**2.1 Regulatory proteins**

*Drought - Detection and Solutions*

economic processes [8].

productivity of native populations and in agricultural systems of arid and semiarid areas [5, 6]. Droughts are increasing in frequency and intensity on large regions of the world due to desertification processes. Desertification as a result of greater world's population, extensive agricultural practices, and the effects of the global climate changes has become a serious problem [7]. Droughts are not unusual when viewed from a geological or evolutionary perspective. Therefore, they represent a permanent, strong, and increasing factor for biological, ecological, agronomic, and

Plants as sessile organisms are under strong selective pressure to adapt to their environment. Along the entire life cycle, plants are frequently subject to a combination of different biotic and abiotic stresses [9]. Drought is one of the major environmental factors that reduce the yield of crops by limiting water availability for plants' growth [10, 11]. Water deficit is one of the most important abiotic stresses that affect development and productivity of crop plants [12]. Water stress induces stomatal closure, restricts gas exchange, and reduces photosynthesis in plants [13]. Plants must cope with their environment through physiological acclimation and evolutionary adaptation. As a result, plants have evolved extraordinary mechanisms to perceive, respond, and survive some abiotic stresses and especially with water stress [8].

Most of the cultivated plants are very sensitive to drought and water stress [14, 15]. Thus, agricultural production is seriously limited by drought, while world population is continuously and increasingly demanding for food [16]. New cultivars that can keep or increase yields while using less water are needed in order to reduce the

Some plant species have evolved morphological, physiological, and biochemical mechanisms that allow them to survive, grow, and reproduce in areas where drought is a common event. Plants may deal with water stress through escape, avoidance, and tolerance strategies [19, 20]. Escape is reached through plastic shifts in phenology in such a way that critical growth periods do not coincide with periods of water deficit. Avoidance includes adaptive responses to keep plant water status during drought. Tolerance strategies protect plant cells, tissues, and organs from water stress letting recovery after periods of water deficit and drought. These strategies are not mutually exclusive and may have different functions depending

Native plants to arid lands must deal with extreme environmental factors such as high temperatures, high evaporation, low precipitation, salinity and high light intensity, and low soil moisture [23, 24]. These plants have evolved numerous mechanisms to survive drought. Some of those mechanisms have been described for model and cultivated plants [25]. However, wild populations frequently contain large pools of genetic and phenotypic diversity that can be useful for identifying new molecular strategies involved in drought tolerance. Detecting and understanding the roles of novel genes in drought tolerance may be the basis to improve or develop cultivars tolerant to drought [26, 27]. In this chapter we review some of the

Plants' drought tolerance is a quantitative trait that is controlled by many genes with additive effects [28, 29]. In native plants to arid areas such as *Cynanchum komarovii* and *Prunus mongolica* Maxim, more than 3000 differentially expressed genes (DEG) under drought stress have been found [30, 31]. These genes and/or their products participate in at least two pathways involved in drought tolerance: ABA-dependent and ABAindependent pathways [25, 32]. According to their roles, these genes have been grouped

risk of grave yield losses and to reach the world food requirement [17, 18].

on species and duration, intensity, and timing of water stress [21, 22].

findings about drought-responsive genes in plants native to arid lands.

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**2. Drought tolerance genes**

Plants must deal with drought by activating a complex signaling network that produces a variety of physiological responses and defense systems [34, 35]. The signaling pathways include a group of stress sensors, cellular signal transduction, and transcriptionally regulated networks. In order to respond to water stress, it must be perceived by specific receptors which transmit the stress signal into the cell and may trigger a series of signal transduction steps. [36]. The signal transduction in ABA-dependent pathways increasing ABA concentrations is sensed by receptors such as GTG1, GTG2, Mg protoporphyrin IX chelatase H subunit (CHLH/ABAR), and pyrabactin resistance 1/PYR1-like/regulatory component of ABA response 1 (PYR/PYL/RCARs) [37]. Once ABA binds to its receptor, ABI1 (ABA insensitive 1) phosphatase activity is blocked causing autophosphorylation and activation of an open serine-threonine kinase (open stomata 1, OST1). Then, transcription factors (TFs) known as ABA-responsive element-binding proteins (ABFs/AREBPs) are phosphorylated and activated by OST1. Besides OST1, calcium-dependent kinases (CDPKs) similarly activate SLAC1 and SLAH3 [38]. Afterwards, ABFs bind to DNA at specific ABA-responsive elements (ABREs). Finally, ABA-dependent gene expression is induced [39].

Gene expression at the transcriptomic level is strongly regulated by transcription factors [40]. TFs can be grouped into several families according to their structure and binding domains [41]. Several TFs belonging to MYB, MYC, NAC, bZIP, HD-ZIP, DREB, and WRKY families are involved in modulation of gene expression of plants in response to drought stress through ABA-dependent or ABA-independent pathways [42]. Even though many TF families have been found in several models and cultivated plants [43, 44], novel TFs are being described from plants native to arid lands. They may be used as a powerful tool for practical approaches for engineering drought stress tolerance in plants. For example, in the desert legume *Eremosparton songoricum*, a novel DREB2B (EsDREB2B) gene was identified. The transcript of EsDREB2B was upregulated by different abiotic stresses, among them drought stress [45]. Other novel DREBs from a desiccation tolerant moss (*Syntrichia caninervis* Mitt.) were isolated and used to transform yeast. The ScDREB enhanced stress tolerance to yeast [46]. In *Larrea tridentata*, a bush native and widely distributed to Northern American deserts, the LtWRKY21 transcription factor that functions downstream of ABI1 to control ABA-dependent expression of genes was found [47]. The PeDREB2L gene from the desert tree *Populus euphratica* Oliva was isolated and transformed into *Arabidopsis thaliana*. Transgenic plants showed an improved tolerance to drought and freeze [48]. Moreover, in *Sophora moorcroftiana*, an endemic Leguminosae shrub species native to arid and semiarid regions of the Qinghai-Tibet Plateau, a total of 1534 TFs were identified. Those TFs were classified into 23 different common families. The major group of TFs was the bZIP family (160, 10.43%), followed by MYB (115, 7.5%), bHLH (107, 6.98%), zinc finger (103, 6.71%), and WRKY (103, 6.71%) [49].

### **2.2 Functional proteins**

Drought stress affects many processes in the plants and cause a variety of physiological and biochemical changes. Some of these changes include loss

of cellular turgor, changes in membrane fluidity and composition, changes in osmotic potential, and protein-protein interaction [10]. Cell turgor loss is perhaps the most evident indicator of water stress which affects integrity of cells, metabolism, and whole plant performance [50]. Maintaining cell turgor is critical for surviving and growth of plants. The changes in osmotic potential play a relevant role for that purpose. Loss of cell turgor, among other effects, may cause stomata closure and limitations of gas exchange which in turn decreases CO2 supply for RuBisCo. Photosynthesis decreases, and the reducing power production exceeds the rate of its use by the Calvin cycle [51]. Consequently, overproduction and accumulation of reactive oxygen species (ROS) alter the redox status [52, 53]. ROS damage all major cell biomolecules impairing their function [53]. Plants may respond to these effects by activating several defense mechanisms that involve participation of numerous proteins such as late embryogenesis abundant (LEA) proteins, osmoprotectants, chaperons, detoxifying enzymes, and various proteases [25, 54].
