## 4 Regulation of ABA Levels and Perception in Crop Species  
Ongoing climate change coupled with the projected increase in human population by over 2.4 billion until 2050 [119; 120] represents a major challenge for the agricultural sector [121]. Over the next decades, climate change is expected to result in considerable rise in the intensity and frequency of drought events. In this water-limited scenario, achieving'more crop per drop' (i.e., crop water productivity that is defined as the mass of agricultural produce per unit of water consumed) is a critical target for food production. Especially when considering that 70% of the available fresh water is used for crop production and that drought is expected to cause the largest yield reductions relative to any other abiotic and biotic stresses [122]. Therefore, enhancing crop WUE is still foreseen as a good means for saving water in agriculture [123] and has proven to be an accurate target in both genotype selection [124; 125; 126] and engineering [127]. The WUE can be determined both at the plant and leaf levels. At the leaf level, the photosynthesis-to-stomatal conductance ratio is defined as the intrinsic water-use efficiency (WUEi). This parameter is typically determined in studies aimed at improving crop yield while saving soil water and mitigating yield limitations by water deficit [127; 128]. In this context, the importance of ABA for WUEi improvement is well-known due to its central role in regulating plant-water relations, specially by modulating stomatal conductance in several crop species [66; 129; 130; 131]. As outlined above, the direct influence of ABA on WUEi in angiosperms has already been demonstrated and several studies have concluded that plant responses driven by ABA depend on the severity and the duration of the drought events [67]. Particularly under short-term water deficit, ABA induces stomatal closure through its direct action on guard cells and modulates mesophyll conductance without restraining CO\({}_{2}\) fixation [8; 12; 38]. During severe and prolonged drought conditions, ABA can trigger further alterations at the transcriptome level, including genes encoding for LEA (late embryogenesis abundant) proteins [132]. Besides, it can alter the levels of osmoprotectcans [100] and cause morphological changes in stomatal size and density [133], \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Metabolite/Enzyme** & **Alteration** & **Organ** & **Plant Species** & **Reference** \\ \hline IVR2 acid vacuolar invertase & Increase & Leaf & _Zea mays_ & [99] \\ activity and expression & Increase & Shoot & _Arabidopsis thaliana_ & [97] \\ Starch levels & Decrease & Leaves & _Arabidopsis thaliana_ & [100] \\ \(\beta\)-AMYLASE1 and \(\alpha\)-AMYLASE3 activity & Increase & Leaves & _Arabidopsis thaliana_ & [100] \\ \(\Delta\)1-pyrroline-5-carboxylate synthase expression & Increase & Whole seedling & _Arabidopsis thaliana_ & [106; 107] \\ Flavononoid biosynthesis and & Increase & Leaves & _Camellia sinensis_ & [112] \\ gene expression & Increase & Leaves and Fruits & _Vitis vinifera_ & [59] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Representative metabolites and enzymes associated with primary and secondary metabolisms that are affected by ABA. leaf size, and shoot and root development [7; 134]. All these alterations strongly affect plant water balance and WUE in the long-term [135]. In addition, ABA have been observed to regulate WUE through a tissue-specific action on hydraulic conductivity. Particularly in leaves, ABA has been found to decrease the hydraulic conductance by reducing the permeability of bundle sheath aquaporins [8; 9; 136] and also indirectly controlling mesophyll hydraulic conductances [11; 137]. In the roots, ABA has been observed to increase hydraulic conductivity, facilitating water uptake under non-transpiring conditions [7; 94]. Therefore, ABA signaling network can be modulated to reduce transpiration and increase crop water productivity under stressful conditions [67; 138].