Exploring the possibility of transforming food crops for salinity tolerance using the TMT gene encoding thiol methyltransferase enzyme
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Soil salinity is a serious environmental stress threatening productivity of major crops worldwide. Among the various biotic and abiotic strategies that exist, transgenic technologies provide a promising avenue to reduce yield losses in crops under saline environments. Recently, transgenic technology involving the TMT gene encoding thiol methyltransferase enzyme has been suggested as an effective solution for engineering a chloride detoxification capability into a high value crops to improve tolerance against chloride ion toxicity under saline environments. This proposed mechanism, however, results in the emission of methyl chloride (CH3Cl) from plants, which has deleterious effects on stratospheric ozone. This study was performed to examine the relationship between salt tolerance and chloride volatilizing capacity of transgenic plants containing TMT gene as well as to explore the possibility of generating transgenic rice crop containing TMT gene for salinity tolerance. To achieve these objectives, transgenic tobacco plants containing TMT gene were grown in comparison with wild type tobacco plants under three levels of sodium chloride (NaCl) salinity (0, 100 and 200 mM), three levels of soil water content (40%, 60% and 80% of the field capacity) and their tolerance to NaCl and water stress was studied. Plant growth parameters recorded included plant height, number of leaves, leaf area, stem dry weight, leaf dry weight, root dry weight, plant dry biomass and root/shoot ratio. Similarly, both types of plants were exposed to five levels of NaCl concentrations (0, 50, 100, 150 and 200 mM) and three levels of soil water content (40%, 60% and 80% of the field capacity), and the quantity of CH3Cl emitted was recorded. Significant decrease in plants growth parameters of both types of plants were recorded upon exposure to salinity and water stress. Under 100 mM NaCl, however, transgenic plants showed better tolerance to salinity by suffering less reduction in growth parameters compared to wild type plants. Under 200 mM NaCl, growth of both types of plants was completely inhibited. The interactive effects of salinity and water stress were more pronounced in wild type plants than in transgenic plants. Results also showed that all engineered plants acquired an ability to efficiently transform chloride ion to CH3Cl, and the rate of such transformation was higher under greater NaCl and soil water content compared to lower NaCl concentrations and soil water content. In order to explore the possibility of generating a transgenic food crop using TMT gene, a hypothetical transgenic rice crop was grown over 27 million hectares of the saline coastal areas of south and southeast Asia and the possible emission of CH3Cl from such ecosystem was inferred based on the CH3Cl emission data obtained from transgenic tobacco plants. The estimates showed that the possible CH3Cl emission from such ecosystem would be 219.21 Gg which is equivalent to 5.36 % of the global atmospheric emissions of CH3Cl.