Abscisic acid antagonizes ethylene-induced hyponastic growth in Arabidopsis

Ethylene induces enhanced differential growth in petioles of Arabidopsis (Arabidopsis thaliana), resulting in an upward movement of the leaf blades (hyponastic growth). The amplitude of this effect differs between accessions, with Columbia-0 (Col-0) showing a large response, while in Landsberg erecta (Ler), hyponastic growth is minimal. Abscisic acid (ABA) was found to act as an inhibitory factor of this response in both accessions, but the relationship between ethylene and ABA differed between the two; the ability of ABA to inhibit ethylene-induced hyponasty was significantly more pronounced in Col-0. Mutations in ABI1 or ABI3 induced a strong ethylene-regulated hyponastic growth in the less responsive accession Ler, while the response was abolished in the ABA-hypersensitive era1 in Col-0. Modifications in ABA levels altered petiole angles in the absence of applied ethylene, indicating that ABA influences petiole angles also independently from ethylene. A model is proposed whereby the negative effect of ABA on hyponastic growth is overcome by ethylene in Col-0 but not in Ler. However, when ABA signaling is artificially released in Ler, this regulatory mechanism is bypassed, resulting in a strong hyponastic response in this accession.     

Abscisic acid coordinates nod factor and cytokinin signaling during the regulation of nodulation in Medicago truncatula

Nodulation is tightly regulated in legumes to ensure appropriate levels of nitrogen fixation without excessive depletion of carbon reserves. This balance is maintained by intimately linking nodulation and its regulation with plant hormones. It has previously been shown that ethylene and jasmonic acid (JA) are able to regulate nodulation and Nod factor signal transduction. Here, we characterize the nature of abscisic acid (ABA) regulation of nodulation. We show that application of ABA inhibits nodulation, bacterial infection, and nodulin gene expression in Medicago truncatula. ABA acts in a similar manner as JA and ethylene, regulating Nod factor signaling and affecting the nature of Nod factor-induced calcium spiking. However, this action is independent of the ethylene signal transduction pathway. We show that genetic inhibition of ABA signaling through the use of a dominant-negative allele of ABSCISIC ACID INSENSITIVE1 leads to a hypernodulation phenotype. In addition, we characterize a novel locus of M. truncatula, SENSITIVITY TO ABA, that dictates the sensitivity of the plant to ABA and, as such, impacts the regulation of nodulation. We show that ABA can suppress Nod factor signal transduction in the epidermis and can regulate cytokinin induction of the nodule primordium in the root cortex. Therefore, ABA is capable of coordinately regulating the diverse developmental pathways associated with nodule formation and can intimately dictate the nature of the plants' response to the symbiotic bacteria.     

Abscisic acid effect on the DNA microgradients of decapped maize roots

Longitudinal and transversal total DNA gradients were analysedin 42-µm thin sections of maize root tips. The optimumDNA concentration was found in the apex (quiescent centre andmeristem). It significantly increased after removal of the rootcap, which can be considered an essential source of abscisicacid (ABA). When the caps were taken off and agar blocks containingABA were immediately placed on the apical cut sections, thetotal DNA level in the root apex decreased greatly. ABA regulationof cellular and nuclear behaviour is discussed. (Received June 3, 1978; )     

Abscisic acid, indole-3-acetic acid and cytokinin changes in buds of Pseudotsuga menziesii during bud quiescence release

Bud quiescence release, considered as the ultimate dormancy breaking phase, was achieved in Pseudotsuga menziesii (Mirb.) Franco by a 9-week cold (5°C) treatment, under short daylength (9 h) followed by a transfer to mild temperature (22°C) under long daylength (16 h). Indole-3-acetic acid (IAA), abscisic acid (ABA), zeatin-type (Z) and isopentenyladenine-type (iPA) cytokinin (CK) levels were measured by means of an ELISA technique performed on HPLC-fractionated extracts of terminal and axillary buds. During the cold period, all hormones except IP-type CK levels decreased, whereas the opposite observation was made after transfer to mild temperature and long daylength, when buds started to grow. Some other immunoreactive compounds were also detected and quantified. The ABA-glucosyl ester (ABA-GE) level pattern was similar to that of ABA, but no accumulation occurred at mild temperatures. A putative IAA conjugate, more polar than IAA, was also detected. Its level increased transiently like IAA in terminal buds and, to a lesser extent, in axillary buds during the 10th week of the experiment. In terminal buds, isopentenyladenosine ([9R]-iP) was released by alkaline hydrolysis of a polar immunoreactive compound detected with anti-[9R]iP antibodies. This compound accumulated during the cold period and quickly dropped at 22°C. Relationships between environmental conditions and endogenous hormones are discussed.     

The effect of aminooxyacetic acid and cytokinin combinations on carnation flower longevity

A simplified method, which utilizes a 50% senescence value (S₅₀) for the measurement of longevity, in cut carnation flowers is used to compare flower longevity. A pulsed treatment using a combination of aminooxyacetic acid (AOA), kinetin and Triton X-100 enhanced flower vase-life relative to a water control. The mixture, however, did not exhibit synergistic effects when S₅₀ values of the individual compounds were compared. A single AOA pulse treatment was as effective as the mixture. These findings held true for three carnation cultivars. With the exception of dihydrozeatin, which greatly enhanced longevity, replacement of kinetin by a range of cytokinins did not produce significantly different results from the AOA/Triton X-100 combination. Holding solution treatments gave similar trends as pulse treatments. S₅₀ values were better units to express longevity than S₁₀₀ values.Abbreviations AOA aminooxyaceticacid - BA benzyladenine - S₅₀ 50% senescence value - STS silverthiosulphate - iP isopentenyladenize - Z zeatin - DHZ dihydrozeatin     

Abscisic acid in the plants-pathogen interaction

Information available concerning the role of ABA in the interaction between plants and pathogenic microorganisms allows a conclusion that this phytohormone is required for plant defense. For the development of plant resistance, short-term increases in the ABA level are of importance during the early stages of plant interaction with pathogens, which trigger anti-stress programs in plants, primarily related to the synthesis of callose. At the same time, high ABA concentrations maintained for a long time reduce efficiency of defense systems controlled by salicylic and jasmonic acids and ethylene. ABA was shown to suppress expression of some genes of defense proteins, including those involved in the synthesis and metabolism of phenolic compounds and lignin. ABA is evidently involved in plant defense mechanisms against pathogens as a regulatory element.     

Abscisic Acid and cytokinin contents of leaves in relation to salinity and relative humidity

The question is raised whether the hormonal modifications in a plant exposed to osmotic root stress result directly from the decrease in water potential of the root environment or from disturbances of the plant's water balance.     

Abscisic acid effects on chromatin thermal denaturation

Extraction in the presence of 10^?6 M abscisic acid (RS—ABA) did not give a detectable alteration in the thermal denaturation profile of radish hypocotyl chromatin. Likewise no significant changes were found when chromatin or chromatin—DNA were thermally denatured in the presence of ABA.     

Abscisic acid in the leaf of Vernonia anthelmintica

Summary A growth inhibitor was isolated in crystalline form from the leaf of Vernonia anthelmintica and was found to be identical with abscisic acid by biological activity and physical measurements namely, melting point, mixed melting point, UV-absorption, mass spectra and paper and thin-layer chromatography.     

Abscisic acid deficiency antagonizes high-temperature inhibition of disease resistance through enhancing nuclear accumulation of resistance proteins SNC1 and RPS4 in Arabidopsis

Plant defense responses to pathogens are influenced by abiotic factors, including temperature. Elevated temperatures often inhibit the activities of disease resistance proteins and the defense responses they mediate. A mutant screen with an Arabidopsis thaliana temperature-sensitive autoimmune mutant bonzai1 revealed that the abscisic acid (ABA)-deficient mutant aba2 enhances resistance mediated by the resistance (R) gene suppressor of npr1-1 constitutive1 (SNC1) at high temperature. ABA deficiency promoted nuclear accumulation of SNC1, which was essential for it to function at low and high temperatures. Furthermore, the effect of ABA deficiency on SNC1 protein accumulation is independent of salicylic acid, whose effects are often antagonized by ABA. ABA deficiency also promotes the activity and nuclear localization of R protein resistance to Pseudomonas syringae4 at higher temperature, suggesting that the effect of ABA on R protein localization and nuclear activity is rather broad. By contrast, mutations that confer ABA insensitivity did not promote defense responses at high temperature, suggesting either tissue specificity of ABA signaling or a role of ABA in defense regulation independent of the core ABA signaling machinery. Taken together, this study reveals a new intersection between ABA and disease resistance through R protein localization and provides further evidence of antagonism between abiotic and biotic responses.     

Abscisic acid and the pre-harvest sprouting in cereals

Pre-harvest sprouting (PHS) leads to loss of grain weight and a reduction in the end use quality of kernels in cereals, especially in wheat, and PHS in rice also becomes a more and more serious problem recent years. Many factors are involved in the controlling this complex trait. Only recently, we have reported the large scale screening and charactersation of the rice phs mutants, providing insight into the molecular mechanism of pre-harvest sprouting in rice. It has been shown that mutations of genes in synthesis of the carotenoid precursors of ABA resulted in the pre-harvest sprouting, which is consequence of ABA deficiency, and photobleaching is likewise due to the absence of photoprotective carotenoids. The further study of all different rice phs mutants will help us to elucidate the complex phenomena and finally capture the target for improving PHS in rice or other cereals.Key words: pre-harvest sprouting, ABA, carotenoids, rice     

Abscisic acid inhibits phloem loading of sucrose

The effect of abscisic acid (ABA) on the uptake of sucrose by discs from castor bean ( Ricinus communis L. cv. Zanzibarensis) cotyledons was investigated. Incubation on ABA solutions for one hour or longer significantly inhibited sucrose uptake. The effect was measurable at ABA concentrations as low as 0.1 μ M . The inhibition was due to a lowering of the apparent V_max of the sucrose-carrier system, the K_m being unaffected. Uptake of sucrose was coupled to proton uptake but ABA had no detectable effect on the latter process. It is suggested that ABA exerts an effect on the phloem loading of sucrose by enhancing the efflux of sucrose. In leaf discs from three other plant species ( Beta vulgaris L., Petunia hybrida L. and Phaseolus vulgaris L.) ABA exerted a similar effect. The results are discussed with respect to source-sink relationships.     

Abscisic acid induced decay of strawberry transpiration

The effect of 0–7.5 × 10⁻³μmol abscisic acid (ABA) on the transpiration of strawberry ( Fragaria grandiflora Duch. ev. Red gauntlet), both under illumination and in darkness, was investigated with the use of an infrared gas analyser. Mathematical analysis of the results suggests that the decrease in transpiration after darkening can be described by a series of first-order processes with continuous distribution of rate constants k. Application of ABA makes the rate constant distribution narrower, so that one single rate constant occurs at the highest doses. ABA accelerates the decrease of transpiration more in darkness than under illumination. This suggests that the process of efflux of osmotically active particles from guard cells is influenced more than influx. For ABA doses greater than 7.5 × 10⁴μmol, the transpiration decreases in two stages, of which only the first one seems to be affected by ABA: but above this dose further acceleration of decay of transpiration after darkening is not observed. The observations may indicate that above this critical dose of ABA the rate determining step consists in guard cell wall relaxation rather than in efflux of osmotica.     

Abscisic acid and gibberellic acid as factors in the translocation of auxin

The effects of abscisic acid (ABA) and gibberellic acid (GA)on the translocation of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T)in beans (Phaseolus vulgaris L. cv. Stringless Greenpd) seedlingswere determined. ¹⁴C-labeled 2,4,5-T was injected in the stemtissue at the cotyledonary node in 1-µl amounts alongwith the AB or GA. ABA caused an enhancement of 2,4,5-T translocationto the lower hypocotyl and roots and promoted a decrease inthe accumulation of 2,4,5-T in the young expanding shoots andprimary leaves. GA promoted the accumulation of 2,4,5-T in youngshoots. The enhanced basipetal translocation of 2,4,5-T wasmeasurable after a few hours of treatment and was partiallyeffective even in the presence of the protein synthesis inhibitorcychloheximide. The GA effects on 2,4,5-T translocation werenullified by cycloheximide and were also noted after only afew hours of treatment. ¹Journal Article 2618 of the Agricultural Experiment Station,Oklahoma State University. (Received October 1, 1973; )     

The effect of auxin concentration on cytokinin stability and metabolism

The stability of [³H]zeatin riboside supplied to freshly excised tobacco pith explants was found to be inversely related to -naphthaleneacetic acid concentration in the incubation medium. At higher concentrations of -naphthaleneacetic acid greater breakdown of [³H]zeatin riboside was indicated by higher levels of degradative metabolites (adenine, adenosine and adenosine nucleotides) formed. This auxin effect on cytokinin metabolism appears to be mediated, at least in part, through cytokinin oxidase. The results of in-vitro assays carried out with partially purified enzyme from corn kernels substantiale this conclusion. These findings are discussed in relation to recent observations of auxin and cytokinin levels in crown-gall tumours with altered morphology.Abbreviations FPLC fast protein liquid chromatography - HPLC high-performance liquid chromatography - IP isopentenyladenine - NAA naphthaleneacetic acid - ZR zeatin riboside     

Abscisic acid relations of plants grown on tungsten enriched substrates

The effect of tungsten as a potential inhibitor of ABA biosynthesis in plants was tested both under controlled conditions in the greenhouse or laboratory and in the field at a site naturally enriched with tungsten. Although as predicted in all cases the activity of aldehyde oxidase was strongly inhibited by tungsten, in well watered, greenhouse-grown maize plants increased concentrations of tungsten did not reduce ABA concentration in tissues. Depending on the time of incubation, mostly an accumulation due to an increased net ABA synthesis could be observed. Under drought stress, however, tungsten may have an inhibitory effect on ABA accumulation. Field experiments with drought-stressed clover and dandelion plants from a habitat close to a former tungsten mine confirm those of the greenhouse and laboratory. The ineffectiveness of tungsten in decreasing ABA levels in well-watered plants and its minimal effects in inhibiting ABA accumulation of droughted plants indicate that it is unsuitable as an inhibitor of ABA biosynthesis.     

Abscisic acid and the after-effect of stress in tobacco plants

Summary Tobacco plants (Nicotiana rustica L.) were exposed to a period of stress of either mineral deprivation or salination of the root medium. Thereafter the plants were transferred back to the pre-stress growth medium, for study of the pattern of recovery. Abscisic acid (ABA) content and the extent of stomatal opening in leaves of tobacco plants were found to be inversely related. The results support the possibility that the phenomenon know as after-effect of stress may not be exclusive to recovery from water stress, but may be typical of the pattern of plant recovery from the effects of several growth restricting environments. It is suggested that the after-affect results from the delay in resumption of the pre-stress hormonal balance in the plant, particularly with regard to ABA, after termination of the stress.Abbreviations ABA abscisic acid - WSD water saturation deficit