Movement of Abscisic Acid into the Apoplast in Response to Water Stress in Xanthium strumarium L.

Abstract

The effect of water stress on the redistribution of abcisic acid (ABA) in mature leaves of Xawthium strumarium L. was investigated using a pressure dehydration technique. In both turgid and stressed laves, the ABA in the xylem exudate, the 'apoplastic' ABA, icased before 'bulk leaf' stress-induced ABA accumulation began. In the initially turgid leaves, the ABA level remained constant in both the apoplast and the leaf as a whole until wilting symptoms appeared. Following turgor loss, sufficient quantities of ABA moved into the apoplast to stimulate stomatal closure. Thus, the initial increase of apoplastic ABA may be relevant to the rapid stomatal closure seen in stressed leaves before their bulk leaf ABA levels rise.Following recovery from water stress, elevated levels of ABA remained in the apoplast after the bulk leaf contents had returned to their prestress values. This apoplastic ABA may retard stomatal reopening during the initial recovery period.ABA is thought to be involved in the regulation of stomatal behavior during water stress (20,26). Application of ABA to intact leaves or epidermal strips induces stomatal closure (8,20) and wilted leaves accumulate large amounts of endogenous ABA (9, 29). There is now convincing evidence, however, that the decline in stomatal conductance, as water stress takes effect, often precedes the stress-induced increase in 'bulk leaf ABA (Fig. 1;4,10,15). Nevertheless, ABA may still be the causal agent of stomatal closure if the stress alters the internal distribution of the ABA already present in the unstressed leaves (18). A physicochemical basis for ABA redistribution has been suggested, based on the relative pH ofthe various leafcompartments (7,(12)(13)(14) It has also been observed that, during recovery from water stress, stomata fully reopen only after bulk leaf ABA levels have declined ( Fig. 1; 3, 4, 16). The ABA present at high concentrations in the apoplast of stressed leaves (2) may not be quickly catabolized or reabsorbed by the mesophyll when the leaves regain turgor. If this is the case, then apoplastic ABA could still influence the guard cells and cause the lag in stomatal opening.In this paper, we present data on the effect of water stress on the redistribution of ABA in leaves and relate this to bulk leaf stress-induced ABA accumulation. We also investigated the relevance of apoplastic ABA to the lack of correlation between stomatal conductance and the decline in bulk leaf ABA during recovery from stress.Xanthium strumarium was chosen for these experiments because mature leaves of this plant are large, have a high ABA content when turgid, and are suitable for manipulation in a pressure chamber. MATERIALS AND METHODSCulture of Plant Material. Mature leaves (laminae 13-16 cm long) were harvested from greenhouse-grown plants ofXanthium strumarium L., Chicago strain, grown as before (32). The percentage of dry weight of turgid leaves, excluding the midrib and major veins, was 18.7%. The xylem volume ofthe 3 cm ofpetiole adjacent to the lamina was esti...

Phenotypic Expression of Wild-Type Tomato and Three Wilty Mutants in Relation to Abscisic Acid Accumulation in Roots and Leaflets of Reciprocal Grafts

“…Thus, the more ABA present, whether arising from intrinsic biosynthesis or from imports, the more slowly the leaves wilted. This is consistent with the recent realization (3,9) that stress-induced ABA plays a minor role in this process: stressinduced ABA does not begin to accumulate until after initiation of stomatal closure. Furthermore, a normal phenotype was generated in the solution culture-grown mutant/wild-type plants, although the leaves retained the rapid wilting characteristic, suggesting that different plant processes require different amounts of ABA.…”

“…The samples were then purified by semi-preparative HPLC as described (2) with some modification (3). The ABA content of the samples was quantified using a Hewlett-Packard 5840A gas chromatograph equipped with a 63Ni-electron capture detector as described (3 Endogenous ABA Levels. The endogenous ABA levels in both turgid and stressed leaflets were unaffected by the culture method under which the plants were grown (Fig.…”

Genetic control of abscisic acid biosynthesis in maize

“…The vp14 mutant evidently targets a pool of ABA involved in stomatal regulation in seedling leaves. Previous studies have established that stress-induced changes in bulk ABA synthesis occur too slowly to account for rapid stomatal responses in leaves and that redistribution of preexisting ABA pools can account for stomatal closure (25). It is therefore unlikely that the partial inhibition of stress-induced ABA synthesis in vp14 mutant leaves would account for the impaired stomatal regulation detected within the first 5 min after leaf excision.…”

“…Given the experimental error of the ABA measurements, the affected pool must comprise less than 20% of the basal ABA content of turgid leaves. Critical sources of ABA may include localized or very rapid synthesis of ABA in cells near the stomatal complex (25) and͞or ABA transported from the roots in the transpiration stream (26). Localized or low-level constitutive expression of Vp14 mRNA in leaf tissues is consistent with our Northern blot analysis of nonstressed leaves.…”

Xylem Sap pH Increase: A Drought Signal Received at the Apoplastic Face of the Guard Cell That Involves the Suppression of Saturable Abscisic Acid Uptake by the Epidermal Symplast

“…Stomata are able to close in response to water stress signals before an increase in bulk leaf ABA is observed (e.g. Loveys, 1977;Dorffling and Tietz, 1984;Cornish and Zeevaart, 1985). These authors suggested that dehydration of detached leaves in air can release ABA sequestered in the symplast so that it accumulates in the apoplast in the vicinity of the guard cells.…”