sensitive plants.

Significance of galactinol and raffinose family oligosaccharide synthesis in plants

Abiotic stress induces differential expression of genes responsible for the synthesis of raffinose family of oligosaccharides (RFOs) in plants. RFOs are described as the most widespread D-galactose containing oligosaccharides in higher plants. Biosynthesis of RFOs begin with the activity of galactinol synthase (GolS; EC 2.4.1.123), a GT8 family glycosyltransferase that galactosylates myo-inositol to produce galactinol. Raffinose and the subsequent higher molecular weight RFOs (Stachyose, Verbascose, and Ajugose) are synthesized from sucrose by the subsequent addition of activated galactose moieties donated by Galactinol. Interestingly, GolS, the key enzyme of this pathway is functional only in the flowering plants. It is thus assumed that RFO synthesis is a specialized metabolic event in higher plants; although it is not known whether lower plant groups synthesize any galactinol or RFOs. In higher plants, several functional importance of RFOs have been reported, e.g., RFOs protect the embryo from maturation associated desiccation, are predominant transport carbohydrates in some plant families, act as signaling molecule following pathogen attack and wounding and accumulate in vegetative tissues in response to a range of abiotic stresses. However, the loss-of-function mutants reported so far fail to show any perturbation in those biological functions. The role of RFOs in biotic and abiotic stress is therefore still in debate and their specificity and related components remains to be demonstrated. The present review discusses the biology and stress-linked regulation of this less studied extension of inositol metabolic pathway.

Inositol methyl tranferase from a halophytic wild rice, Porteresia coarctata Roxb. (Tateoka): regulation of pinitol synthesis under abiotic stress

Methylated inositol D-pinitol (3-O-methyl-D-chiro-inositol) accumulates in a number of plants naturally or in response to stress. Here, we present evidence for accumulation and salt-enhanced synthesis of pinitol in Porteresia coarctata, a halophytic wild rice, in contrast to its absence in domesticated rice. A cDNA for Porteresia coarctata inositol methyl transferase 1 (PcIMT1), coding for the inositol methyl transferase implicated in the synthesis of pinitol has been cloned from P. coarctata, bacterially overexpressed and shown to be functional in vitro. In silico analysis confirms the absence of an IMT1 homolog in Oryza genome, and PcIMT1 is identified as phylogenetically remotely related to the methyl transferase gene family in rice. Both transcript and proteomic analysis show the up-regulation of PcIMT1 expression following exposure to salinity. Coordinated expression of L-myo-inositol 1-phosphate synthase (PcINO1) gene along with PcIMT1 indicates that in P. coarctata, accumulation of pinitol via inositol is a stress-regulated pathway. The presence of pinitol synthesizing protein/gene in a wild halophytic rice is remarkable, although its exact role in salt tolerance of P. coarctata cannot be currently ascertained. The enhanced synthesis of pinitol in Porteresia under stress may be one of the adaptive features employed by the plant in addition to its known salt-exclusion mechanism.

Porteresia coarctata (Roxb.) Tateoka, a wild rice: a potential model for studying salt-stress biology in rice.

Porteresia coarctata (Syn = Oryza coarctata) is a tetraploid wild rice growing abundantly in the coastal region of India and some other Asian countries. The salt tolerance property of this mangrove associate has been dealt with by a number of workers earlier. The distinct morphology and leaf architecture enabling the plant to exclude salt is a characteristic feature of Porteresia in comparison with Oryza sp. A number of genes have been isolated and characterized from Porteresia that are related to the salt-tolerance property of the plant. Evidence have accumulated that some pathways critical to salt tolerance are in operation in Porteresia of which the inositol metabolic pathway has been recently elaborated. Some of the enzymes of Porteresia have been shown to function as salt-tolerant under in vitro studies giving a clue that this wild halophytic rice may have evolved genes and proteins capable of functioning under a salt environment. Bioprospecting of such genes and proteins coupled with genomic and proteomic approaches remain an exciting area of research in evaluating this plant as a model for salt tolerance for the rice plant.

Osmolyte Regulation in Abiotic Stress

To withstand osmotic stress induced by salinity, drought or extreme temperatures, all organisms have evolved a machinery to synthesize metabolites, termed “compatible solutes” or “osmo-protectants”, which help in raising the osmotic pressure and thereby maintaining both the turgor pressure and the driving gradient for water uptake. In addition, these compounds also help in maintaining the structural integrity of enzymes, membranes and other cellular components during the stress regime. Of special importance among these metabolites is nitrogen containing compounds (e.g., quaternary amino compounds and proline) and hydroxyl compounds (e.g., polyols and oligosaccharides). These compounds are distributed throughout the biological kingdom and are generally products of stress-induced pathway extensions, although normal metabolites such inositols may also act as osmolytes. Chemically, different osmolytes function through a common mechanism of stabilization of proteins under stress or by osmotic adjustments, and these mechanisms seem to be universal among the biological system. Over-expression of genes for the synthesis of different osmolytes in transgenics enables the plants to cope better with the stress due to higher accumulation of the concerned osmolytes. However, in several cases, such as trehalose and inositol, the accumulation is far below the required amount and it is conjectured that these metabolites might function in a manner unrelated to their osmolyte role and are hence more involved in the general growth and development of the plants under abiotic stress conditions.

Galactinol synthase across evolutionary diverse taxa: functional preference for higher plants?

Galactinol synthase (GolS), a GT8 family glycosyltransferase, synthesizes galactinol and raffinose series of oligosaccharides (RFOs). Identification and analysis of conserved domains in GTs among evolutionarily diverse taxa, structure prediction by homology modeling and determination of substrate binding pocket followed by phylogenetic analysis of GolS sequences establish presence of functional GolS predominantly in higher plants, fungi having the closest possible ancestral sequences. Evolutionary preference for a functional GolS expression in higher plants might have arisen in response to the need for galactinol and RFO synthesis to combat abiotic stress, in contrast to other organisms lacking functional GolS for such functions.

Targeted expression of L-myo- inositol 1-phosphate synthase from Porteresia coarctata (Roxb.) Tateoka confers multiple stress tolerance in transgenic crop plants

Improving crop tolerance to osmotic stresses is a longstanding goal of agricultural biotechnology. In the present work the PcINO1 gene coding for a salt-tolerant L-myo-inositol-1-phosphate synthase (MIPS) from Porteresia coarctata (Roxb.) Tateoka, a halophytic wild rice was introgressed into cultivated mustard, Brassica juncea var B85. The transgenic plants demonstrate increased tolerance to salinity and oxidative stress with elevated level of inositol in both roots and shoots. The yield and crop quality of transgenic Brassica plants remain uncompromised and the plants were able to stably grow, set seeds and germinate in saline environments. When targeted to seeds of Nicotiana, PcINO1 was able to improve the seed survival rate under salinity and dehydration. Inositol and its derivatives regulate stress responses in various ways, serving as compatible solutes or signaling molecules. It is implicated that engineering inositol metabolism may affect the plant metabolic network leading to a stress tolerant phenotype as enumerated here in transgenic crop plants. How inositol itself or its derivatives affect the overall metabolic pathways leading to a stress-tolerant phenotype remains an intriguing question for future investigations.

Physiological and genomic basis of mechanical-functional trade-off in plant vasculature.

Some areas in plant abiotic stress research are not frequently addressed by genomic and molecular tools. One such area is the cross reaction of gravitational force with upward capillary pull of water and the mechanical-functional trade-off in plant vasculature. Although frost, drought and flooding stress greatly impact these physiological processes and consequently plant performance, the genomic and molecular basis of such trade-off is only sporadically addressed and so is its adaptive value. Embolism resistance is an important multiple stress- opposition trait and do offer scopes for critical insight to unravel and modify the input of living cells in the process and their biotechnological intervention may be of great importance. Vascular plants employ different physiological strategies to cope with embolism and variation is observed across the kingdom. The genomic resources in this area have started to emerge and open up possibilities of synthesis, validation and utilization of the new knowledge-base. This review article assesses the research till date on this issue and discusses new possibilities for bridging physiology and genomics of a plant, and foresees its implementation in crop science.

Manipulation of inositol metabolism for improved plant survival under stress: a “network engineering approach”

Emergence of high-throughput sequencing tools and omics technologies paved the way for systems biology in last decade. While we have started to look at the biology of the plant in a more unified manner, the integration of such knowledge in agricultural biotechnology has become an arena of potential interest. The network of several central molecules operating in various life and developmental processes are now more adequately known, and fine tuning of such molecule pools, if connected to stress response, can result in enhanced stress tolerance of plants.This review interprets the potential of manipulation of myo-inositol and its derivatives in generation of transgenic crop plants. Being a molecule of central importance in plant life, inositol is connected to numerous life processes. The exploration of such pathways indicates that inositol itself and many of its derivatives can impart abiotic stress tolerance (against salinity, dehydration, chilling or oxidative stress) to plants when overexpressed. We propose that engineering inositol metabolic network is a potential approach towards stress-tolerant transgenic crop plant generation and thus its exploitation in agricultural biotechnology is the call of time.

Improvement of Plants through Biotechnology: An Assessment of the Present Status

Advancement in transgenic research has proved its potential by allowing introduction of different genes into the plant genome for better agronomic traits and thus presenting successfully the first generation of genetically modified crop plants. World agriculture has experienced the cultivation of genetically modified crops since last twenty years. Inspiration from the success of first generation GM crops, academia and Industrial research reorient themselves towards the development of second generation GM plants by introducing other value added traits like nutritional value, improved insect and disease resistance and better adaptive features in adverse environmental conditions with special reference to drought, cold and saline environment. In planta production of valuable vaccines and many other pharmaceutical products against different human diseases has also been achieved successfully. Introduction of multiple traits together in some cases has created an example of generating transgenic plants in a composed way. However, there is no end of improving techniques and identifying different beneficial genes of our interest. The togetherness of these two factors always leaves scopes behind to improve the trends of transgenic research further. Nonetheless, appropriate measures to tackle the biosafety and regulatory issues are to be kept in mind prior to commercialization of the genetically modified (GM) products. As science breaths well in social acceptability, production of the GM products need to address consumers’ query in the most scientific manner to allow further use of this technology for human welfare.