## 3 ABA and Drought Tolerance: From Stomata Closure to the Modulation of Plant Metabolism
### Linking ABA with Stomatal Aperture and Xylem Embolism  
Stomatal closure during drought is one of the first attempts of vascular plants to reduce transpiration, preventing tissue desiccation and damage [60; 61]. The signaling pathway for ABA-induced stomatal closure in angiosperm species has been the subject of detailed investigations in recent years and it is now reasonably well characterized [62; 63; 64; 65]. As angiosperms often exhibit stomatal conductances higher than the minimum amount sufficient to allow maximum rates of photosynthesis, declines in stomatal conductances during drought driven by increased foliar ABA levels often result in optimized WUE [66; 67]. Aiming to increase WUE in economically important crop species, genetic and molecular tools have been applied to generate plants with increased ABA biosynthesis and signaling in recent years [66; 68; 69]. See topic 4.0 for further details.  
Although increasing levels of foliar ABA has long been associated with stomatal closure and increases in WUE in angiosperm species during drought [1; 38; 67; 70; 71], it has recently been discovered that the stomata of ferns and lycopbytes do not close in response to both endogenous and exogenous ABA [72; 73; 74]. Instead, in these early-diverging species, changes in leaf water status regulate stomatal aperture by an ABA-independent, hydrolysate mechanism [72; 75; 76]. In a similar way, recent findings demonstrate a number of hornwort species that lack stomatal responses to ABA [77]. These findings indicate a gradualistic model of stomatal control, which has been the focus of considerable debate in the last decade [78; 79; 80; 81; 82; 83]. For a recent and comprehensive review on ABA signaling pathway and function across the evolution of land plants, see [PERSON] and [PERSON] [84].  
Early closure of stomata has been demonstrated to occur prior to leaf and stem xylem embolism in a number of species from ferns to angiosperms, supporting the idea of the water-saving function of stomata and xylem protection during drought [61; 76; 85; 86; 87]. This protective mechanism is of paramount importance given that leaf xylem embolism has been associated with foliar mortality across the canopy [88; 89], potentially affecting plant growth and productivity. In addition, extensive embolism in the stem xylem and consequent hydraulic failure have been largely associated with drought-induced mortality of plants [90; 91; 92; 93]. Curiously, ABA signaling has very recently been associated with the construction of conduits of stem xylem more prone to suffer embolism in a study comparing a transgenic line of tomato overproducing ABA with its wild-type [94]. The specific ABA over-expressing transgenic line utilized in this study, however, exhibits several differences from the wild-type plants regarding stem xylem anatomy traits that are closely linked to xylem resistance to embolism (i.e., xylem conduits are much less numerous and narrower in the transgenic line compared with the wild-type). Whether this represents that ABA signaling modulates xylem anatomy, or if this arises from pleiotropic effects is of major importance to confirm the association between ABA and xylem vulnerability to embolism. Besides, the transgenic line utilized in this study presents a very similar hydraulic resistance to other cultivars of tomato that do not overproduce ABA [95] (and personal observations), which might suggest that the wild-type utilized, rather than the transgenic line itself, presents a different xylem vulnerability from other cultivars potentially due to differences in the xylem anatomy. In any case, the idea of hydraulic vulnerable plants when ABA biosynthesis is increased might challenge the use of transgenic plants with increased ABA biosynthesis and signaling to improve crop production under drought and thus should be further investigated.