Initiation of the synthesis of ‘stress’ ABA by (+)‐[²H₆]ABA infiltrated into leaves of Commelina communis

Abstract

The 'fettered' fraction of abscisic acid (ABA) that is held within the chloroplasts of unwilted bean and Commelina communis leaves is released when the leaves wilt and it is this 'free' ABA that is now proposed to cause the stomata to close within 2 or 3 min, well before the rise in total ABA can be detected. The large increase in 'stress' ABA begins 2-3 h later. The fettered ABA in a centrifuged homogenate is released by hyperosmotic solutions of mannitol (0.8 M) and NaCl (0.4 M). Dilute solution of halothane (10 mM) and colchicine (1 mM), the detergent sodium dodecyl sulphate (1 mM) the herbicide 2,4-d (0.1 mM) and dithiothreitol (0.01 mM) also caused ABA to be released. Zeatin (0.01 mM), cumene hydroperoxide (0.01 mM) and CaCl(2) (1 mM) had negligible effects. It was postulated that the ABA released from the chloroplasts by wilting could be the signal that initiates the synthesis of the dioxygenase and other enzymes necessary to produce the rise (up to 40-fold) in the amount of stress ABA that is seen 2-3 h later. To test this hypothesis, a solution of (+)-[(2) H(6) ]ABA was vacuum infiltrated into unwilted Commelina leaves to mimic the rise in ABA caused by wilting and gas chromatography/mass spectrometry of the ABA in the extract after 3 h showed that concentrations of (+)-[(2) H(6) ]ABA of up to 0.3 µM stimulated synthesis of endogenous [(1) H]ABA by 15-fold in the unwilted leaves. A 0.5 µM solution blocked the increase in the amount of ABA formed and also reduced the amount of ABA formed in response to a 0.8 M mannitol solution.

Stomatal responses to vapour pressure deficit are regulated by high speed gene expression in angiosperms

“…Alternative explanations, including the release of ABA from internal stores (Georgopoulou & Milborrow ) or the hydrolysis of the conjugate ABA‐GE (Dietz et al ), cannot explain our observations of rapid increases in foliar ABA levels in angiosperms in response to increased VPD. While not relevant for stomatal responses to VPD transitions, the release of fettered‐ABA from the chloroplasts may play a major role in triggering the substantial biosynthesis of foliar ABA levels when plants are drought stressed beyond turgor loss point (Loveys ; Pierce & Raschke ; Georgopoulou & Milborrow ). Our data on ABA‐GE levels, though, indicate that the hydrolysis of ABA‐GE to ABA is only ever likely to contribute to a very small portion of increased ABA levels during water stress.…”

“…During an increase in VPD, increased flux, driven by a higher transpiration rate, leads to the concentration of root-sourced ABA around the guard cells causing stomata to close (Tardieu & Davies 1993). As an alternative to the flux model, stomatal responses to changes in VPD have also been attributed to the rapid release of fettered-ABA from internal stores in the leaf (Georgopoulou & Milborrow 2012), most likely held in the chloroplasts (Loveys 1977).…”

Molecular characterization of a mutation affecting abscisic acid biosynthesis and consequently stomatal responses to humidity in an agriculturally important species

“…Recently we have shown that stomatal responses to changes in humidity or VPD in angiosperms are driven by functionally significant changes in foliar ABA level; this is particularly evident in angiosperm herbs ( McAdam and Brodribb 2015 ). Previously, some have argued that ABA responsible for the rapid (<1 h) responses of stomata to changes in water status does not have an origin in de novo synthesis but rather release from internal stores in the chloroplasts ( Georgopoulou and Milborrow 2012 ) or via a single step that converts the catabolite ABA–glucose ester to ABA ( Lee et al . 2006 ).…”

Surviving a Dry Future: Abscisic Acid (ABA)-Mediated Plant Mechanisms for Conserving Water under Low Humidity

“…ABA plays a major role in regulating stomatal responses to VPD in angiosperms, as evidenced by the wilty phenotypes of ABA biosynthetic and signaling mutants at high VPD, combined with significantly impaired stomatal responses to increased VPD in sextuplet ABA-receptor mutants and mutants in the key ABA signaling gene OPEN STOMATA1 ( OST1 ) [ 31 , 32 ]. The speed of the stomatal response to VPD has previously led to the suggestion that ABA levels rise rapidly due to the release of fettered ABA [ 33 ], by the single-step hydrolyzation of stored, conjugated ABA–glucose ester (ABA–GE) [ 34 , 35 ]. While this hydrolysis pathway appears important for plants to respond to sustained dehydration stress [ 35 ], it does not appear to play a significant role in fast VPD responses across diverse angiosperm species, as ABA–GE levels do not change sufficiently, or even in the right direction (i.e., decrease) in some species, to account for the rapid increase in ABA levels under transient (20 min) VPD transitions [ 21 ].…”