Salt Tolerance in Rice: Focus on Mechanisms and Approaches

CRISPR-Cas9 directed genome engineering for enhancing salt stress tolerance in rice.

Salt affected soil inhibits plant growth, development and productivity, especially in case of rice crop. Ion homeostasis is a candidate defense mechanism in the salt tolerant plants or halophyte species, where the salt toxic ions are stored in the vacuoles. The aim of this investigation was to determine the OsNHX1 (a vacuolar Na+/H+ exchanger) and OsHKT2;1 (Na+/K+ transporter) regulation by salt stress (200 mM NaCl) in two rice cultivars, i.e. Pokkali (salt tolerant) and IR29 (salt susceptible), the accumulation of Na+ in the root and leaf tissues using CoroNa Green® staining dye and the associated physiological changes in test plants. Na+ content was largely increased in the root tissues of rice seedlings cv. Pokkali (15 min after salt stress) due to the higher expression of OsHKT2;1 gene (by 2.5 folds) in the root tissues. The expression of OsNHX1 gene in the leaf tissues was evidently increased in salt stressed seedlings of Pokkali, whereas it was unchanged in salt stressed seedlings of IR29. Na+ in the root tissues of both Pokkali and IR29 was enriched, when subjected to 200 mM NaCl for 12 h and easily detected in the leaf tissues of salt stressed plants exposed for 24 h, especially in cv. Pokkali. Moreover, the overexpression of OsNHX1 gene regulated the translocation of Na+ from root to leaf tissues, and compartmentation of Na+ into vacuoles, thereby maintaining the photosynthetic abilities in cv. Pokkali. Overall growth performance, maximum quantum yield (Fv/Fm), photon yield of PSII (ΦPSII) and net photosynthetic rate (Pn) was improved in salt stressed leaves of Pokkali than those in salt stressed IR29.

Expression levels of the Na+/K+ transporter OsHKT2;1 and vacuolar Na+/H+ exchanger OsNHX1, Na enrichment, maintaining the photosynthetic abilities and growth performances of indica rice seedlings under salt stress.

Rice is an important cereal crop responsible for world’s food security. The sensitivity of rice plants towards a range of abiotic stresses is a prime challenge for its overall growth and productivity. Among these, salinity is a major stress which results in a significant loss of global rice yield annually. For finding straightforward and strict future solutions in order to assure the food security to growing world population, understanding of the various salt stress tolerance mechanisms in rice is of paramount importance. Plants combat the effects of salt stress through various mechanisms that operate at different levels. Diverse strategies have been adopted for understanding and unraveling the key components involved in these complex mechanisms. In classical studies, identification of salt tolerant cultivars and the genetic markers linked to salt tolerance and breeding approaches have been give emphasis for developing salt tolerance in rice. However, only limited success has been achieved in these approaches as salt tolerance is a complex process and is governed by multiple factors. Hence, for better understanding of salt tolerance mechanisms, a comprehensive approach involving physiological, biochemical and molecular studies is much warranted. Modern experimental and genetic resources have provided a momentum in this direction. Integrating the knowledge of classical and modern research in understanding of salt stress adaptive pathways will facilitate for designing effective strategies to combat salt stress. Hence, the present review focuses on the interdependency of classical and modern approaches and their integration for better understanding of salt stress tolerance mechanisms in rice.

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Integrating Classical with Emerging Concepts for Better Understanding of Salinity Stress Tolerance Mechanisms in Rice

Abstract Background Plant growth-promoting endophytic fungi (PGPEF) that are associated with halophytes have the potential to boost crop salinity tolerance and productivity. This in turn has the potential of enabling and improving cultivation practices in coastal lands affected by salt stress. Methods Endophytic fungi from the wild halophytic rice Oryza coarctata were isolated, characterized, identified, and studied for their effects on all developing stages of rice plant growth and their yields both with and without salt stress. Key results In this study, three different fungal endophytes were isolated from the halophytic wild rice Oryza coarctata . Two isolates were identified as Talaromyces adpressus (OPCRE2) and Talaromyces argentinensis (OPCRh1) by ITS region sequencing. The remaining isolate NPCRE2 was confirmed as a novel strain named Aspergillus welwitschiae Oc streb1 ( AwOc streb1) by whole genome sequencing. These endophytes showed various plant growth-promoting (PGP) abilities in vitro (e.g., IAA, ACC-deaminase and siderophore production, phosphate, and zinc solubilization as well as nitrogen fixation), where AwOc streb1 was significantly more efficient compared to the other two isolates at high salinity (900 mm). Independent application of these fungi in commercial rice ( Oryza sativa ) showed significant elevation in plant growth, especially in the case of the AwOc streb1 inoculants, which had enhanced metabolite and chlorophyll content at the seedling stage in both no-salt control and 100-m m salt-stressed plants. At the same time, AwOc streb1-treated plants had a significantly lower level of H 2 O 2 , electrolyte leakage, and Na + /K + ratio under saline conditions. Higher expression (1.6 folds) of the SOS1 (salt overly sensitive 1) gene was also observed in these plants under salinity stress. This strain also improved percent fertility, tillering, panicle number, and filled grain number in both no-salt control and 45-m m salt-stressed inoculated plants at the reproductive stage. Consequently, the differences in their yield was 125.16% and 203.96% ( p &lt; 0.05) in colonized plants in normal and saline conditions, respectively, compared to uninoculated controls. Conclusions We propose that AwOc streb1 is a potential candidate for an eco-friendly biofertilizer formula to improve the cultivation and yield of rice or any other crop in the highly saline coastal regions of Bangladesh. 

Plant growth-promoting endophytic fungi of the wild halophytic rice Oryza coarctata

At the cellular level, the Salt Overly Sensitive (SOS) signaling pathway comprising SOS3 , SOS2 , and SOS1 has been proposed to mediate cellular signaling under salt stress to maintain ion (Na+) homeostasis. In this regulatory pathway, both OsSOS1 encoding plasma membrane and OsNHX1 encoding vacuolar Na+/H+ antiporters are regulated by SOS3 ‐ SOS2 protein kinase complex. In the present study, the rice variety BRRI dhan28 - which is popular with farmers and high yielding, but salt sensitive, was transformed with the OsSOS1 gene isolated from salt tolerant Pokkali rice and driven by the constitutive promoter, CaMV35S. The construct was transformed through a tissue culture-independent Agrobacterium mediated in planta transformation method that circumvents the problems associated with tissue culture-based indica rice transformation methods. Integration of the foreign genes ( OsSOS1 ) into the genome of transgenic plants was confirmed by gene-specific PCR and Southern blot analysis. The level of transgene expression ( SOS1 ) was also quantified by semi-quantitative RT PCR and real time PCR. Genetic segregation ratio for T 1 progenies was calculated and found to follow the Mendelian law of inheritance in case of positive transformants. The transformants were shown to be salt tolerant compared to wild type in molecular analysis as well as physiological screening. Future work will involve transformation of both the OsSOS1 and OsNHX1 genes together; with the expectation for enhancing the tolerance level compared to currently available transgenic rice. Plant Tissue Cult. & Biotech. 25(2): 257-272, 2015 (December)

/pdf/constitutive-overexpression-of-the-plasma-membrane-na-h-2fc8727z4n.pdf

Constitutive Overexpression of the Plasma Membrane Na+/H+ Antiporter for Conferring Salinity Tolerance in Rice

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Constitutive Overexpression of the Plasma Membrane Na+/H+ Antiporter for Conferring Salinity Tolerance in Rice