Physiological and molecular responses to water-stress in local Saudi wheat cultivars

Abstract

The kingdom of Saudi Arabia (KSA) is among the top countries in terms of agriculture in desert areas, the country uses highly developed agricultural technologies to grow different crops under challenging environmental conditions. The global climate change and the consequent increase in temperature and drought especially in the arid and semi-arid regions made the situation even more challenging. This work aimed at determining the physiological and molecular mechanisms underpinning drought-tolerance in wheat using six local Saudi wheat cultivars. Understanding the phycological responses and gene regulations under water stress could contribute to improving wheat cultivation in Saudi Arabia. My thesis was divided into four main experimental chapters, each chapter describes one experiment. The first experiment was to assess drought tolerance in a collection of six known wheat Cultivar grown in different regions of the KSA by comparing their growth and yield under well-watered conditions and water-stress conditions. Shoot weight and length, Root weight and length, Root: shoot ratio, RWC, proline content, soluble sugar content and protein content in addition to yield were used as selection criteria for drought resistance. The results allowed to group the studied wheats into two groups, drought resistant (193 Najran (Cv2) and 357 Sama (Cv3) ) and drought sensitive (181 Jizan (Cv1), 377 Rafha (Cv4), 562 Ma’ayah (Cv5) and 981 Najd (Cv6)). In the second experiment, RNA sequencing was performed in leaf samples harvested from water control and PEG-treated plants of one drought resistant 193 Najran (Cv2) and one drought sensitive cultivar (377 Rafha (Cv4)) at the vegetative, flowering and grain filling stage. Transcriptomic analysis aimed at finding differentially expressed genes and associated metabolic pathways in the two Saudi wheat cultivars under water stress at the three growth stages. This yielded 24.2 GB of sequence data. At least 40 million reads of 85 bp each were obtained per sample, Although genes from different pathways changed expression under water-stress, the increase in expression of genes associated with Photosynthesis, Amino acid metabolism and Secondary metabolism such as phenylpropanoid biosynthesis seemed to be the most important. The third experiment investigated the drought induced changes in the proteome in drought resistant 193 Najran (Cv2) and drought sensitive cultivar (377 Rafha (Cv4)) at the vegetative, flowering and grain filling stage, Proteomics analysis showed tangible changes in protein levels indicated a general regulation trend of plant defence under water stress, such as Stress/defence/detoxification proteins, Photosynthesis proteins, Carbohydrate metabolism proteins and Amino acid metabolism proteins. In a fourth experiment, based on the results of the transcriptomics and proteomics results together with the literature, the expression of four drought related genes The four genes included Dehydrin gene (DHn3), Bidirectional sugar transporter (Sweet), Phenylalanine ammonia-lyase (Pal5) and Serine hydroxy methyl transferase (Shmt) found to be various regulation in the six wheat cultivars based on their response to water stress. using qRT-PCR. The qRT-PCR analysis confirmed increased expression levels of these genes known to be up-regulated under water stress. From this work, I could conclude the following: 1. There is a big difference in the speed of response to water-stress between wheat Cultivars, this difference is associated with variation in transcript and protein expression levels at three growth stages. 2. The vulnerability of wheat plants to water stress is higher at the flowering stage compared to the vegetative and grain filling stages. Attempts to improve drought tolerance in wheat should be targeted to this growth stage. 3. The phenylpropanoid metabolic pathway plays a key role in resistance to water-stress in wheat and might be a target for improving drought resistance in this crop.