The drought response of the extremophile Eutrema salsugineum (Thellungiella salsuginea) was studied using an experimental protocol involving two progressive drought exposures separated by a recovery period. Accessions from the Yukon Territory, Canada, and Shandong Province, China, were distinguished with respect to their responses to the initial drought, their recovery from wilting, and their response to a subsequent drought following recovery. Eutrema cauline leaves and rosettes were sampled at different stages of the drought treatment for water status and biomass measurements and this information guided tissue selection for transcriptome sequencing by RNA-Seq. For Yukon plants, the initial drought led to a 46% reduction in stomatal conductance (from 122.3 to 66.3 mol m-2s-1) and 25% reduction in rosette water loss relative to unstressed control plants, evidence of drought avoidance to conserve water. Yukon leaf solute potentials decreased to -1.83 MPa compared to -1.54 MPa for Shandong leaves indicating that more solutes accumulated in Yukon leaves in response to drought. Upon wilting, Yukon plants re-established turgor at significantly lower leaf solute potentials than the level for well-watered Yukon plants consistent with osmotic adjustment. In contrast, leaf solute potentials in re-watered Shandong plants returned to pre-drought levels (-1.6 MPa). During the second drought exposure, leaf water content and specific leaf area measurements were significantly higher in Yukon plants compared to plants experiencing the initial drought and wilting was delayed relative to Shandong plants. At the transcriptional level, the initial drought exposure resulted in over 2000 differentially expressed genes in leaves of Yukon plants compared to only two in Shandong plants. Following exposure to a second drought only 45 genes were differentially expressed in leaves of Yukon plants while Shandong plants underwent substantial transcriptional re-programming with nearly 500 genes showing differential expression. Studies of Eutrema grown under controlled conditions were supplemented by physiological measurements made using Eutrema plants found on saline soils in the Yukon. The average stomatal conductance for field plants was 84.8 mol m-2s-1, a rate similar to that of drought-treated Yukon plants in the cabinet. Leaf solute potentials of field plants ranged from -2.0 MPa to -3.5 MPa. RT-qPCR showed the relative expression of four dehydrin-encoding genes, EsRAB18, EsRD22, EsRD29A, and EsERD1, was high in the field plants and levels of expression were comparable to drought-stressed cabinet plants. In summary, Eutrema salsugineum has a naturally high tolerance to water deficits. Between the two accessions studied, Yukon plants have a superior capacity to withstand drought relative to Shandong plants. The heightened capacity for Yukon plants to recover from drought and tolerate repeated drought exposures makes this accession a particularly valuable model for studying many mechanisms underlying innate and inducible plant tolerance to drought. / Thesis / Doctor of Philosophy (PhD)

>Magister Scientiae - MSc / Drought is a major abiotic stress which causes not only differences between the mean yield and the potential yield but also yield variation from year to year. Although selection for genotypes with improved productivity under drought environments has been a central goal of numerous plant breeding programs, the molecular basis for plant tolerance towards drought stress is still poorly understood. Exposure of plants to this abiotic stress is known to trigger excessive formation of reactive oxygen species (ROS), which induce cell death and reduce growth. Part of the mechanism of plant responses to drought involves alterations in the expression of antioxidant enzymes and biosynthesis of different compatible solutes such as proline. Sorghum is regarded as generally more drought tolerant than maize, and it is a potential key model system for investigating the physiological and molecular mechanisms conferring drought tolerance. Comparative studies in crop plants to decipher differences in drought tolerance are essential for crop improvement to sustain a higher level of production, which in turn will improve food security, under severe drought conditions resulting from climate change. On this basis, the aim of this study is to determine molecular differences between Zea mays and Sorghum bicolor in response to drought stress in an attempt to identify novel biomarkers for drought tolerance. The physiological and molecular responses of maize and sorghum were studied for changes in growth, chlorophyll content, relative water content, ROS content, lipid peroxidation level, proline content, and antioxidant enzyme activity. Spectral Count Label-free Quantitation analysis was conducted to reveal the changes in protein profiles under drought in attempt to identify drought-responsive molecular mechanisms in the leaves of the two plant species. In this study, water deficit triggered mechanisms that resulted in overproduction of ROS in both Zea mays and Sorghum bicolor. However, Sorghum bicolor showed less oxidative damage under water stress compared to Zea mays. Drought-induced proline accumulation in the roots of Sorghum bicolor was associated with enhanced water retention. Significant changes were identified in the antioxidant enzyme activity between the two plant species in response to drought conditions. Proteomics results showed differing patterns for drought-responsive proteins in the two species. Together with the physiological, biochemical and proteomic profiling results between Zea mays and Sorghum bicolor, potential proteins and/or metabolic pathways underlying drought tolerance were identified. The findings obtained through this study provide insight towards understanding the molecular basis of crop drought tolerance.

Bread wheat (Triticum aestivum L.) is one of the most important cereal crops grown in the world. It has great importance because it constitutes a major source of carbohydrate for more than one third of the world’s population (Budak et al., 2013). In the last three decades, drought conditions are becoming more widespread in wheat production areas including Europe, Australia and Asia, and it is considered a major cause of reduced wheat growth and productivity in most developing countries with semi-arid climates. Drought constitutes the most important threat for wheat production in Iraq and especially for the Kurdistan Regional Governate owing to the limited source of water during at least some part of the growing period. Because wheat is considered a staple food and has economic importance for the Kurdistan Regional Government research is needed to determine the production capacity of Iraqi wheat varieties under drought stress and the potential for the maximization of the drought tolerance response. The soil moisture holding capacity of the intended growing medium was measured gravimetrically in pots with and without wheat plants and correlated with the soil capacitance measured using a TDR Theta Probe (Delta-T Devices). This was used to determine the available water content of the soil (AWC) and to control and manage the watering regimes during drought studies. The results of a study of the response of different cultivars of Iraqi wheat (Triticum aestivum L.) to watering regimes of 70% and 50% showed that drought stress had a significant effect on the biomass and yield traits especially tiller number and stem bundle weight compared to normal conditions. The highest significant difference was observed for cv. Tamooz 2 in comparison to Adana 99, but there was a little difference between cvs. Rizgary and Sham 6. The effect of the exogenous application of salicylic acid (SA) and molybdenum (Mo) on drought tolerance of cvs. Tamooz 2 and Adana 99, showed that Tamooz 2 had higher values for growth characteristics and higher yield potential when sprayed with a lower concentration of SA (1.44 mM) under well-watered conditions in comparison with Adana 99. The effect of spraying variety Tamooz 2 with SA at different growth stages indicated that biomass production and yield components (the number of spikes/pot, grain dry weight and average 1000 grain dry weight) significantly increased at both stem+flower as well as leaf+stem+flower sprayings for plants subjected to drought. Also, SA treatments at stem extension and flowering had a positive effect on the up-regulation of the drought response gene CBF/DREB under drought stress conditions. These findings indicate that agronomic treatments with exogenous applications of salicylic acid and molybdenum could help to reduce the effects of drought in the field.

>Magister Scientiae - MSc / Drought is a major stress that affects the productivity of plants across the globe. The consequences of drought are posed to continue reducing agricultural productivity as world climate changes. This makes it imperative to improve tolerance of staple food crops to drought. Under drought stress, reactive oxygen species (ROS) accumulate, which may lead to oxidative damage of DNA, proteins and lipids.

The response of plants to environmental stress spans several orders of magnitude in time and space, causing system-wide changes. These changes comprise of both protective responses and adverse reactions in the plant. Stresses like water deficit or drought cause a drastic effect in crop yield, while concomitantly agriculture consumes 1/3rd of the fresh water available to us and there is widespread water scarcity around the world. It is, hence, a fundamental goal of modern biology and applied biotechnology to unravel this complex stress response in laboratory model plants like Arabidopsis and crop models like rice. Such an understanding, especially at the cellular level, will aid in informed engineering of stress tolerance in plants. We have developed and used integrative functional genomics approaches to characterize environmental stress response at various levels of organization including genes, modules and networks in Arabidopsis and rice. We have also applied these methods in problems concerning bioenergy. Since the poor knowledge of the cellular roles of a large portion of plant genes remains a fundamental barrier to using such approaches, we have further explored the problem of 'gene function prediction'. And, finally, as a contribution to the community, we have curated a large mutant resource for the crop model, rice, and established a web resource for exploratory analysis of abiotic stress in this model. All together, this work presents insights into several facets of stress response, offers numerous novel predictions for experimental validation, and provides principled analysis frameworks for systems level analysis of environmental stress response in plants. / Ph. D.

Drought is the major environmental factor affecting crop growth, yield and quality. Two experiments were conducted to understand rice genotypic variability responses to drought stress conditions during seed germination and early-seedling growth. In Experiment I, the influence of wide range of osmotic stress on seed germination properties of 15 rice cultivars were studied using polyethylene glycol media. In Experiment II, seedling morph-physiological parameters including root traits were quantified by subjecting rice seedlings to three different soil moisture treatments, 100, 66 and 33% field capacity. Rice cultivars differed in their response to drought at both the stages. Cultivars were classified into different drought tolerant groups based cumulative drought response indices. Based on seed- and early-season growth and developmental responses, RU1104122, Rex, CL111 and RU1304154 were identified as drought tolerant among the rice cultivars tested. The identified tolerant cultivars will be a source for rice breeders to develop new drought tolerant cultivars.