Genetic Engineering Of Osmolytes For Improving Abiotic Stress Tolerance

GENETIC ENGINEERING OF PROLINE

Scientists advanced transgenic cultivars overexpressing the P5CS gene from Vigna aconitifolia (moth bean). The converted cultivars showed enlarged proline accumulation compared with the nontransformed wild plants by 10- to 18-fold. The enhanced levels of proline contributed to flower development and enlargement of root and plant dry biomass.

The rice OsP5CS1 and OsP5CS2 genes were expressed together in tobacco.The transgenic cultivars of the second generation (T2) recorded 3.2-fold increased proline accumulation along with improved root length and mean fresh weight in comparison with its wild type counterparts treated with 200 mM NaCl.

Coexpression of the Phaseolus vulgaris P5CS (PvP5CS1 and PvP5CS2) genes in the model plant, A. thaliana. The transgenic lines were subjected to different salt concentrations (0, 100, and 200 mM NaCl) and they accumulated 1.6 times and 1.9 times more proline at 100 and 200 mM NaCl concentration respectively.

GENETIC ENGINEERING OF GLYCINE BETAINE

Scientists exploited the genes involved in the GB biosynthetic pathway (bacterial codA, CMO, and BADH) to develop transgenic varieties of agriculturally important crops such as rice, potato, soybean, groundnut, and maize, among other plants. Significant increase in yield and growth parameters was observed in barley, soybean, wheat, maize, tobacco, beans, and sunflower exposed to abiotic stresses.

The codA gene of Arthobacter spp. has been introduced in many plant species such as A. thaliana, Solanum tuberosum, Zea mays, S. lycopersicum, and Lycopersicon. The transgenic lines showed increased GB accumulation, which enhanced photosynthetic activity, plant development, and crop yield under abiotic stress conditions.

The A.globiformis codA gene in potato and exposed the plants to salt (100 mM NaCl) and low temperature (4C) stress. The transformed varieties were characterized by normal growth, improved plant biomass, and high tuber yield both under salt and low temperature stress. The transgenic potato tuber yield was reported to be 44 g/plant while it was only 33.6 g/plant in the wild type (WT) plants.

Transformatio of tomato with the same A. globiformis codA gene. The transgenic varieties exhibited increased GB accumulation in comparison with the nonaccumulator wild types. Moreover there was significant improvement in photosynthetic rates due to restoration of chlorophyll content, and activity of antioxidant enzymes in lowering ROS content.

The BADH gene coding for BADH enzyme that regulates the oxidation of betaine aldehyde to GB has also been exploited to enhance GB accumulation in sweet potato. Development of transgenic sweet potato harboring the SoBADH gene isolated from Spinacia oleracea. The amount of GB accumulation in the transgenic plants doubled conferring tolerance to stress.

Transgenic tomato carrying the SoBADH gene exhibited high photosynthetic capacity even at 420C. Enhanced GB accumulation increased the D1 protein content thereby preventing photosystem II from denaturation. Moreover, there was significant decrease in H2O2 content and superoxide radical in the transgenic tomato plants.

GB accumulation in chloroplast of transgenic wheat lines harboring the BADH gene isolated from cyanobacteria is reported about170.7 μmol/g dry weight under salt and drought stress. These experiments highlight the regulatory role of GB in improving plant growth and development under abiotic stress conditions.

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GENETIC ENGINEERING OF MANNITOL

Introduction of the Cucurbita ficifolia spermidine synthase gene (CsSPDS) in the model plant A. thaliana under the regulation of aconstitutive cauliflower mosaic virus 35S (CaMV 35S) promoter. The transformed A. thaliana lines exhibited twofold more accumulation of spermidine under salt (200 mM NaCl), low temperature (40C), and drought stress when compared with its wild types.

Scientists engineered rice with the Datura stramonium ADC gene (DsADC) and expressed it under the inducible monocot Ubi-1 promoter. The transformed rice varieties showed threefold increase in endogenous putrescine levels and also facilitated spermidine and spermine synthesis under drought stress.

Development of transgenic tobacco expressing the D. stramonium SPDS gene (DsSPDS) and reported high spermidine accumulation, restoration of tissue water, and chlorophyll content facilitating plant growth and development under 200 mM NaCl stress. The transgenic lines exhibited high ROS scavenging that helped in shielding membrane and protein structures from denaturation.

GENETIC ENGINEERING OF TREHALOSE

The rapid progresses in the field of genetic engineering has enabled researchers to harness the potential trehalose biosynthetic genes (TPP and TPS) from plants as well as prokaryotes and develop transgenic varieties. These transgenic lines are designed to enhance endogenous trehalose accumulation, which in turn protects the plants from the deleterious effects of abiotic stress.

The ScTPS1 gene was isolated from yeast and used to develop the first transgenic tobacco plants expressing the TPS enzyme. The trehalose accumulation in the transgenic cultivars was recorded as 0.17 mg/g fresh weight in leaves. Though the plants exhibited stunted growth and lancet-shaped leaves they were tolerant to prolonged drought exposure.

The yeast (ScTPS1) gene was overexpressed in transgenic potato and tomato respectively under the regulation of a 35S CaMV promoter and exposed to salt, drought, and cold stress. Both the transgenic plants showed enhanced trehalose accumulation leading to abiotic stress tolerance.

Development of transgenic rice varieties expressing the E. coli trehalose biosynthetic genes (otsAand otsB) under the control of a stress responsive promoter. The transformed plants accumulated 3- to 10-fold more trehalose resulting in improved growth, less photooxidative damage, and maintenance of mineral homeostasis under salt, drought, and low temperature. Leaf rolling and wilting were common in nontransformed plants whereas the transformed plants exhibited significant shoot development.

Scientists expressed Porphyra yezoensis (PyTPS) gene in a particular rice variety (TP309). The transgenic plants were characterized with improved seed germination rate and yield parameters due to increased trehalose accumulation.

GENETIC ENGINEERING OF MANNITOL

Introduction of the E. coli (mtlD) gene in potato and exposed both the transformed and the control varieties to100 mM NaCl stress. The transformed plants only observed 17.3% reduction in shoot fresh weight while the reduction was 76.5% weight in the nontransformed potato lines.

Scientists designed a gene construct of Hordeum vulgare HVA1 (a group 3 LEA protein) and E. coli(mtlD) genes under the control of rice actine promoter (Act1). This construct was expressed in maize exposed to high salinity (300 mM NaCl) and drought stress. The successfully transformed varieties expressing the HVA1 1 mtlD construct exhibited better shoot and root biomass and sustained throughout the stress period while their wild type counterparts failed to survive.

REFERANCE

1. DUTTA, T., NEELAPU, N.R.R., WANI, SHABIR, H. AND SUREKHA, C., 2019, Role and Regulation of Osmolytes as Signaling Molecules to Abiotic Stress Tolerance. Plant signaling molecules.,30: 459-477.
2. HASANUZZAMAN, M., ANEE, TAUFIKA. I., BHUIYAN, TASMIN.F., NAHAR, K. AND FUJITA, M., 2019, Emerging role of osmolytes in enhancing abiotic stress tolerance in rice. Advances in Rice Research for Abiotic Stress Tolerance., 33: 677-708.
3. JOGAWAT, A., 2019, Osmolytes and their Role in Abiotic Stress Tolerance in Plants. Molecular Plant Abiotic Stress: Biology and Biotechnology, First Edition 5: 99-104.
4. SUPRASANNA, P., NIKALJE, G.C. AND RAI, A.N., 2016, Osmolyte Accumulation and Implications in Plant Abiotic Stress Tolerance. Emerging Omics Technologies: 1-13.