Enhancement of ethylene biosynthesis and germination with thidiazuron and some selected auxins in Striga asiatica seeds

Witchweed [ Striga asiatica (L.) Kuntze], an economically important parasitic weed on several poaceous crops, is difficult to control. In nature, germination and subsequent morphogenesis of Striga are cued to specific host-derived chemical signals. Seeds (approximately 2.4 mg) treated with thidiazuron (TDZ) or the auxins 2,4-dichlorophenoxy-acetic acid (2,4-D), 1-naphthalene acetic acid (NAA), or 2-(4-chloro- o -tolyloxy) propionic acid (MCPP) produced little ethylene (66-138 nl l⁻¹). Combinations of TDZ with the auxins increased ethylene production by 4- to 18-fold. Ethylene production was strongly inhibited (86–92%) by aminoethoxyvinylglycine (AVG), inhibitor of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase. Ethylene evolved from seeds treated with TDZ in combination with 2,4-D increased after a lag period and was promoted by a pretreatment in 2,4-D. TDZ or any of the auxins, at the rates tested, effected negligible to low levels of germination (0 to 16%), whereas mixtures of TDZ with the above auxins stimulated 38 to 84% germination. Test solutions containing TDZ and indole-3-acetic acid (IAA) were, however, less effective. TDZ/auxin-induced germination was inhibited by AVG and the ethylene action inhibitor silver thiosulfate (STS). The inhibitory effect of the former was reversed by treatment with ACC. In vitro studies revealed negligible germination (< 1%) on control medium. Seeds germinating on media containing TDZ alone developed into seedlings with distinct shoots and rudimentary roots. Seeds germinating on media containing 2,4-D, irrespective of TDZ concentration, were induced to form calli. The results are consistent with a model in which both germination and subsequent morphogenesis in Striga are associated with exogenous and endogenous phytohormones.     

Effects of Cytokinins on In Vitro Seed Germination and Early Seedling Morphogenesis in Lotus corniculatus L.

We determined the effects of zeatin (ZEA), isopentenyl adenine (2iP), kinetin (KIN), benzyladenine (BA), and thidiazuron (TDZ) on seed germination, elongation of seedling shoots and roots, frequency of regeneration, and the number of regenerants per seedling in Lotus corniculatus L. Sterilized seeds were cultured in vitro on Murashige and Skoog (1962) medium containing 3% sucrose, 0.7% agar, and various cytokinins (0, 0.08, 0.22, 0.35, 0.80, 2.20, and 3.50 μM). After 30 days, seedlings were transferred to cytokinin-free medium for another 60 days. All cytokinins stimulated the rate and percentage of seed germination at least twofold in optimum concentrations; TDZ and ZEA were the most active, followed closely by BA, whereas KIN and 2iP stimulated germination in higher concentrations only. Elongation of shoots and roots was strongly inhibited at the lowest TDZ and BA concentrations, whereas ZEA, KIN, and 2iP exerted moderate, dose-dependent inhibition. The frequency of regenerant-producing seeds was highest on ZEA and BA, whereas the greatest number of regenerants per seedling was found on TDZ. It is concluded that the culture of seeds on cytokinin-containing media, followed by transfer to cytokinin-free medium, is a suitable procedure for rapid production of a large number of uniform regenerants. The presumed role of particular cytokinins is discussed.     

Synergistic enhancement of ethylene production and germination with kinetin and 1-aminocyclopropane-1-carboxylic Acid in lettuce seeds exposed to salinity stress

Relief of salt (0.1 molar NaCl) stress on germination of lettuce (Lactuca sativa L., cv Mesa 659) seeds occurred with applications of 0.05 millimolar kinetin (KIN) and 1 to 10 millimolar 1-aminocyclopropane 1-carboxylic acid (ACC). Treatment with KIN enhanced the pregermination ethylene production under saline condition. A synergistic or an additive enhancement of pregermination ethylene production and germination occurred under saline condition in the presence of KIN and a saturating dose (10 millimolar) of ACC. No KIN-ACC synergism was noted in ethylene production or germination under nonsaline condition. Addition of 1 millimolar aminoethoxyvinylglycine (AVG) inhibited the KIN-enhanced pregermination ethylene production (85 to 89%) and germination (58%) under saline condition but not the synergistic effect of KIN + ACC on ethylene production. Under nonsaline condition, AVG had no effect on germination even though ethylene production was strongly inhibited. Alleviation of salt stress by KIN was inhibited in a competitive manner by 2,5-norbornadiene (NBD) (0.02-0.2 milliliter per liter), and the addition of ACC and/or ethylene reduced this inhibition. An increase in the pregermination ethylene production and germination occurred also by cotylenin E (CN) under saline condition. However, neither AVG (1 millimolar) nor NBD (0.02 to 0.2 milliliter per liter) prevented the relief of salt stress by CN. Thus, KIN may alleviate salt stress on germination by promoting both ACC production and its conversion to ethylene. Rapid utilization of ACC may be the basis for the synergistic or the additive effect of KIN plus ACC. The need for ethylene production and action for the relief of salt stress is circumvented by a treatment with CN.     

Conditioning period, CO2 and GR24 influence ethylene biosynthesis and germination of Striga hermonthica

Germination of witchweed ( Striga hermonthica [Del.] Benth), an important root parasite on poaceous crops, requires pretreatment 'conditioning' in a warm moist environment and a subsequent exposure to a stimulant. The roles of conditioning period, CO₂ and a strigol analogue (GR24) in ethylene biosynthesis and germination of the parasite were investigated. Conditioning increased the seeds' capacity to oxidize exogenous 1-aminocyclopropane-1-carboxylic acid (ACC). Exogenous CO₂ increased the seeds capacity to oxidize ACC by 3- to 9-fold. A combination of GR24 and ACC increased ethylene production by more than 3-fold in comparison with the rates obtained using these compounds separately. Aminoethoxyvinylglycine (AVG) completely inhibited ethylene induction by GR24, but not by ACC. A GR24 treatment, made subsequent to conditioning in GR24, did not induce ethylene. However, seeds conditioned in GR24 and then given 1 m M ACC produced 293 nl l⁻¹ ethylene. ACC oxidase (ACCO) activity in crude extracts was increased by conditioning and CO₂. The enzyme displayed an absolute requirement for ascorbate. Absence of exogenous Fe²⁺ reduced enzyme activity only by 14%. GR24 applied during conditioning reduced germination in response to a subsequent GR24 treatment. ACC was, invariably, less effective in inducing S. hermonthica germination than GR24 even at concentrations which induce more ethylene than concurrent GR24 treatments. The results are consistent with a model in which conditioning removes a restriction on the ethylene biosynthetic pathway in S. hermonthica seeds. GR24 modulates the key enzymes in ethylene biosynthesis. The stimulant suppresses ethylene biosynthesis in unconditioned seeds and promotes it in conditioned ones. Germination of S. hermonthica results from the joint action of GR24 and the ethylene it induces.     

Germination strategy of Striga hermonthica involves regulation of ethylene biosynthesis

Ethylene involvement in germination of Striga hermonthica (Del.) Benth., an important root parasitic weed on poaceous crops, was investigated at the physiological and molecular levels. Seeds, conditioned at 30°C for 14 days, were treated with ethylene, ethephon or 1-aminocyclopropane-1-carboxylic acid (ACC). Ethylene consistently induced low germination. Ethephon and ACC effectively stimulated germination at concentrations of 0.01 and 1 m M , respectively. In contrast to ethylene, both ethephon and ACC acted in a concentration-dependent manner. Germination induced by the synthetic strigolactone GR24 was inhibited by aminoethoxyvinylglycine (AVG) and 1-methylcyclopropene. ACC reversed the inhibition caused by AVG. When seeds were treated with GR24 in sealed vials, ethylene concentration in headspace gas increased prior to the onset of germination. Total RNA extracted from germinating seeds 12 h after GR24 treatment was used for PCR-based amplification of cDNA fragments encoding the ACC synthase- and oxidase-active site domains. Two distinct cDNA fragments encoding ACC synthase ( SHACS1 and SHACS2 ) and one encoding ACC oxidase ( SHACO1 ) were cloned and sequenced. Southern analysis suggested that each of the cloned genes was present as a single copy in the genome of S. hermonthica . Northern analyses showed that SHACS1 exhibited a temporal change in expression peaking at 10 h after GR24 treatment, which coincided with a steady increase in ethylene concentration. SHACS2 was expressed at a low level with a similar trend. SHACO1 exhibited a temporal change in expression peaking at 15 days during conditioning, when seed response to GR24 was maximal. In summary, expression of ACC synthase and ACC oxidase genes was found to be responsive to a germination stimulant and to conditioning, respectively. The implications of these findings with respect to germination of S. hermonthica under field conditions are discussed.     

Requirement for Ethylene Synthesis and Action during Relief of Thermoinhibition of Lettuce Seed Germination by Combinations of Gibberellic Acid, Kinetin, and Carbon Dioxide

Application of exogenous ethylene in combination with gibberellic acid (GA₃), kinetin (KIN), and/or CO₂ has been reported to induce germination of lettuce seeds at supraoptimal temperatures. However, it is not clear whether endogenous ethylene also plays a mediatory role when germination under these conditions is induced by treatment regimes that do not include ethylene. Therefore, possible involvement of endogenous ethylene during the relief of thermoinhibition of lettuce (Lactuca sativa L. cv Grand Rapids) seed germination at 32°C was investigated. Combinations of GA₃ (0.5 millimolar), KIN (0.05 millimolar), and CO₂ (10%) were used to induce germination. Little germination occurred in controls or upon treatment with ethylene, KIN, or CO₂. Neither KIN nor CO₂ affected the rate of ethylene production by seeds. Both germination and ethylene production were slightly promoted by GA₃. Treatments with GA₃+CO₂, GA₃+KIN, or GA₃+CO₂+KIN resulted in approximately 10-to 40-fold increases in ethylene production and 50 to 100% promotion of germination as compared to controls. Initial ethylene evolution from the treated seeds was greater than from the controls and a major surge in ethylene evolution occurred at the time of visible germination. Application of 1 millimolar 2-aminoethoxyvinyl glycine (AVG), an inhibitor of ethylene synthesis, in combination with any of above three treatments inhibited the ethylene production to below control levels. This was accompanied by a marked decline in germination percentage. Germination was also inhibited by 2,5-norbornadiene (0.25-2 milliliters per liter), a competitive inhibitor of ethylene action. Application of exogenous ethylene (1-100 microliters per liter) overcame the inhibitory effects of AVG and 2,5-norbornadiene on germination. The results demonstrate that endogenous ethylene synthesis and action are essential for the alleviation of thermoinhibition of lettuce seeds by combinations of GA₃, KIN, and CO₂. It also appears that these treatment combinations do not act exclusively via promotion of ethylene evolution as the application of exogenous ethylene alone did not promote germination.     

Kinetin Enhanced 1-Aminocyclopropane-1-Carboxylic Acid Utilization during Alleviation of High Temperatures Stress in Lettuce Seeds

The thermoinhibition at 35 and 32°C of pregermination ethylene production and germination in lettuce (Lactuca sativa L. cv Mesa 659) seeds was synergistically or additively alleviated by 0.05 millimolar kinetin (KIN) and 10 millimolar 1-aminocyclopropane-1-carboxylic acid (ACC). The synergistic effect of KIN + ACC on ethylene production and germination at 35°C was inhibited by Co²⁺ (44-46%) but not by aminoethoxyvinyl glycine (AVG). The uptake of ACC by the seed was not influenced by KIN. Upon slitting of the seed coats (composed of pericarp, testa and endosperm), following the uptake of chemicals, ACC was readily converted into ethylene at all temperatures, and the synergistic effects of KIN + ACC at 35°C were lost. At 35°C, KIN acted synergistically with ACC or ethephon (ETH) in alleviating the osmotic restraint. At 25°C, ETH was more active than KIN or KIN + ACC in overcoming the osmotic restraint. Thus, the integrity of the seed coats, the KIN-enhanced ACC utilization, and an interaction of KIN with the ethylene produced may be the basis for the synergistic or additive effects of KIN + ACC at high temperature.     

Cytokinin-induced ethylene biosynthesis in nonsenescing cotton leaves

The influence of cytokinins on ethylene production was examined using cotton leaf tissues. Treatment of intact cotton (Gossypium hirsutum L. cv LG 102) seedlings with both natural and synthetic cytokinins resulted in an increase in ethylene production by excised leaves. The effectiveness of the cytokinins tested was as follows: thidiazuron BA isopentyladenine ≥ zeatin kinetin. Using 100 micromolar thidiazuron (TDZ), an initial increase in ethylene production was observed 7 to 8 hours post-treatment, reached a maximum by 24 hours and then declined. Inhibitors of 1-aminocyclopropane-1-carboxylic acid (ACC) synthesis and its oxidation to ethylene reduced ethylene production 24 hours post-treatment; however, by 48 hours only inhibitors of ACC oxidation were effective. The increase in ethylene production was accompanied by a massive accumulation of ACC and its acid-labile conjugate. TDZ treatment resulted in a significant increase in the capacity of tissues to oxidize ACC to ethylene. Endogenous levels of methionine remained constant following TDZ treatment. It was concluded that the stimulation of ethylene production in cotton leaves following cytokinin treatment was the result of an increase in both the formation and oxidation of ACC.     

Ethylene biosynthesis and strigol-induced germination of Striga asiatica

Germination of witchweed [ Striga asiatica (L.) Kuntze], an important parasite on cereal crops, is stimulated by several natural and synthetic compounds. In the present study the role of ethylene in germination of Striga asiatica in response to strigol was examined. Unconditioned seeds and those conditioned for 3 days produced negligible amounts of ethylene in response to strigol. However, extending the conditioning period to 5 and 8 days increased ethylene evolution by more than 10-fold. Ethylene production preceded radicle protrusion and was detectable within 3h after treatment. No germination was observed in the first 6 h of exposure to strigol. Germination and ethylene production increased with strigol concentration. Strigol-induced germination was considerably reduced by the ethylene action inhibitors. 2. 5-norbornadiene, silver thiosulphate and CO₂. The ethylene precursor 1-aminocyclopropane-1-carboxyac acid (ACC) at 5 to 200 μ M elicited neither germination nor ethylene production. However, a combination of strigol and ACC resulted in a high germination rate and copious ethylene production. Both germination and ethylene production were reduced by CoCl₂ and cyclobeximide, inhibitors of the ethylene-forming enzyme and of protein synthesis, respectively. The results are consistent with a model in which conditioning and strigol are required to remove a restriction on the ethylene biosynthetic pathway and in which the ethylene-forming enzyme is rate limiting.     

Inhibition of Amaranthus caudatus seed germination by polyethylene glycol-6000 and abscisic acid and its reversal by ethephon or 1-aminocyclopropane-1-carboxylic acid

The effect of polyethylene glycol (PEG-6000), abscisic acid (ABA), 2-chloroethylphosphonic acid (ethephon) and 1-aminocyclopropane-1-carboxylic acid (ACC) on the germination of Amaranthus caudatus L. seeds was examined. Both PEG-6000 and ABA inhibited the rate and percentage of seed germination. ABA potentiated the effect of PEG. Ethephon was highly effective in reversing the inhibitory effect of PEG and ABA or combinations of both. ACC relieved inhibition by ABA and the combined effect of ABA and PEG. Aminoethoxyvinylglycine (AVG) increased the inhibition of seed germination caused by ABA. The inhibition of seed germination by ABA seems to be related more or less to ethylene biosynthesis or is associated with a change of tissue sensitivity to ethylene. The possibility of ethylene control of water uptake by seeds is also considered.     

Ethylene in seed dormancy and germination

The role of ethylene in the release of primary and secondary dormancy and the germination of non-dormant seeds under normal and stressed conditions is considered. In many species, exogenous ethylene, or ethephon – an ethylene-releasing compound - stimulates seed germination that may be inhibited because of embryo or coat dormancy, adverse environmental conditions or inhibitors (e.g. abscisic acid, jasmonate). Ethylene can either act alone, or synergistically or additively with other factors. The immediate precursor of ethylene biosynthesis, 1-aminocyclopropane-1-carboxylic acid (ACC), may also improve seed germination, but usually less effectively. Dormant or non-dormant inhibited seeds have a lower ethylene production ability, and ACC and ACC oxidase activity than non-dormant, uninhibited seeds. Aminoethoxyvinyl-glycine (AVG) partially or markedly inhibits ethylene biosynthesis in dormant or non-dormant seeds, but does not affect seed germination. Ethylene binding is required in seeds of many species for dormancy release or germination under optimal or adverse conditions. There are examples where induction of seed germination by some stimulators requires ethylene action. However, the mechanism of ethylene action is almost unknown.
The evidence presented here shows that ethylene performs a relatively vital role in dormancy release and seed germination of most plant species studied.     

The influence of cytokinins on apple embryo photosensitivity and acid phosphatase activity during stratification

Kinetin (KIN) and benzyladenine (BA) stimulate to different extent the germination of apple embryos isolated from dormant seeds or seeds submitted to stratification. KIN is much more active in the replacement of light requirement in apple embryos germination. Both cytokinins decrease the photosensitivity of embryos isolated from the seeds stratified less than one month, but only BA accelerates the appearance of the second photosensitivity maximum, normally occuring on the 70th day of stratification. Both cytokinins stimulate the activity of acid phosphatase between the 30th and 50th day of apple seed stratification. The stimulation between the 50th day and the end of stratification is exerted only by KIN. These differences allow to discuss the specificity of action of particular cytokinins during the after-ripening and germination of apple embryos.     

Requirement for the action of endogenous ethylene during germination of non-dormant seeds of Amaranthus caudatus

The role of endogenous ethylene during germination of non-dormant seeds of Amaranthus caudatus L. was investigated. The seeds readily germinated in water and darkness at 24°C. Application of ethylene or of its precursor I-aminocyclopropane-I-carboxylic acid (ACC) slightly increased the rate of germination. Both compounds effectively antagonized osmotic inhibition by polyethyleneglycol. Application of aminoethoxyvinylglycine (AVG) reduced ethylene production by 90% but did not inhibit germination. However, germination was inhibited by 2,5-norbornadiene, a competitive inhibitor of ethylene action. This inhibition was counteracted by ethylene, ethephon or ACC and enforced by AVG. It is concluded that the action of endogenous ethylene is an indispensable factor during germination of non-dormant seeds of A. caudatus. Ethylene action is required from the start of imbibition on. In water, low levels of endogenous ethylene are sufficient for this action. PEG increased the ethylene requirement considerably.     

Implication of Ethylene in the Release of Secondary Dormancy in Amaranthus caudatus L. Seeds by Gibberellins or Cytokinin

Ethephon (Eth), gibberellin A₃, A_{4 + 7} (GA₃, GA_{4 + 7}), and 6-benzyladenine (BA) removed secondary dormancy of Amaranthus caudatus seeds. The GAs and BA potentiated the effect of ethephon or 1-aminocyclopropane-1-carboxylic acid (ACC), an ethylene biosynthesis precursor, in terms of the rate or final percent of germination. Aminoethoxyvinylglycine (AVG), an ACC synthase activity inhibitor, was observed to simultaneously inhibit the release from dormancy effected by GA₃ or BA as well as the ethylene production stimulated by these regulators. Breaking of secondary dormancy by GA₃, GA_{4 + 7} or BA was prevented by 2,5-norbornadiene (NBD), an inhibitor of ethylene binding. Ethylene completely or markedly reversed the inhibitory effect of NBD. We thus conclude that the removal of secondary dormancy in Amaranthus caudatus seeds by gibberellin or benzyladenine involves ethylene biosynthesis and action.     

Action of ferric and aluminium ions on the dormancy breakage of <Emphasis Type="Italic">Stylosanthes humilis</Emphasis> seeds

Dormancy of scarified seeds of Stylosanthes humilis was broken by acidic Al³⁺ and Fe³⁺ solutions. Fe⁺³-stimulated seeds exhibited a high activity of 1-aminocyclopropane-1-carboxylate (ACC) oxidase and produced great amounts of ethylene, which showed correlated with the germination process. In addition, specific inhibitors of ethylene biosynthesis and action largely depressed the Fe³⁺-stimulated germination, leading to the conclusion that the ion broke dormancy by triggering ethylene production by the seeds. By contrast, inhibitors of ethylene biosynthesis and action did not impair germination of Al³⁺-stimulated dormant seeds. Moreover, ethylene production and activity of ACC oxidase of Al³⁺-treated seeds was substantially decreased by inhibitors of ethylene biosynthesis, but germination kept large. Together these data suggest that ethylene biosynthesis was not required in the chain of events triggered by Al³⁺ leading to dormancy breakage. Methyl viologen (MV), a reactive oxygen species-generating compound, broke dormancy of seeds to the same extent as Al³⁺ did. Germination of both Al³⁺- and MV-stimulated dormant seeds was inhibited by sodium selenate, an antioxidant compound; selenate, however had no effect on germination of Fe³⁺-stimulated seeds. Together these data indicate that the mechanisms underlying the germination of Al³⁺- and Fe³⁺-treated seeds are not the same.     

Ethylene biosynthesis in Amaranthus caudatus seeds in response to methyl jasmonate

Methyl jasmonate (JA-Me) at 10^–3 M completely inhibited Amaranthus caudatus seed germination. Exogenous ethylene could totally reverse this inhibition. The inhibitor of ethylene action, 2,5-norbornadiene (NBD), increased the sensitivity of seeds to JA-Me. Methyl jasmonate inhibited ethylene production and also decreased both 1-aminocyclopropane-1-carboxylic acid (ACC) and malonyl ACC (MACC) content. Likewise, ACC oxidase activity in vivo was decreased by jasmonate. Similarly ACC oxidase activity in vitro isolated from seeds incubated in the presence of JA-Me was lower than that isolated from untreated seeds.The inhibitory JA-Me action on seed germination seems to be mainly associated with the inhibition of ethylene biosynthesis. Both inhibition of ACC synthase and ACC oxidase activity and/or synthesis can be involved.     

Ethylene-dependent action of gibberellin in seed germination of Amaranthus caudatus

The role of gibberellins in the germination of seeds of Amaranthus caudatus L. was examined. Tetcyclacis (BAS 106), an inhibitor of gibberellin biosynthesis, inhibited germination of the seeds. The inhibition caused by BAS 106 was antagonised by gibberellin A₄₊₇ (GA₄₊₇). Ethephon (2-chloroethylphosphonic acid) and 1-aminocyclopropane-1-carboxylic acid (ACC) could replace GA₄₊₇. Ethephon and ACC counteracted also the side effects of BAS 106 that are not reversible by GA₄₊₇. The rate of seed germination was not increased by gibberellin in the presence of aminoethoxyvinylglycine (AVG). AVG increased the effect of BAS 106. GA₄₊₇ could not reverse the effect of BAS 106 when AVG was simultaneously applied. The results indicate that the biosynthesis of endogenous gibberellins may be required for germination of A. caudatus seeds and that main physiological effects of GA₄₊₇ on seed germination may depend on ethylene biosynthesis.     

Endogenous ethylene and reversing methyl jasmonate inhibition of Amaranthus caudatus seed germination by benzyladenine or gibberellin

BA at 10^–5 M, GA₃ at 3×10^–4 M or GA₄₊₇ at 3×10^–5 M partially or largely reversed the inhibition of Amaranthus caudatus seed germination due to JA-Me. BA or GA₃ did not affect ethylene production and ACC oxidase activity in vivo in the presence of JA-Me before radicle protrusion. However, both increased ethylene production after 72 h of incubation, when the reversal of the JA-Me inhibition of seed germination was observed. AVG at 3×10^–4 M decreased ethylene production when it was applied simultaneously with BA and JA-Me or GA₃ and JA-Me, but it had no effect on seed germination. NBD almost completely reversed the stimulatory effect of BA, GA₃ or GA₄₊₇ on the germination of seeds in the presence of JA-Me. Exogenous ethylene reversed the inhibitory effect of NBD. The results indicate that action of endogenous ethylene is involved in the response of JA-Me inhibited seeds to BA or GA_s.     

Regulation of Auxin-induced Ethylene Biosynthesis. Repression of Inductive Formation of 1-Aminocyclopropane-1-carboxylate Synthase by Ethylene

Changes in the 1-aminocyclopropane-1-carboxylate (ACC) synthaseactivity which regulates auxin-induced ethylene production werestudied in etiolated mung bean hypocotyl segments. Increasesboth in ethylene production and ACC synthase activity in tissuetreated with IAA and BA were severely inhibited by cycloheximide(CHI), 2-(4-methyl-2,6-dinitroanilino)-N-methylpropionamide,actinomycin D and -amanitin. Aminoethoxyvinylglycine (AVG),a potent inhibitor of the ACC synthase reaction, increased theactivity of the enzyme in the tissue 3- to 4-fold. This stimulationalso was severely inhibited by the above inhibitors. Stimulationof the increase in the enzyme content by AVG was partially suppressedby an exogenous supply of ACC or ethylene. Suppression of theincrease in the enzyme took place with 0.3 µl/liter ethylene,and inhibition was increased to 10 µl/liter, which caused65% suppression. Air-flow incubation of the AVG-treated tissue,which greatly decreased the ethylene concentration surroundingthe tissue, further increased the amount of enzyme. Thus, oneeffect of AVG is to decrease the ethylene concentration insidethe tissue. The apparent half life of ACC synthase activity,measured by the administration of CHI, was estimated as about25 min. AVG lengthened the half life of the activity about 2-fold.Feedback repression by ethylene in the biosynthetic pathwayof auxin-induced ethylene is discussed in relation to the effectof AVG. (Received January 22, 1982; Accepted March 26, 1982)