Drought Tolerance and Recovery in Transgenic and Wild Type Tobacco Plants

                  Genetic engineering of plants that are remarkably adept at flourishing in conditions of drought has become a common practice. A variety of genes have been identified as contributors to this trait in many different plants. However, there is currently great demand for studies measuring the effectiveness of specific genes in specific crop plants with quantitative data. Arif et al. (2012) performed a study on the effect of one gene, Arabidopsis Vacuolar Pyrophosphatase (AVP1), on the drought resistance of tobacco plants. The researchers examined growth performance of transgenic plants overexpressing AVP1 and non-transgenic plants in varying water treatments. Additionally, the cell structures of these two plant types were compared. Interestingly, the data collected on height, mass, seed number, and more from the plants confirmed that transgenic plants overexpressing AVP1 exhibit significantly greater growth under water shortage stress. What’s more, while no statistical significance was established, structural differences were evident between the transgenic and wild-type (WT) plants.—Chad Redman
                  Arif, A., Zafar, Y., Arif, M., Blumwald, E., 2012. Improved Growth, Drought Tolerance, and Ultrastructural Evidence of Increased Turgidity in Tobacco Plants Overexpressing Arabidopsis Vacuolar Pyrophosphatase (AVP1). Molecular Biotechnology
                  Arif et al. examined phenotypic differences between genetically engineered tobacco plants that were designed to over express the gene AVP1 and WT tabacco plants. The objective of this study was to identify the growth difference between this specific transgenic line of tobacco and WT tobacco under differing water stresses. The basic procedural design was to grow both transgenic and WT plants in three different conditions: fully watered, partially limited water, and fully desiccated.
                  The researchers began by breeding their genetically modified tobacco plants, producing several generations so as to ensure that all test plants were expressing the AVP1 gene at the desired level. Tests were conducted to confirm that these plants were producing the correct proteins as an added security against impure transgenic lines. Once these test organisms were prepared, several of the transgenic plants and an equal number of the WT plants were planted in large, identical pots and grown for six weeks under ideal growing conditions. At this point five transgenic tobacco plants and five WT plants were designated as “fully watered” subjects. Similarly, Arif et al.designated five plants of each genotype as “less-watered” and five others as “desiccated.” Over the course of eighteen days, the fully watered plants were maintained at ideal growing conditions, while the less-watered plants received a greatly reduced supply of water. Finally, the desiccated treatment entailed no watering whatsoever. After the eighteen day test period, ideal conditions were resumed for all test groups for five weeks, until harvesting. This period was intended to test for recovery capacity.
                   In order to measure the results of their study, Arif et al. measured the fresh (hydrated) and dry biomass of shoots and capsules from all test tobacco plants, both transgenic and WT. Still more, the mass of seeds produced from all plants was recorded. In order to compare more fundamental differences between the two phenotypes, the researchers used electron microscopy to examine the cell structures of photosynthesizing leaves from each.
                  Results from this straightforward and well-designed experiment were decisive and clear. First, it is important to note that several blotting techniques confirmed the overexpression of the AVP1 gene in test transgenic plants. During the growth period of the experiment, it was clear to researchers that transgenic lines in general produced more numerous capsules and larger leaves than WT plants, and that the two limiting conditions caused far greater wilting and stunting in WT plants. After the recovery period, all plants except the desiccated WT plants survived, and all subjects were harvested. The quantitative data collected on fresh and dry mass of shoots and capsules, dry capsules alone, and dry seeds alone provided Arif et al. with fascinating statistically significant differences. The transgenic and WT plants grown at ideal conditions demonstrated no significant differences, but the other conditions were more disparate. All mass measurements on less-watered and desiccated plants showed that transgenic plants were significantly more successful. In short, the researchers demonstrated that AVP1 overexpression helps tobacco plants cope with drought conditions.
                  Finally, Arif et al. did not identify a significant difference in the number or size of photosynthesizing cells between transgenic and WT tobacco plants. However, they insist that transgenic cells generally displayed larger vacuoles and smoother outline than WT cells, with guard cells boasting thicker cell walls. While this result is not decisive, it provides some notion of what is the structural difference may be that allows plants overexpressing AVP1 to survive water shortage more successfully than WT plants.

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