Inhibition of the Conversion of 1-Aminocyclopropane-1-carboxylic Acid to Ethylene by Structural Analogs, Inhibitors of Electron Transfer, Uncouplers of Oxidative Phosphorylation, and Free Radical Scavengers

Non-specificity of ethylene inhibitors: 'double-edged' tools to find out new targets involved in the root morphogenetic programme

In the last decade, genetic and pharmacological approaches have been used to explore ethylene biosynthesis and perception in order to study the role of ethylene and ethylene/auxin interaction in root architecture development. However, recent findings with pharmacological approaches highlight the non-specificity of commonly used inhibitors. This suggests that caution is required for interpreting these studies and that the use of pharmacological agents is a 'double-edged' tool. On one hand, non-specific effects make interpretation difficult unless other experiments, such as with different mutants or with multiple diversely acting chemicals, are conducted. On the other hand, the non-specificity of inhibitors opens up the possibility of uncovering some ligands or modulators of new receptors such as plant glutamate-like receptors and importance of some metabolic hubs in carbon and nitrogen metabolism such as the pyridoxal phosphate biosynthesis involved in the regulation of the root morphogenetic programme. Identification of such targets is a critical issue to improve the efficiency of absorption of macronutrients in relation to root the morphogenetic programme.

Four alleles of AtCESA3 form an allelic series with respect to root phenotype in Arabidopsis thaliana

Plant cell shape is determined by the orientation of cellulose microfibrils in the primary cell wall. Consequently, mutations that affect genes encoding the enzymes responsible for the synthesis of cellulose, namely, the cellulose synthase catalytic subunits, can alter cell shape substantially, particularly in the roots of affected plants. The multiple response expansion1 (mre1) mutant of Arabidopsis thaliana results from a point mutation in the AtCESA3 gene, which encodes one of the three isoforms of the cellulose synthase catalytic subunit required for synthesis of cellulose in the primary cell wall. Phenotypic comparison of the mre1 mutant with three other alleles (ectopic lignification1-1, ectopic lignification1-2 and constitutive expression of vsp1) showed that these four alleles form an allelic series with respect to their root phenotypes, with mre1 being the weakest allele identified to date. These analyses demonstrated that sucrose affects a significant alteration of cell shape in the roots of these mutants and likely suppresses root cell division in them as well, and that the chemical aminoisobutyric acid can suppress these effects of sucrose. Interestingly, the cell walls in the roots of these four AtCESA3 alleles contain different percentages of cellulose, and these percentages correlate with the lengths of the roots and cortex cells in these roots when grown on media containing high levels of sucrose.

Delayed leaf senescence by exogenous lyso-phosphatidylethanolamine: towards a mechanism of action

Exogenous application of the lysophospholipid, lyso-phosphatidylethanolamine (LPE) is purported to delay leaf senescence in plants. However, lyso-phospholipids are well known to possess detergent-like activity and application of LPE to plant tissues might be expected to rather elicit a wound-like response and enhance senescence progression. Since phosphatidic acid (PA) accumulation and leaf cell death are a consequence of wounding, PA- and hormone-induced senescence was studied in leaf discs from Philodendron cordatum (Vell.) Kunth plants in the presence or absence of egg-derived 18:0-LPE and senescence progression quantified by monitoring both lipid peroxidation (as the change in malondialdehyde concentration), and by measuring retention of total chlorophyll (Chl(a+b)) and carotenoids (C(c+x)). Only abscisic acid (ABA) stimulated lipid peroxidation whereas ABA, 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor to ethylene (ETH), and 16:0-18:2-PA stimulated loss of chloroplast pigments. Results using primary alcohols as attenuators of the endogenous PA signal confirmed a role for PA as an intermediate in both ABA- and ETH-mediated senescence progression. Exogenous 18:0-LPE did not appear to influence senescence progression and was unable to reverse hormone-induced senescence progression. However, when supplied together with 16:0-18:2-PA at 1:1 (mol:mol), activity of phosphatidylglycerol (PG) hydrolase, chlorophyllase (E.C. 3.1.1.14), and progression of leaf senescence were negated. This apparent anti-senescence activity of exogenous 18:0-LPE was associated with induction of the pathogenesis-related protein, extracellular acid invertase (Ac INV, E.C. 3.2.1.26) suggesting that 18:0-LPE like 16:0-18:2-PA functions as an elicitor.

Dose-response relationships for N7-(2-hydroxyethyl)guanine induced by low-dose [14C]ethylene oxide: evidence for a novel mechanism of endogenous adduct formation

Ethylene oxide (EO) is widely used in the chemical industry and is also formed in humans through the metabolic oxidation of ethylene, generated during physiologic processes. EO is classified as a human carcinogen and is a direct acting alkylating agent, primarily forming N7-(2-hydroxyethyl)guanine (N7-HEG). To conduct accurate human risk assessments, it is vital to ascertain the relative contribution of endogenously versus exogenously derived DNA damage and identify the sources of background lesions. We have therefore defined in vivo dose-response relationships over a concentration range relevant to human EO exposures using a dual-isotope approach. By combining liquid chromatography-tandem mass spectrometry and high-performance liquid chromatography-accelerator mass spectrometry analysis, both the endogenous and exogenous N7-HEG adducts were quantified in tissues of [(14)C]EO-treated rats. Levels of [(14)C]N7-HEG induced in spleen, liver, and stomach DNA increased in a linear manner from 0.002 to 4 adducts/10(8) nucleotides. More importantly, the extent of damage arising through this route was insignificant compared with the background abundance of N7-HEG naturally present. However, at the two highest doses, [(14)C]EO exposure caused a significant increase in endogenous N7-HEG formation in liver and spleen, suggesting that EO can induce physiologic pathways responsible for ethylene generation in vivo and thereby indirectly promote N7-HEG production. We present evidence for a novel mechanism of adduct formation to explain this phenomenon, involving oxidative stress and 1-aminocyclopropane-1-carboxylic acid as a potential biosynthetic precursor to ethylene in mammalian cells. Based on the proposed pathway, N7-HEG may have potential as a biomarker of cellular oxidative stress.

Mechanistic studies of 1-aminocyclopropane-1-carboxylic acid oxidase: single turnover reaction

The final step in the biosynthesis of the plant hormone ethylene is catalyzed by the non-heme iron-containing enzyme 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACCO). ACC is oxidized at the expense of O(2) to yield ethylene, HCN, CO(2), and two waters. Continuous turnover of ACCO requires the presence of ascorbate and HCO(3)(-) (or an alternative form), but the roles played by these reagents, the order of substrate addition, and the mechanism of oxygen activation are controversial. Here these issues are addressed by development of the first functional single turnover system for ACCO. It is shown that 0.35 mol ethylene/mol Fe(II)ACCO is produced when the enzyme is combined with ACC and O(2) in the presence of HCO(3)(-) but in the absence of ascorbate. Thus, ascorbate is not required for O(2) activation or product formation. Little product is observed in the absence of HCO(3)(-), demonstrating the essential role of this reagent. By monitoring the EPR spectrum of the sample during single turnover, it is shown that the active site Fe(II) oxidizes to Fe(III) during the single turnover. This suggests that the electrons needed for catalysis can be derived from a fraction of the initial Fe(II)ACCO instead of ascorbate. Addition of ascorbate at 10% of its K(m) value significantly accelerates both iron oxidation and ethylene formation, suggesting a novel high-affinity effector role for this reagent. This role can be partially mimicked by a non-redox-active ascorbate analog. A mechanism is proposed that begins with ACC and O(2) binding, iron oxidation, and one-electron reduction to form a peroxy intermediate. Breakdown of this intermediate, perhaps by HCO(3)(-)-mediated proton transfer, is proposed to yield a high-valent iron species, which is the true oxidizing reagent for the bound ACC.

Immunocytolocalization of 1-aminocyclopropane-1-carboxylic acid oxidase in tomato and apple fruit

The subcellular localization of 1-aminocyclopropane-1-carboxylic acid oxidase (ACC oxidase), an enzyme involved in the biosynthesis of ethylene, has been studied in ripening fruits of tomato (Lycopersicum esculentum Mill.). Two types of antibody have been raised against (i) a synthetic peptide derived from the reconstructed pTOM13 clone (pRC13), a tomato cDNA encoding ACC oxidase, and considered as a suitable epitope by secondary-structure predictions; and (ii) a fusion protein overproduced in Escherichia coli expressing the pRC13 cDNA. Immunoblot analysis showed that, when purified by antigen affinity chromatography, both types of antibody recognized a single band corresponding to ACC oxidase. Superimposition of Calcofluor white with immunofluorescence labeling, analysed by optical microscopy, indicated that ACC oxidase is located at the cell wall in the pericarp of breaker tomato and climacteric apple (Malus x domestica Borkh.) fruit. The apoplasmic location of the enzyme was also demonstrated by the observation of immunogold-labeled antibodies in this region by both optical and electron microscopy. Transgenic tomato fruits in which ACC-oxidase gene expression was inhibited by an antisense gene exhibited a considerable reduction of labeling. Immunocytological controls made with pre-immune serum or with antibodies pre-absorbed on their corresponding antigens gave no staining. The discrepancy between these findings and the targeting of the protein predicted from sequences of ACC-oxidase cDNA clones isolated so far is discussed.

Involvement of mixed function oxidase systems in polychlorinated biphenyl metabolism by plant cells

Nineteen different polychlorinated biphenyl (PCB) congeners ranging in chlorine content from 2 to 6 chlorine atoms were provided to nonphotosynthetic suspension cultures of rose (Rosa sp. cv. Paul's Scarlet). After a 96 h incubation period, 11 individual congeners had been metabolized by > 10%. Provision of mixed function oxidase inhibitors (10 mM metyrapone or 0.5 mM 7,8-benzoflavone) either stopped or severely reduced the metabolism of individual congeners; whereas (inhibitors of peroxidase) (1 mM benzoate or 1 mM n-propylgallate) had minimal influence on PCB metabolism. The metabolism of PCBs by rose cultures appears to be catalyzed by a cytochrome P-450-and/or P-448-dependent enzyme system.

Identification of a naturally occurring inhibitor of the conversion of 1-aminocyclopropane-1-carboxylic Acid to ethylene by carnation microsomes

During cell-free experiments with membranes isolated from carnation petals (Dianthus caryophillus L. cv White Sim), the conversion of 1-aminocyclopropane-1-carboxylic acid into ethylene was blocked by a factor derived from the cytosol. Subsequent characterization of the inhibitor revealed that its effect was concentration dependent, that it was water soluble, and that it could be removed from solution by dialysis and addition of polyvinyl-polypyrrolidone. Activity profiles obtained after solvent partitioning over a range of pH values and after chromatography on silica gel, size exclusion gel, and ion exchange resins revealed that the inhibitor was a highly polar, low molecular weight species that was nonionic at low pH and anionic at pH values above 8. Use of selected solvent systems during paper and thin layer chromatography combined with specific spray reagents tentatively identified the compound as a hydroxycinnamic acid derivative. Base hydrolysis and subsequent comparison with known standards by high performance liquid chromatography, gas-liquid chromatography, and ultraviolet light spectroscopy established that the inhibitor was a conjugate with a ferulic acid moiety. Release of ferulic acid following treatment with beta-glucosidase also indicated the presence of a glucose moiety, and unequivocal identification of the inhibitor as 1-O-feruloyl-beta-d-glucose was confirmed by gas chromatography-mass spectroscopy and by ultraviolet light, (1)H-, and (13)C- nuclear magnetic resonance spectroscopy. Feruloylglucose constituted about 0.1% of the dry weight of stage III (preclimacteric) carnation petals, but concentrations fell sharply during stage IV (climacteric), when ethylene production peaks and the flowers senesce. In a reaction mixture containing microsome-bound ethylene forming enzyme system, 98% of all ethylene production was abolished in the presence of 50 mum concentrations of the inhibitor.

Ethylene production by sunflower cell suspensions : effects of plant growth retardants

From a variety of undifferentiated plant cell suspensions, 2,4-dichlorophenoxyacetic acid-dependent cells of sunflower (Helianthus annuus L. Spanners Allzweck) produced large quantities of ethylene. The maximum rate was about 1 nanomole x gram fresh weight(-1) x hour(-1) during the exponential growth phase. The action of various compounds known to interfere with ethylene formation in plant tissue was studied in sunflower cell suspensions. The influence on ethylene, 1-aminocyclopropanecarboxylic acid (ACC), and N-malonyl-ACC (MACC) levels suggested that the final steps in ethylene synthesis resemble those of other plant systems. This makes sunflower cells suitable for analyzing the effects of biologically active compounds on cellular ethylene biosynthesis. In particular, plant growth retardants of the norbornenodiazetine and triazole type inhibited ethylene production of sunflower cells. On the other hand, the ACC level was considerably elevated while that of MACC did not change significantly. It is assumed that the conversion of ACC to ethylene catalyzed by the ethylene-forming enzyme was influenced.

Inhibition of pear fruit ripening by mannose

Softening of the flesh and the rise in ethylene evolution and respiration associated with ripening in pear (Pyrus communis L.) fruit was delayed when mannose was vacuum infiltrated into intact fruit. The extent of delay could be modified by altering the concentration or the volume of mannose applied to the fruit. Inhibition of ripening was associated with phosphorylation of mannose to mannose 6-phosphate (M6P), and accumulation of M6P was associated with lowered levels of inorganic phosphate (Pi), glucose 6-phosphate (G6P), and ATP in the fruit tissue. Subsequently, however, as the M6P was metabolized, the levels of Pi, G6P, and ATP increased and ripening processes were concomitantly released from inhibition. Hence, the degree of inhibition by mannose or the release from inhibition was related to the level of M6P in the fruit and its rate of metabolism. The data provide correlative evidence to support a view that one inhibitory effect of mannose is depletion of Pi in the cell as a result of phosphorylation of mannose to M6P. Inhibition of ripening by mannose was not alleviated by co-application of glucose as a competitive substrate for the hexokinase(s), or by Pi, presumably the depleted metabolite. Also, incubation of tissue disks with M6P resulted in inhibition of ethylene production and respiration. The structural analogs of mannose, glucosamine, and 2-deoxyglucose, which have been shown to mimic mannose action in several plant tissues, did not cause inhibition of ripening of pear fruit comparable with that associated with mannose. Both analogs stimulated respiration, and glucosamine caused only a small inhibition of softening and ethylene evolution. Another mannose analog, alpha-methylmannoside, did inhibit fruit ripening though to a lesser extent than mannose. Its influence was also associated with accumulation of M6P and a decrease of Pi levels. We conclude that the mannose effect may, in part, be due to M6P toxicity, as well as by depletion of Pi.

Superoxide dismutase mimetic activity of cytokinin-copper(II) complexes

Dissociation constants of cytokinins, derivatives of purine which form complexes with cupric ion, were determined by spectrophotometry and the stability constants of their copper complexes by pH titration. The values found for kinetin were 3.76, 9.96, 7.8, and 15.3 for pK1, pK2, logk1, and log beta 2, respectively, and those for 6-benzylaminopurine were, in the same order, 3.90, 9.84, 8.3, and 15.9. The copper(II) complexes with kinetin and 6-benzylaminopurine had superoxide dismutase mimetic activity, and the reaction rate constants with superoxide, which were determined by polarography, were 2.3 X 10(-7) M-1 s-1 for kinetin and 1.5 X 10(-7) M-1 s-1 for 6-benzylaminopurine at pH 9.8 and 25 degrees C.

Ethylene biosynthesis in isolated vacuoles of Vicia faba L. - requirement for membrane integrity

The ability of vacuoles prepared from V. faba leaves to convert 1-aminocyclopropane-1-carboxylic acid to C2H4 was destroyed when vacuoles were lysed by passage through a hypodermic needle, freezing and thawing, osmotic shock, treatment with ethanol or with a detergent. Ethylene synthesis in the vacuolar fraction was also inhibited by the uncouplers carbonyl cyanide m-chlorophenyl hydrazone and dinitrophenol and by the ionophores valinomycin, nigericin, and A23187. Ethylene formation increased with increasing pH of the incubation medium over the pH range of 5.0-7.5. These observations support the hypothesis that C2H4 biosynthesis in vacuolar preparations is dependent on membrane integrity, possibly because of the requirement for a transmembrane ion gradient.

Deferral of senescence and abscission by chemical inhibition of ethylene synthesis and action in bean explants

Three compounds known to inhibit ethylene synthesis and/or action were compared for their ability to delay senescence and abscission of bean explants (Phaseolus vulgaris L. cv Contender). Aminoethoxyvinyl-glycine (AVG), AgNO(3), and sodium benzoate were infiltrated into the petiole explants. Their effect on abscission was monitored by measuring the force required to break the abscission zone, and their effect on senescence was followed by measuring chlorophyll and soluble protein in the distal (pulvinus) sections. AVG at concentrations between 1 and 100 micromolar inhibited ethylene synthesis by about 80 to 90% compared to the control during sampling periods of 24 and 48 hours after treatment. This compound also delayed the development of abscission and senescence. Treatment with AgNO(3) at concentrations between 1 and 100 micromolar progressively reduced ethylene production, but to a lesser extent than AVG. The effects of AgNO(3) on senescence and abscission were quite similar to those of AVG. Sodium benzoate at 50 micromolar to 5 millimolar did not inhibit ethylene synthesis during the first 24 hours, but appreciably inhibited ethylene synthesis 48 hours after treatment. It also delayed the development of abscission and senescence. The effects of AVG, Ag(+), and sodium benzoate suggest that ethylene could play a major role in both the senescence induction phase and the separation phase in bean explants.

Cell-free ethylene-forming systems lack stereochemical fidelity

In-vitro systems for the conversion of 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene have been reported using pea supernatants, carnation petal microsomes, olive leaf protein and, most recently, pea mitochondria. It has also been shown, in intact tissues of apple, mung bean and pea, that the system responsible for conversion of ACC to ethylene can produce 1-butene from isomers of 1-amino-2-ethylcyclopropane-1-carboxylic acid (AEC). This conversion shows a high degree of steroselectivity, and isomer discrimination is therefore a valuable criterion by which to judge the validity of subcellular systems. It is shown here that all in-vitro ethylene-forming systems so far described fail by a wide margin to match the AEC-isomer preference of the corresponding intact tissues with respect to 1-butene generation. This work supports and extends recent reports by McKeon and Yang (1984, Planta 160, 84-87) and by Guy and Kende (1984, Planta 160, 281-287) on the characteristics of ethylene formation by pea homogenates. The vacuolar conversion described by the latter authors is the simplest system yet described that retains appropriate sterochemical fidelity.

Conversion of 1-aminocyclopropane-1-carboxylic acid to ethylene by isolated vacuoles of Pisum sativum L

We compared the distribution of 1-aminocyclopropane-1-carboxylic acid (ACC) between the vacuole of isolated pea (Pisum sativum L.) protoplasts and the remainder of the cell and found that over 80% of the ACC was localized in the vacuole. Isolated protoplasts and vacuoles evolved ethylene. Over 80% of the ethylene production by protoplasts could be accounted for as originating from the vacuole. Ethylene synthesis by isolated vacuoles was saturated at ACC concentrations above 1 mM, and the apparent Km for the conversion of ACC to ethylene was 61 μM. Ethylene production in isolated vacuoles was inhibited by Co(2+), n-propyl-gallate, in a N2 atmosphere, and following lysis of the vacuoles. The ethylene-forming enzyme in pea vacuoles exhibited stereospecificity inasmuch as it catalyzed the conversion of (±)-allocoronamic acid to 1-butene but not that of (±)-coronamic acid. The same stereospecificity was also shown by leaf tissue. Based on competition studies with ACC and (±)-allocoronamic acid, we conclude that conversion of ACC to ethylene and (±)-allocoronamic acid to 1-butene is mediated by the same enzyme in isolated vacuoles and in intact leaf tissue. Vacuoles isolated from Vicia faba L. leaves showed essentially the same characteristics with regard to ACC-dependent ethylene synthesis as did pea vacuoles.

Antifungal antibiotics and Siba inhibit 1-aminocyclopropane-1-carboxylic acid synthase activity

The antifungal antibiotics Sinefungin and A9145C isolated from Streptomyces griseolus and the synthetic nucleoside Siba, which are analogs of S-adenosylmethionine, inhibit the activity of 1-aminocyclopropane 1-carboxylic acid synthase from tomato fruits. Sinefungin and Siba were shown to be more potent inhibitors than A9145C. In extracts of green fruits, the enzyme activity was inhibited by Sinefungin with an I50 of 1 microM, which was similar to that caused by aminoethoxyvinylglycine, and by Siba with an I50 of 100 microM; in extracts from red tomatoes, the I50's were 25 microM and 100 microM, respectively. The inhibition of ACC synthase by these analogs could be reversed by gel filtration chromatography.

Inhibition of ethylene biosynthesis in carnation petals by cytokinin

Pretreatment of detached carnation petals (Dianthus caryophyllus cv White Sim) for 24 hours with 0.1 millimolar of the cytokinins n(6)-benzyl-adenine (BA), kinetin, and zeatin blocked the conversion of externally supplied 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene and delayed petal senescence by 8 days. The normal enhanced wilting and increase in endogenous levels of ACC and ethylene production following exposure of petals to ethylene (16 mul/l for 10 hours), were not observed in BA-pretreated petals. In carnation foliage leaves pretreated with 0.1 mm BA, a reduction rather than inhibition of the conversion of exogenous ACC to ethylene was observed. This indicates that foliage leaves respond to cytokinins in a different way than petals. A constant 24-hour treatment with BA (0.1 mm) was not able to reduce ethylene production of senescing carnation petals, while 2 mm aminoxyacetic acid, a known inhibitor of ACC synthesis, or 10 mm propyl gallate, a free radical scavenger, decreased ethylene production significantly.

Inhibition by polyamines of macromolecular synthesis and its implication for ethylene production and senescence processes

Applied diamines and polyamines inhibited the incorporation of radioactively labeled leucine and uridine into trichloroacetic acid-insoluble material in apple (Malus domestica Borkh, cv Golden Delicious) fruit tissue. The inhibitory effect was in general more pronounced with the higher molecular weight amines. Putrescine at 5 millimolar inhibited leucine incorporation by 37% and uridine by 44%. Spermidine and spermine at the same concentration inhibited uridine incorporation by 60%. The polyamines at concentrations between 0.1 and 1.0 millimolar inhibited leucine incorporation by 55 to 90%. The inhibitory effect of 0.1 to 10 millimolar polyamines on dark- and wound-induced senescence or ethylene production, is discussed in the light of interference with macromolecular synthesis.

Some Characteristics of the System Converting 1-Aminocyclopropane-1-carboxylic Acid to Ethylene

The rate of C(2)H(4) production in plant tissue appears to be limited by the level of endogenous 1-aminocyclopropane-1-carboxylic acid (ACC). Exogenous ACC stimulated C(2)H(4) production considerably in plant tissues, but this required 10 to 100 times the endogenous concentrations of ACC before significant increases in C(2)H(4) production were observed. This was partially due to poor penetration of ACC into the tissues. Conversion of ACC to C(2)H(4) was inhibited by free radical scavengers, reducing agents, and copper chelators, but not by inhibitors of pyridoxal phosphate-mediated reactions. The system for converting ACC to C(2)H(4) may be membrane-associated, for it did not survive treatment with surface-active agents and cold or osmotic shock reduced the capacity of the system to convert ACC to C(2)H(4). The reaction rate was sensitive to temperatures above 29 and below 12 C, which suggests that the system may be associated with membrane-bound lipoproteins. The data presented support the possibility that the conversion of exogenous ACC to C(2)H(4) proceeds via the natural physiological pathway.