Phenotyping transgenic wheat for drought resistance

Drought is one of the largest environmental stress factors that agriculturalist struggle with because it limits plant growth and crop productivity globally. Plant responses to drought often have complex mechanisms which are normally under multigenic control. Time, intensity, duration, plant-soil-atmosphere interactions, and frequency of drought stress can all affect the plants response. St. Pierre et al. (2012) evaluated field performances of 14 transgenic wheat lines previously selected under greenhouse conditions for survival to severe drought and high water use efficiency. The study was conducted under various water regimes in field conditions to compare biomass production and yield performance between transgenic lines and control lines. Researchers over-expressed dehydration-responsive element-binding (DREB) transcription factors that were previously found to enhance drought resistance in transgenic plants like tomatoes, peanuts, rice, barley, and wheat (Kasuga et al., 2009). The paper found some significant conclusions by transforming DREB1A from Arabidopsis thaliana under the stress-inducible promoter rd29A. Researchers found that the survival rate of transgenic plants was increased without growth retardation and a positive association between WUE and total biomass. Moreover, researchers found that some transgenic plants produced a higher yield under well-irrigated field conditions. Finally, the 14 transgenic lines were evaluated in the field and showed no pleiotropic effects or unpredictable unwanted events. This study suggests that plants can be transformed to recover better after severe water stress. Thus, a greater resilience towards droughts could be extremely useful when growing in dry climates or in drought prone areas. —Paloma Medina
Saint Pierre, Carolina, et al., 2012. Phenotyping transgenic wheat for drought resistance. Journal of experimental botany 63, 1799–1808.

Researchers transformed bread wheat (Triticum aestivum), or Bobwhite, with the DREB1A gene from Arabidopsis thaliana under the stress-inducible promoter rd29A. Under this promoter, the target DREB1A gene would only be expressed under stress conditions. The transgenic lines were screened for the DREB1A gene by polymerase chain reaction. The integration of the DREB1A gene was further confirmed by self-pollination and examining the T1 generation to find that DREB1A expression. Thus, the researchers confirmed that DREB1A was integrated into the genome of the transgenic plants.
Fourteen transgenic wheat lines with the DREB1A gene were examined to assess biomass production (BM) and yield performance (YLD) of transgenic plants relative to control lines under different water regimes in field conditions. Three water deficit treatments: severe stress (DEF), terminal water deficit starting at anthesis (ANT), and terminal water deficit starting in grain filling (GF). Anthesis is the point at which a flower is open and functional. These treatments were applied by reducing irrigation starting at different phonological stages. Well-irrigated plots (IRR) received a total of 260 mm of irrigation water throughout the growing season. Conversely, plots under the most severe water deficit treatment (DEF) received a total of 68 mm from irrigation, only 26% of the amount applied to IRR. Plots undergoing the ANT treatment were well-irrigated with a total of 121 mm of water until the booting stage (occurs shortly after flag leaf emergence) of the plant. Lastly, the effect of terminal stress starting in grainfill was evaluated by stopping the irrigation at the early grainfill stage in the GF group.
The evaluation of above-ground biomass and water use efficiency (WUE) took place shortly after anthesis on drought-stressed and well-irrigated plants grown in small pots under greenhouse conditions. On average, drought reduced biomass by 42% after repeated drought cycles imposed from the 5-leaf stage to anthesis. The lowest biomass reduction occurred at 19% in the transgenic line WUE-12 between the well-irrigated and the water-deficit treatments. The control (Bobwhite) line had a biomass reduction of 46%. Moreover, all the transgenic plants selected for high WUE had higher biomass prodction than the null and control line.
In addition to a larger biomass in the transgenic lines, significant differences were observed for water use efficiency (WUE) under water-deficit. All WUE-selected events tended to have higher WUE than controls under well-irrigated conditions, but only one out of the five events was significant. A significant positive correlation was drawn between BM and WUE bother under well-irrigated conditions (r=0.72, P
Plant performance was evaluated under field conditions and found that irrigation regimes had a significant main effect for both biomass (BM) and grain yield (YLD). The total biomass was reduced from 504 g m–2at well-irrigated conditions (IRR) to 325 g m–2 at severe stress conditions (DEF). The transgenic event WUE-11 outperformed the control lines for YLD under IRR conditions. This line had a yield of 366 g m–2under IRR while Bobwheat and the null event yielded 297 g m–2 and 310 g m–2 respectively.
In conclusion, screening genetic lines for drought resistance characteristics can be fundamental to the identification of high performance lines. The researchers of this study suggest that several studies involving transgenic crops may be misleading due to the fact that the plants were grown under artificial stress conditions. The importance of this study is that the plants were tested under field conditions with aims to mimic natural conditions as closely as possible. The results of this experiment support the function of the DREB gene in which transgenic crops were observed to have a higher survival rate and recovery after severe water deficit. Wheat growers can benefit tremendously from the grain yielding potential and drought resistant crop growth under a warming climate. Moreover, the consumer population would also benefit from this transgenic drought-resistant line of crops. Further research is suggested to determine and identify appropriate gene and gene-promoter combinations to maximize drought-resistance and crop yield.

Works Citied
Kasuga M, Liu Q, Miura S, Yamaguchi-Shinozaki K, Shinozaki K. 1999. Improving plant drought, salt, and freezing tolerance by gene transfer of a single stress-inducible transcriptionfactor. Nature Biotechnology 17, 287–291.

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