ACC Synthase Activity Research Articles

Expression of Ethylene Biosynthetic Pathway Transcripts in Senescing Carnation Flowers

We have examined the expression of mRNAs for S-adenosylmethionine synthetase (EC 2.5.1.6), 1-aminocyclopropane-1-carboxylate (ACC) synthase (EC 4.4.1.14), and the ethylene-forming enzyme (EFE) in various floral organs of carnation (Dianthus caryophyllus) during the increase in ethylene biosynthesis associated with petal senescence. The abundance of ACC synthase and EFE mRNAs increased and S-adenosylmethionine synthetase transcripts decreased concomitantly with the ethylene climacteric in senescing petals. The increase in abundance of ACC synthase and EFE mRNAs in aging flowers was prevented by treatment with the ethylene action inhibitor 2,5-norbornadiene. Furthermore, an increase in ACC synthase and EFE transcripts was detected in petals from presenescent flowers within 3 to 6 hours of exposure to 2 microliters per liter of ethylene. The increase in ethylene production by senescing petals was associated with a concomitant increase in ethylene biosynthesis in styles, ovary, and receptacle tissues. In all tissues, this increase was associated with increased activities of ACC synthase and EFE. The increase in EFE activities by all floral organs examined was correlated with increased abundance of EFE transcripts. In contrast, the level of ACC synthase mRNA, as detected by the cDNA probe pCARACC3, did not always reflect enzyme activity. The combined tissues of the pistil exhibited high rates of ACC synthase activity but contained low levels of ACC synthase mRNAs homologous to pCARACC3. In addition, pollinated styles exhibited a rapid increase in ethylene production and ACC synthase activity but did not accumulate detectable levels of ACC synthase mRNA until several hours after the initiation of ethylene production. These results suggest that transcripts for ACC synthase leading to the early postpollination increase in ACC synthase activity and ethylene production are substantially different from the mRNA for the ethylene-responsive gene represented by pCARACC3.

ROLES AND INTERACTIONS OF ETHYLENE AND POLYAMINES DURING THE SENESCENCE OF HIBISCUS(HIBISCUS SYRIACUS CV.YEONGGWANG) FLOWER.

During the aging of H. syriacus flower, biosynthetic pathways of ethylene and polyamines, their interactions, and their effects on senescence were investigated. The evolution of ethylene in ephemeral flower was rapidly increased immediately after initiation of in-rolling of corolla at which EFE activity became maximum peak. After that, EFE activity was gradually decreased even though the aging was continued. Ethylene production was, however, slightly inhibited by the treatments of AOA and putrescine. The activity of ACC synthase and SAM decarboxylase were most rapidly increased at the time of 36th hrs. The contents of ACC and MACC were gradually increased from the early stage. However, ACC contents was decreased at the final stage but MACC was continuously increased. In normal condition, endogenous level of polyamines exists in the order of putrescine>spermidine>spermine. Putrescine was reduced from the initial point of aging, but spermidine and spermine were reduced from the middle and final stage of aging, respectively.

THE EFFECT OF POLYAMINES ON THE SENESCENCE OF CARNATION PETALS.

The interaction between polyamines and ethylene is still not clear for floral tissues. The aim of the present paper is to examine the senescence on the isolated petals of carnation (Dianthus caryophyllus cv. Desio) but not the whole flower in an attempt to clarify the exact role of polyamines. Petals were treated with putrescine (Put; 0.0, 1.0, 10mM), spermidine (Spd; 0.0, 1.0, 10mM), spermine (Spn; 0.0, 1.0, 10mM), Put+Spd (1.0mM), Put+Spn (1.0mM). The fresh weight of petals in all 10mM treatment was decreased less than those in the other treatments at all times but there were no significant differences. The differences in ethylene production were significant. In petals maintained in 10mM of polyamines, ethylene production was completely inhibited until 13 days and senescence was considerably retarded. However, ethylene productions in 1.0mM polyamines treatments were delayed 2-3 days with reduced amounts. These results suggest that high concentrations of polyamines retard senescence and completely inhibit ethylene production. ACC content, activities of ACC synthase and SAM decarboxylase were analyzed. Finally, the role of SAM in ethylene and polyamines biosynthesis will be discussed.

ENHANCED SYNTHESIS AND ACCUMULATION OF ACC AND MACC DURING RELIEF OF THERMOINHIBITION WITH KINETIN IN LETTUCE SEED

The level of 1-aminocyclopropane-1-carboxylic acid (ACC) was 0.55 nmol.g-1 in dry lettuce (Lactuca sativa cv. Emperor) seeds. After 4h soak at 25°, 35° and 35°C+ KIN (kinetin, 50μM), the levels were 0, 0.2 and 1.14 nmol.g-1 seeds, respectively. The level of ACC was higher at 35°+KIN than at 35°C for up to 16h soak. No ACC was detectable at 25°C during 2 to 16h soak. In the presence of 50μM ABA, ACC level decreased to 0.2 nmol.g-1 at 4h soak and to zero level during 8 to 16h soak. The level of l-(malonylamino) cyclopropane-1-carboxylic acid (MACC), in dry seeds was 14 nmol.g-1. Exposure to 35°C in the presence or absence of KIN increased the level to 40-42 nmol.g-1 within 2h soaking, while at 25° only a slight increase (23 nmol.g-1) occurred. As in the case of ACC, the level of MACC was higher at 35°C+ KIN than at 35° or 25° for up to 16h soak.When seeds were soaked in ABA, the pattern of MACC produced was similar to that produced at 35°C. The results indicate that ACC synthase activity is enhanced by the addition of KIN at 35°C resulting in increased synthesis and/or accumulation of ACC and MACC. The relationship of ethylene biosynthesis to changes during stress imposition and alleviation by various factors will be discussed.

Metabolism of 1-Aminocyclopropane-1-Carboxylic Acid in Etiolated Maize Seedlings Grown under Mechanical Impedance

We investigated the metabolism of 1-aminocyclopropane-1-carboxylic acid (ACC) in etiolated maize (Zea mays L.) seedlings subjected to mechanical impedance by applying pressure to the growing medium. Total concentrations of ACC varied little in unimpeded seedlings, but impeded organs accumulated ACC. Roots had consistently higher concentrations of ACC than shoots or seeds, regardless of treatment. The concentration of ACC in the roots increased more than 100% during the first hour of treatment irrespective of the pressure applied; in shoots, total ACC concentration increased 46% at either low or high pressure during the first hour of treatment. The bulk of ACC synthesized under impeded and unimpeded conditions was present in a conjugated form, presumably, 1-(malonylamino)-cyclopropane-1-carboxylic acid. However, 1-(malonylamino)-cyclopropane-1-carboxylic acid increased 73% over controls after 10 hours at 25 kilopascals of pressure. Unimpeded tissue had about 77% ACC as the conjugate and 17% as free ACC, and less than 6% was used in ethylene production. Increased amounts of ACC were converted into ethylene under stress. In vivo ACC synthase activity in roots became six and seven times higher only 1 hour after initiation of treatment at 25 and 100 kilopascals of pressure, respectively, and remained high for at least 6 hours. However, the immediate and massive conjugation of mechanically induced ACC suggests that ACC N-malonyltransferase may play an important role in the regulation of mechanically induced ethylene production. After 8 hours, in vivo activity of the ethylene-forming enzyme complex increased 100 and 50% above normal level at 100 and 25 kilopascals, respectively. Furthermore, ethylene-forming enzyme complex activity was significantly greater at 100 kilopascals than in controls as early as 1 hour after treatment initiation. These data suggest that regulation of ethylene production under mechanical impedance involves the concerted action of ACC synthase, the ethylene-forming enzyme complex, and ACC N-malonyltransferase.

Development of ACC oxidase activity during maturation and ripening of ‘Paulared’, ‘Empire’ and ‘Law Rome’ apples

Abstract The development of ACC oxidase activity was investigated in three cultivars of apple ( Malus domestica Borkh.), ‘Paulared’, ‘Empire’ and ‘Law Rome’, as typifying early, mid-season and late cultivars, respectively, during maturation and ripening. Fruits were selected at progressive stages of development of the autogenous ethylene climacteric during maturation and ripening on the tree. The level of 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene was used as a relative measure of the activity of ACC synthase and the ability of fruit tissue to convert ACC to ethylene was used as a measure of the activity of ACC oxidase, the ethylene forming enzyme. ‘Paulared’ fruits accumulated high levels of ACC as the autogenous ethylene climacteric began and this was associated with slow development of ACC oxidase activity. ‘Law Rome’ apples showed ACC oxidase activity early in the development of the ethylene climacteric but this was associated with accumulation of low levels of ACC in fruits. ‘Empire’ fruits were found to be intermediate between the early and late cultivars. ACC oxidase activity of fruits of the three cultivars was found to increase concurrently with or subsequent to the accumulation of ethylene to ca. 0.1 μl 1 −1 in the internal atmosphere of the fruit. Thus, the ACC content of the fruit was found to be a function of the development of ACC synthase activity as well as the relative activity of ACC oxidase. The levels of these activities were cultivar dependent. Differential sequestering of ACC into malonyl ACC by malonyl Co A-ACC transferase among the three cultivars may also account for the wide disparity in changes in ACC content with maturation but this was not investigated.

Accumulation of wound-inducible ACC synthase transcript in tomato fruit is inhibited by salicylic acid and polyamines.

Regulation of wound-inducible 1-aminocyclopropane-1-carboxylic acid (ACC) synthase expression was studied in tomato fruit (Lycopersicon esculentum cv. Pik-Red). A 70 base oligonucleotide probe homologous to published ACC synthase cDNA sequences was successfully used to identify and analyze regulation of a wound-inducible transcript. The 1.8 kb ACC synthase transcript increased upon wounding the fruit as well as during fruit ripening. Salicylic acid, an inhibitor of wound-responsive genes in tomato, inhibited the wound-induced accumulation of the ACC synthase transcript. Further, polyamines (putrescine, spermidine and spermine) that have anti-senescence properties and have been shown to inhibit the development of ACC synthase activity, inhibited the accumulation of the wound-inducible ACC synthase transcript. The inhibition by spermine was greater than that caused by putrescine or spermidine. The transcript level of a wound-repressible glycine-rich protein gene and that of the constitutively expressed rRNA were not affected as markedly by either salicylic acid or polyamines. These data suggest that salicylic acid and polyamines may specifically regulate ethylene biosynthesis at the level of ACC synthase transcript accumulation.

Effects of Low O2 Root Stress on Ethylene Biosynthesis in Tomato Plants (Lycopersicon esculentum Mill cv Heinz 1350)

Low O(2) conditions were obtained by flowing N(2) through the solution in which the tomato plants (Lycopersicon esculentum Mill cv Heinz 1350) were growing. Time course experiments revealed that low O(2) treatments stimulated 1-aminocyclopropane-1-carboxylate (ACC) synthase production in the roots and leaves. After the initiation of low O(2) conditions, ACC synthase activity and ACC content in the roots increased and reached a peak after 12 and 20 hours, respectively. The conversion of ACC to ethylene in the roots was inhibited by low levels of O(2), and ACC was apparently transported to the leaves where it was converted to ethylene. ACC synthase activity in the leaves was also stimulated by low O(2) treatment to the roots, reaching a peak after 24 hours. ACC synthase levels were enhanced by cobalt chloride and aminooxyacetic acid (AOA), although they inhibited ethylene production. Cobalt chloride enhanced ACC synthase only in combination with low O(2) conditions in the roots. Under aeration, AOA stimulated ACC synthase activity in both the roots and leaves. However, in combination with low O(2) conditions, AOA caused a stimulation in ACC synthase activity in the leaves and no effect in the roots.

Ethylene Biosynthesis and Polyamine Accumulation in Apples with Watercore

Ethylene biosynthesis and polyamine content were determined in normal and watercore-affected apple (Malus domestics Borkh. cv. Delicious). Fruit with watercore produced more ethylene and contained higher amounts of putrescine (PUT), spermidine (SPD), 1-aminocyclopropane-1-carboxylic acid (ACC), and 1-(malonylamino) cyclo-propane-1-carboxylic acid (MACC). The activities of ACC synthase and ethylene-forming enzyme (EFE) in watercore-affected fruit were also higher than in normal fruit. The EFE activity in severely affected flesh was inhibited, resulting in ACC accumulation and low ethylene production. S-adenosylmethionine (AdoMet) was maintained at a steady-state level even when C2 H4 and polyamides were actively synthesized in normal and affected fruit.

Relationship between Stress Ethylene Production Induced by Gamma Irradiation and Ripening of Cherry Tomatoes

Changes in ACC metabolism induced by gamma irradiation have been studied during ripening of cherry tomatoes (Lycopersicon esculentum L. cv. Sweet 100) treated at the mature-green stage. Irradiation caused a sharp and transient dose-dependent increase in ethylene production during the first 24 hours that was associated with an increase in ACC synthase activity. The activity of ethylene-forming enzyme (EFE) was also stimulated but was never limiting. Nine days following irradiation, ACC metabolism was more active in irradiated fruits, with ACC being mainly directed to ethylene [80% at 3 kilogray (kGy; 1 Gray = 100 Rad = 1 J·kg-1)] rather than to malonyl ACC (MACC). As a consequence, fruit ripening was accelerated. For doses <1 kGy, the time required for 50% of the fruits to reach breaker stage (the onset of climacteric ethylene production) was inversely correlated with radiation dose and the amount of stress ethylene produced during the first 24 hours. At doses >1 kGy, in spite of a continuous stimulation of stress ethylene production, no additional acceleration of ripening occurred. At 3- to 5-kGv doses. fruit ripening was impaired transiently with a fast subsequent recovery. As a result a significant synchronizȧtioṅ 'ȯḟ fruit ripening (presumed to be caused by enhanced ethylene production) was observed. Chemical, names use: l-aminocyclopropane-l-carboxylic acid (ACC).

Polyamines Promote the Biosynthesis of Ethylene in Detached Rice Leaves

The effects of polyamines (putrescine, spermidine, spermine and diaminopropane) on the production of ethylene in detached rice leaves were investigated. Polyamines effectively promoted the production of ethylene in detached rice leaves under both light and dark conditions. Putrescine stimulated the production of ethylene within 4 hours of its application, a result suggests that putrescine enhances the production of ethylene directly. Putrescine also stimulated the production of ethylene in detached leaves that had been aged for 2 and 4 days. The stimulatory effect of putrescine resulted from the enhancement of the synthesis of 1-aminocyclopropane-lcarboxylic acid (ACC) and the conversion of ACC to ethylene. • The activity of S-adenosylmethionine decarboxylase in segments of rice leaves was inhibited by the application of putrescine. Thus, the enhancement of the synthesis of ACC by putrescine seems to be mediated by increases in the activity of ACC synthase and in the level of the substrate (S-adenosylmethionine) for ACC synthase.

The effect on Cd contents and ethylene biosynthesis in tomato plants of adding cadmium sulphate to soil

An experiment was carried out with increasing concentrations of Cd in soil on tomato (Lycopersicon esculentum Mill.) plants. The Cd contents of soil and in tomato roots, leaves and fruit have been determined during the growing cycle. The effect of Cd on ethylene production has been investigated by monitoring hormone biosynthesis in whole fruit during different ripening stages. Furthermore, 1-Aminocyclopropane-1-carboxylic acid (ACC) content, ACC synthase activity in vitro and Ethylene-Forming Enzyme (EFE) activity in vivo have been studied after incubation of fruit discs at different Cd concentrations. A clear relationship existed between Cd in the soil and its concentration in the leaves. Roots showed the highest Cd concentration, whereas fruit did not accumulate the element. Ethylene evolution of whole fruit was not affected by the Cd treatment. High Cd concentration (80 ug mL−1) induced a slight increase of EFE activity in fruit discs.

Regulation of Ethylene Biosynthesis in Gravistimulated Kniphofia (Hybrid) Flower Stalks

Summary Gravistimulation of cut flowering stems of Kniphofia (hybrid) causes rapid upward bending due to a decreased growth rate at the upper half and an increased growth rate at the lower half of the flower stalk. Within a few hours of gravistimulation, the level of 1-aminocyclopropane-1-carboxylic acid (ACC) and the ethylene production in the lower half of the stem increased, apparently due to a rapid increase in ACC-synthase activity. Later, the level of N-malonyl-ACC also increased in the lower half of the stalk. Ethylene, ACC and MACC levels in the upper half of the gravistimulated stem did not change during the experimental period. The activity of the ethylene-forming enzyme was similar in all stem sections. The asymmetric production of ethylene in gravistimulated stems resulted in seven times higher internal ethylene levels in the lower half of the stem. Treatment of the stems with ethylene or inhibitors of ethylene synthesis (aminooxy-acetic acid) and of ethylene action (silver thiosulphate, CO 2 , 2,5 norbornadiene) inhibited elongation growth in vertical controls but had no appreciable effect on the response to gravity. The results therefore indicate that endogenous ethylene plays no pivotal role in flower stalk gravitropism.

Role of Ethylene on de Novo Shoot Regeneration from Cotyledonary Explants of Brassica campestris ssp. pekinensis (Lour) Olsson in Vitro

The promotive effect of AgNO(3) and aminoethoxyvinylglycine (AVG) on in vitro shoot regeneration from cotyledons of Brassica campestris ssp. pekinensis in relation to endogenous 1-amino-cyclopropane-1-carboxylic acid (ACC) synthase, ACC, and ethylene production was investigated. AgNO(3) enhanced ACC synthase activity and ACC accumulation, which reached a maximum after 3 to 7 days of culture. ACC accumulation was concomitant with increased emanation of ethylene which peaked after 14 days. In contrast, AVG was inhibitory to endogenous ACC synthase activity and reduced ACC and ethylene production. The promotive effect of AVG on shoot regeneration was reversed by 2-chloroethylphosphonic acid at 50 micromolar or higher concentrations, whereas explants grown on AgNO(3) medium were less affected by 2-chloroethylphosphonic acid. The distinctive effect of AgNO(3) and AVG on endogenous ACC synthase, ACC, and ethylene production and its possible mechanisms are discussed.

Ethylene Biosynthesis during Peach Fruit Development

Ethylene evolution and ACC levels were determined throughout the growth and development of peach fruit (Prunus persica L. Batsch cv. Redhaven). In the four stages of growth (I, II, III, IV), as indicated by weekly monitoring of fresh (FW) and dry (DW) weight accumulation, ethylene biosynthesis in whole fruit decreased during FWI and remained almost undetectable during FWII and FWIII. In pericarp disks, ethylene evolution followed the same trend, although a peak at 78 days after full bloom and a slight increase before the onset of the climacteric were observed. The high rates of ethylene evolution were associated with a concurrent increase in ACC content. Enhancement of ACC synthase and ethylene-forming enzyme (EFE) activities was responsible for the peak of ethylene evolution detected before the beginning of FWIII and DWIII. At the climacteric, which occurred at the FWIII-FWIV transition, sequential events were observed in different fruit tissues. An increase of ethylene production in the mesocarp preceded the onset of the climacteric rise in whole fruit. The high amount of ethylene detected during the climacteric appeared to be related to increased EFE activity in the epicarp. Chemical name used: 1-aminocyclopropane-1-carboxylic acid (ACC).

Cloning and sequence of two different cDNAs encoding 1-aminocyclopropane-1-carboxylate synthase in tomato.

1-Aminocyclopropane-1-carboxylate synthase (ACC synthase; S-adenosyl-L-methionine methylthioadenosine-lyase, EC 4.4.1.14), the key enzyme in ethylene biosynthesis, was purified 5000-fold from induced tomato pericarp. ACC synthase activity was unambiguously correlated with a 45-kDa protein by two independent methods. Peptide sequences were obtained both from the N terminus after electroblotting and from tryptic peptides separated by reversed-phase chromatography. Mixed oligonucleotide probes were used to screen a lambda gt11 library prepared from RNA of induced pericarp tissue. Putative ACC synthase clones were isolated with a frequency of 0.01%. One of these contained a 1.9-kilobase insert with a single open reading frame encoding a polypeptide of 55 kDa. A second, partial cDNA clone was found that differed from the first one in 18% of its bases. Genomic Southern blotting suggests possible tandem organization of the two genes in tomato. The entire coding region was expressed in Escherichia coli and the denatured recombinant polypeptide was used to raise polyclonal antibodies. The antibody preparation both immunoinhibits and immunoprecipitates ACC synthase activity from an enriched tomato extract, confirming the identity of the clone. Northern blot analysis demonstrates that the ACC synthase messenger accumulation is coordinated with fruit ripening.

Short-Term Effects of γ-Irradiation on 1-Aminocyclopropane-1-Carboxylic Acid Metabolism in Early Climacteric Cherry Tomatoes

gamma-Irradiation of early climacteric (breaker) cherry tomatoes (Lycopersicon pimpinellifollium L.) caused a sharp burst in ethylene production during the first hour. The extent of ethylene production was dose dependent and was maximum at about 3 kilograys. The content of 1-aminocyclopropane-1-carboxylic acid (ACC), followed the same evolution as ethylene production, while malonyl ACC increased steadily with time in irradiated fruits. The burst in ethylene production was accompanied by a sharp stimulation of ACC synthase activity which began 15 minutes after irradiation. The stimulation was completely prevented by cycloheximide, but not by actinomycin d or cordycepin. In contrast with irradiation, mechanical wounding continuously stimulated ethylene production over several hours. gamma-Irradiation and cordycepin applied to wounded tissues both caused the cessation of this continuous increase, but the initial burst was still persisting. These data suggest that gamma-irradiation, like wounding, stimulates the translation of preexisting mRNAs. It also reduces, at least temporarily, the subsequent transcription-dependent stimulation of ethylene production. gamma-Irradiation greatly inhibited the activity of ethylene-forming enzyme at doses higher than 1 kilogray. Such sensitivity is in accordance with a highly integrated membranebound enzyme.

追熟性トマト ′Rutgers′ および追熟変異性トマト nor と rin のエチレン生成における制御機構

This experiment was done to clarify the influence of the absence of the genes responsible for ripening and ethylene production using three types of tomato fruits; cv. Rutgers as ripening type, nor mutant as slight ripening type and rin mutant as non-ripening type. Changes in 1-aminocyclopropane-1-carboxylic acid (ACC) content, ACC synthase activity and ethylene forming enzyme (EFE) activity in these tomatoes were measured at several stages. ′Rutgers′ tomato produced ethylene in ripening stage, whereas nor tomato produced ethylene slightly and rin hardly produced it except for production due to decay of fruits. It seems that ethylene production of ′Rutgers′ is attributed to accumulation of ACC and increase in EFE activity during climacteric, and insufficient ethylene production of nor tomato fruit is due to accumulation of ACC only, without increased EFE activity in the tissue. And very little ethylene production of rin is due to the small amount of ACC content without increased EFE activity.

Exogenous Polyamines Stimulate Ethylene Synthesis by Soybean Leaf Tissues

Exogenous supply of spermine (Spm) markedly stimulated ethylene evolution from intact soybean leaves of leaf discs, strongly increased the level of free 1-aminocyclopropane-1-carboxylic acid (ACC), and slightly stimulated ethylene forming-enzyme (EFE) activity Spm treatment also resulted in leaf epinasty and accelerated leaf senescence Ethylene stimulation was depressed, but not abolished, by light, and was suppressed by inhibitors of ACC synthase and EFE activity Spermidine had a less pronounced stimulatory effect on ethylene production whereas the diamines putrescine and diaminopropane were without effect These results contrast with other reports indicating that di- and polyamines inhibit ethylene biosynthesis in plants, and extend our previous results on detached tobacco leaves exogenously treated with polyamines

Decreased Ethylene Biosynthesis, and Induction of Aerenchyma, by Nitrogen- or Phosphate-Starvation in Adventitious Roots of Zea mays L.

Plants of Zea mays L. cv TX5855 were grown in a complete, well oxygenated nutrient solution then subjected to nutrient starvation by omitting either nitrate and ammonium or phosphate from the solution. These treatments induced the formation of aerenchyma close to the apex of the adventitious roots that subsequently emerged from the base of the shoot, a response similar to that shown earlier to be induced by hypoxia. Compared with control plants supplied with all nutrients throughout, N- or P-starvation consistently depressed the rates of ethylene release by excised, 25 mm apical segments of adventitious roots. Some enzymes and substrates of the ethylene biosynthetic pathway were examined. The content of 1-amino cyclopropane-1-carboxylic acid (ACC) paralleled the differences in ethylene production rates, being depressed by N or P deficiency, while malonyl-ACC showed a similar trend. Activity of ACC synthase and of ethylene forming enzyme (g(-1) fresh weight) was also greater in control roots than in nutrient starved ones. These results indicate that much of the ethylene biosynthetic pathway is slowed under conditions of N- or P-starvation. Thus, by contrast to the effects of hypoxia, the induction of aerenchyma in roots of Zea mays by nutrient starvation is not related to an enhanced biosynthesis and/or accumulation of ethylene in the root tips.