Contrasting interactions between ethylene and abscisic acid in Rumex species differing in submergence tolerance

Complete submergence of flooding-tolerant Rumex palustris plants strongly stimulates petiole elongation. This escape response is initiated by the accumulation of ethylene inside the submerged tissue. In contrast, petioles of flooding-intolerant Rumex acetosa do not increase their elongation rate under water even though ethylene also accumulates when they are submerged. Abscisic acid (ABA) was found to be a negative regulator of enhanced petiole growth in both species. In R. palustris, accumulated ethylene stimulated elongation by inhibiting biosynthesis of ABA via a reduction of RpNCED expression and enhancing degradation of ABA to phaseic acid. Externally applied ABA inhibited petiole elongation and prevented the upregulation of gibberellin A(1) normally found in submerged R. palustris. In R. acetosa submergence did not stimulate petiole elongation nor did it depress levels of ABA. However, if ABA concentrations in R. acetosa were first artificially reduced, submergence (but not ethylene) was then able to enhance petiole elongation strongly. This result suggests that in Rumex a decrease in ABA is a prerequisite for ethylene and other stimuli to promote elongation.     

A comparative study of the effects of abscisic acid and methyl jasmonate on seedling growth of rice

The effects of abscisic acid (ABA) and methyl jasmonate (MJ) on growth of rice seedlings were compared. The lowest tested concentration of ABA and MJ that inhibited seedling growth was found to be 4.5 and 0.9 µM, respectively. Growth inhibition by ABA is reversible, whereas that by MJ is irreversible. GA₃ was found to be more effective in reversing inhibition of shoot growth by ABA than by MJ. KCl partially relieved MJ-inhibited, but not ABA-inhibited, growth of rice seedlings. The beneficial effect of K⁺ on growth of rice seedlings in MJ medium could not be replaced by Li⁺, Na⁺ or Cs⁺. MJ treatment caused a marked release of K⁺ into the medium. In order to understand whether cell wall-bound peroxidase activity was inversely related to rice seedling growth, effects of ABA and MJ on cell wall-bound peroxidase activity were also examined. Results indicated that both ABA and MJ increased cell wall-bound peroxidase activity in roots and shoots of rice seedlings. Although MJ (4.5 µM) was less effective in inhibiting root growth than ABA (9 µM), MJ was found to increase more cell wall-bound peroxidase activity in roots than ABA.     

Current advances in abscisic acid action and signalling

Abscisic acid (ABA) participates in the control of diverse physiological processes. The characterization of deficient mutants has clarified the ABA biosynthetic pathway in higher plants. Deficient mutants also lead to a revaluation of the extent of ABA action during seed development and in the response of vegetative tissues to environmental stress. Although ABA receptor(s) have not yet been identified, considerable progress has been recently made in the characterization of more downstream elements of the ABA regulatory network. ABA controls stomatal aperture by rapidly regulating identified ion transporters in guard cells, and the details of the underlying signalling pathways start to emerge. ABA actions in other cell types involve modifications of gene expression. The promoter analysis of ABA-responsive genes has revealed a diversity of cis-acting elements and a few associated trans-acting factors have been isolated. Finally, characterization of mutants defective in ABA responsiveness, and molecular cloning of the corresponding loci, has proven to be a powerful approach to dissect the molecular nature of ABA signalling cascades.     

Modulation of carrier-mediated uptake of abscisic acid by methyl jasmonate in Phaseolus coccineus L

The effects of methyl jasmonate and jasmonic acid on uptake of abscisic acid (ABA) by suspension-cultured runner-bean cells and subapical runner-bean root segments have been investigated. Increasing concentrations of methyl jasmonate inhibit ABA uptake by the cultured cells with a K _i of 22±3 M. This is not due to cytoplasmic acidification or to effects on metabolism of ABA, and is not additive with inhibition of radioactive ABA uptake by nonradioactive ABA. Uptake of indol-3-yl acetic acid (IAA) is unaffected by methyl jasmonate. The maximum effect of nonradioactive ABA in inhibiting uptake of radioactive ABA, previously shown to reflect saturation of an ABA carrier, is generally greater than the effect of maximally inhibitory concentrations of methyl jasmonate. Similar results were obtained with root segments, but longer incubation times were necessary to observe inhibitory effects of methyl jasmonate. Demethylation of methyl jasmonate to jasmonic acid does not appear to be required since similar concentrations of jasmonic acid had no observable direct effect on ABA uptake other than that attributable to cytoplasmic acidification. Histidine reagents, a proton ionophore and acidic external pH all affect in parallel the inhibition by methyl jasmonate and nonradioactive ABA of uptake of radioactive ABA by the cultured cells. There is no effect of ABA or nonradioactive methyl jasmonate on uptake of radioactive methyl jasmonate by the cultured cells. It is proposed that methyl jasmonate interacts with the ABA carrier. Various models for this interaction are discussed.Abbreviations ABA abscisic acid - DMO 5,5-dimethyloxazolidine-2,4-dione - IAA indol-3-yl acetic acid     

Differential effects of abscisic acid and methyl jasmonate on endoproteinases in senescing barley leaves

Endoproteinase activity was analyzed in chloroplasts isolated from barley leaf segments incubated in the dark with various hormonal senescence effectors. As a control, the endoproteinase activity of the supernatant fraction obtained during chloroplast preparation was also analyzed. Measured against azocaseine as substrate, the endoproteinase activity in chloroplasts increased 18 fold during the induction of senescence. This rise in activity was inhibited by kinetin (the activity increased only 10 fold) and very strongly stimulated by abscisic acid (ABA) (117 fold) and methyl jasmonate (Me-JA) (57 fold). Although less so, the endoproteinase activity of the supernatant fraction, mainly vacuolar and with acid pH optimum, was affected in the same way by all three effectors. Among the five endoproteinases (EC) found in chloroplasts, EC2 and EC4 were induced after incubation in water. ABA increased the levels of EC2 and EC4 (5 fold), and induced the development of EC3 and EC5, while Me-JA totally inhibited EC2 and EC4, and induced the development of EC1. At least one of the endoproteinases, EC2, is synthesized in chloroplasts. Among the six endoproteinases found in the supernatant fraction (E), E1, E2, E3 and E5, which are very probably extrachloroplastic endoproteinases, are stimulated by ABA to varying degrees. However, Me-JA stimulates E1 to a greater extent and totally inhibits E3. The differential effects of ABA and Me-JA on chloroplast and supernatant fraction endoproteinases suggest different action mechanisms for both senescence promotors.Abbreviations ABA abscisic acid - DTT dithiothreitol - E supernatant fraction endoproteinase - EC chloroplast endoproteinase - Me-JA methyl jasmonate - PNP p-nitrophenol - SDS-PAGE polyacrylamide gel electrophoresis containing sodium dodecyl sulphate - TCA trichloroacetic acid     

Zooming into sub-organellar localization of reactive oxygen species in guard cell chloroplasts during abscisic acid and methyl jasmonate treatments

Regulation of stomata movements is crucial for plants ability to cope with their changing environment. Guard cell’s (GC) water potential directs water flux inside/outside this cell, which eventually is causing the stoma to open or close, respectively. Some of the osmolytes which accumulates in the GC cytoplasm and are known to play a role in stomata opening are sugars, arising from chloroplast starch degradation. During stomata closure, the accumulated osmolytes are removed from the GC cytoplasm. Surprisingly little is known about prevention of starch degradation and forming additional sugars which may interfere with osmotic changes that are necessary for correct closure of stomata.   One of the early events leading to stomata closure is production of reactive oxygen species (ROS) in various sub-cellular sites and organelles of the stoma. Here we report that ROS production during abscisic acid (ABA) and methyl jasmonate (MJ) stimuli in Arabidopsis GC chloroplasts were more than tripled. Moreover, ROS were detected on the sub-organelle level in compartments that are typically occupied by starch grains. This observation leads us to suspect that ROS function in that particular location is necessary for stomata closure. We therefore hypothesize that these ROS are involved in redox control that lead to the inactivation of starch degradation that takes place in these compartments, thus contributing to the stoma closure in an additional way.     

A tomato cDNA inducible by salt stress and abscisic acid: nucleotide sequence and expression pattern

We have characterized a new tomato cDNA, TAS14, inducible by salt stress and abscisic acid (ABA). Its nucleotide sequence predicts an open reading frame coding for a highly hydrophilic and glycine-rich (23.8%) protein of 130 amino acids. Southern blot analysis of tomato DNA suggests that there is one TAS14 structural gene per haploid genome. TAS14 mRNA accumulates in tomato seedlings upon treatment with NaCl, ABA or mannitol. It is also induced in roots, stems and leaves of hydroponically grown tomato plants treated with NaCl or ABA. TAS14 mRNA is not induced by other stress conditions such as cold and wounding. The sequence of the predicted TAS14 protein shows four structural domains similar to the rice RAB21, cotton LEA D11 and barley and maize dehydrin genes.     

The effects of abscisic acid and methyl jasmonate on 1-aminocyclopropane 1-carboxylic acid conversion to ethylene in hypocotyl segments of sunflower seedlings,and their control by calcium and calmodulin

The conversion of 1-aminocyclopropane 1-carboxylic acid (ACC) to ethylene by hypocotyl segments of sunflower (Helianthus annuus L.) seedlings was inhibited by abscisic acid (ABA) and methyl jasmonate (Me-Ja), and this inhibitory effect increased with increasing concentration of both growth regulators. On the contrary, CaCl, enhanced ACC conversion to ethylene at the concentrations of 10^-4 M and 5 x 10^-4 M, however lower and higher concentrations had no significant action. CaCl, (5 x 10^-4M) seemed to magnify the inhibition of the reaction induced by ABA, whereas it reduced (5 x 10^-4M) and even abolished (10^-3M) the inhibitory action of Me-Ja. The results obtained with a Ca²⁺ chelator (EGTA), a Ca²⁺ channel blocker (nifedipine) and calmodulin antagonists (W7 and TFP), given in association with ABA or Me-Ja, suggested that calcium was involved in the inhibition of ACC conversion to ethylene by ABA and Me-Ja through an interaction with calmodulin. However, the mechanism of action of the two growth regulators seemed to be different, since all treatments which resulted in a decrease in cytosolic Ca²⁺ concentration or in calmodulin action induced a decrease in the effect of ABA and an increase in the effect of Me-Ja.Abbreviations ABA abscisic acid - ACC 1-aminocyclopropane 1-carboxylic acid - EFE ethylene for enzyme - EGTA ethylene glycol-bis-2-aminoethyl tetraacetic acid - Me-Ja methyl jasmonate - NIF nifedipine - TFP trifluoperazine dihydrochloride - W7 N-(6-aminohexyl)5-chloro-l-naphthalenesulfonamide hydrochloride     

Involvement of abscisic acid and ethylene in the responses of rice grains to water stress during filling

This study was to test the hypothesis that the interaction between abscisic acid (ABA) and ethylene may be involved in mediating the effects of water stress on grain filling. Two high lodging‐resistant rice (Oryza sativa L.) cultivars were pot‐grown. Three treatments, well‐watered, moderate water‐stressed (MD), and severe water‐stressed (SD), were imposed from 9 d post‐anthesis until maturity. Grain filling rate and grain weight were significantly increased under MD but decreased under SD. The two cultivars behaved the same. ABA concentration in the grains was very low during the grain filling stage, reaching a maximum when the grain filling rate was highest. Both the grain filling rate and ABA concentration were substantially enhanced by water stress. In contrast to ABA, concentrations of ethylene and 1‐aminocylopropane ‐1‐carboxylic acid (ACC) in the grains were very high at early grain filling stage and sharply decreased during the linear period of grain growth. MD reduced, whereas SD remarkably increased, their accumulation. The ratio of ABA to ACC was increased in MD grains but decreased in SD grains, indicating that there was a greater enhancement of ABA concentration than ethylene production in the MD treatment only. Application of cobalt ion (inhibitor of ethylene synthesis) or ABA at the early grain filling stage significantly increased grain filling rate. Spraying with ethephon (ethylene‐releasing agent) or fluridone (inhibitor of ABA synthesis) had the opposite effect. The results suggest that antagonistic interactions between ABA and ethylene mediate the grain filling rate, and a high ratio of ABA to ethylene enhances grain filling rate.     

Regulation of genes associated with auxin, ethylene and ABA pathways by 2,4-dichlorophenoxyacetic acid in Arabidopsis

The chemical 2,4-dichlorophenoxyacetic acid (2,4-D) regulates plant growth and development and mimics auxins in exhibiting a biphasic mode of action. Although gene regulation in response to the natural auxin indole acetic acid (IAA) has been examined, the molecular mode of action of 2,4-D is poorly understood. Data from biochemical studies, (Grossmann (2000) Mode of action of auxin herbicides: a new ending to a long, drawn out story. Trends Plant Sci 5:506–508) proposed that at high concentrations, auxins and auxinic herbicides induced the plant hormones ethylene and abscisic acid (ABA), leading to inhibited plant growth and senescence. Further, in a recent gene expression study (Raghavan et al. (2005) Effect of herbicidal application of 2,4-dichlorophenoxyacetic acid in Arabidopsis. Funct Integr Genomics 5:4–17), we have confirmed that at high concentrations, 2,4-D induced the expression of the gene NCED1, which encodes 9-cis-epoxycarotenoid dioxygenase, a key regulatory enzyme of ABA biosynthesis. To understand the concentration-dependent mode of action of 2,4-D, we further examined the regulation of whole genome of Arabidopsis in response to a range of 2,4-D concentrations from 0.001 to 1.0 mM, using the ATH1-121501 Arabidopsis whole genome microarray developed by Affymetrix. Results of this study indicated that 2,4-D induced the expression of auxin-response genes (IAA1, IAA13, IAA19) at both auxinic and herbicidal levels of application, whereas the TIR1 and ASK1 genes, which are associated with ubiquitin-mediated auxin signalling, were down-regulated in response to low concentrations of 2,4-D application. It was also observed that in response to low concentrations of 2,4-D, ethylene biosynthesis was induced, as suggested by the up-regulation of genes encoding 1-aminocyclopropane-1-carboxylic acid (ACC) synthase and ACC oxidase. Although genes involved in ethylene biosynthesis were not regulated in response to 0.1 and 1.0 mM 2,4-D, ethylene signalling was induced as indicated by the down-regulation of CTR1 and ERS, both of which play a key role in the ethylene signalling pathway. In response to 1.0 mM 2,4-D, both ABA biosynthesis and signalling were induced, in contrast to the response to lower concentrations of 2,4-D where ABA biosynthesis was suppressed. We present a comprehensive model indicating a molecular mode of action for 2,4-D in Arabidopsis and the effects of this growth regulator on the auxin, ethylene and abscisic acid pathways. Experiment station: Plant Biotechnology Centre, Primary Industries Research Victoria, Department of Primary Industries, La Trobe University, Bundoora, Victoria 3086, and the Victorian Microarray Technology Consortium (VMTC).     

Thiol‐based redox proteins in abscisic acid and methyl jasmonate signaling in Brassica napus guard cells

Reversibly oxidized cysteine sulfhydryl groups serve as redox sensors or targets of redox sensing that are important in various physiological processes. However, little is known about redox‐sensitive proteins in guard cells and how they function in stomatal signaling. In this study, Brassica napus guard‐cell proteins altered by redox in response to abscisic acid (ABA) or methyl jasmonate (MeJA) were identified by complementary proteomics approaches, saturation differential in‐gel electrophoresis and isotope‐coded affinity tagging. In total, 65 and 118 potential redox‐responsive proteins were identified in ABA‐ and MeJA‐treated guard cells, respectively. All the proteins contain at least one cysteine, and over half of them are predicted to form intra‐molecular disulfide bonds. Most of the proteins fall into the functional groups of ‘energy’, ‘stress and defense’ and ‘metabolism’. Based on the peptide sequences identified by mass spectrometry, 30 proteins were common to ABA‐ and MeJA‐treated samples. A total of 44 cysteines were mapped in the identified proteins, and their levels of redox sensitivity were quantified. Two of the proteins, a sucrose non‐fermenting 1‐related protein kinase and an isopropylmalate dehydrogenase, were confirmed to be redox‐regulated and involved in stomatal movement. This study creates an inventory of potential redox switches, and highlights a protein redox regulatory mechanism in ABA and MeJA signal transduction in guard cells.     

Salt stress in cultured rice cells: effects of proline and abscisic acid

Abstract. The presence of 1 and 10 mol m⁻³ proline in media containing 100 and 200 mol m⁻³ of NaCl, had little effect on the growth of salt-adapted callus of rice. However, in such callus proline accumulation was stimulated by 10 mol m⁻³ proline in the presence of 100 mol m⁻³ NaCl. On the other hand, with 100 mol m⁻³ NaCl, both 1 and 10 mol m⁻³ proline significantly increased both the growth and proline content of salt-unadapted callus. On replacing NaCl with KCl (100 and 200 mol m⁻³), growth of saltadapted as well as unadapted callus was inhibited, but the presence of 10 mol m⁻³ proline had an ameliorating effect. Abscisic acid (ABA) supressed the growth of both salt-adapted and unadapted callus of rice in the absence of salt stress. ABA inhibited the growth of callus adapted to and grown in 100 and 200 mol m⁻³ of NaCl or when it was replaced by equimolar concentrations of KCl. Growth of 100 mol m⁻³ NaCl adapted cells was inhibited when they were transferred to a medium containing 200 mol m⁻³ of NaCl, but in the presence of ABA it was stimulated. ABA increased the growth of unadapted cells when subjected to different salts. Also, ABA accelerated the adaptation of cells exposed to salt but not to water deficits imposed by nonionic solutes.