## 4 Regulation of ABA Levels and Perception in Crop Species
The ABA signal transduction pathway has recently been well documented, consisting of pyrabactin resistance/pyrabactin resistance-like/regulatory component of ABA receptors (PYR/PYL/RCAR), clade A type 2C protein phosphatases (PP2 Cs), and SNFI related protein kinase 2 (SnRK2s) [139; 140]. The first attempts to use ABA in large scale agriculture were constrained by the chemical instability of ABA (e.g., under UV-light), its relatively expensive production, and its rapid cellular catabolism [141]. The modulation of ABA signaling, however, can be efficiently achieved through genetically engineered plants with an overexpression of ABA receptors [54; 69; 127; 142] and signal transduction components [143; 144], as well as through chemical intervention with ABA agonists [145; 146; 147; 148; 149]. Over the last years, studies on ABA overexpression receptors have led to promising results both in the field [150; 151] and under controlled conditions [127; 142]. The overexpression of PYL/RCAR receptors induces a higher plant sensitivity to ABA, leading to a globally enhanced WUE and drought resistance, as previously demonstrated in rice, poplar, and wheat [69; 142; 153; 152]. The actual challenge, however, is to balance the improvements in WUE against the negative effects on growth due to ABA oversensitivity. Recent studies on plants overexpressing PYL receptors have shown an opposite effect on growth and stress adaptation and this may be due to the specific receptor choice. Indeed, the overexpression of some specific subfamilies result in plants with less transpiration for a similar leaf area and biomass compared to the wild-types, thus leading to an increased WUE [138]. In particular, PYL12/RCAR6 and PYL4/RCAR10 (subfamily I and II) overexpressing lines in Arabidopsis possessed a balance of reduced water use with negligible effects on growth and a \"water-saving\" phenotype that resulted in a significant improvement in total biomass relative to water use gains [127]. Similar observations have been made in transgenic wheat and in rice, in which the overexpression of specific PYL receptors has led to a reduced transpiration and a concomitant increase in photosynthetic activity compared to the wild-type, improving grain production per liter of water and protecting productivity during water deficit [69; 142; 151]. A faster and a more manageable alternative to the expression of genes for increasing crop's WUE and conferring drought resistance is the use of ABA agonists [147; 154], which are small synthetic molecules binding ABA receptors and activating ABA signaling pathway [155]. Among the ABA agonists, the first synthetized molecule Pyrabactin has been identified in a chemical genetic screen for seed germination inhibitors and it has been instrumental in showing how ABA binds the PYR/PYL receptor family [140]. The direct application of this molecule is not practical for agricultural purposes since its major effects are in seeds rather than in vegetative tissues. However, the examination of the structure of Pyrobactin and its interaction with ABA receptors has provided a framework for the design of novel ABA agonists [154]. As a consequence, several ABA agonists have been developed during the last ten years, with promising results in increasing crop water productivity [68]. An example is represented by Quinabactin, which appeared to be an overall better ABA agonist than Pyrabactin, inducing ABA responses both in seeds and vegetative tissues. Application of Quinabactin in plants resulted in improved water use upon drought stress [145]. This molecule targets all of the dimeric subfamily III ABA receptors, affecting guard cell closure to prevent water loss from detached leaves and conferring drought tolerance in both Arabidopsis and soybean during drought [145]. In addition, Quinabactin has shown effect in rapeseed and tomato, inhibiting germination and eliciting drought stress responses [156; 157; 158].