## 2 Drought-Induced Biosynthesis of ABA  
Given the key role played by ABA in enhancing plant survival during drought, a comprehensive understanding of how the biosynthesis of ABA occurs, how it is regulated within the plant, and where it mostly takes place is of major importance. In recent years, the pathway by which ABA is synthesized in plants has been extensively investigated through the use of mutants deficient in the synthesis and perception of ABA [16; 17; 18; 19; 20]. It has been demonstrated that the biosynthetic pathway for ABA accumulation begins in the chloroplasts with the hydroxylation of \(\beta\)-carotene to zeaxanthin, which is then converted to violaxanthin through the xanthophyll cycle. Next, neoxanthin synthase converts violaxanthin to neoxanthin, which is then isomerized from 9-trans to 9-cis (neoxanthin). The last step of the biosynthetic pathway that occurs in the chloroplasts is the oxidative cleavage of 9-cis-neoxanthin and/or 9-cis-violaxanthin to xanthoxin, which is further catalyzed by 9-cis-epoxycarotenoid dioxygenase (NCED). NCED is known to be the key rate-limiting enzyme in the ABA biosynthesis and it is responsible for catalyzing the first non-reversible step in the pathway [21]. The formed xanthoxin then moves from the chloroplasts to the cytosol, where it is converted to abscisic aldehyde, and finally to ABA. ABA accumulation in plants occurs not only through activation of ABA biosynthesis but also by the hydrolysis of its glycosyl ester (i.e., ABA-GE) [22; 23], which can be utilized to maintain the high levels of ABA in plants during drought [24; 25; 26]. Since the first studies linking ABA with stomatal closure under drought, a number of results have led to the critical assumption that the pool of ABA resulting in stomatal closure in leaves is mostly synthesized in the root tips [27; 28]. However, this conventional view of root-to-shoot ABA signaling  
Figure 1: Current understanding of how abscisic acid (ABA) regulates plant form and function during drought. has recently been challenged by studies using a number of experimental approaches, including the reciprocal grafting between ABA biosynthetic mutant and wild-type plants and exogenous labeled ABA [29; 30; 31; 32; 33]. In fact, these studies clearly demonstrate that although multiple organs and tissues can accumulate small amounts of ABA, leaves are the predominant site for ABA biosynthesis, and they are even responsible to export ABA to the roots, maintaining normal root levels of ABA levels and determining root architecture and growth [30; 32; 33]. Inside leaves, phloem companion cells, guard cells, and mesophyll cells have been demonstrated to be capable of synthesizing ABA [34; 35; 36; 37]. In addition, ABA biosynthesis has been recently observed to occur in parenchyma cells around the vascular tissues [38]. Regarding the ABA transport within plants, the recent identification of a number of transmembrane ABA transporters strongly suggests that the movement of this hormone is actively regulated in an intercellular network [15]. However, the fine regulation of ABA biosynthesis and transport among different plant tissues remains a matter of debate [15]. The major production of ABA in leaves, not roots, during drought seems considerably more advantageous for plants to avoid excessive declines in water potential as leaves represent the main organ exchanging (and mostly losing) water with the atmosphere. For instance, a de novo biosynthesis of ABA over an extremely short time-frame following leaf exposure to high vapor pressure deficit (VPD) has been demonstrated to allow an efficient stomatal closure in angiosperm species [39; 40]. Declines in leaf water potential to a threshold leaf water potential results in major increases in foliar ABA [41; 42] and such threshold water potential strongly coincides with bulk leaf turgor loss point [20; 43; 44; 40].