Inhibition of ethylene production by 2,4-dinitrophenol and high temperature

Systematic identification and analysis of heat-stress-responsive lncRNAs, circRNAs and miRNAs with associated co-expression and ceRNA networks in cucumber (Cucumis sativus L.)

Researchers have shown that long non-coding RNAs (lncRNAs) and circular RNAs (circRNAs) act as competitive endogenous RNAs (ceRNAs) and are mutually regulated by competition for binding to common microRNA response elements (MREs). However, a comprehensive identification and analysis of lncRNAs and circRNAs as ceRNAs have not yet been completed in cucumber (Cucumis sativus L.) exposed to high-temperature stress. In our study, 32 663 coding transcripts, 2085 lncRNAs, 2477 circRNAs and 348 differentially expressed miRNAs were identified using RNA sequencing. In addition, six heat-stress-responsive miRNAs (five known and one novel miRNAs) and eight lncRNAs were selected for qPCR to confirm their expression profiles. By analyzing the cis effects of lncRNAs, we constructed a lncRNA-mRNA co-expression network. Based on the results, the corresponding lncRNAs play a regulatory role in the stress response in cucumber plants. In our study, the PatMatch software was used to predict the potential function of lncRNAs and circRNAs as ceRNAs. A total of 18 lncRNAs and seven circRNAs were predicted to bind to 114 differentially expressed miRNAs and compete with 359 mRNAs for miRNA binding sites. These mRNAs are predicted to be involved in various pathways, such as plant hormone signal transduction, plant-pathogen interaction and glutathione metabolism. Among them, TCONS_00031790, TCONS_00014332, TCONS_00014717, TCONS_00005674, novel_circ_001543 and novel_circ_000876 may interact with miR9748 by plant hormone signal transduction pathways in response to high-temperature stress. Moreover, indole-3-acetic acid (IAA) and 1-aminocyclopropane-l-carboxylic acid (ACC) levels decreased in the high-temperature treatment group, indicating that IAA and ethylene signaling might be involved in response to high-temperature stress. In this study, we conducted a full transcriptomic analysis in response to high-temperature stress in cucumber and, for the first time, integrated the potential ceRNA functions of lncRNAs/circRNAs. The results provide a basis for studying the potential functions of lncRNAs/circRNAs in response to high-temperature stress.

Elevated carbon dioxide decreases the adverse effects of higher temperature and drought stress by mitigating oxidative stress and improving water status in Arabidopsis thaliana

This study revealed that elevated carbon dioxide increases Arabidopsis tolerance to higher temperature and drought stress by mitigating oxidative stress and improving water status of plants. Few studies have considered multiple aspects of plant responses to key components of global climate change, including higher temperature, elevated carbon dioxide (ECO₂), and drought. Hence, their individual and combinatorial effects on plants need to be investigated in the context of understanding climate change impact on plant growth and development. We investigated the interactive effects of temperature, CO₂, watering regime, and genotype on Arabidopsis thaliana (WT and ABA-insensitive mutant, abi1-1). Plants were grown in controlled-environment growth chambers under two temperature regimes (22/18 °C and 28/24 °C, 16 h light/8 h dark), two CO₂ concentrations (400 and 700 μmol mol^-1), and two watering regimes (well-watered and water-stressed) for 18 days. Plant growth, anatomical, physiological, molecular, and hormonal responses were determined. Our study provided valuable information about plant responses to the interactive effects of multiple environmental factors. We showed that drought and ECO₂ had larger effects on plants than higher temperatures. ECO₂ alleviated the detrimental effects of temperature and drought by mitigating oxidative stress and plant water status, and this positive effect was consistent across multiple response levels. The WT plants performed better than the abi1-1 plants; the former had higher rosette diameter, total dry mass, leaf and soil water potential, leaf moisture, proline, ethylene, trans-zeatin, isopentyladenine, and cis-zeatin riboside than the latter. The water-stressed plants of both genotypes accumulated more abscisic acid (ABA) than the well-watered plants; however, higher temperatures decreased the ability of WT plants to produce ABA in response to drought. We conclude that drought strongly, while higher temperature to a lesser extent, affects Arabidopsis seedlings, and ECO₂ reduces the adverse effects of these stressors more efficiently in the WT plants than in the abi1-1 plants. Findings from this study can be extrapolated to other plant species that share similar characteristics and/or family with Arabidopsis.

Heat reduces nitric oxide production required for auxin-mediated gene expression and fate determination in tree tobacco guard cell protoplasts

Tree tobacco (Nicotiana glauca) is an equatorial perennial with a high basal thermotolerance. Cultured tree tobacco guard cell protoplasts (GCPs) are useful for studying the effects of heat stress on fate-determining hormonal signaling. At lower temperatures (32°C or less), exogenous auxin (1-naphthalene acetic acid) and cytokinin (6-benzylaminopurine) cause GCPs to expand 20- to 30-fold, regenerate cell walls, dedifferentiate, reenter the cell cycle, and divide. At higher temperatures (34°C or greater), GCPs expand only 5- to 6-fold; they do not regenerate walls, dedifferentiate, reenter the cell cycle, or divide. Heat (38°C) suppresses activation of the BA auxin-responsive transgene promoter in tree tobacco GCPs, suggesting that inhibition of cell expansion and cell cycle reentry at high temperatures is due to suppressed auxin signaling. Nitric oxide (NO) has been implicated in auxin signaling in other plant systems. Here, we show that heat inhibits NO accumulation by GCPs and that L-N(G)-monomethyl arginine, an inhibitor of NO production in animals and plants, mimics the effects of heat by limiting cell expansion and preventing cell wall regeneration; inhibiting cell cycle reentry, dedifferentiation, and cell division; and suppressing activation of the BA auxin-responsive promoter. We also show that heat and L-N(G)-monomethyl arginine reduce the mitotic indices of primary root meristems and inhibit lateral root elongation similarly. These data link reduced NO levels to suppressed auxin signaling in heat-stressed cells and seedlings of thermotolerant plants and suggest that even plants that have evolved to withstand sustained high temperatures may still be negatively impacted by heat stress.

Formation of cyanide from carbon 1 of 1-aminocyclopropane-1-carboxylic acid during its conversion to ethylene

It has been shown that 1-aminocyclopropane-1-carboxylic acid (ACC) is the immediate precursor of ethylene, which is derived from C-2 and C-3 of ACC. When [1-(14)C]ACC was administered to etiolated mungbean (Vigna radiata) hypocotyls, approximately 16% of the ACC was converted to ethylene and about 10% of the radioactivity was converted to [(14)C]asparagine in 7 hr. In etiolated epicotyls of common vetch (Vicia sativa), after 7 hr about 14% of the ACC was converted to ethylene and 16% of the radioactivity was converted to beta-cyanoalanine plus gamma-glutamyl-beta-cyanoalanine. Itis known that in most plants cyanide is metabolized to asparagine via the intermediate beta-cyanoalanine, whereas in a fewplants such as V. sativa, beta-cyanoalanine is converted to the conjugate gamma-glutamyl-beta-cyanoalanine. We confirmed that [(14)C]cyanide was metabolized into [(14)C]asparagine in mungbean and into [(14)C]cyanoalanine plus its conjugate in V. sativa. Moreover, after feeding plant tissue with [1-(14)C]ACC, [(14)C]asparagine isolated from mungbean and beta-[(14)C]cyanoalanine from V. sativa were labeled in the C-4 position, as would be expected if these two compounds were derived from [(14)C]cyanide. When the conversion of ACC to ethylene in V. sativa tissue was inhibited by high temperature (41 degrees C), the conversion of [1-(14)C]ACC to beta-[(14)C]cyanoalanine and gamma-glutamyl-beta-[(14)C]cyanoalanine was similarly inhibited. When [carboxyl-(14)C]ACC was administered to mungbean and V. sativa, (14)CO(2) was recovered in an amount equivalent to the amount of ethylene produced. These data indicate that in the conversion of ACC to ethylene the carboxyl group yields CO(2), and C-1 is released as HCN.

Turnover of 1-aminocyclopropane-1-carboxylic Acid synthase protein in wounded tomato fruit tissue

Ethylene production in plant tissues declines rapidly following induction, and this decline is due to a rapid decrease in the activity of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase, a key enzyme in ethylene biosynthesis. To study the nature of the rapid turnover of ACC synthase in vivo, proteins in wounded ripening tomato (Lycopersicon esculentum) fruit discs were radiolabeled with [(35)S]methionine, followed by a chase with nonradioactive methionine. Periodically, the radioactive ACC synthase was isolated with an immunoaffinity gel and analyzed. ACC synthase protein decayed rapidly in vivo with an apparent half-life of about 58 min. This value for protein turnover in vivo is similar to that previously reported for activity half-life in vivo and substrate-dependent enzyme inactivation in vitro. Carbonylcyanide-m-chlorophenylhydrazone and 2,4-dinitrophenol, potent uncouplers of oxidative phosphorylation, strongly inhibited the rapid decay of ACC synthase protein in the tissue. Degradation of this enzyme protein was moderately inhibited by the administration of aminooxyacetic acid, a competitive inhibitor of ACC synthase with respect to its substrate S-adenosyl-l-methionine, alpha,alpha'-dipyridyl, and phenylmethanesulfonyl fluoride or leupeptin, serine protease inhibitors. These results support the notion that the substrate S-adenosyl-l-methionine participates in the rapid inactivation of the enzyme in vivo and suggest that some ATP-dependent processes, such as the ubiquitin-requiring pathway, are involved in the degradation of ACC synthase proteins.

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.

Inhibition of ethylene biosynthesis by salicylic Acid

Salicylic acid inhibited ethylene formation from ACC in self-buffered (pH 3.8) pear (Pyrus communis) cell suspension cultures with a K(1) (app) of about 10 micromolar after 1 to 3 hours incubation. Inhibition appeared noncompetitive. Among 22 related phenolic compounds tested, only acetylsalicylic acid showed similar levels of inhibition. Inhibition by salicylic acid was inversely dependent on the pH of the culture medium and did not require a continuous external supply of salicylate. When compared to known inhibitors of the ethylene forming enzyme, cobalt, n-propyl gallate, and dinitrophenol, inhibition by salicylic acid most closely resembled that by dinitrophenol but salicylic acid did not produce the same degree of respiratory stimulation. Results are discussed in terms of other known effects of salicylic acid on plants, pH-dependency, and the possible influence of salicylic acid on electron transport.

Ethylene regulation of β-1,3-glucanase in tobacco

Ethylene treatment (approx. 20 μl ·1(-1) in air for 2 d) of tobacco (Nicotiana tabacum L. cv. Havana 425) plants markedly increases the endo-β-1,3-glucanase (EC 3.2.1.39) content of leaves. The antigenic form of the enzyme induced is the same one whose production is blocked by treating cultured cells with combinations of auxin (1.1 · 10(-5) M α-naphthaleneacetic acid) and cytokinin (1.4 · 10(-6) M kinetin). Evidence is presented that cultured tobacco cells require ethylene for β-1,3-glucanase accumulation: i) ethylene treatment increased the accumulation of \-1,3-glucanase in callus tissues >10 d after subculturing and in cell-suspension cultures; ii) callus tissues can produce ethylene; iii) conditions known to inhibit ethylene production (1 mM CoCl2; 33° C treatment) or ethylene action (approx. 1.6 mmol · 1(-1) norbornadiene in air) inhibited β-1,3-glucanase accumulation by callus tissues treated for 4 d following subculturing; and, these inhibitory effects were prevented by exogenous ethylene. Combinations of auxin and cytokinin blocked ethylene-induced accumulation of β-1,3-glucanase by cell-suspension cultures. The results favor a model in which ethylene induces results favor a model in which ethylene induces β 1,3-glucanase accumulation, and auxin and cytokinin inhibit this induction process.

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.

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.

Metabolism of α-aminoisobutyric acid in mungbean hypocotyls in relation to metabolism of 1-aminocyclopropane-1-carboxylic acid

1-Aminocyclopropane-1-carboxylic acid (ACC) is known to be converted to ethylene and conjugated into N-malonyl-ACC in plant tissues. When α-amino[1-(14)C]isobutyric acid (AIB), a structural analog of ACC, was administered to mungbean (Vigna radiata L.) hypocotyl segments, it was metabolized to (14)CO2 and conjugated to N-malonyl-AIB (MAIB). α-Aminoisobutyric acid inhibited the conversion of ACC to ethylene and also inhibited, to a lesser extent, N-malonylation of ACC and D-amino acids. Although the malonylation of AIB was strongly inhibited by ACC as well as by D-amino acids, the metabolism of AIB to CO2 was inhibited only by ACC but not by D-amino acids. Inhibitors of ACC conversion to ethylene such as anaerobiosis, 2,4-dinitrophenol and Co(2+), similarly inhibited the conversion of AIB to CO2. These results indicate that the malonyalation of AIB to MAIB is intimately related to the malonylation of ACC and D-amino acids, whereas oxidative decarboxylation of AIB is related to the oxidative degradation of ACC to ethylene.

Comparative analysis of physical stress responses in soybean seedlings using cloned heat shock cDNAs

Soybean seedlings were subjected to a wide range of physical (abiotic) or environmental stresses. Cloned cDNAs to heat shock (hs)-induced mRNAs were used to assess whether these diverse stresses induced the accumulation of poly(A)RNAs in common with those induced by hs. Northern blot hybridization analyses indicated that a wide range of stress agents lead to the accumulation of detectable levels of several of the hs-induced poly(A)RNAs; the relative concentration of those RNAs 'induced' by the wide range of stress agents (e.g. water stress, salt stress, anaerobiosis, high concentrations of hormones, etc.), was generally in the order of 100-fold lower than that induced by hs. There are two notable exceptions to that pattern of response to the stress agents. First, arsenite treatment resulted in accumulation of the 'hs poly(A)RNAs' to levels similar to those induced by hs. Cadmium also induced a somewhat normal spectrum of the 'hs poly(A)RNAs', but generally lower levels accumulated than in hs- and arsenite0treated tissues. Second, one set of poly(A)RNAs which are present at low and variable levels in control (non-stressed tissue) tissue, and which are increased some 5- to 10-fold by hs, increased in relative concentration in response to a wide range of the stress agents similarly to the response to hs. The physiological significance of the accumulation of this set of poly(A)RNAs (which translate into four electrophoretically different 27 kd proteins) is not known, but they certainly seem to serve as a monitor (or barometer) of physiological stress conditions. Cadmium treatment results in the accumulation of those same poly(A)RNAs and an additional band of higher molecular weight poly(A)RNA homologous to the same hs cDNA clone (clone pCE 54). Ethylene seems to have no obvious causal relationship to the hs response, even though hs-treated seedlings display some symptoms similar to those exhibited by ethylene-treated seedlings.

Intermediates in the recycling of 5-methylthioribose to methionine in fruits

The recycling of 5-methylthioribose (MTR) to methionine in avocado (Persea americana Mill, cv Hass) and tomato (Lycopersicum esculentum Mill, cv unknown) was examined. [(14)CH(3)]MTR was not metabolized in cell free extract from avocado fruit. Either [(14)CH(3)]MTR plus ATP or [(14)CH(3)]5-methylthioribose-1-phosphate (MTR-1-P) alone, however, were metabolized to two new products by these extracts. MTR kinase activity has previously been detected in these fruit extracts. These data indicate that MTR must be converted to MTR-1-P by MTR kinase before further metabolism can occur. The products of MTR-1-P metabolism were tentatively identified as alpha-keto-gamma-methylthiobutyric acid (alpha-KMB) and alpha-hydroxy-gamma-methylthiobutyric acid (alpha-HMB) by chromatography in several solvent systems. [(35)S]alpha-KMB was found to be further metabolized to methionine and alpha-HMB by these extracts, whereas alpha-HMB was not. However, alpha-HMB inhibited the conversion of alpha-KMB to methionine. Both [U-(14)C]alpha-KMB and [U-(14)C]methionine, but not [U-(14)C]alpha-HMB, were converted to ethylene in tomato pericarp tissue. In addition, aminoethoxyvinylglycine inhibited the conversion of alpha-KMB to ethylene. These data suggest that the recycling pathway leading to ethylene is MTR --> MTR-1-P --> alpha-KMB --> methionine --> S-adenosylmethionine --> 1-aminocyclopropane-1-carboxylic acid --> ethylene.

Ethylene Biosynthesis and Cadmium Toxicity in Leaf Tissue of Beans (Phaseolus vulgaris L.)

Stress ethylene production in bean (Phaseolus vulgaris L., cv. Taylor's Horticultural) leaf tissue was stimulated by Cd(2+) at concentrations above 1 micromolar. Cd(2+)-induced ethylene biosynthesis was dependent upon synthesis of 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase. Activity of ACC synthase and ethylene production rate peaked at 8 h of treatment. The subsequent decline in enzyme activity was most likely due to inactivation of the enzyme by Cd(2+), which inhibited ACC synthase activity in vitro at concentrations as low as 0.1 micromolar. Decrease in ethylene production rate was accompanied by leakage of solutes and increasing inhibition of ACC-dependent ethylene production. Ca(2+), present during a 2-hour preincubation, reduced the effect of Cd(2+) on leakage and ACC conversion. This suggests that Cd(2+) exerts its toxicity through membrane damage and inactivation of enzymes. The possibility of an indirect stimulation of ethylene biosynthesis through a wound signal from injured cells is discussed.

Stereospecific conversion of 1-aminocyclopropanecarboxylic Acid to ethylene by plant tissues : conversion of stereoisomers of 1-amino-2-ethylcyclopropanecarboxylic Acid to 1-butene

Inasmuch as the molecule of 1-aminocyclopropanecarboxylic acid (ACC) possesses reflective symmetry but lacks rotational symmetry, the two chemically alike methylene groups can be distinguished by a stereospecific enzyme. To determine whether ACC conversion to ethylene by plant tissues proceeds in a stereospecific fashion, the four stereoisomers of 1-amino-2-ethylcyclopropanecarboxylic acid (AEC) were administered to postclimacteric apple (Malus sylvestris Mill., var. Golden Delicious), excised preclimacteric cantaloupe (Cucumis melo L., var. reticulatis Naud cv. PMR-45), and etiolated mung bean (Vigna radiata L., Wilczek, var. Berken) hypocotyls. In each case (1R,2S)-AEC was the preferred substrate yielding 1-butene. In contrast, all AEC isomers were converted equally well to butene by chemical oxidation using NaOCl. Both ACC and AEC appear to be substrates for the same enzyme since both reactions are inhibited in parallel by N(2) or Co(2+), both reactions are induced in parallel by excision, and when both substrates are present simultaneously each will act as an inhibitor with respect to the other. The aforementioned observations indicate that ACC is stereospecifically converted to ethylene. For AEC to be the most active precursor of 1-butene, the ethyl substituent should be trans to the carboxyl group and the pro-(S) methylene group should be unsubstituted. This observation leads to the suggestion that the enzyme interacts with amino, carboxyl, and pro-(S) methylene groups, a configuration corresponding to a l-amino acid. This view is consistent with the observation that the l-forms of alanine and methionine inhibit the conversion of ACC to ethylene more than the corresponding d-amino acids in the mung bean hypocotyl system.

Chilling-Induced Ethylene Production in Cucumbers (Cucumis sativus L.)

1-Aminocyclopropane-1-carboxylic acid (ACC) level, ACC synthase activity, and ethylene production in cucumbers (Cucumis sativus L.) remain low while the fruit are held at a temperature which causes chilling injury (2.5 degrees C) and increase rapidly only upon transfer to warmer temperatures. The increase in ACC synthase activity during the warming period is inhibited by cycloheximide but not cordycepin or alpha-amanitin. Our data indicate that the synthesis of ACC synthase, which results in increased ACC levels and accelerated ethylene production, occurs only upon warming, possibly from a message produced or unmasked during the chilling period. Ethylene production by chilled (2.5 degrees C) cucumbers increased very little upon transfer to 25 degrees C if the fruit were chilled for more than 4 days. The fruit held for 4 days or longer showed a large increase in ACC levels but little ethylene production even in the presence of exogenous ACC. This suggests that the system which converts ACC to ethylene is damaged by prolonged exposure to the chilling temperature. Cucumbers stored at a low but nonchilling temperature (13 degrees C) showed very little change in ACC level, ethylene production, or ACC synthase activity even after transfer to 25 degrees C.

Ethylene formation from 1-aminocyclopropane-1-carboxylic acid by microsomal membranes from senescing carnation flowers

Isolated membranes from the petals of senescing carnation flowers (Dianthus caryophyllus L. cv. White-Sim) catalyze the conversion of 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene. A microsomal membrane fraction obtained by centrifugation at 131,000 g for 1 h proved to be more active than the membrane pellet isolated by centrifugation at 10,000 g for 20 min. The ethylene-producing activity of the microsomal membranes is oxygen-dependent, heat-denaturable, sensitive to n-propyl gallate, and saturable with ACC. Corresponding cytosol fractions from the petals are incapable of converting ACC to ethylene. Moreover, the addition of soluble fraction back to the membrane fraction strongly inhibits the ACC to ethylene conversion activity of the membranes. The efficiency with which isolated membranes convert ACC to ethylene is lower than that exhibited by intact flowers based on the relative yield of membranes per flower. This may be due to the presence of the endogenous soluble inhibitor of the reaction, for residual soluble fraction inevitably remains trapped in membrane vesicles isolated from a homogenate.