# Below versus above Ground Plant Sources of
## 2 Metabolism of Abscisic Acid (ABA)
### Two Scenarios of ABA Biosynthesis with Distinct Environmental Controls  
In the literature, two scenarios related to ABA biosynthesis have been described (Figure 1). The increase in ABA content of root, xylem sap and leaves of drought-stressed plants has been extensively reported [68]. Although the majority of research to-date has focused on ABA production in roots followed by transport to leaves via the transpiration stream (Scenario 1), ABA production is now acknowledged to also occur in leaves (Scenario 2) [69]. Transgenic plants overexpressing key enzymes in the ABA biosynthetic pathway show elevated ABA tissue levels and reduced stomatal conductance [70] with an increased tolerance to drought [71]. Changes in stomatal conductance by guard cells are linked with ABA signaling arriving in the xylem [72], and numerous studies have reported negative correlations between concentrations of ABA in xylem sap and stomatal conductance [73]. However, girdling and decapitation experiments revealed ABA gradients were at least partially determined by local biosynthesis rather than root to leaf transport [74]. As reviewed previously [75], historical studies concluded that the primary plant source of ABA are roots [76] with this biochemical model propagated throughout the literature including its incorporation into widely used leaf gas-exchange models which allow root-derived ABA to be transported to leaves where it impacts stomatal conductance, and therefore fluxes of net photosynthesis and transpiration [77; 78]. However, a series of recent studies using foliar application of labeled ABA, reciprocal grafting between ABA biosynthetic mutant and wild-type plants, and stem girdling to block basipetal phloem transport, it was concluded that foliage-derived ABA is readily transported to the roots where it is critical for maintaining normal roots ABA levels and determining root architecture and growth [79; 80]. As summarized in the recent review article [75], the results of the two experimental studies concluded that not only is the majority of leaf ABA produced locally in the leaf tissues, leaf-sourced ABA followed by transport to roots dominates root sources of ABA. Thus, the emerging view is that ABA biosynthesis in roots is considered minimal. This conclusion is supported by a previous study which found that leaf response to limited soil water supply was not affected by the capacity to generate ABA in the root, but instead requires ABA biosynthesis and signaling within leaves [81]. Furthermore, these authors concluded that the long-distance communication signal between the roots and leaves is not ABA, but rather a hydraulic signal, which proceeds ABA signaling and stomatal closure. Isohydric plants are able to rapidly respond to transient water shortages in leaves manifested by decreases in leaf water potential during the late morning to early afternoon by closing their stomata to  
Figure 1: Simplified diagram of the 2-C-methyl-D-erythriitol-4-phosphate (MEP) pathway for phytohormone abscisic acid (ABA) biosynthesis with components occurring in the Chloroplast and Cytosol. ABA is derived from C\({}_{40}\) epoxycarotenoid precursors through an oxidative cleavage reaction in chloroplasts. The C\({}_{15}\) intermediate xanthoxin is converted to ABA by a two-step reaction via ABA-aldehyde in the cytosol. The green intermediates (CO\({}_{2}\) and glyceraldehyde-3-phosphate, G3P) represent the substrate and product of photosynthesis in the Calvin Cycle) with blue photorespiratory intermediates (glycolate and glycerate). reduce transpiration leading to a suppression of net photosynthesis [82]. Such a short-term control is difficult to reconcile with the long-distance transport of ABA in trees between roots and leaves and this difficulty is further amplified in the tropics due to low sap velocities (<30 cm\(\cdot\)h\({}^{-1}\)) and large tree heights (up to 45 m).