Abscisic Acid Status Research Articles

Potential of hydrogen (pH) differentially modulates cadmium stress response in abscisic acid-deficient sitiens tomato mutant

ABSTRACT Uptake of nutrients and cadmium (Cd) are dependent upon many factors, including plant species, ions concentration and pH. Tolerance to Cd-induced oxidative stress can be modulated by phytohormones such as abscisic acid (ABA), which induce the production of reactive oxygen species, activating proteins and enzymes involved in stress response and possibly stress tolerance. The present study aimed to evaluate the biochemical variations induced by Cd in ABA-deficient sitiens tomato mutant (sit) and its wild-type counterpart, Micro-Tom (MT), grown at different pH conditions. The plants were cultivated in nutrient solution (pH 5, 6 and 7; 20-days) and were then further grown over a 48-h period in 0 or 50 ?M CdCl2 at pH 6. Before Cd addition, the concentrations of nitrogen, sulfur, copper, iron and zinc were determined and variations in nutrients concentrations were observed. After Cd addition, sit roots grown at pH 5 and 7 did not exhibit differences in ascorbate peroxidase (APX) activity in 0 or 50 ?M CdCl2, and sit root grown at pH 6 exhibited lower glutathione reductase (GR) activity in the presence of Cd. Moreover, sit shoot grown at pH 5 showed decreased activities of superoxide dismutase (SOD), Mn-SOD II and Cu/Zn-SOD V, in 0 and 50 ?M CdCl2. The results indicated that pH modulates the plant nutrition in a complex way and may involve multiple ABA signaling pathways. Likewise, ABA status seems to be related with the Cd-translocation within the plant, suggesting that Cd, ABA and pH responses cannot be evaluated as isolated systems.

Prolonged drying cycles stimulate ABA accumulation in Citrus macrophylla seedlings exposed to partial rootzone drying

Partial rootzone drying (PRD) establishes discrete wet and dry parts of the rootzone (for example using parallel drip lines on either side of the crop row), and alternates them to stimulate root growth and root-to-shoot ABA signalling. To assess whether alternation frequency affects plant physiological responses, Citrus macrophylla Wester seedlings were grown with the root system split between two pots and 5 irrigation treatments applied: Control, PRD-Fixed (where wet and dry parts of the rootzone were not alternated) and three alternate PRD treatments where the wet and dry parts were swapped at 3 (PRD1), 6 (PRD2) and 12 (PRD3) days intervals, to dry the soil to different degrees before alternating the irrigation. Water was equally distributed between both pots in Control plants, whereas only one pot was watered and the other allowed to dry in PRD plants, with all plants receiving the same irrigation volume. After 24 days, soil water content (θv), leaf water potential (Ψleaf), root water potential (Ψroot), abscisic acid (ABA) concentration in roots ([ABA]root), leaves ([ABA]leaf) and shoot xylem sap ([X-ABA]shoot), biomass allocation and leaf area were measured. Higher soil water availability of the dry side (PRD1 and PRD2) had no significant effects on leaf water relations, ABA status and plant biomass allocation. However, increasing the duration of exposure of part of the root system to dry soil (PRD3 and PRD-Fixed) further decreased Ψroot and stimulated root ABA accumulation, while decreasing Ψleaf and increasing [ABA]leaf of PRD3 plants compared to the other treatments. Differences in physiological response between PRD3 and PRD-Fixed plants were attributed to differences in the proportion of root mass exposed to drying soil: PRD3 plants had a lower Ψleaf and a higher [ABA]leaf with a smaller proportion of their root mass in wet soil. Since long drying cycles were required to alter plant biomass allocation and physiological responses in PRD plants, these should be implemented in designing suitable PRD strategies for field application.

Manipulation of ABA Content in Arabidopsis thaliana Modifies Sensitivity and Oxidative Stress Response to Dickeya dadantii and Influences Peroxidase Activity.

The production of reactive oxygen species (ROS) is one of the first defense reactions induced in Arabidopsis in response to infection by the pectinolytic enterobacterium Dickeya dadantii. Previous results also suggest that abscisic acid (ABA) favors D. dadantii multiplication and spread into its hosts. Here, we confirm this hypothesis using ABA-deficient and ABA-overproducer Arabidopsis plants. We investigated the relationships between ABA status and ROS production in Arabidopsis after D. dadantii infection and showed that ABA status modulates the capacity of the plant to produce ROS in response to infection by decreasing the production of class III peroxidases. This mechanism takes place independently of the well-described oxidative stress related to the RBOHD NADPH oxidase. In addition to this weakening of plant defense, ABA content in the plant correlates positively with the production of some bacterial virulence factors during the first stages of infection. Both processes should enhance disease progression in presence of high ABA content. Given that infection increases transcript abundance for the ABA biosynthesis genes AAO3 and ABA3 and triggers ABA accumulation in leaves, we propose that D. dadantii manipulates ABA homeostasis as part of its virulence strategy.

Significant contribution from foliage-derived ABA in regulating gas exchange in Pinus radiata.

The complex regulatory system controlling stomata involves physical and chemical signals that affect guard cell turgor to bring about changes in stomatal conductance (gs). Abscisic acid (ABA) closes stomata, yet the mechanisms controlling foliar ABA status in tree species remain unclear. The importance of foliage-derived ABA in regulating gas exchange was evaluated under treatments that affected phloem export through girdling and reduced water availability in the tree species, Pinus radiata (D. Don). Branch- and whole-plant girdling increased foliar ABA levels leading to declines in gs, despite no change in plant water status. Changes in gs were largely independent of the more transient increases in foliar non-structural carbohydrates (NSC), suggesting that gradual accumulation of foliar ABA was the primary mechanism for reductions in gs and assimilation. Whole-plant girdling eventually reduced root NSC, hindering root water uptake and decreasing foliar water potential, causing a dramatic increase in ABA level in leaves and concentrations in the xylem sap of shoots (4032 ng ml-1), while root xylem sap concentrations remained low (43 ng ml-1). Contrastingly, the drought treatment caused similar increases in xylem sap ABA in both roots and shoots, suggesting that declines in water potential result in relatively consistent changes in ABA along the hydraulic pathway. ABA levels in plant canopies can be regulated independently of changes in root water status triggered by changes by both phloem export and foliar water status.

Increasing abscisic acid levels by immunomodulation in barley grains induces precocious maturation without changing grain composition.

Abscisic acid (ABA) accumulates in seeds during the transition to the seed filling phase. ABA triggers seed maturation, storage activity, and stress signalling and tolerance. Immunomodulation was used to alter the ABA status in barley grains, with the resulting transgenic caryopses responding to the anti-ABA antibody gene expression with increased accumulation of ABA. Calculation of free versus antibody-bound ABA reveals large excess of free ABA, increasing signficantly in caryopses from 10 days after fertilization. Metabolite and transcript profiling in anti-ABA grains expose triggered and enhanced ABA-functions such as transcriptional up-regulation of sucrose-to-starch metabolism, storage protein synthesis and ABA-related signal transduction. Thus, enhanced ABA during transition phases induces precocious maturation but negatively interferes with growth and development. Anti-ABA grains display broad constitutive gene induction related to biotic and abiotic stresses. Most of these genes are ABA- and/or stress-inducible, including alcohol and aldehyde dehydrogenases, peroxidases, chaperones, glutathione-S-transferase, drought- and salt-inducible proteins. Conclusively, ABA immunomodulation results in precocious ABA accumulation that generates an integrated response of stress and maturation. Repression of ABA signalling, occurring in anti-ABA grains, potentially antagonizes effects caused by overshooting production. Finally, mature grain weight and composition are unchanged in anti-ABA plants, although germination is somewhat delayed. This indicates that anti-ABA caryopses induce specific mechanisms to desensitize ABA signalling efficiently, which finally yields mature grains with nearly unchanged dry weight and composition. Such compensation implicates the enormous physiological and metabolic flexibilities of barley grains to adjust effects of unnaturally high ABA amounts in order to ensure and maintain proper grain development.

A balanced JA/ABA status may correlate with adaptation to osmotic stress in Vitis cells

Water-related stress is considered a major type of plant stress. Osmotic stress, in particular, represents the common part of all water-related stresses. Therefore, plants have evolved different adaptive mechanisms to cope with osmotic-related disturbances. In the current work, two grapevine cell lines that differ in their osmotic adaptability, Vitis rupestris and Vitis riparia, were investigated under mannitol-induced osmotic stress. To dissect signals that lead to adaptability from those related to sensitivity, osmotic-triggered responses with respect to jasmonic acid (JA) and its active form JA-Ile, abscisic acid (ABA), and stilbene compounds, as well as the expression of their related genes were observed. In addition, the transcript levels of the cellular homeostasis gene NHX1 were examined. The data are discussed with a hypothesis suggesting that a balance of JA and ABA status might correlate with cellular responses, either guiding cells to sensitivity or to progress toward adaptation.

Abscisic acid metabolizing rhizobacteria decrease ABA concentrations in planta and alter plant growth

Although endogenous phytohormones such as abscisic acid (ABA) regulate root growth, and many rhizobacteria can modulate root phytohormone status, hitherto there have been no reports of rhizobacteria mediating root ABA concentrations and growth by metabolising ABA. Using a selective ABA-supplemented medium, two bacterial strains were isolated from the rhizosphere of rice (Oryza sativa) seedlings grown in sod-podzolic soil and assigned to Rhodococcus sp. P1Y and Novosphingobium sp. P6W using partial 16S rRNA gene sequencing and phenotypic patterns by the GEN III MicroPlate test. Although strain P6W had more rapid growth in ABA-supplemented media than strain P1Y, both could utilize ABA as a sole carbon source in batch culture. When rice seeds were germinated on filter paper in association with bacteria, root ABA concentration was not affected, but shoot ABA concentration of inoculated plants decreased by 14% (strain P6W) and 22% (strain P1Y). When tomato (Solanum lycopersicum) genotypes differing in ABA biosynthesis (ABA deficient mutants flacca – flc, and notabilis – not and the wild-type cv. Ailsa Craig, WT) were grown in gnotobiotic cultures on nutrient solution agar, rhizobacterial inoculation decreased root and/or leaf ABA concentrations, depending on plant and bacteria genotypes. Strain P6W inhibited primary root elongation of all genotypes, but increased leaf biomass of WT plants. In WT plants treated with silver ions that inhibit ethylene perception, both ABA-metabolising strains significantly decreased root ABA concentration, and strain P6W decreased leaf ABA concentration. Since these changes in ABA status also occurred in plants that were not treated with silver, it suggests that ethylene was probably not involved in regulating bacteria-mediated changes in ABA concentration. Correlations between plant growth and ABA concentrations in planta suggest that ABA-metabolising rhizobacteria may stimulate growth via an ABA-dependent mechanism.

Wheat ferritins: Improving the iron content of the wheat grain

The characterization of the full complement of wheat ferritins show that the modern hexaploid wheat genome contains two ferritin genes, TaFer1 and TaFer2, each represented by three homeoalleles and placed on chromosome 5 and 4, respectively. The two genes are differentially regulated and expressed. The TaFer1 genes are, except in the endosperm, the most abundantly expressed and regulated by iron and abscisic acid status. The promoter of TaFer1, in contrast to TaFer2, has iron- and ABA-responsive elements, supporting the expression data. The TaFer1 and TaFer2 genes encode two isoforms, probably functional different and acting in heteropolymer structures of ferritin in cereals. Iron biofortification of the wheat grain is possible. Endosperm targeted intragenic overexpressing of the TaFer1-A gene results in a 50–85% higher iron content in the grain.

Electrophysiological responses of maize roots to low water potentials: relationship to growth and ABA accumulation

The maintenance of root elongation is an important adaptive response to low water potentials (psi(w)), but little is known about its regulation. An important component may be changes in root cell electrophysiology, which both signal and maintain growth maintenance processes. As a first test of this hypothesis, membrane potentials (E(m)) were measured within the cell elongation zone of maize (Zea mays L.) primary roots. Seedlings were grown in oxygenated solution culture, and low psi(w) was imposed by the gradual addition of polyethylene glycol. Cells hyperpolarized approximately 25 mV in response to low psi(w), and after 48 h resting potentials remained significantly hyperpolarized at psi(w) lower than -0.3 MPa compared with roots at high psi(w). Inhibitor experiments showed that the hyperpolarization was dependent on plasma membrane H(+)-ATPase activity. Previous work showed that accumulation of abscisic acid (ABA) is required for the maintenance of maize primary root elongation at low psi(w). To determine if the mechanism of action of ABA involves changes in root electrophysiology, E(m) measurements were made during long-term exposure to low psi(w). Steady-state resting E(m) were measured in regions in which maintenance of cell elongation was dependent on ABA accumulation (2-3 mm from the apex), or in which elongation was inhibited regardless of ABA status (6-8 mm from the apex). E(m) was substantially more negative in ABA-deficient roots specifically in the 2-3 mm region. The results suggest that set-points for ion homeostasis shifted in association with the maintenance of root cell elongation at low psi(w), and that ABA accumulation plays a role in regulating the ion transport processes involved in this response.

The control of proline consumption by abscisic acid during osmotic stress recovery of canola leaf discs

Some aspects of the function of abscisic acid (ABA) in the control of proline (PRO) metabolism in canola leaf discs (CLD) subjected in vitro to consecutive hyper‐osmotic (stress) and hypo‐osmotic (recovery) treatments have been investigated. PRO accumulation in response to stress conditions relies on both stimulation of its synthesis via enhancement of transcription of the gene and activity of Δ1‐pyrroline‐5‐carboxylate synthetase (P5CS) and inhibition of its degradation via inactivation of PRO dehydrogenase (PDH). These changes were partly reversed under recovery conditions. Thus both PDH mRNA and PDH activity increased while P5CS mRNA decreased. Surprisingly P5CS activity remained high even after a 20‐h period of rehydration. Exogenously supplied ABA at recovery inhibited net PRO consumption and this could be associated with downregulation of PDH gene expression and PDH activity. Under these conditions ABA hardly upregulated P5CS gene expression while P5CS activity first transiently decreased and then reached a value close to that found under stressing conditions. Experiments with CLD supplied with either methionine sulphoximine or gabaculine, brought preliminary evidence for a significant synthesis of PRO from glutamate during recovery that replenished the proline pool(s) and provoked a negative effect on the net rate of PRO consumption. Consequently, it is suggested that the availability of both PRO precursors and ABA could be determinant in the control of the amount of residual PRO present in CLD after the period of recovery. This level also seemed to depend on the amount of P5CS transcripts induced under stress conditions. However, the results obtained with turgid leaf discs treated with ABA indicate that the ABA status of the tissues, necessary for inducing the proline response, is not sufficient to determine their PRO content because it remained relatively low despite the stimulation of P5CS expression and P5CS activity.

ABA, ethylene and the control of shoot and root growth under water stress

The question of whether abscisic acid (ABA) acts as an inhibitor or promoter of shoot growth in plants growing in drying soil is examined, drawing on current understanding of the role of ABA in root growth maintenance. Particular consideration is given to studies of endogenous ABA deficiency, which have shown that an important role of ABA is to limit ethylene production, and that this interaction is involved in the effects of ABA status on shoot and root growth.

Quantitative relationship between indole-3-acetic acid and abscisic acid during leaf growth in Coleus blumei

Quantitative determinations by gas chromatography-mass spectrometry ofindole-3-acetic acid (IAA) and abscisic acid (ABA) in growing leaves ofColeusblumei plants show parallel declines in leaf concentrations of bothhormones,except in leaf number 3 (about three-fourths of full size) where IAA level wasthe lowest of those measured. Expansion of the most recently unfurled leaf tofull size serves, in effect, to dilute both IAA and ABA about 1.7 to 1.Althoughabsolute levels of leaf IAA varied as much as an order of magnitude from onebatch of plants to another, ABA levels were proportional to the IAA level withan overall correlation coefficient of 0.91. Evidence, both correlative andcausal, for the determination of ABA status by IAA—and of IAA status byABA—in leaves and other developing organs is summarized.

Confirmation that abscisic acid accumulation is required for maize primary root elongation at low water potentials

Abstract Previous work showed that primary root elongation in maize (Zeamays L.) seedlings at low water potentials was severely inhibited when accumulation of abscisic acid (ABA) was decreased using either fluridone or the vp5 mutant to inhibit carotenoid (and ABA) biosynthesis. The objective of this study was to confirm that the inhibition of root elongation resulting from these treatments was indeed attributable to the decrease in ABA levels. Seedlings were transplanted after germination to vermiculite at water potentials of −1.6 or − 0.3 MPa. ABA was mixed at various concentrations with the vermiculite to test whether the effects of fluridone and the vp5 mutation on root elongation could be overcome. In both treatments, restoration of ABA levels in the root apical 10 mm, which encompassed the elongation zone, resulted in recovery of root elongation rate at both −1.6 MPa and −0.3 MPa. Analysis of the spatial distribution of elongation rate showed that the recovery of overall root elongation resulted from restoration of the profile of elongation, and did not involve over-promotion of local elongation rate at any position. The recovery of root elongation was shown to be independent of effects of ABA status on shoot growth. When ABA was applied at high water potential, such that levels of ABA in the root tip reached those associated with maintenance of elongation at low water potentials, root elongation was inhibited. Thus, the response of root elongation to bulk tissue ABA content varied with the tissue water status. The results confirm that accumulation of ABA is required for the maintenance of maize primary root elongation at low water potentials.

Root Growth Maintenance at Low Water Potentials (Increased Activity of Xyloglucan Endotransglycosylase and Its Possible Regulation by Abscisic Acid).

Previous work suggested that an increase in cell wall-loosening contributes to the maintenance of maize (Zea mays L.) primary root elongation at low water potentials ([psi]w). It was also shown that root elongation at low [psi]w requires increased levels of abscisic acid (ABA). In this study we investigated the effects of low [psi]w and ABA status on xyloglucan endotransglycosylase (XET) activity in the root elongation zone. XET is believed to contribute to wall-loosening by reversibly cleaving xyloglucan molecules that tether cellulose microfibrils. The activity of XET per unit fresh weight in the apical 10 mm (encompassing the elongation zone) was constant at high [psi]w but increased by more than 2-fold at a [psi]w of -1.6 MPa. Treatment with fluridone to decrease ABA accumulation greatly delayed the increase in activity at low [psi]w. This effect was largely overcome when internal ABA levels were restored by exogenous application. Spatial distribution studies showed that XET activity was increased in the apical 6 mm at low [psi]w whether expressed per unit fresh weight, total soluble protein, or cell wall dry weight, corresponding to the region of continued elongation. Treatment with fluridone progressively inhibited the increase in activity with distance from the apex, correlating with the pattern of inhibition of elongation. Added ABA partly restored activity at all positions. The increase in XET activity at low [psi]w was due to maintenance of the rate of deposition of activity despite decreased deposition of wall material. The loss of activity associated with decreased ABA was due to inhibition of the deposition of activity. The results demonstrate that increased XET activity is associated with maintenance of root elongation at low [psi]w and that this response requires increased ABA.

Abscisic acid relationships in the chill-related dormancy mechanism in apple seeds

Abscisic acid (ABA) levels in seeds from three cultivars of apple (Malus domestica Borkh.) which have substantially different chilling requirements were investigated by gas chromatography mass-spectrometry selected ion monitoring (GCMS-SIM) during stratification. The ABA content of dormant unchilled seeds was similar in the three cultivars, suggesting no relationship between the chilling requirement of those seeds and their ABA status. That chilling is not related to ABA changes during stratification was confirmed by warm (20°C) and cold (5°C) stratification experiments. ABA content dropped rapidly and nearly identically under both temperature regimes, but only cold stratification promoted germination. The decline in ABA during stratification was due in large part to leaching from the seed coat and nucellar membrane; the ABA content of the embryo remained nearly constant. The radicle in intact seeds stratified at 5°C began growing 20–30 days after the ABA in the seed coat and nucellar membrane had nearly disappeared. Radicle growth did not occur in unchilled seeds, even though ABA had leached from them as well. It is possible that the leaching of ABA from the seed allows certain promotive forces to develop, but if so, these can develop only at chilling temperatures. Studies were also conducted on 2-trans ABA relationships to apple seed dormancy, but no association was evident.