Dormancy and Impotency of Cocklebur Seeds. IX. Changes in ACC-Ethylene Conversion Activity and ACC Content of Dormant and Nondormant Seeds during Soaking

Differences in ethylene production between dormant (D) and nondormant(ND) lower seeds of cocklebur (Xanthium pennsylvanicum Wallr.)were studied with respect to changes in the activity of conversionof 1-aminocyclopropane-l-carboxylic acid (ACC) to ethylene andin the contents of ACC and malonyl-ACC in their axial-tissuesduring soaking. Superior ethylene production in ND seeds ascompared to D seeds became evident during a soaking period rangingfrom 12–24 h, when the radicle protrusion in ND seedshad not yet occurred. Ethylene production in ND seeds increasedabruptly after the radicle protrusion. The inhibitors of ethyleneproduction, aminoethoxyvinyglycine, cobaltous ion and -aminoisobutyricacid, inhibited the germination of ND seeds, whereas ACC enabledD seeds to germinate. Activity of ACC-ethylene conversion was absent in dry axialtissues and developed with soaking. After 24 h, this activityin ND axes was superior to that in D axes. Under hypoxia, however,the difference in the ACC-conversion activity appeared before24 h. On the other hand, the contents of ACC in both D and NDaxes remained almost unchanged until 24 h of soaking. It isthus suggested that the inferior ethylene production in D seedsis associated mainly with their low activity of ACC-ethyleneconversion, though partly with their low activity of ACC supply. Activity of ACC-ethylene conversion in the axes of ND seedsincreased sharply after radicle protrusion which occurred after24 h of soaking. Correspondingly, the contents of both ACC andmalonyl-ACC increased in the axes of germinated ND seeds. Theseimply that the high ethylene production in the ND seeds in thepost-germination period comes from the increasing activitiesof ACC supply as well as ACC-ethylene conversion in their axes. Key words: Cocklebur seeds, Dormancy, Ethylene production, 1-aminocyclopropane-1-carboxylic acid, Germination, Xanthium     

Ethylene Production in Pea and Cocklebur Seeds of Differing Vigour

Relationships between seed vigour and ethylene (C₂H₄) productionwere studied using C₂H₄-responsive fatty cocklebur seeds (Xanthiumpennsyhanicum Wallr.) and C₂H₄-insensitive starchy pea seeds(Pisum sativum L. cv. Alaska), which had been harvested in differentyears and subjected to different storage conditions. In bothspecies, the seeds with the highest vigour evolved the largestamounts of C₂H₄ during a period of water imbibition. The reductionof C₂H₄ production in cocklebur seeds occurred concomitantlywith the reduction in the growth potentials of both axial andcotyledonary tissues. Similarly, the activity of ACC-C₂H₄ conversionincreased with soaking, and was greater in seeds of high vigourcompared with those of low vigour. However, the change in ACCcontent in pea seeds differed from that in cocklebur seeds.That is, pea seeds with high vigour accumulated less ACC duringan imbibition period than those with low vigour. From theseresults it was suggested that the inferior C₂H₄ production bylow vigour pea seeds is mainly attributable to low ACC-C₂H₄conversion, whereas that by low vigour cocklebur seeds is dueto the shortage of ACC supply in addition to the reduced ACC-C₂H₄conversion. However, germination of deteriorated cocklebur seedswas not restored by exposure to ACC or C₂H₄, suggesting thatthe loss of seed vigour reduces the responsiveness of seedsto C₂H₄ as well as C₂H₄ production. Key words: Pea, cocklebur, seed vigour, ethylene production, 1-aminocyclopropane-1-carboxylic acid     

Changes in Ethylene Production and in 1-Aminocyclopropane-1-carboxylic Acid (ACC) and Malonyl-ACC Contents of Cocklebur Seeds during Their Development

Ethylene production in developing cocklebur (Xanthium pennsyluanicumWallr.) seeds peaked when the dry weight of the seeds beganto increase in the early period of development. The productionthen began to decrease and stopped when the dry weight increasewas completed. The upsurge of ethylene production in the earlydevelopmental period paralleled increases in ACC synthase activityand the 1-aminocyclopropane-1-carboxylic acid (ACC) contentof the seeds, both of which rapidly decreased later. Malonyl-ACC (MACC) accumulated in developing cocklebur seedsduring the early period of development, before the ACC contentand ethylene production increased. Although the ACC synthaseactivity, ACC content and ethylene production showed markeddecreases, the MACC content remained almost unchanged duringthe middle period of seed development, with a pronounced decreaseoccurring in the late period. Exogenous application of MACCdid not promote ethylene production of seeds collected at thelate developmental stage. Aminoethoxyvinylglycine, an inhibitorof ACC synthase, strongly inhibited the ethylene productionof the same lot of seeds. Therefore, the decrease in the MACCcontent in developing cocklebur seeds was not due to reuse ofMACC for ethylene production. (Received May 24, 1984; Accepted August 15, 1984)     

Inhibitory Effects of Methyl Jasmonate on the Germination and Ethylene Production in Cocklebur Seeds

Methyl jasmonate (JA-Me) inhibited the germination of cocklebur (Xanthium pennsylvanicum Wallr.) seeds. The inhibition of the germination of cocklebur seeds treated with JA-Me at concentrations less than 300 μm was nullified by ethylene applied exogenously, although the inhibitory effect of 1,000 μm JA-Me was not recovered completely even by high concentrations of ethylene (10,000 μL/liter). JA-Me inhibited ethylene production before seed germination. The level of 1-aminocyclopropane-1-carboxylic acid (ACC) in the cotyledonary tissues treated with JA-Me decreased but not the level of 1-(malonylamino)cyclopropane-1-carboxylic acid (MACC). JA-Me inhibited the conversion of ACC to ethylene in the tissues. These results suggested that JA-Me inhibits ethylene production by prevention of ACC oxidation in addition to ACC synthesis. We believe that the inhibition of ethylene production by JA-Me results in the retardation of the germination of cocklebur seeds. Received June 4, 1997; accepted October 23, 1997     

In vivo formation of 1-malonylaminocyclopropane-1-carboxylic acid and its relationship to ethylene production in cocklebur seed segments: A tracer study with 1-amino-2-ethylcyclopropane-1-carboxylic acid

The in vivo formation of 1-malonylaminocyclopropane-1-carboxylic acid (malonyl-ACC) and its relationship to ethylene production in the axial tissue of cocklebur (Xanthium pennsylvanicum) seeds were investigated using the stereoisomers of the 2-ethyl derivative of ACC (AEC), as tracers of ACC. Of the four AEC isomers, the (1R, 2S)-isomer was converted most effectively to a malonyl conjugate as well as to 1-butene. Malonyl-AEC, once formed, was not decomposed, supporting the view that malonyl-ACC does not liberate free ACC for ethylene production in this tissue. d-Phenylalanine inhibited the formation of malonyl-AEC and, at the same time, promoted the evolution of 1-butene, whereas l-phenylalanine did not. Possibly, the d-amino-acid-stimulated ethylene production in cocklebur seed tissues is due to an increase in the amount of ACC available for ethylene production which results from the decrease of ACC malonylation in the tissues treated with d-amino acid. 2-Aminoisobutyric acid, a competitive inhibitor of ACC-ethylene conversion, did not affect the malonylation of AEC.     

Rapid Ethylene Production in Soybean in Response to the Cupric Ion

Very rapid accumulation of free 1-aminocyclopropane-1-carboxylicacid (ACC), followed by stimulation of ethylene production wereinduced by a Cu²⁺ in soybean cuttings. The accumulation mustbe attributed to an increase in ACC synthesis, because: (1)it was completely inhibited by aminoethoxyvinylglycine (AVG);and (2) the ethylene stimulation was inhibited by AVG, indicatingthat free ACC cannot be released from its conjugated form. Theactivity of the ethylene-forming enzyme slightly decreased followingthe Cu²⁺ pulse, and this event was accompanied by a slight increasein electrolyte leakage from the treated soybean tissues. Glycine max L., soybean, ethylene, cupric ion     

Ethylene Production in Sweet Potato Root Tissue Infected by Ceratocystis fimbriata

The rate of ethylene production by sweet potato (Ipomoea batatasLam. cv. Norin No. 1) root tissue infected with Ceratocystisfimbriata Ell. & Halst. increased markedly during incubationat 29?C under high relative humidity. During incubation thefungus progressively invaded root tissue. The rate of ethyleneproduction reached a peak two days after inoculation when thebrowning region that contained the penetrating mycelia had expandedinward about 0.3 mm from the surface, followed by a declinein ethylene production. Apparently, the 1-aminocyclopropane-1-carboxylicacid (ACC) synthase activity was not high enough, and the amountof ACC in the infected tissue was too low to account for thehigh rate of ethylene production throughout the incubation period.Ethylene production by the infected tissue showed scarcely anyinhibition by amino-ethoxyvinylglycine, a specific inhibitorof ACC synthase. These findings suggest that the pathway ofethylene biosynthesis that operates in infected sweet potatoroot tissue may differ from the methionine pathway in whichACC serves as an intermediate. (Received March 24, 1984; Accepted June 27, 1984)     

Role of Aminocyclopropane-1-carboxylic Acid (ACC) in Control of Ethylene Production in Fresh and Cold-stored Rose Flowers

The relationships between ethylene production, aminocyclopropane-1-carboxylicacid (ACC) content and ethylene-forming-enzyme (EFE) activityduring ageing and cold storage of rose flower petals (Rose hybridaL. cv. Gabriella) were investigated. During flower ageing at20 °C there was a climacteric rise in petal ethylene production,a parallel increase in ACC content, but a continuous decreasein EFE activity. Applied ACC increased petal ethylene productionc. 200-fold. During cold storage of flowers at 1 °C therewere parallel increases in petal ethylene production and ACCcontent, to levels greater than those reached in fresh flowersheld at 20 °C. EFE activity decreased during storage. Immediatelyafter cold-stored flowers were transferred to 20 °C ethyleneproduction and ACC levels were c. four times greater than infreshly cut flowers. These levels increased to maximum valuesof two to four times the maximum values reached during ageingof fresh, unstored, flowers. It was concluded that in rose petalsethylene synthesis is probably regulated by ACC levels and thatcold storage stimulates ethylene synthesis because it increasesthe levels of ACC in the petals. Key words: Rose flower, senescence, ethylene     

The Effects of Oxygen, Carbon Dioxide and Ethylene on Ethylene Biosynthesis in Relation to Shoot Extension in Seedlings of Rice (Oryza sativa) and Barnyard Grass (Echinochloa oryzoides)

Exposing dark-grown seedlings for 3 d to oxygen deficiency (0or 5 kPa) or to additions of carbon dioxide (10 kPa) or ethylene(0·1 Pa) slowed shoot extension in Echinochloa oryzoides,while in rice it was promoted by these treatments, except that5 kPa oxygen was without effect. In E. oryzoides this was dueto reduced growth of the mesocotyl, and in rice to enhancedgrowth of the coleoptile. These responses to carbon dioxideand oxygen deficiency were not consequences of increased ethyleneproduction, since this remained unchanged by carbon dioxideand depressed by oxygen shortage in both species. Furthermore,exogenous ethylene and the ethylene action inhibitor 2,5-norbornadieneeach failed to influence extension in anoxic seedlings, indicatingno regulatory role for ethylene in the absence of oxygen. However,concentrations of the ethylene precursor 1 -aminocyclopropane-1-carboxylic acid (ACC) were increased by carbon dioxide and0 kPa or 5 kPa oxygen, although after 72 h without oxygen totalACC production (i.e. changes in ethylene + ACC + MACC) was suppressedin both species. There was little effect on bound ACC [putativemalonyl-ACC (MACC)] formation. Transferring anaerobic (0 kPa)seedlings to oxygenated conditions (21 kPa) resulted in abnormallyfast rates of ethylene formation, possibly due to the accumulationof ACC under anoxia. This post-anoxic ethylene may have contributedto the faster extension by rice coleoptiles and slower extensionby mesocotyls of E. oryzoides compared with those of seedlingsmaintained continuously in air. Echinochloa oryzoides [Ard.] Fritsch, barnyard grass, Oryza sativa L, rice, oxygen shortage, carbon dioxide, ethylene biosynthesis, shoot extension, 1-aminocyclopropane-1-carboxylic acid (ACC), malonyl-ACC, GC-MS     

{alpha}-Aminoisobutyric acid : A probable competitive inhibitor of conversion of 1-aminocyclopropane-1-carboxylic acid to ethylene

Of 16 compounds related to 1-aminocyclopropane-1-carboxylicacid (ACC), aminoisobutyric acid (AIB) inhibited the productionof endogenous ethylene in the cotyledonary segments of cocklebur(Xanthium pennsylvanicum Wallr.) seeds most strongly. AIB at4 mM inhibited the formation of ethylene by about 50%, althoughthe O₂ uptake of the segments was not affected even at 20 mM.AIB also inhibited ethylene formation in the stem segments ofetiolated pea (Pisum sativum L. cv. Alaska) seedlings. Kineticanalysis with cell free extracts from etiolated pea shoots revealedthat AIB competitively inhibits the conversion of ACC into ethylene. (Received May 26, 1980; )     

Inhibition of Auxin-Induced Ethylene Production by Lycoricidinol

Lycoricidinol, a natural growth inhibitor isolated from bulbsof Lycoris radiata Herb. strongly suppressed auxin-induced ethyleneproduction from the hypocotyl segments of etiolated mung bean(Vigna radiata Wilczek) seedlings. The inhibitor did not significantlyinhibit ethylene formation from its immediate precursor, 1-aminocyclopropane-1-carboxylicacid (ACG), during short-term (up to 4 h) incubation. The ACCcontent in tissue treated with IAA was reduced by lycoricidinolin close parallel with the inhibition of ethylene production.Examination of radioactive metabolites in tissues labeled with3,4-¹⁴C-methionine indicated that reduction of the ACC contentwas not due to any possible promotive effect of lycoricidinolon conjugation of ACC with malonate. Lycoricidinol showed noinhibitory effect on the activity of ACC synthase if appliedin vitro, but it almost completely abolished the increase inthe enzyme activity when applied in vivo during incubation ofthe tissue with IAA. Lycoricidinol also strongly inhibited incorporationof ¹⁴C-leucine into protein in the tissue. The suppression ofthe enzyme induction and, in turn, that, of ethylene productionby lycoricidinol were interpreted as being due to the inhibitionof protein synthesis. (Received September 30, 1983; Accepted December 8, 1983)     

Ethylene Production by the Lichen Ramalina duriaei

The lichen Ramalina duriaei evolved ethylene when in a wettedstate, the rate of ethylene evolution being constant for atleast the first 20 h. Inhibitors of the ACC (I-aminocyclopropane-I-carboxylicacid) pathway did not inhibit ethylene production. Metal ionsstimulated the production, with Fe²⁺ being the most effective.This stimulation was not affected by inhibitors of the ACC pathwaybut was inhibited by free radical scavengers such as propylgallateand quercitin. Endogenous ACC content was similar whether thelichens were producing ethylene at a basal rate or during Fe²⁺-stimulatedethylene formation. Malondialdehyde and aldehyde contents werehigher in the presence of Fe²⁺. The results are discussed interms of known pathways of ethylene production by micro-organisms. ACC, ethylene, metal ions, methionine, 2-oxo-methylthiobutyric acid, Ramalina duriaei (De Not.) Bagl     

α-Aminoisabutyric acid, propyl gallate and cobalt ion and the mode of inhibition of ethylene production by cotyledonary segments of cocklebur seeds

Using cotyledonary segments of cocklebur ( Xanthium pennsylvanicum Wallr. ) seeds, the inhibitory effect of α-aminoisobutyric acid (AIB) on ethylene production was compared with that of propyl gallate and CoCl₂. Of these inhibitors only AIB was effective in causing the accumulation of endogenous free 1-aminocyclopropane-l-carboxylic acid (ACC) in the tissue. The degree of inhibition of ethylene production by AIB decreased markedly with increasing concentrations of pre-loaded ACC, while the inhibition by propyl gallate and CoCl₂ changed little. Kinetic analysis showed that AIB competitively inhibited the conversion of pre-loaded ACC to ethylene, but propyl gallate and CoC_l2 did not. Short-chain organic acids and analogues of AIB, such as acetic, propionic, butyric and cyclopropanecarboxylic acids, did not inhibit ethylene production by the segments. Thus, additional support for the competitive mode of inhibitory action of AIB on the conversion of free ACC to ethylene was provided.
A conjugated hydrolysable ACC was found to be present in abundance in cotyledons of this seed. However, its content in the tissue was hardly affected by treatment with the three inhibitors and by administration of exogenous ACC, suggesting that the conjugated ACC was not directly involved in ethylene production.     

The Synthesis of Ethylene in Melon Fruit during the Early Stage of Ripening

The levels of mRNA and polypeptide for a 1-aminocyclopropane-1-carboxylate(ACC) oxidase were studied to identify the tissues in whichthe synthesis of ethylene first occurs during the initial stageof ripening. RNA and immunoblot analysis showed that the levelsof the mRNA and polypeptide for ACC oxidase were very low inunripe fruit. They first became detectable in the placentaltissue at the pre-climacteric stage, and then their levels increasedin the mesocarp tissue during the climacteric increase in theproduction of ethylene. Two mRNAs for ACC synthase (transcribedfrom ME-ACS1 and ME-ACS2) were detected in the placental tissueand seeds at the pre-climacteric stage, but only the level ofME-ACS1 mRNA, which has been characterized as the mRNA for awound-inducible ACC synthase, increased in mesocarp, placentaltissues and seeds during ripening. The level of ME-ACS2 mRNAthat was isolated from etiolated seedlings of melon, did notchange markedly during ripening. These results suggest thatthe central region of melon fruit (placental tissue and seeds)plays a major role in the production of ethylene during theearly stage of ripening. ³These three authors made equal contribution to this study.     

Stimulation of ethylene production by hydroxamic acid, in particular, by benzohydroxamic acid

The effects on the ethylene production of known inhibitors ofa cyanide-insensitive, alternative respiration in plants wereinvestigated using cotyledonary segments of cocklebur (Xanthiumpennsylvanicum Wallr.) seeds. Benzohydroxamic acid (BHAM) at3 mM stimulated ethylene production 4- to 8-fold over the control,but respiration of the segments was hardly affected at thatconcentration. The stimulatory effects of 3-chlorobenzohydroxamicacid (CLAM) and salicylhydroxamic acid were far smaller thanthat of BHAM. BHAM at 3 mM also markedly stimulated the ethyleneformation in the epicotyl or hypocotyl sections of etiolatedpea (Pisum sativum L.) and mung bean (Vigna radiata [L.] Wilczek)seedlings. Moreover, 3 mM BHAM further promoted the increasedethylene formation which was caused by administration of 1-aminocyclopro-pane-1-carboxylicacid (ACC) to the cotyledonary segments. The promoting effectsby BHAM and CLAM were also found in the conversion of ACC intoethylene in pea stem homogenates. (Received July 26, 1980; )     

Stimulation of ethylene production by hydroxamic acid, in particular, by benzohydroxamic acid

The effects on the ethylene production of known inhibitors ofa cyanide-insensitive, alternative respiration in plants wereinvestigated using cotyledonary segments of cocklebur (Xanthiumpennsylvanicum Wallr.) seeds. Benzohydroxamic acid (BHAM) at3 mM stimulated ethylene production 4- to 8-fold over the control,but respiration of the segments was hardly affected at thatconcentration. The stimulatory effects of 3-chlorobenzohydroxamicacid (CLAM) and salicylhydroxamic acid were far smaller thanthat of BHAM. BHAM at 3 mM also markedly stimulated the ethyleneformation in the epicotyl or hypocotyl sections of etiolatedpea (Pisum sativum L.) and mung bean (Vigna radiata [L.] Wilczek)seedlings. Moreover, 3 mM BHAM further promoted the increasedethylene formation which was caused by administration of 1-aminocyclopro-pane-1-carboxylicacid (ACC) to the cotyledonary segments. The promoting effectsby BHAM and CLAM were also found in the conversion of ACC intoethylene in pea stem homogenates. (Received July 26, 1980; )     

Ethylene biosynthesis in oilseed rape pods in relation to pod shatter

Ethylene production was studied during the development and senescence of seeds and pericarp tissues of oilseed rape (Brassica napus L.) pods (siliquae). In the course of the rise to a pre-senescence climacteric, little change in 1-aminocyclopropane-1-carboxylic acid (ACC) was recorded in the seeds, indicating a rapid conversion to ethylene. In contrast, very small amounts of ethylene were produced by the pod wall (PW) tissues, which included the dehiscence zone (DZ), while levels of free and conjugated ACC in the PW increased consistently. As climacteric thylene production by the seeds declined, biosynthesis of ethylene by the PW increased. Effects of reducing ethylene production by various means were examined in relation to cell separation in the dehiscence zone. Aminoethoxyvinylglycine (AVG) applied during the pre-senescence climacteric reduced ACC levels and ethylene production by the seeds, but did not affect subsequent values in the PW. The production of -1,4-glucanase and the separation of the cells of the DZ were delayed for 3-4 d by AVG, but the force required to open fully mature pods was unaltered. In parthenocarpic (seedless) pods, ethylene was produced during senescence. Cell separation in the DZ took place as in seeded pods, although it was also delayed by 3-4 d. The results are related to changes in indole-3-acetic acid (IAA) levels in oilseed rape pods which decline in PW and DZ tissues during senescence. It is concluded that separation in the cells of the dehiscence zone requires only small amounts of ethylene to trigger the process when IAA levels are low.     

Regulation of ACC-Dependent Ethylene Production by Excised Leaves from Normal and Albino Zea mays L. Seedlings

Dunlap, J. R. 1988. Regulation of ACC-dependent ethylene productionby excised leaves from normal and albino Zea mays L. seedlings.—J.exp. Bot. 39: 1079–1089. Albino corn (Zea mays L.) seedlings lacking natural leaf pigmentswere obtained by germinating seeds treated with fluridone, aninhibitor of carotenoid biosynthesis. Basal rates of ethyleneproduction were less than 2.0 nl g^–1 fr. wt h^–1in both treated (albino) and untreated (normal) leaves but increasedby 10- to 20-fold in the presence of added ACC. ACC-dependentethylene production (ADEP) was inhibited by cobalt or cyanideions and stimulated by NaHCO₃, CO₂ and light. ADEP in both tissueswas stimulated by glucose, fructose, galactose and sucrose.The accumulation of respiratory CO₂ did not account for thecarbohydrate response. The decline in the ADEP characteristicof albino leaf tissue was slowed by incubation in the presenceof sucrose. IAA and ABA stimulated ADEP in normal leaves butinhibited ADEP in albino leaves. Sucrose-stimulated ADEP wasinhibited in albino leaf tissue treated with IAA or ABA indicatinga possible role for the chloroplast in carbohydrate-facilitatedADEP. However, results from this study suggest that chloroplastsperform a function in the regulation of ethylene productionby leaf tissue that extends beyond merely influencing internallevels of CO₂. In the absence of detectable ACC, EFE was responsiblefor the entire series of responses expressed in regulation ofethylene biosynthesis by corn seedling leaf tissue. Key words: Corn, ethylene, sugars, phytohormones     

Initiation of Rapid Ethylene Synthesis by Apple and Pear Fruits in Relation to Storage Temperature

The influence of storage temperature on the onset of rapid ethyleneproduction was investigated for fruits of Conference pear (Pyruscommunis L.) and five cultivars of apple (Malus domestica Borkh.).The time taken from harvest to rapid ethylene production wasshorter and more uniform at 3 ?C than at 18–20 ?C forConference pears and Golden Delicious apples. Increases in internalethylene concentration, 1-amino cyclopropane-1-carboxylic acidconcentration and ethylene production were simultaneous in GoldenDelicious apples at 3 ?C. When Golden Delicious apples wereheld at 3 ?C for 48 h and then kept at 20 ?C the mean time ofonset of ethylene production was similar to that for applesheld continuously at 20 ?C. However, two periods of 48 h at3 ?C caused earlier ethylene production. Conversely, ethyleneproduction at 3 ?C was delayed by transfer to 20 ?C for twoperiods of 48 h. Cox's Orange Pippin and other apple cultivarstended to show more synchronous ethylene production at 3 ?Cthan at higher temperatures but the mean time of onset was eitherunaffected by temperature or slighdy delayed at lower temperature.Acceleration of the onset of ethylene production by low temperaturewas never observed in Cox's Orange Pippin apples harvested atweekly intervals from 10 August to 17 September. Key words: Ethylene, Storage temperature, Pyrus communis, Malus domestica     

Differential Expression of Genes Involved in the Biosynthesis and Perception of Ethylene during Ripening of Passion Fruit (Passiflora edulis Sims)

An increase in the enzyme activity of 1-aminocyclopropane-1-carboxylicacid (ACC) synthase and ACC oxidase induces the evolution ofethylene during the ripening of passion fruit. A much higherlevel of ethylene is produced in arils than in seeds or peelsduring ripening. The pattern of expression of two ACC synthasegenes (PE-ACS1 and PE-ACS2), one ACC oxidase gene (PE-ACO1),and two ethylene receptor genes (PE-ETR1 and PE-ERS1) revealedthat the expression of these genes is differentially regulated.Expression of PE-ACS1 and PE-ACO1 was enhanced during ripeningand after ethylene treatment. However, prominent expressionof PE-ACS1 was delayed compared to that of PE-ACO1. Much largerquantities of PE-ACS1 mRNA and PE-ACO1 mRNA were seen in arilsthan in seeds; this corresponds well with an increase in theamount of ethylene produced by the plant tissue itself. Thelevel of PE-ACS2 mRNA was detectable in arils of the preclimactericfruit, although it decreased during ripening. These resultssuggest that expression of PE-ACS1 and PE-ACO1 is required toincrease the activity of ethylene biosynthetic enzymes duringripening. The level of expression of PE-ETR1 and PE-ERS1 didnot significantly change over the course of ripening; however,the mRNA levels of PE-ETR1 and PE-ERS1 were much higher in arilsthan in seeds. ⁴Present address: Center forMolecular Genetics Research, Shizuoka University, Shizuoka, 422-8529 Japan.