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PubMed2002

Salt-stress-induced ABA accumulation is more sensitively triggered in roots than in shoots.

Salt-stress-induced ABA accumulation in maize root tissues was compared with that in leaf tissues. While salt stress with NaCl resulted in a significant ABA accumulation in root tissues (up to 10-fold), the same stress only led to a small ABA accumulation in leaf tissues (about 1-fold). Pretreatment with ethylene glycol (EG), a permeable and inert monomer of PEG, could prevent the shrinkage of cell volume and completely block the ABA accumulation in leaf tissues under salt stress, but substantial salt-induced ABA accumulation was still observed in root tissues following such pretreatment. Hypotonic salt solutions, i.e. below 100 mM NaCl, still induced a significant ABA accumulation (more than 3-fold) in roots, but showed no effect on that in leaf tissues. Results suggest that the salt-stress-induced ABA accumulation in roots may also be triggered by an osmosensing mechanism, which is in addition to the perception of the changes in reduced cellular volume or plasmalemma tension that leads to ABA accumulation in leaves. When leaf and root tissues were immersed into salt solutions, salt entered into the cells as a function of time and salt concentrations. Such entrance apparently led to a loss of sensitivity of leaf tissues to accumulate ABA under the salt stress, and also prevented the leaf tissues from responding to further air-drying in terms of ABA accumulation. Roots showed no such responses. Results suggest that the entrance of salt into leaf cells brought about some toxic effect that might have reduced the capability of leaf cells to produce ABA under dehydration.

Abscisic Acid↗
PubMed2004

Abscisic acid (ABA) flows from Hordeum vulgare to the hemiparasite Rhinanthus minor and the influence of infection on host and parasite abscisic acid relations.

Using the facultative root hemiparasite Rhinanthus minor and Hordeum vulgare as a host, the flows, depositions, and metabolism of abscisic acid (ABA) within the host, within the parasite, and between host and parasite have been studied. When the plants were supplied with 5 mM NO(3)(-), there were weak or no effects of parasitism on ABA flows, biosynthesis, and ABA degradation in barley. However, ABA deposition was significantly affected in the leaf laminae (3-fold) and in the leaf sheath (2.4-fold), but not in roots. Dramatic changes in ABA flows, metabolism, and deposition on a per plant basis, however, have been observed in Rhinanthus. Biosynthesis in the roots was 12-fold higher after attachment, resulting in 14-fold higher ABA flows in the xylem. A large portion of this ABA was metabolized, a small portion was deposited. Phloem flows of ABA were increased 13-fold after attachment. The concentrations of ABA in tissues and transport fluids were higher in attached Rhinanthus by an order of magnitude than in host tissues and xylem sap. The same tendency was also found in a comparison between single Rhinanthus and unparasitized barley. As compared with 5 mM NO(3)(-), lower NO(3)(-) or 1 mM NH(4)(+) supply doubled the ABA concentrations in barley leaf laminae, while having only small or non-significant effects in the other organs. The possible function of ABA for the parasite is discussed.

Abscisic Acid↗
PubMed2007

Ethylene promotes submergence-induced expression of OsABA8ox1, a gene that encodes ABA 8'-hydroxylase in rice.

A rapid decrease of the plant hormone ABA under submergence is thought to be a prerequisite for the enhanced elongation of submerged shoots of rice (Oryza sativa L.). Here, we report that the level of phaseic acid (PA), an oxidized form of ABA, increased with decreasing ABA level during submergence. The oxidation of ABA to PA is catalyzed by ABA 8'-hydroxylase, which is possibly encoded by three genes (OsABA8ox1, -2 and -3) in rice. The ABA 8'-hydroxylase activity was confirmed in microsomes from yeast expressing OsABA8ox1. OsABA8ox1-green fluorescent protein (GFP) fusion protein in onion cells was localized to the endoplasmic reticulum. The mRNA level of OsABA8ox1, but not the mRNA levels of other OsABA8ox genes, increased dramatically within 1 h after submergence. On the other hand, the mRNA levels of genes involved in ABA biosynthesis (OsZEP and OsNCEDs) decreased after 1-2 h of submergence. Treatment of aerobic seedlings with ethylene and its precursor, 1-aminocyclopropane-1-carboxylate (ACC), rapidly induced the expression of OsABA8ox1, but the ethylene treatment did not strongly affect the expression of ABA biosynthetic genes. Moreover, pre-treatment with 1-methylcyclopropene (1-MCP), a potent inhibitor of ethylene action, partially suppressed induction of OsABA8ox1 expression under submergence. The ABA level was found to be negatively correlated with OsABA8ox1 expression under ACC or 1-MCP treatment. Together, these results indicate that the rapid decrease in ABA levels in submerged rice shoots is controlled partly by ethylene-induced expression of OsABA8ox1 and partly by ethylene-independent suppression of genes involved in the biosynthesis of ABA.

Base Sequence↗
PubMed1987

Isolation and Characterization of ABA-Insensitive Cell Lines of Carrot.

While abscisic acid (ABA) exerts multiple effects on somatic embryogenesis, the most pronounced of these effects is the arrestment of torpedo-stage embryos, preventing them from developing into plantlets. In order to understand the mechanism of ABA inhibition of plantlet formation, we have isolated seven ABA-insensitive cell lines capable of developing into plantlets in the presence of ABA. These ABA-insensitive cell lines, whose frequency of appearance is 7 x 10(-6), have been isolated from a haploid cell line of Daucus carota L. var Juwarot. Surprisingly, all seven cell lines exhibit auxin insensitivity as evidenced by their ability to produce heart-stage embryos in various auxins including 2,4-dichlorophenoxyacetic acid (2,4-D), naphthalene acetic acid, and indolacetic acid. Three of the cell lines, ABA 1, ABA 15, and ABA 17, have been further characterized. We found that all three showed lower levels of ABA uptake which may be the cause of ABA insensitivity. However, the uptake of 2,4-D is higher in the three cell lines than in the wild type. The basis of the interaction between ABA and 2,4-D responses is discussed.

Journal Article↗
PubMed2007

Regulation and manipulation of ABA biosynthesis in roots.

Overexpression of 9-cis-epoxycarotenoid dioxygenase (NCED) is known to cause abscisic acid (ABA) accumulation in leaves, seeds and whole plants. Here we investigated the manipulation of ABA biosynthesis in roots. Roots from whole tomato plants that constitutively overexpress LeNCED1 had a higher ABA content than wild-type (WT) roots. This could be explained by enhanced in situ ABA biosynthesis, rather than import of ABA from the shoot, because root cultures also had higher ABA content, and because tetracycline (Tc)-induced LeNCED1 expression caused ABA accumulation in isolated tobacco roots. However, the Tc-induced expression led to greater accumulation of ABA in leaves than in roots. This demonstrates for the first time that NCED is rate-limiting in root tissues, but suggests that other steps were also restrictive to pathway flux, more so in roots than in leaves. Dehydration and NCED overexpression acted synergistically in enhancing ABA accumulation in tomato root cultures. One explanation is that xanthophyll synthesis was increased during root dehydration, and, in support of this, dehydration treatments increased beta-carotene hydroxylase mRNA levels. Whole plants overexpressing LeNCED1 exhibited greatly reduced stomatal conductance and grafting experiments from this study demonstrated that this was predominantly due to increased ABA biosynthesis in leaves rather than in roots. Genetic manipulation of both xanthophyll supply and epoxycarotenoid cleavage may be needed to enhance root ABA biosynthesis sufficiently to signal stomatal closure in the shoot.

Abscisic Acid↗
PubMed2006

ABA-induced NO generation and stomatal closure in Arabidopsis are dependent on H2O2 synthesis.

Nitric oxide (NO) and hydrogen peroxide (H(2)O(2)) are key signalling molecules produced in response to various stimuli and involved in a diverse range of plant signal transduction processes. Nitric oxide and H(2)O(2) have been identified as essential components of the complex signalling network inducing stomatal closure in response to the phytohormone abscisic acid (ABA). A close inter-relationship exists between ABA and the spatial and temporal production and action of both NO and H(2)O(2) in guard cells. This study shows that, in Arabidopsis thaliana guard cells, ABA-mediated NO generation is in fact dependent on ABA-induced H(2)O(2) production. Stomatal closure induced by H(2)O(2) is inhibited by the removal of NO with NO scavenger, and both ABA and H(2)O(2) stimulate guard cell NO synthesis. Conversely, NO-induced stomatal closure does not require H(2)O(2) synthesis nor does NO treatment induce H(2)O(2) production in guard cells. Tungstate inhibition of the NO-generating enzyme nitrate reductase (NR) attenuates NO production in response to nitrite in vitro and in response to H(2)O(2) and ABA in vivo. Genetic data demonstrate that NR is the major source of NO in guard cells in response to ABA-mediated H(2)O(2) synthesis. In the NR double mutant nia1, nia2 both ABA and H(2)O(2) fail to induce NO production or stomatal closure, but in the nitric oxide synthase deficient Atnos1 mutant, responses to H(2)O(2) are not impaired. Importantly, we show that in the NADPH oxidase deficient double mutant atrbohD/F, NO synthesis and stomatal closure to ABA are severely reduced, indicating that endogenous H(2)O(2) production induced by ABA is required for NO synthesis. In summary, our physiological and genetic data demonstrate a strong inter-relationship between ABA, endogenous H(2)O(2) and NO-induced stomatal closure.

Abscisic Acid↗
PubMed2006

CYP707A3, a major ABA 8'-hydroxylase involved in dehydration and rehydration response in Arabidopsis thaliana.

Abscisic acid (ABA) catabolism is one of the determinants of endogenous ABA levels affecting numerous aspects of plant growth and abiotic stress responses. The major ABA catabolic pathway is triggered by ABA 8'-hydroxylation catalysed by the cytochrome P450 CYP707A family. Among four members of Arabidopsis CYP707As, the expression of CYP707A3 was most highly induced in response to both dehydration and subsequent rehydration. A T-DNA insertional cyp707a3-1 mutant contained higher ABA levels in turgid plants, which showed a reduced transpiration rate and hypersensitivity to exogenous ABA during early seedling growth. On dehydration, the cyp707a3-1 mutant accumulated a higher amount of stress-induced ABA than the wild type, an event that occurred relatively later and was coincident with slow drought induction of CYP707A3. The cyp707a3 mutant plants exhibited both exaggerated ABA-inducible gene expression and enhanced drought tolerance. Conversely, constitutive expression of CYP707A3 relieved growth retardation by ABA, increased transpiration, and a reduction of endogenous ABA in both turgid and dehydrated plants. Taken together, our results indicate that CYP707A3 plays an important role in determining threshold levels of ABA during dehydration and after rehydration.

Abscisic Acid↗
PubMed2000

Analysis of Arabidopsis glucose insensitive mutants, gin5 and gin6, reveals a central role of the plant hormone ABA in the regulation of plant vegetative development by sugar.

Sugars have signaling roles in a wide variety of developmental processes in plants. To elucidate the regulatory components that constitute the glucose signaling network governing plant growth and development, we have isolated and characterized two Arabidopsis glucose insensitive mutants, gin5 and gin6, based on a glucose-induced developmental arrest during early seedling morphogenesis. The T-DNA-tagged gin6 mutant abrogates the glucose-induced expression of a putative transcription factor, ABI4, previously shown to be involved in seed-specific abscisic acid (ABA) responses. Thus, ABI4 might be a regulator involved in both glucose- and seed-specific ABA signaling. The characterization of the gin5 mutant, on the other hand, reveals that glucose-specific accumulation of ABA is essential for hexokinase-mediated glucose responses. Consistent with this result, we show that three ABA-deficient mutants (aba1-1, aba2-1, and aba3-2) are also glucose insensitive. Exogenous ABA can restore normal glucose responses in gin5 and aba mutants but not in gin6 plants. Surprisingly, only abi4 and abi5-1 but not other ABA-insensitive signaling mutants (abi1-1, abi2-1, and abi3-1) exhibit glucose insensitivity, indicating the involvement of a distinct ABA signaling pathway in glucose responses. These results provide the first direct evidence to support a novel and central role of ABA in plant glucose responses mediated through glucose regulation of both ABA levels by GIN5 and ABA signaling by GIN6/ABI4.

Abscisic Acid↗
PubMed1985

Arsonate-specific murine T cell clones. II. Delayed-type hypersensitivity induced by P-azobenzenearsonate-L-tyrosine (ABA-Tyr).

To study T cell idiotype expression at the functional level, we developed a hapten-specific delayed-type hypersensitivity (DTH) system by which we avoid the complication of anti-hapten antibody and which is specific only for the immunizing hapten, and not for conjugate specific determinants. Immunization with ABA-Tyr and challenge with ABA diazonium induced footpad swelling with the characteristics of DTH. Anti-ABA antibodies did not contribute to this reaction, as they were undetectable in mice immunized with ABA-Tyr. Furthermore, this ABA-Tyr-specific DTH was under Ir gene control identical to that reported for ABA-Tyr-specific lymphocyte proliferation. All mouse strains tested responded to ABA-Tyr except those of the b haplotype across the entire Ia region. In contrast, contact sensitivity induced by ABA diazonium was not under apparent Ir gene control, probably reflecting 1) different specificities of the induced T cells and 2) the production of anti-ABA antibodies that contribute to the footpad swelling via an Arthus reaction. Having shown that ABA-Tyr can induce T cells mediating DTH, we then examined ABA-Tyr-reactive T cell clones, propagated in vitro, for their ability to mediate DTH. Such clones elicited a response identical to that seen with in vivo immunization with respect to dose dependency, I-Ak restriction, and antigen specificity.

Animals↗
PubMed1983

Hapten-specific T cell response to azobenzenearsonate-N-acetyl-L-tyrosine in the Lewis rat. II. Induction of helper activity demonstrated by TNP-haptenated ABA-peptide-Ficoll.

In an effort to study T cell functions in Lewis rats immunized with ABA-N-acetyl-L-tyrosine (ABA-tyr), we developed an antigen that provides a sensitive assay of ABA-specific helper function that is read as an increase in TNP-specific plaque-forming cells (PFC). This antigen has ABA coupled to AECM-Ficoll by virtue of a tripeptide (tyr-ala-ala) spacer and TNP coupled to the AECM side chains. At subimmunogenic doses, this antigen induced 400 anti-TNP PFC/10(6) spleen cells in ABA-tyr-immunized rats. As many as 8000 PFC/10(6) spleen cells were induced with larger doses of antigen (200 micrograms). By contrast, only 490 PFC/10(6) spleen cells could be induced with 1 mg of the conventional doubly haptenated protein carriers such as ABA-BSA-TNP. Both direct and indirect PFC were induced by this antigen in primed rats. The use of this antigen and passive transfer techniques to study ABA-specific helper activity revealed some differences from ABA-specific delayed-type hypersensitivity (DTH) and in vitro proliferation, which were studied previously. Cells responsible for helper activity appeared sooner after immunization and were found most prominently in peritoneal exudates but also significantly in spleen where the cells responsible for DTH or in vitro proliferative responses were never found. By contrast, helper cells were not seen in lymph nodes, where some proliferative activity could be found. Of these three ABA-specific T cell functions, helper activity was least easily suppressed by the previously used regimens of ABA-tyr in incomplete freunds adjuvant (IFA). Moreover, helper activity appears after injection of ABA-tyr in IFA, a method that has never in our hands yielded detectable DTH or in vitro proliferative responses. Despite these differences, phenotyping with monoclonal antibodies indicated that cells responsible for helper and proliferative activities were both W3/25+ and OX8-.

Acetylmuramyl-Alanyl-Isoglutamine↗
PubMed2006

Regulation of ABA level and water-stress tolerance of Arabidopsis by ectopic expression of a peanut 9-cis-epoxycarotenoid dioxygenase gene.

The oxidative cleavage of cis-epoxycarotenoids catalyzed by 9-cis-epoxycarotenoid dioxygenase (NCED) is considered to be the rate-limiting step in abscisic acid (ABA) biosynthesis. Here we demonstrate that the expression of AhNCED1 gene in peanut plants is significantly up-regulated by dehydration and high salinity. The AhNCED1 transcript and endogenous ABA both accumulate predominantly in leaves and stems of peanut in response to dehydration. The considerable up-regulation of AhNCED1 expression by exogenous application of ABA suggests a positive feedback control of ABA biosynthesis in peanut. NAA at high concentration increases ABA biosynthesis in peanut plants through up-regulation of the AhNCED1 gene expression. The constitutive expression of the AhNCED1 gene in wild-type Arabidopsis results in an increase of ABA accumulation in transgenic plants in response to drought stress. Ectopic expression of AhNCED1 gene in 129B08/nced3 mutant Arabidopsis (with impaired AtNCED3 gene involved in ABA biosynthesis under water stress) driven by the AtNCED3 promoter restores its ability to accumulate ABA during drought stress, and reverts its hypersensitivity to nonionic osmotic stress and soil drought. These results indicate that the expression of AhNCED1 gene plays an important role in the regulation of ABA level during water stress, and that water-stress tolerance of Arabidopsis plants can be improved by ectopic expression of the AhNCED1 gene causing accumulation of endogenous ABA.

Abscisic Acid↗
PubMed1999

Expression of a dehydrin gene during embryo development and drought stress in ABA-deficient mutants of sunflower (Helianthus annuus L.).

The synthesis of a particular class of proteins, the dehydrins, is a common response to drought in plants. Dehydrins are known to be synthesized by the cell in response to abscisic acid, which represents a link between environment and nuclear activity, though dehydrin genes may be expressed even constitutively. We have investigated the relationship between abscisic acid (ABA) and accumulation of a dehydrin mRNA in sunflower, in which a dehydrin cDNA (HaDhnla) was isolated. In particular, we studied changes in the steady-state level of dehydrin transcripts in two mutants for ABA synthesis and accumulation: nd-1 (an albino, non-dormant and lethal mutant with a very low ABA content and no ABA accumulation in response to stress) and w-1 (a wilty mutant, with reduced ABA accumulation) during embryo and plantlet development and drought stress. Differences between genotypes were observed through embryogenesis: w-1 shows a lower content of dehydrin transcripts in the early stages compared to control plants, indicating that ABA affects dehydrin mRNA accumulation; however, dehydrin transcripts level appears independent of ABA content in late embryogenesis. Also during drought stress in w-1 adult leaves, ABA is not quantitatively related to the steady-state level of the HaDhn1a transcripts. Finally, data on nd-1 mutant show a high level of dehydrin transcripts after drought stress in plantlet cotyledons and leaflets. These results indicate the existence of two regulation pathways of HaDhn1a transcripts accumulation, an ABA-dependent and an ABA-independent one, which may have cumulative effects.

Abscisic Acid↗
PubMed1999

An Arabidopsis mutant with deregulated ABA gene expression: implications for negative regulator function.

The physiological acclimation of plants to osmotic stresses involves a complex programme of gene regulation. In one signalling pathway, elevated levels of abscisic acid (ABA) activate a subset of stress genes. Because ABA responses lack a definable morphological phenotype, we have screened for mutants that exhibit deregulated ABA-responsive gene expression. To monitor this ABA response, a line of Arabidopsis thaliana carrying a transgene composed of the ABA-responsive Arabidopsis kin2 promoter fused to the coding sequence for the firefly luciferase gene, kin2::luc, was generated. Patterns of ABA-responsive luciferase activity were monitored by photon counting. In contrast to wild-type plants which display a transient activation of kin2::luc, an ABA deregulated gene expression mutant (ade1) exhibits both sustained and enhanced levels of transgene activity. Levels of kin2, cor47 and rab18 expression in ade1 plants are also enhanced and prolonged indicating that the molecular mechanism(s) altered in ade1 plants affects the regulation of other ABA-responsive genes. The mutant phenotype is specific for the ABA response as cold-inducible kin2 expression is unaltered in ade1 plants. Genetic analyses indicate that the ade1 mutant is a monogenic recessive trait. A role for negative regulator function in ABA signalling is discussed.

Abscisic Acid↗
PubMed2003

Increased dehydrin promoter activity caused by HvSPY is independent of the ABA response pathway.

A barley SPINDLY protein, HvSPY, is a negative regulator of gibberellin (GA) action. It is also found to be a positive regulator of the promoter of a barley dehydrin (Dhn) gene which is abscisic acid (ABA) upregulated. To investigate whether HvSPY acts through the ABA signaling pathway to upregulate the Dhn promoter, functional characterization was carried out by co-bombardment experiments. These experiments used Dhn promoter-GUS reporter constructs and an effector construct to overexpress HvSPY protein in barley aleurone. ABA dose-response experiments with and without HvSPY overexpression showed that the induction by HvSPY occurred in addition to the ABA effect. Gibberellic acid (GA3) did not reduce the induction by ABA, but it had a small, although significant, effect on the ability of HvSPY to upregulate. The induction of promoter activity of Dhn by HvSPY required the intact protein, and a small deletion in the tetratricopeptide repeat (TPR) region reduced this ability significantly. When a promoter region containing an element for ABA responsiveness was mutagenized or deleted, the mutant promoters lost ABA responsiveness but remained responsive to HvSPY. In addition, HvSPY did not increase promoter activities of other ABA-upregulated genes. Taken together, these results indicate that HvSPY and ABA both regulate promoter activity of Dhn, and that HvSPY acts independently of the ABA signaling pathway.

Abscisic Acid↗
PubMed2005

The modulating effect of the perisperm-endosperm envelope on ABA-inhibition of seed germination in cucumber.

Abscisic acid (ABA) markedly reduced the germination of developing seeds at much lower concentrations (ABA50=0.1 mM) compared with that of mature seeds (ABA50=1.6 mM) in cucumber (Cucumis sativus L. cv. Green long). The perisperm-endosperm (PE) envelope in developing seeds showed partly differentiated lipid and callose layers, considerable ABA biosynthetic activity in endosperm cells, and appreciable permeability to applied ABA. The decrease in the sensitivity of seeds to applied ABA was coincident with the complete development of lipid and callose layers, diminished ABA biosynthetic activity in endosperm cells in imbibed mature seeds, and moderate permeability of the PE envelope to applied ABA. Decoated seeds pretreated with chloroform showed decreased germination (ABA50=0.4 mM) in response to applied ABA and increased ABA permeation through the PE envelope. ABA thus allowed to permeate into embryonic tissues substantially reduced the pregerminative activity of beta-glucanase in the radicles. The structure and biophysical/biochemical properties of the PE envelope seem to modulate the effect of ABA on the germination of developing and mature cucumber seeds.

Abscisic Acid↗
PubMed2006

Anti-transpirant activity in xylem sap from flooded tomato (Lycopersicon esculentum Mill.) plants is not due to pH-mediated redistributions of root- or shoot-sourced ABA.

In flooded soils, the rapid effects of decreasing oxygen availability on root metabolic activity are likely to generate many potential chemical signals that may impact on stomatal apertures. Detached leaf transpiration tests showed that filtered xylem sap, collected at realistic flow rates from plants flooded for 2 h and 4 h, contained one or more factors that reduced stomatal apertures. The closure could not be attributed to increased root output of the glucose ester of abscisic acid (ABA-GE), since concentrations and deliveries of ABA conjugates were unaffected by soil flooding. Although xylem sap collected from the shoot base of detopped flooded plants became more alkaline within 2 h of flooding, this rapid pH change of 0.5 units did not alter partitioning of root-sourced ABA sufficiently to prompt a transient increase in xylem ABA delivery. More shoot-sourced ABA was detected in the xylem when excised petiole sections were perfused with pH 7 buffer, compared with pH 6 buffer. Sap collected from the fifth oldest leaf of "intact" well-drained plants and plants flooded for 3 h was more alkaline, by approximately 0.4 pH units, than sap collected from the shoot base. Accordingly, xylem [ABA] was increased 2-fold in sap collected from the fifth oldest petiole compared with the shoot base of flooded plants. However, water loss from transpiring, detached leaves was not reduced when the pH of the feeding solution containing 3-h-flooded [ABA] was increased from 6.7 to 7.1 Thus, the extent of the pH-mediated, shoot-sourced ABA redistribution was not sufficient to raise xylem [ABA] to physiologically active levels. Using a detached epidermis bioassay, significant non-ABA anti-transpirant activity was also detected in xylem sap collected at intervals during the first 24 h of soil flooding.

Abscisic Acid↗
PubMed2026

Ultraviolet-B-induced OsKOL4 promotes ABA accumulation by inhibiting OsABA8ox1 and OsABA8ox2 expression.

Ultraviolet-B (UV-B) light is a component of sunlight that influences plant survival and adaptation. UV-B induces plants to regulate their phenotypes and metabolism to increase resistance to UV-B and associated stresses. Abscisic acid (ABA) metabolism and signaling are important for plant responses to UV-B. However, the mechanisms underlying UV-B-induced ABA accumulation and signaling in rice remain poorly understood. Here, we report that ENT-KAURENE OXIDASE LIKE 4 (OsKOL4) regulates UV-B-induced responses and ABA biosynthesis. UV-B activates OsKOL4 expression via OsbZIP48, an ELONGATED HYPOCOTYL 5 (HY5) homolog that directly binds to the OsKOL4 promoter. Rice plants overexpressing OsKOL4 exhibit UV-B-induced phenotypes under normal conditions, along with ABA overaccumulation phenotypes resulting from increased ABA levels. Moreover, UV-B promotes ABA accumulation by inhibiting the expression of the ABA 8'-HYDROXYLASE1 and ABA 8'-HYDROXYLASE 2 (OsABA8ox1/2) genes through OsKOL4. OsKOL4 interacts with the transcription factor AP2/ERF ON CHROMOSOME 3 (OsAPE3), which in turn represses the transcription of OsABA8ox1/2. Furthermore, both UV-B and OsKOL4 enhance the binding of OsAPE3 to the OsABA8ox1/2 promoters. Collectively, our findings demonstrate that the OsKOL4-OsAPE3 module regulates ABA homeostasis in response to UV-B signaling by reducing ABA catabolism.

Abscisic Acid↗
PubMed1992

The aba Mutant of Arabidopsis thaliana (L.) Heynh. Has Reduced Chlorophyll Fluorescence Yields and Reduced Thylakoid Stacking.

It has been shown that the aba mutant of Arabidopsis thaliana (L.) Heynh. is impaired in epoxy-carotenoid biosynthesis and accumulates the epoxy-carotenoid precursor, zeaxanthin (C.D. Rock, J.A.D. Zeevaart [1991] Proc Natl Acad Sci USA 88: 7496-7499). In addition to providing conclusive evidence for the indirect pathway of abscisic acid biosynthesis from epoxy-carotenoids, the aba mutation offers a powerful means to study the function of xanthophylls (oxygenated carotenoids) in photosynthesis. We measured in vivo the chlorophyll (Chl) fluorescence parameters F(o) (initial), F(m) (maximum), F(v) (variable = F(m) - F(o)), and t((1/2)) (half-rise time of fluorescence induction) of wild-type (WT) and three allelic aba mutants. The mutant genotypes had significantly lower F(o) and F(m) values relative to those of WT. The F(v)/F(m) ratio and t((1/2)), which are parameters affected by photochemical efficiency, photosystem II (PSII), and plastoquinone pool sizes, were similar in the aba alleles and WT. Because the aba genotypes accumulate high levels of zeaxanthin, which is involved in nonphotochemical quenching of Chl fluorescence, we propose that the reduced fluorescence yields in the aba genotypes are a consequence of the accumulated zeaxanthin. Measurement of PSII oxygen evolution rates in isolated thylakoid membranes of WT and aba-4 confirmed that quantum efficiency was not altered in aba-4 but indicated that the mutant had reduced PSII activity in vitro. Electron microscopy revealed an abnormal chloroplast ultrastructure in the aba plants: the mutants had significantly fewer thylakoid lamellae per granum stack but significantly more grana per chloroplast, as well as more chloroplasts per cell than WT. Immunoblot analysis established that aba-4 had normal levels of the Chl a/b-binding core polypeptide of PSII (CP29) and the PSII light-harvesting Chl a/b-binding complex. These results provide evidence for the role of zeaxanthin in nonphotochemical fluorescence quenching and suggest involvement of epoxy-carotenoids and/or zeaxanthin in thylakoid stacking and PSII activity.

Journal Article↗