**1. Introduction**

### **1.1. Drought stress**

A world in which average temperatures reach 4°C above preindustrial levels would likely see unprecedented heat waves, severe drought, major flooding, and up to 1 m of sea-level rise (http://

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www.worldbank.org/). The aforementioned change in environment clearly is because of the careless human activities. In nature, plants as sessile organisms must respond to different stimuli throughout their life cycle; otherwise, fluctuations of these abiotic stresses outside their normal ranges can cause serious consequences on plant growth and reproduction. Many physiologi‐ cal and biochemical processes will be changed in plants under stress by affecting RNA stability, protein, and ion transport [1].

Stress is a specific environmental condition herein plant growth and development disrupted and its full genetic potential defeated [1]. Up to 70% of food crops yield is negatively influenced by biotic stresses, such as drought, cold, high salinity, and heat, which obviously threaten food security worldwide. Many organic compounds such as proline, glycine betaine, and polya‐ mines, a variety of sugars (mainly fructose and sucrose), sugar alcohols, complex sugars (such as trehalose and fructans), and organic acids (oxalate, malate) can be aggregated inside the cell, while plants have faced with salt, drought, and cold to prevent cellular dehydration and protect cellular proteins. These osmoprotectants presumably accumulate several folds more than unstressed condition, without disturbing the intracellular biochemistry. Drought stress is by far the most important environmental factor contributing to yield losses in crops [2]. Plants experienced drought stress either when the water supply to roots becomes difficult or when the transpiration rate becomes very high. These two conditions often coincide under arid and semiarid climates. Imbalance between light capture and its utilization caused by drought stress inhibits photosynthetic activity [3]. The changes in the photochemistry of chloroplasts due to fluctuation of electrons in the leaves of drought-stressed plants result in the dissipation of excess light energy in the PSII core and antenna, thus generating reactive oxygen species (ROS), which are potentially dangerous under drought stress conditions [4]. Due to multigenic and quantitative properties of aforementioned stresses, advanced plant breeding purposes have been challengeable. High throughput sequencing and functional genomics tools provide new window to underlie signal perception and transduction of the molecular regulatory networks. The mechanism by which plants perceive and transmit the signals down to active adaptive response inside the cell is of great importance to mimic this strategy for manipulating organelle to convey tolerance properties under stress conditions. To do this, merging physiological, biochemical, and gene regulatory network knowledge is inevitable [5]. The objectives of the present work were as follows:


### **1.2. Turfgrasses importance**

The *Poaceae* family comprise over 9000 monocot species in which environmental grasses distribution is pretty diverse. Perennial grass has a specific role in providing food for livestock, bioenergy, fiber products, soil and water improvement and conservation, habitats for wildlife population, and recreation and beautification [6]. Over 50 grass species are cultivated as turfgrass, which have a major role in our daily life by providing a ground cover in parks, home lawns, sports fields, and golf courses [7]. Turfgrasses are also used along roadsides, edges of waterways, or in preserved land, where they may act as barriers to reduce pollution, water runoff, and wind erosion of the underlying soil [7]. The turfgrass industry potentially is a multibillion-dollar-a-year business. Turfgrass provide attractive vegetative ground cover, which is consistently mowed.

www.worldbank.org/). The aforementioned change in environment clearly is because of the careless human activities. In nature, plants as sessile organisms must respond to different stimuli throughout their life cycle; otherwise, fluctuations of these abiotic stresses outside their normal ranges can cause serious consequences on plant growth and reproduction. Many physiologi‐ cal and biochemical processes will be changed in plants under stress by affecting RNA stability,

Stress is a specific environmental condition herein plant growth and development disrupted and its full genetic potential defeated [1]. Up to 70% of food crops yield is negatively influenced by biotic stresses, such as drought, cold, high salinity, and heat, which obviously threaten food security worldwide. Many organic compounds such as proline, glycine betaine, and polya‐ mines, a variety of sugars (mainly fructose and sucrose), sugar alcohols, complex sugars (such as trehalose and fructans), and organic acids (oxalate, malate) can be aggregated inside the cell, while plants have faced with salt, drought, and cold to prevent cellular dehydration and protect cellular proteins. These osmoprotectants presumably accumulate several folds more than unstressed condition, without disturbing the intracellular biochemistry. Drought stress is by far the most important environmental factor contributing to yield losses in crops [2]. Plants experienced drought stress either when the water supply to roots becomes difficult or when the transpiration rate becomes very high. These two conditions often coincide under arid and semiarid climates. Imbalance between light capture and its utilization caused by drought stress inhibits photosynthetic activity [3]. The changes in the photochemistry of chloroplasts due to fluctuation of electrons in the leaves of drought-stressed plants result in the dissipation of excess light energy in the PSII core and antenna, thus generating reactive oxygen species (ROS), which are potentially dangerous under drought stress conditions [4]. Due to multigenic and quantitative properties of aforementioned stresses, advanced plant breeding purposes have been challengeable. High throughput sequencing and functional genomics tools provide new window to underlie signal perception and transduction of the molecular regulatory networks. The mechanism by which plants perceive and transmit the signals down to active adaptive response inside the cell is of great importance to mimic this strategy for manipulating organelle to convey tolerance properties under stress conditions. To do this, merging physiological, biochemical, and gene regulatory network knowledge is

inevitable [5]. The objectives of the present work were as follows:

genotypes during midsummer

tolerance of this species

**1.2. Turfgrasses importance**

**1.** Elucidating cellular enzymatic and nonenzymatic antioxidative defense strategies in the drought stress response in leaves of four genotypically distinct *F. arundinacea* Schreb.

**2.** Studying the regulatory role of *NYE/*Stay GReen (*SGR*) transcript during drought stress

**3.** Overexpressing of the *P5CS1* gene in tall fescue background aimed at improving drought

The *Poaceae* family comprise over 9000 monocot species in which environmental grasses distribution is pretty diverse. Perennial grass has a specific role in providing food for livestock,

protein, and ion transport [1].

64 Water Stress in Plants

Turf species are divided into two groups based on origin and geographical distribution: coolseason grasses and warm-season grasses. Cool-season turf species are typically adapted to the cool–humid and cool–arid zones, and warm-season turf species are best adapted to the warm– arid and warm–humid regions. Most of the cool-season grasses are propagated by seeds, while a majority of the warm-season grasses are vegetatively propagated. Maintaining the quality of turf under adverse environmental conditions is of prime importance and actually a big challenge in turf growth and production. This challenge could be addressed through molecular breeding to create stress-tolerant germplasm. Traditional breeding techniques resulted in the production of many genotypes with an improved stress tolerance; however, traditional breeding progress is limited mostly due to the lack of superior stress-tolerant grass germplasm and because of poor understanding of the different aspects of physiological and molecular mechanisms for stress tolerance in perennial grass species. Finding and overexpressing of some drought-related candidate genes, either transcription factors, key enzymes, or channel proteins, recognized by RNA seq and other high-throughput techniques resulted in improving plant adaptation to drought stress [8]. The molecular mechanism and genetic information underlying drought stress have not yet been well addressed in turfgrass species.

### **1.3. Origin, taxonomy, and cytological features of** *Festuca arundinacea* **Schreb.**

An increasing interest in tall fescue (*Festuca arundinacea* Schreb.) in western Europe and elsewhere is mainly because of its better drought resistance and yield potential in contrast to that of perennial ryegrass (*Lolium perenne* L.) [9]. Tall fescue is native to Europe; it also grows on the Baltic coasts throughout the Caucasus, in western Siberia, and extends into China [10].

The genus *Festuca* encompasses over 80 species, including two agriculturally important forage crops *Festuca arundinacea*, which is one of the most widely used cool-season species, and *F. pratensis* Huds [11]. Tall fescue is a wind-pollinated species with a high degree of selfincompatibility. Its genome size is approximately 6 × 103 Mbp (mega base pair) and contains three genomes (P, G1, and G2). The P (2*x*) genome is from the diploid species, meadow fescue (*F. pratensis*), while the G1 and G2 (4*x*) genomes are from the tetraploid species, *F. arundina‐ cea* var. "glaucescens" [12, 13]. In comparison to other cool-season perennial grasses, tall fescue exhibits a high degree of stability when confronting drought stress. In contrast to having low rate of water use, tall fescue's deep and expansive root system allows this species to avoid drought conditions by continually having access to water [14]; even under continuous dry periods, the cells remain turgid. Transpiration is another strategy for tall fescue in order to cool itself via vast leaves [15]. This relative drought tolerance makes it an ideal option for cultivation in urban landscapes throughout transitional climates. Tall fescue is a coarsetextured vigorous perennial bunchgrass. This species reproduces through tillering and seed. Its desirable agronomic characteristics encompass high yields of herbage, excellent persistence, adaptation to a wide range of soil conditions, compatibility with various management practices, long grazing season, and low incidence of pest problems [16, 17]. Given the contin‐ uous drought stress in many areas of the world, the necessity of highly tolerant turf species for landscaping, xeriscaping, and other usage is inevitable. Selection and/or production of high-tolerant transgenic turfgrass would be one of the key ways that leads to ameliorate their drought tolerance threshold in the realm of landscaping.
