Inhibitory Effect Of Abscisic Acid Research Articles

PBS3: a versatile player in and beyond salicylic acid biosynthesis in Arabidopsis.

AVRPPHB SUSCEPTIBLE 3 (PBS3) belongs to the GH3 family of acyl acid amido synthetases, which conjugates amino acids to diverse acyl acid substrates. Recent studies demonstrate that PBS3 in Arabidopsis plays a key role in the biosynthesis of plant defense hormone salicylic acid (SA) by catalyzing the conjugation of glutamate to isochorismate to form isochorismate-9-glutamate, which is then used to produce SA through spontaneous decay or ENHANCED PSEUDOMONAS SUSCEPTIBILITY (EPS1) catalysis. Consistent with its function as an essential enzyme for SA biosynthesis, PBS3 is well known to be a positive regulator of plant immunity in Arabidopsis. Additionally, PBS3 is also involved in the trade-off between abiotic and biotic stress responses in Arabidopsis by suppressing the inhibitory effect of abscisic acid on SA-mediated plant immunity. Besides stress responses, PBS3 also plays a role in plant development. Under long-day conditions, PBS3 influences Arabidopsis flowering time by regulating the expression of flowering regulators FLOWERING LOCUS C and FLOWERING LOCUS T. Taken together, PBS3 functions in the signaling network of plant development and responses to biotic and/or abiotic stresses, but the molecular mechanisms underlying its diverse roles remain obscure.

Different pathways for exogenous ABA-mediated down-regulation of cadmium accumulation in plants under different iron supplies

Exogenous abscisic acid (ABA) could inhibit cadmium (Cd) accumulation in plants; however, its performance in an uneven iron (Fe) background remains unknown. Here, we found that the inhibitory effects of ABA on Cd accumulation in plants were optimal under nonlimiting Fe availability (25 and 50 µM), causing a reduction of 25–50 %, whereas only a 0–29 % decrease was observed in a Fe-free or -deficient (5 µM) medium. Although ABA significantly inhibited the expression of IRT1 under different Fe supplies, the inhibitory effects of ABA on Cd accumulation were lower (or absent) in irt1-mutants than in wild-type plants growing under nonlimiting Fe availability, whereas no significant difference was found under Fe deficiency. The mechanisms by which ABA reduces Cd accumulation under different Fe environments may differ. Furthermore, under Fe sufficiency, ABA increased Fe levels of root apoplasts by 91 % without changing the activity level of root ferric reductase (FCR). In contrast, ABA resulted in a 17 % decrease in Fe concentration in apoplasts and a 37 % decrease in FCR activity under Fe-deficient conditions. Thus, under Fe sufficiency, plants may show a reduced accumulation of Cd by accumulating more Fe in the apoplasts, which in turn inhibits the expression of IRT1. However, plants are more prone to redirect apoplastic Fe to prevent Cd accumulation under Fe deficiency. The different mechanisms of inhibition of Cd accumulation by ABA under different Fe supplies revealed in this study may provide guidelines for the precise regulation of Cd accumulation in crops via ABA-based strategies.

Abscisic acid is involved in several processes associated with root system architecture in maize

Studies concerning abscisic acid (ABA) involvement in root system architecture (RSA) and the interaction of ABA with auxin have reported contrasting results. In this study, the effects of exogenous ABA application and withdrawal as well as a combined treatment of ABA with the synthetic auxin 1-naphthaleneacetic acid (NAA) were thoroughly investigated in maize. The results showed that ABA reduced both the primary root (PR) elongation and the lateral root density (LRD), whereas NAA inhibited PR elongation but increased LRD. The combined treatment involving ABA and NAA inhibited PR elongation. Regarding ABA withdrawal, PR elongation was restored when ABA was removed from the growth media, but LRD was not restored after ABA withdrawal. However, the results of the combined treatment showed that auxin can reverse the inhibitory effect of ABA on LRD. A more in-depth analysis revealed that the inhibitory effect of ABA on lateral root (LR) formation depends on the stage of development. Exogenously added ABA blocked the development of lateral root primordia (LRPs) in the early stages, but was unable to inhibit the elongation of developed LRPs. These results suggest that ABA arrests the formation of LRPs rather than the growth and emergence of LRPs and their subsequent elongation.

Quick screening of true tyrosinase inhibitors from natural products using tyrosinase‐immobilized magnetic nanoparticles and a magnetic microplate: Part II melanogenesis bioactivity

AbstractWe previously reported a rapid method for screening true tyrosinase inhibitors. Here, the identified active component from litchi flower extract was further confirmed using a tyrosinase inhibitory assay and inhibitory effects on melanogenesis of murine melanoma B16‐F1 cell to prove its bioactivity. This is the first report regarding the inhibitory effect of abscisic acid on melanogenesis. The crude extraction of litchi flower showed some degree of cell toxicity. This report would also like to remind people who want to use crude extraction of herbs to carefully check their toxicological test reports.

Nuclear-Ecoded Plastid RNA Polymerases Are Components of Anterograde Control in Hormonal Regulation of Chloroplast Gene Expression

Phytohormones are known to play an important role in the regulation of chloroplast biogenesis. However, information on the pathways of hormonal signal perception by chloroplasts is limited. In order to analyze the possible mechanisms of hormone action, the response to abscisic acid (ABA) and cytokinin (CKs) treatment of Arabidopsis thaliana (L.) Heynh. scabra3-2 (sca3-2) and rpotmp mutants with defective genes for nuclear-encoded plastid RNA polymerases was studied. The reaction of the rpotmp mutant usually did not differ from the response of wild-type plants: trans-zeatin activated the accumulation of transcripts of genes encoding the transcription apparatus and genes encoded by the plastid genome, while ABA weakly suppressed their expression or had no significant effect. However, the steady-state levels of gene templates of the plastid-encoded RNA polymerase (PEP-A) enzyme complex in the rpotmp mutant were higher than in wild-type plants, probably due to a compensatory mechanism that allows the mutant to enhance PEP-dependent transcription in the absence of the RPOTmp enzyme. Dysfunction of the plastid RNA polymerase RPOTp enzyme in the sca3-2 mutant induced an altered hormonal response of the genes for plastid transcriptional complex and plastid-encoded genes in comparison with the response of wild-type plants. The absence of a significant up-regulating effect of CKs on the expression of genes encoding the transcription apparatus correlated with the retention of the steady-state levels of transcripts for chloroplast genes and plastid proteins. In addition, the sca3-2 mutant had an increased steady-state expression of genes for CK metabolism: IPT3, IPT5, and CKX5. However, the response to the inhibitory effect of ABA in this mutant, on the contrary, was more pronounced, which is possibly caused by the changes in the steady-state levels of the transcripts of a number of genes encoding of ABA signaling and metabolism proteins. Thus, changes in the expression of NEP (nuclear-encoded RNA polymerases) genes under the action of hormones can be transformed into activation or inhibition in the expression of chloroplast genes of nuclear or plastome coding, providing the regulation of chloroplast functions through the mechanisms of anterograde signaling.

OsMADS18, a membrane-bound MADS-box transcription factor, modulates plant architecture and the abscisic acid response in rice.

The APETALA1 (AP1)/FRUITFULL (FUL)-like transcription factor OsMADS18 plays diverse functions in rice development, but the underlying molecular mechanisms are far from fully understood. Here, we report that down-regulation of OsMADS18 expression in RNAi lines caused a delay in seed germination and young seedling growth, whereas the overexpression of OsMADS18 produced plants with fewer tillers. In targeted OsMADS18 genome-edited mutants (osmads18-cas9), an increased number of tillers, altered panicle size, and reduced seed setting were observed. The EYFP-OsMADS18 (full-length) protein was localized to the nucleus and plasma membrane but the EYFP-OsMADS18-N (N-terminus) protein mainly localized to the nucleus. The expression of OsMADS18 could be stimulated by abscisic acid (ABA), and ABA stimulation triggered the cleavage of HA-OsMADS18 and the translocation of OsMADS18 from the plasma membrane to the nucleus. The inhibitory effect of ABA on seedling growth was less effective in the OsMADS18-overexpressing plants. The expression of a set of ABA-responsive genes was significantly reduced in the overexpressing plants. The phenotypes of transgenic plants expressing EYFP-OsMADS18-N resembled those observed in the osmads18-cas9 mutants. Analysis of the interaction of OsMADS18 with OsMADS14, OsMADS15, and OsMADS57 strongly suggests an essential role for OsMADS18 in rice development.

Rational Discovery of (+) (S) Abscisic Acid as a Potential Antifungal Agent: a Repurposing Approach

Fungal infections are spreading widely worldwide, and the types of treatment are limited due to the lack of diverse therapeutic agents and their associated side effects and toxicity. The discovery of new antifungal classes is vital and critical. We discovered the antifungal activity of abscisic acid through a rational drug design methodology that included the building of homology models for fungal chorismate mutases and a pharmacophore model derived from a transition state inhibitor. Ligand-based virtual screening resulted in some hits that were filtered using molecular docking and molecular dynamic simulations studies. Both in silico methods and in vitro antifungal assays were used as tools to select and validate the abscisic acid repurposing. Abscisic acid inhibition assays confirmed the inhibitory effect of abscisic acid on chorismate mutase through the inhibition of phenylpyruvate production. The repositioning of abscisic acid, the well-known and naturally occurring plant growth regulator, as a potential antifungal agent because of its suggested action as an inhibitor to several fungal chorismate mutases was the main result of this work.

Effects of methyl jasmonate and abscisic acid on anthocyanin biosynthesis in callus cultures of red-fleshed apple (Malus sieversii f. niedzwetzkyana)

We researched the effects of methyl jasmonate, abscisic acid, and a combination of the two on anthocyanin metabolism in callus material induced from the leaves of a red-fleshed apple individual, which was a hybrid offspring of Malus sieversii f. niedzwetzkyana and Malus domestica cv. ‘Fuji’. The results showed that methyl jasmonate promoted anthocyanin accumulation with increasing methyl jasmonate concentration in the range of 10−6 to 10−3 mol/L. Abscisic acid significantly inhibited anthocyanin biosynthesis, and its inhibitory effect was highest at 6 μmol/L among all treatments. In addition, methyl jasmonate alleviated the inhibitory effect of abscisic acid on anthocyanin biosynthesis. Gene expression analysis indicated that methyl jasmonate induced the expression of MdMYB9 and MdMYB10, which are positive regulator of anthocyanin biosynthesis, and inhibited MdMYB11 expression. It also upregulated the expression of the anthocyanin structural genes MdCHS, MdF3H and MdUFGT. Abscisic acid inhibited the expression of MdMYB3 and MdMYB10, as well as the expression of the structural genes MdF3H, MdDFR, MdLDOX and MdUFGT. Methyl jasmonate relieved the inhibition by abscisic acid by decreasing the expression of MdMYB11 and MdMYB16, and increasing the expression of the anthocyanin regulatory genes MdMYB3, MdMYB9 and MdMYB10, and the structural genes MdCHS, MdDFR, MdF3H and MdUFGT. Thus, we elucidated the roles of methyl jasmonate and abscisic acid in the regulation of anthocyanin production at the gene transcription level.

An ancient and conserved function for Armadillo‐related proteins in the control of spore and seed germination by abscisic acid

Summary Armadillo‐related proteins regulate development throughout eukaryotic kingdoms. In the flowering plant Arabidopsis thaliana, Armadillo‐related ARABIDILLO proteins promote multicellular root branching. ARABIDILLO homologues exist throughout land plants, including early‐diverging species lacking true roots, suggesting that early‐evolving ARABIDILLOs had additional biological roles.Here we investigated, using molecular genetics, the conservation and diversification of ARABIDILLO protein function in plants separated by c. 450 million years of evolution.We demonstrate that ARABIDILLO homologues in the moss Physcomitrella patens regulate a previously undiscovered inhibitory effect of abscisic acid (ABA) on spore germination. Furthermore, we show that A. thaliana ARABIDILLOs function similarly during seed germination. Early‐diverging ARABIDILLO homologues from both P. patens and the lycophyte Selaginella moellendorffii can substitute for ARABIDILLO function during A. thaliana root development and seed germination.We conclude that (1) ABA was co‐opted early in plant evolution to regulate functionally analogous processes in spore‐ and seed‐producing plants and (2) plant ARABIDILLO germination functions were co‐opted early into both gametophyte and sporophyte, with a specific rooting function evolving later in the land plant lineage.

Genetic Variation for Thermotolerance in Lettuce Seed Germination Is Associated with Temperature-Sensitive Regulation of ETHYLENE RESPONSE FACTOR1 (ERF1).

Seeds of most lettuce (Lactuca sativa) cultivars are susceptible to thermoinhibition, or failure to germinate at temperatures above approximately 28°C, creating problems for crop establishment in the field. Identifying genes controlling thermoinhibition would enable the development of cultivars lacking this trait and, therefore, being less sensitive to high temperatures during planting. Seeds of a primitive accession (PI251246) of lettuce exhibited high-temperature germination capacity up to 33°C. Screening a recombinant inbred line population developed from PI215246 and cv Salinas identified a major quantitative trait locus (Htg9.1) from PI251246 associated with the high-temperature germination phenotype. Further genetic analyses discovered a tight linkage of the Htg9.1 phenotype with a specific DNA marker (NM4182) located on a single genomic sequence scaffold. Expression analyses of the 44 genes encoded in this genomic region revealed that only a homolog of Arabidopsis (Arabidopsis thaliana) ETHYLENE RESPONSE FACTOR1 (termed LsERF1) was differentially expressed between PI251246 and cv Salinas seeds imbibed at high temperature (30°C). LsERF1 belongs to a large family of transcription factors associated with the ethylene-signaling pathway. Physiological assays of ethylene synthesis, response, and action in parental and near-isogenic Htg9.1 genotypes strongly implicate LsERF1 as the gene responsible for the Htg9.1 phenotype, consistent with the established role for ethylene in germination thermotolerance of Compositae seeds. Expression analyses of genes associated with the abscisic acid and gibberellin biosynthetic pathways and results of biosynthetic inhibitor and hormone response experiments also support the hypothesis that differential regulation of LsERF1 expression in PI251246 seeds elevates their upper temperature limit for germination through interactions among pathways regulated by these hormones. Our results support a model in which LsERF1 acts through the promotion of gibberellin biosynthesis to counter the inhibitory effects of abscisic acid and, therefore, promote germination at high temperatures.

Nitric oxide suppresses the inhibitory effect of abscisic acid on seed germination by S-nitrosylation of SnRK2 proteins

Nitric oxide (NO) plays important roles in plant development, and biotic and abiotic stress responses. In a recent study, we showed that endogenous NO negatively regulates abscisic acid (ABA) signaling in guard cells by inhibiting sucrose nonfermenting 1 (SNF1)-related protein kinase 2.6 (SnRK2.6)/open stomata 1(OST1) through S-nitrosylation. Application of NO breaks seed dormancy and alleviates the inhibitory effect of ABA on seed germination and early seedling growth, but it is unclear how NO functions at the stages of seed germination and early seedling development. Here, we show that like SnRK2.6, SnRK2.2 can be inactivated by S-nitrosoglutathione (GSNO) treatment through S-nitrosylation. SnRK2.2 and the closely related SnRK2.3 are known to play redundant roles in ABA inhibition of seed germination in Arabidopsis. We found that treatment with the NO donor SNP phenocopies the snrk2.2snrk2.3 double mutant in conferring ABA insensitivity at the stages of seed germination and early seedling growth. Our results suggest that NO negatively regulates ABA signaling in germination and early seedling growth through S-nitrosylation of SnRK2.2 and SnRK2.3.

SIS8, a putative mitogen‐activated protein kinase kinase kinase, regulates sugar‐resistant seedling development in Arabidopsis

Sugar signaling pathways have been evolutionarily conserved among eukaryotes and are postulated to help regulate plant growth, development and responses to environmental cues. Forward genetic screens have identified sugar signaling or response mutants. Here we report the identification and characterization of Arabidopsis thaliana sugar insensitive8 (sis8) mutants, which display a sugar-resistant seedling development phenotype. Unlike many other sugar insensitive mutants, sis8 mutants exhibit wild-type responses to the inhibitory effects of abscisic acid and paclobutrazol (an inhibitor of gibberellin biosynthesis) on seed germination. Positional cloning of the SIS8 gene revealed that it encodes a putative mitogen-activated protein kinase kinase kinase (MAPKKK; At1g73660). SIS8mRNA is expressed ubiquitously among Arabidopsis organs. A UDP-glucosyltransferase, UGT72E1 (At3g50740), was identified as an interacting partner of SIS8 based on a yeast two-hybrid screen and in planta bimolecular fluorescence complementation. Both SIS8-yellow fluorescent protein (YFP) and UGT72E1-YFP fusion proteins localize to the nucleus when transiently expressed in tobacco leaf cells. T-DNA insertions in At3g50740 cause a sugar-insensitive phenotype. These results indicate that SIS8, a putative MAPKKK, is a regulator of sugar response in Arabidopsis and interacts with a UDP-glucosyltransferase in the nucleus.

Effects of exogenous hormones on leaf photosynthesis of Panax ginseng

Ginseng (Panax ginseng) is a typical perennial shade plant. Aim of this study was to investigate the effects of exogenous hormones on photosynthesis of P. ginseng. At different growth stages, the aerial parts of P. ginseng plants were cut at the stem base and they were inserted into the nutrient solutions containing different exogenous hormones. Then the leaf photosynthesis and water absorbing capacity (absorbing water mass) of the excised plants were measured. The results showed that exogenous abscisic acid (ABA) decreased significantly net photosynthetic rate (P N), stomatal conductance, transpiration rate, and absorbed water mass of excised P. ginseng at all growth stages, while both cytokinin (CTK) and indole-3-acetic acid (IAA) enhanced those parameters. Comparing different growth stages, ABA caused more severe inhibition of leaf photosynthesis at the early growth stage, while CTK and IAA showed significant enhancement of leaf photosynthesis at later growth stage. ABA reduced highly intercellular CO2 concentration of P. ginseng at the flowering and green fruit stages, but it had only a small effect at red fruit early and red fruit stages. During the early growth stage, the inhibitory effect of ABA on leaf P N might be caused mainly due to the stomatal limitation. However, the reason for this reduction was complex at the later growth stage and it included stomatal and other factors.

SUGAR-INSENSITIVE3, a RING E3 Ligase, Is a New Player in Plant Sugar Response

Sugars, such as sucrose and glucose, have been implicated in the regulation of diverse developmental events in plants and other organisms. We isolated an Arabidopsis (Arabidopsis thaliana) mutant, sugar-insensitive3 (sis3), that is resistant to the inhibitory effects of high concentrations of exogenous glucose and sucrose on early seedling development. In contrast to wild-type plants, sis3 mutants develop green, expanded cotyledons and true leaves when sown on medium containing high concentrations (e.g. 270 mm) of sucrose. Unlike some other sugar response mutants, sis3 exhibits wild-type responses to the inhibitory effects of abscisic acid and paclobutrazol, a gibberellic acid biosynthesis inhibitor, on seed germination. Map-based cloning revealed that SIS3 encodes a RING finger protein. Complementation of the sis3-2 mutant with a genomic SIS3 clone restored sugar sensitivity of sis3-2, confirming the identity of the SIS3 gene. Biochemical analyses demonstrated that SIS3 is functional in an in vitro ubiquitination assay and that the RING motif is sufficient for its activity. Our results indicate that SIS3 encodes a ubiquitin E3 ligase that is a positive regulator of sugar signaling during early seedling development.

Involvement of Arabidopsis thaliana phospholipase Dζ2 in root hydrotropism through the suppression of root gravitropism

Root hydrotropism is the phenomenon of directional root growth toward moisture under water-deficient conditions. Although physiological and genetic studies have revealed the involvement of the root cap in the sensing of moisture gradients, and those of auxin and abscisic acid (ABA) in the signal transduction for asymmetric root elongation, the overall mechanism of root hydrotropism is still unclear. We found that the promoter activity of the Arabidopsis phospholipase Dzeta2 gene (PLDzeta2) was localized to epidermal cells in the distal root elongation zone and lateral root cap cells adjacent to them, and that exogenous ABA enhanced the activity and extended its area to the entire root cap. Although pldzeta2 mutant root caps did not exhibit a morphological phenotype in either the absence or presence of exogenous ABA, the inhibitory effect of ABA on gravitropism, which was significant in wild-type roots, was not observed in pldzeta2 mutant roots. In root hydrotropism experiments, pldzeta2 mutations significantly retarded or disturbed root hydrotropic responses. A drought condition similar to that used in a hydrotropism experiment enhanced the PLDzeta2 promoter activity in the root cap, as did exogenous ABA. These results suggest that PLDzeta2 responds to drought through ABA signaling in the root cap and accelerates root hydrotropism through the suppression of root gravitropism.

Endo-β-mannanase and β-mannosidase activities in rice grains during and following germination, and the influence of gibberellin and abscisic acid

Grains of indica rice (Oryza sativa cv. Peiza 67) exhibit an increase in endo-β-mannanase activity, mostly after the completion of germination. According to tissue blots, the initial increase occurs in association with the embryo, and possibly the scutellum, although the largest sustained increase in activity is in the peripheral regions of the endosperm. The aleurone layer, being the only living region of the endosperm, is presumably the site of synthesis and secretion of the enzyme into the non-living, starch-laden region. β-Mannosidase activity is low throughout germination and subsequent seedling growth, particularly in the endosperm regions. Its activity profile does not mimic that of endo-β-mannanase. In the intact grain, gibberellin (GA) causes a relatively small increase in endo-β-mannanase activity, while abscisic acid (ABA) causes a large decrease; this inhibition is overcome to a considerable extent when GA is supplied along with ABA. β-Mannosidase activity is little affected by either GA or ABA. Embryoless half-grains imbibed in water exhibit only a small increase in endo-β-mannanase activity with time of imbibition, showing the necessity for a stimulus from the embryo for this to occur. Incubating half-grains in the presence of GA results in a large increase in enzyme activity; ABA reduces the amount of activity compared to the water controls. GA is capable of reversing the inhibitory effect of ABA with respect to endo-β-mannanase activity. As in the intact grains, β-mannosidase activity in the half-grains is unaffected by either GA or ABA. It is concluded that the major site for the production of endo-β-mannanase activity is the aleurone layer, and this event is influenced by the presence of the embryo; in the absence of the latter, the increase in enzyme activity is stimulated by GA. β-Mannosidase activity is low throughout germination and post-germination, it is not influenced by GA and ABA, and thus its activity is not regulated in a coordinated manner with that of endo-β-mannanase.

The Arabidopsis thaliana homeobox gene ATHB5 is a potential regulator of abscisic acid responsiveness in developing seedlings.

ATHB5 is a member of the homeodomain-leucine zipper (HDZip) transcription factor gene family of Arabidopsis thaliana. In this report we show that increased expression levels of ATHB5 in transgenic Arabidopsis plants cause an enhanced sensitivity to the inhibitory effect of abscisic acid (ABA) on seed germination and seedling growth. Consistent with this finding we demonstrate in northern blot experiments that the ABA-responsive gene RAB18 is hyperinduced by ABA in transgenic overexpressor lines as compared to the wild type. Northern blot and promoter-GUS fusion analyses show that ATHB5 gene transcription is initiated rapidly after the onset of germination and localized primarily to the hypocotyl of germinating seedlings. Moreover, analysis of ATHB5 gene expression during post-germinative growth in different ABA response mutants shows that ATHB5 gene activity is down-regulated in the abil-1, abi3-1 and abi5-1 mutant lines, but not in abi2-1 or abi4-1. The identification of a T-DNA insertion mutant line of ATHB5 is described and no phenotypic alterations could be discerned, suggesting that ATHB5 may act redundantly with other HDZip genes. Taken together, these data suggest that ATHB5 is a positive regulator of ABA-responsiveness, mediating the inhibitory effect of ABA on growth during seedling establishment.

Inhibitory Effect of Some Plant Growth Regulators and Chlorsulfuron on Growth, Protein Composition and Proteolytic Activity of Maize Seedlings

ABSTRACTThe well known inhibitory effect of abscisic acid (ABA) on growth and protein mobilization was confirmed. In general benzylaminopurine (BAP) is known as a stimulator of plant growth. However in very high concentration BAP provoked a little suppression of plant growth and of activity of some proteolytic enzymes. Irrespective of this a faster hydrolysis of storage proteins in BAP treated seeds was detected. Similar results to BAP action after N1-(2-chloro-4-pyridyl)-N2 phenylurea (4PU-30) treatment were obtained.Data for growth, proteolytic activity and protein content of maize seedlings treated with 4PU-30 and chlorsulfuron (CS) in an identical high concentration enlarged our knowledge for their relationship and supported the assumption for antagonistic action of these agents with very strong structural similarity. As CS is newly developed and widely used herbicide these results have not only theoretical but also practical significance.

Xylem abscisic acid accelerates leaf abscission by modulating polyamine and ethylene synthesis in water-stressed intact poplar

We investigated the effects of endogenous and exogenous abscisic acid (ABA) on polyamines (PAs) and ethylene synthesis and their relevance to leaf senescence in 1-year-old intact cuttings of a drought-sensitive poplar genotype Populus × euramericana cv. I-214 (cv. Italica) and a drought-tolerant genotype P. popularis 35–44 (P. popularis). P. popularis exhibited a transitory and moderate increase in xylem ABA concentrations in response to water stress and no leaf abscission during the period of drought (30% of field capacity; the soil water potential was –2.108 MPa). In contrast, leaf shedding occurred in stressed cv. Italica following a sustained increase in xylem ABA concentrations (up to 1.73 µM). Application of ABA to the transpiration stream accelerated leaf abscission in both genotypes with a threshold of approximately 1.7–1.8 µM. Initial elevation of xylem ABA concentrations reduced PA levels but enhanced ethylene synthesis simultaneously at the onset of water stress or the ABA treatment. Moreover, sharp increases in ABA concentrations, i.e. over 1.7 µM, severely restricted PA synthesis in aged leaves in the longer term (within 3 days). Leaves abscised following a progressive decline of PA levels, when putrescine decreased below approximately 0.5 µmol g–1 fresh weight, and spermidine and spermine became almost undetectable by HPLC. Therefore, we concluded that a drastic reduction in the level of PAs might increase the sensitivity of the leaf to ethylene, thus accelerating defoliation, even though the ethylene emission of stressed plants returned to pre-stress values by day 3. Young leaves were not shed during the period corresponding to the increase in xylem ABA, which appeared to result from the lesser reductions of PA. Compared with P. popularis, the inhibitory effect of ABA on PA synthesis was more pronounced in the drought-sensitive genotype cv. Italica. On the other hand, cv. Italica plants typically produced more ethylene than P. popularis. Taken together, these observations might explain our previous finding that ABA-treated plants of cv. Italica experienced more leaf loss than P. popularis (Chen et al. 1997).

Effects of Growth Regulators on H+Translocation across the Membranes of Plasma Membrane Vesicles from Potato Tuber Cells

Effects of the growth regulators epibrassinolide-694 (EB), gibberellic acid (GA), and abscisic acid (ABA) on the ATP-dependent translocation of H+through the membranes of plasma membrane vesicles of potato (Solanum tuberosumL.) tuber cells were studied. The ATP-dependent accumulation of H+in the plasma membrane vesicles from dormant tubers was inhibited by EB and ABA and stimulated by GA. After the break of dormancy, the stimulatory effect of GA increased, the inhibitory effect of ABA decreased, and EB stimulated the accumulation of H+in the vesicles. The data suggest that the plasma membrane H+ATPase is a target of phytohormones that regulate the dormancy of potato tubers.