Ethylene in plant growth

Control by ethylene of arginine decarboxylase activity in pea seedlings and its implication for hormonal regulation of plant growth

Activity of arginine decarboxylase in etiolated pea seedlings appears 24 hours after seed imbibition, reaches its highest level on the 4th day, and levels off until the 7th day. This activity was found in the apical and subapical tissue of the roots and shoots where intensive DNA synthesis occurs. Exposure of the seedlings to ethylene greatly reduced the specific activity of this enzyme. The inhibition was observed within 30 min of the hormone application, and maximal effect-90% inhibition-after 18 hours. Ethylene at physiological concentrations affected the enzyme activity; 50% inhibitory rate was recorded at 0.12 microliters per liter ethylene and maximal response at 1.2 microliters per liter. Ethylene provoked a 5-fold increase in the K(m) (app) of arginine decarboxylase for its substrate and reduced the V(max) (app) by 10-fold. However, the enzyme recovered from the inhibition and regained control activity 7 hours after transferral of the seedlings to ethylene-free atmosphere. Reducing the endogenous level of ethylene in the tissue by hypobaric pressure, or by exposure to light, as well as interfering with ethylene action by treatment with silver thiosulfate or 2,5-norbornadiene, caused a gradual increase in the specific activity of arginine decarboxylase in the apical tissue of the etiolated seedlings. On the basis of these findings, the possible control of arginine decarboxylase activity by endogenous ethylene, and its implication for the hormone effect on plant growth, are discussed.

Stimulation of ethylene production and gas-space (aerenchyma) formation in adventitious roots of Zea mays L. by small partial pressures of oxygen

Adventitious roots of two to four-weekold intact plants of Zea mays L. (cv. LG11) were shorter but less dense after extending into stagnant, non-aerated nutrient solution than into solution continuously aerated with air. Dissolved oxygen in the non-aerated solutions decreased from 21 kPa to 3-9 kPa within 24 h. When oxygen partial pressures similar to those found in non-aerated solutions (3, 5 and 12 kPa) were applied for 7 d to root systems growing in vigorously bubbled solutions, the volume of gas-space in the cortex (aerenchyma) was increased several fold. This stimulation of aerenchyma was associated with faster ethylene production by 45-mm-long apical root segments. When ethylene production by roots exposed to 5 kPa oxygen was inhibited by aminoethoxyvinylglycine (AVG) dissolved in the nutrient solution, aerenchyma formation was also retarded. The effect of AVG was reversible by concomitant applications of 1-aminocyclopropane-1-carboxylic acid, an immediate precursor of ethylene. Addition of silver nitrate, an inhibitor of ethylene action, to the nutrient solution also prevented the development of aerenchyma in roots given 5 kPa oxygen. Treating roots with only 1 kPa oxygen stimulated ethylene production but failed to promote gas-space formation. These severely oxygen-deficient roots seemed insensitive to the ethylene produced since a supplement of exogeneous ethylene that promoted aerenchyma development in nutrient solution aerated with air (21 kPa oxygen) failed to do so in nutrient solution supplied with 1 kPa oxygen. Both ethylene production and aerenchyma formation were almost completely halted when roots were exposed to nutrient solutions devoid of oxygen. Thus both processes require oxygen and are stimulated by oxygen-deficient surroundings in the 3-to 12-kPa range of oxygen partial pressures when compared with rates observed in air (21 kPa oxygen).

Ethylene-induced lateral expansion in etiolated pea stems : the role of Acid secretion

Ethylene-induced inhibition of elongation and promotion of lateral expansion in the stems of etiolated pea (Pisum sativum L. var Alaska) seedlings is not associated with any alteration of auxin-stimulated proton extrusion. Indeed, lateral expansion in response to ethylene apparently requires an acidified wall since it is prevented by strong neutral buffers and by the ATPase inhibitor orthovanadate. Ethylene treatment reduces the capacity of live and frozen-thawed sections to extend in the longitudinal direction in response to acid. The effect of ethylene on lateral acid growth capacity is more complicated. Ethylene-treated internodes do not exhibit acid-induced lateral expansion. Ethylene-treated segments which have been frozen-thawed do show an enhanced capacity to extend in the transverse direction at acid pH, but only when the inner tissues have been removed by coring. We conclude that two of the factors which control the directionality of expansion during ethylene treatment are a decrease in the sensitivity of the walls to acid longitudinally and an increase in the sensitivity of the outer cortical parenchyma walls to acid in the transverse direction.

Ethylene-induced lateral expansion in etiolated pea stems : kinetics, cell wall synthesis, and osmotic potential

Treatment of etiolated pea (Pisum sativum L.) internode tissue with ethylene gas inhibits elongation and induces lateral expansion. Precise kinetics of the induction of this altered mode of growth of excised internode segments were recorded using a double laser optical monitoring device. Inhibition of elongation and promotion of lateral expansion began after about 1 hour of treatment and achieved a maximum by 3 hours. Similar induction kinetics were observed after treating internodes with colchicine and 2,6-dichlorobenzonitrile, an inhibitor of cellulose synthesis. In sealed flask experiments, ethylene had no detectable effect on incorporation of label from [(14)C]glucose into any of the classical pectin, hemicellulose, or cellulose wall fractions. Ethylene inhibited fresh weight increase (total cell expansion) of both excised internode segments (in sealed flasks) and intact seedlings. Ethylene treatment resulted in an increase in cell sap osmolality in those tissues (intact and excised) which are inhibited by the gas. A model for ethylene-induced inhibition of elongation and induction of lateral expansion is presented.

Light or ethylene treatments induce transverse cell enlargement in etiolated maize mesocotyls

Dark-grown maize seedlings (hybrid WF-9 x 38-11) exposed for 1 or more hours to white light and then returned to darkness developed mesocotyls with enlarged apical diameters. This swelling response was an all-or-none response, and the fraction of the seedling population that showed the response depended on seedling age at irradiation. Irradiation of the coleoptile alone was nearly as effective in causing this response as was irradiation of the nodal region of the epicotyl, but irradiation of the mesocotyl base was ineffective. Removal of the coleoptile prior to irradiation did not prevent the formation of the light-induced swelling. Exogenously applied C(2)H(4) (10 microliters per liter) for 24 hours in dark also induced swelling of the mesocotyl. The swelling induced in the intact seedlings was localized in the apical mesocotyl tissues with either light or C(2)H(4) treatment, and maximal response to both treatments occurred with 3- to 4-day-old seedlings. Swelling of the mesocotyl was the result of transverse cell enlargement, not increase of cell numbers. The evidence suggests that light and C(2)H(4) induce mesocotyl swelling in intact maize shoots by a common mechanism.

Hormones are no causal links in phytochrome-mediated adventitious root formation in mustard seedlings (Sinapis alba L.)

The question of whether or not hormones are causal links in the realization of phytochrome control during photomorphogenesis was investigated using the phytochrome-dependent formation of adventitious roots in hypocotyl cuttings excised from mustard seedlings as a test system. Histological examination of regenerating "rest" seedlings revealed that phytochrome (operationally, continuous far-red light) mediates the de novo formation of root primordia in the pericycle region of the hypocotyl near the cutting surface withing 12-24 h after excision.Auxin (IAA), gibberellin (GA3), Cytokinin (kinetin), abscisic acid (ABA), and ethylene had no promotive effect on primordium formation in dark-grown or far-red irradiated rest seedlings. Depending on concentration, the application of these hormones was either ineffective or inhibitory in the rooting response. It is concluded that phytochrome does not operate through changes of hormone (auxin, gibberellin, cytokinin, ABA, ethylene) levels.While externally applied ethylene had no specific effect on primordium formation, the number of primordia produced in darkness could be increased to the far-red light level by removing the endogenously formed ethylene. Since the stimulatory effect of light could not be related to a lower ethylene level, it is concluded that ethylene interferes with primordium formation by modulating the susceptibility of this process to phytochrome control. This ethylene effect takes place in a concentration range below the range that can be manipulated by external application of the hormone.

Effect of 1-Aminocyclopropane-1-Carboxylic Acid on the Production of Ethylene in Senescing Flowers of Ipomoea tricolor Cav

Application of 1-aminocyclopropane-1-carboxylic acid (ACC) to rib segments excised from flowers of Ipomoea tricolor Cav. resulted in the formation of C(2)H(4) in greater quantities than produced under natural conditions. The ability of ACC to enhance C(2)H(4) production was independent of the physiological age of the tissue and its capacity to synthesize C(2)H(4) without applied ACC. When ACC was fed to rib segments that had been treated with [(14)C]methionine, incorporation of radioactivity into C(2)H(4) was reduced by 80%. Aminoethoxyvinylglycine and aminooxyacetic acid inhibited C(2)H(4) production in rib segments of I. tricolor but had no effect on ACC-enhanced C(2)H(4) production. Protoplasts obtained from flower tissue of I. tricolor did not form C(2)H(4), even when incubated with methionine or selenomethionine. They produced C(2)H(4) upon incubation with ACC, however. ACC-dependent C(2)H(4) production in protoplasts was inhibited by n-propyl gallate, AgCl, CoCl(2), KCN, Na(2)S, and NaN(3). ACC-dependent C(2)H(4) synthesis in rib segments and protoplasts was dependent on O(2), the K(m) for O(2) being 1.0 to 1.4% (v/v). These results confirm the following pathway for C(2)H(4) biosynthesis in I. tricolor. methionine [selenomethionine] --> S-adenosylmethionine [selenoadenosylmethionine] --> ACC --> C(2)H(4).

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

2,4-Dinitrophenol (DNP) and high temperature (35 to 40 C) are known to inhibit C(2)H(4) production in various plant tissues. The present study was made to determine the step in the C(2)H(4) biosynthetic pathway (methionine --> S-adenosylmethionine [SAM] --> 1-aminocyclopropane-1-carboxylic acid [ACC] --> C(2)H(4)) at which these treatments exert their inhibitory effect. In mung bean hypocotyls the dose-inhibition curves for the effect of DNP on auxin-dependent C(2)H(4) production (in which auxin exerts its effect by stimulating the conversion of SAM to ACC) and on ACC-dependent C(2)H(4) production (in which ACC is directly utilized as precursor) were similar. It was concluded, therefore, that DNP at low concentrations (below 50 micromolar) exerted its effect primarily on the conversion of ACC to C(2)H(4), a step which is common to both systems. This view was further substantiated by quantitative analysis of the intermediates in the biosynthetic sequence. DNP exerted little influence on the content of SAM but caused a significant increase in the ACC content and marked inhibition in C(2)H(4) production, indicating that the conversion of ACC to C(2)H(4) is the crossover point. At higher concentrations (above 100 micromolar), DNP inhibited the conversion of methionine to ACC and to C(2)H(4), and this effect could be attributed to the inhibition of SAM synthesis.The optimal temperature for maximal C(2)H(4) production by apple tissue and mung bean hypocotyl is about 30 C. An increase in temperature to 35 C caused an accumulation of endogenous ACC, whereas C(2)H(4) production was greatly reduced. These results suggest that the conversion of ACC to C(2)H(4) is highly vulnerable to high temperature inhibition.

Formation of ethylene from methionine. Reactivity of radiolytically produced oxygen radicals and effect of substrate activation

Ethylene was determined by gas chromatography after reaction of radiolytically produced OH and O2- radicals with methionine, methionine + pyridoxal phosphate and S-adenosyl-methionine (SAM). Both oxygen radicals, alone or in combination, liberate ethylene from methionine and methionine/pyridoxal phosphate. From SAM ethylene was primarily produced by the combined attack of OH nad H2O2 or O2-.

Interactions of Methionine and Selenomethionine with Methionine Adenosyltransferase and Ethylene-generating Systems

Since selenomethionine appears to be a better precursor of ethylene in senescing flower tissue of Ipomoea tricolor and in indole acetic acid-treated pea stem sections than is methionine (Konze JR, N Schilling, H Kende 1978 Plant Physiol 62: 397-401), we compared the effectiveness of selenomethionine and methionine to participate in reactions which may be connected to ethylene biosynthesis. Evidence is presented that selenomethionine is also a better substrate of methionine adenosyltransferase (ATP: methionine S-adenosyltransferase, EC 2.5.1.6) from I. tricolor, the V(max) for selenomethionine being twice as high as that for methionine. The affinity of the enzyme is higher for methionine than for selenomethionine, however. Methionine added to flower tissue together with selenomethionine inhibits the enhancement of ethylene synthesis by the seleno analog. Likewise, methionine reduces the high, selenomethionine-dependent reaction rates of methionine adenosyltransferase from I. tricolor flower tissue. On the other hand, selenomethionine is less effective as an ethylene precursor than is methionine in model systems involving oxidation by free radicals. It was concluded that activation of methionine by methionine adenosyltransferase and formation of S-adenosylmethionine are more likely to be involved in ethylene biosynthesis than is oxidation of methionine by free radicals.

Ethylene formation from 1-aminocyclopropane-1-carboxylic acid in homogenates of etiolated pea seedlings

Homogenates of etiolated pea (Pisum sativum L.) shoots formed ethylene upon incubation with 1-aminocyclopropane-1-carboxylic acid (ACC). In-vitro ethylene formation was not dependent upon prior treatment of the tissue with indole-3-acetic acid. When homogenates were passed through a Sephadex column, the excluded, high-molecular-weight fraction lost much of its ethylene-synthesizing capacity. This activity was largely restored when a heat-stable, low-molecular-weight factor, which was retarded on the Sephadex column, was added back to the high-molecular-weight fraction. The ethylene-synthesizing system appeared to be associated, at least in part, with the particulate fraction of the pea homogenate. Like ethylene synthesis in vivo, cell-free ethylene formation from ACC was oxygen dependent and inhibited by ethylenediamine tetraacetic acid, n-propyl gallate, cyanide, azide, CoCl3, and incubation at 40°C. It was also inhibited by catalase. In-vitro ethylene synthesis could only be saturated at very high ACC concentrations, if at all. Ethylene production in pea homogenates, and perhaps also in intact tissue, may be the result of the action of an enzyme that needs a heat-stable cofactor and has a very low affinity for its substrate, ACC, or it may be the result of a chemical reaction between ACC and the product of an enzyme reaction. Homogenates of etiolated pea shoots also formed ethylene with 2-keto-4-mercaptomethyl butyrate (KMB) as substrate. However, the mechanism by which KMB is converted to ethylene appears to be different from that by which ACC is converted.

Induction of swelling in pea internode tissue by ethylene

Ethylene inhibits polar cell expansion of etiolated pea (Pisum sativum) internode tissue and results in a radial cell expansion, swelling. A special gas-tight growth chamber was developed to monitor continuously the induction of cell expansion inhibition of excised internode segments in the presence of ethylene. Following a complex induction period which lasts about 3 hours, a low, but steady-state growth rate is achieved. Ethylene removal experiments indicate that the gas induces an irreversible change in the cell expansion mode. If the tissue was transferred to an ethylene-free chamber after the steady-state rate had been achieved, this rate continued in the absence of the gas. If the gas was removed during an intermediate phase, that intermediate rate of growth continued after removal. In other experiments, segments were incubated with intact apical hooks and the induction period was much shorter than with isolated segments. Changes in imino acid metabolism have been correlated with the induction of swelling. Although total proline and hydroxyproline levels were not affected by ethylene treatment, incorporation of [(14)C]proline into a wall-associated protein was inhibited during the period when swelling occurred. These results suggest that ethylene is affecting proline pool sizes.

Rapidly Induced Wound Ethylene from Excised Segments of Etiolated Pisum sativum L., cv. Alaska: II. Oxygen and Temperature Dependency

Wound-induced ethylene synthesis by subapical stem sections of etiolated Pisum sativum L., cv. Alaska seedlings, as described by Saltveit and Dilley (Plant Physiol 1978 61: 447-450), was half-saturated at 3.6% (v/v) O(2) and saturated at about 10% O(2). Corresponding values for CO(2) production during the same period were 1.1% and 10% O(2), respectively. Anaerobiosis stopped all ethylene evolution and delayed the characteristic pattern of wound ethylene synthesis. Exposing tissue to 3.5% CO(2) in air in a flow-through system reduced wound ethylene synthesis by 30%. Enhancing gas diffusivity by reducing the total pressure to 130 mm Hg almost doubled the rate of wound ethylene synthesis and this effect was negated by exposure to 250 mul liter(-1) propylene. Applied ethylene or propylene stopped wound ethylene synthesis during the period of application, but unlike N(2), no lag period was observed upon flushing with air. It is concluded that the characteristic pattern of wound-induced ethylene synthesis resulted from negative feedback control by endogenous ethylene.No wound ethylene was produced for 2 hours after excision at 10 or 38 C. Low temperatures prolonged the lag period, but did not prevent induction of the wound response, since tissue held for 2 hours at 10 C produced wound ethylene immediately when warmed to 30 C. In contrast, temperatures above 36 C prevented induction of wound ethylene synthesis, since tissue cooled to 30 C after 1 hour at 40 C required 2 hours before ethylene production returned to normal levels. The activation energy between 15 and 36 C was 12.1 mole kilocalories degree(-1).

Investigations on the role of ethylene in phytochrome-mediated photomorphogenesis : I. Anthocyanin Synthesis

The etiolating, intact mustard (Sinapis alba L.) seedling exhibits a distinct temporal pattern of ethylene production. Light, operating through phytochrome, increases the rate of ethylene production without changing the pattern. Ethylene production of the isolated plant parts (segments), added together, exceed the production of the intact system even if the wound effect is taken into account. There is no significant light effect on ethylene production of the segments. Phytochrome-mediated anthocyanin synthesis in the cotyledons is inhibited by ethylene. The responsiveness towards ethylene of the anthocyanin producing metabolic chain is decreased by phytochrome. As anthocyanin synthesis is only partly inhibited under saturating ethylene concentrations in the atmosphere around the seedlings (100 μl l(-1)), a twofactor analysis becomes feasible. This analysis leads to the result that phytochrome and ethylene show multiplicative behavior, meaning that phytochrome and ethylene act on the same metabolic sequence (leading to anthocyanin) but independently of each other, and at different sites. Therefore, the hypothesis that ethylene mediates the action of phytochrome in anthocyanin synthesis and photomorphogenesis in general appears to be inapplicable.

Investigations on the role of ethylene in phytochrome-mediated photomorphogenesis : II. Enzyme levels and chlorophyll synthesis

The concept (Burg, 1973) that ethylene mediates the action of phytochrome in seedling photomorphogenesis was tested in the intact mustard (Sinapis alba L.) seedling. The effect of exogenous ethylene (100 μl l(-1)) on five distinct, phytochrome-mediated photoresponses of the cotyledons was investigated. It was found that anthocyanin contents (see Bühler et al., 1978) and phenylalanine ammonia-lyase levels (EC 4.3.1.5) are strongly reduced by ethylene while the capacity of chlorophyll synthesis is considerably enhanced. Levels of glutathione reductase (EC 1.6.4.2) and pools of photoconvertible protochlorophyll(ide) are unaffected by ethylene. It is concluded that these findings are incompatible with the idea that ethylene plays the role of a mediator in phytochrome-induced photomorphogenesis.

Methionine metabolism in apple tissue

A metabolic intermediate isolated from apple tissue fed either methionine or 5'-methylthioadenosine has been tentatively identified as a methionine-pyridoxal Schiff base. The formation of this compound is discussed in relation to ethylene biosynthesis.

Methionine metabolism in apple tissue: implication of s-adenosylmethionine as an intermediate in the conversion of methionine to ethylene

If S-adenosylmethionine (SAM) is the direct precursor of ethylene as previously proposed, it is expected that 5'-S-methyl-5'-thioadenosine (MTA) would be the fragment nucleoside. When [Me-(14)C] or [(35)S]methionine was fed to climacteric apple (Malus sylvestris Mill) tissue, radioactive 5-S-methyl-5-thioribose (MTR) was identified as the predominant product and MTA as a minor one. When the conversion of methionine into ethylene was inhibited by (l)-2-amino-4-(2'-aminoethoxy)-trans-3-butenoic acid, the conversion of [(35)S] or [Me(14)C]methionine into MTR was similarly inhibited. Furthermore, the formation of MTA and MTR from [(35)S]methionine was observed only in climacteric tissue which produced ethylene and actively converted methionine to ethylene but not in preclimacteric tissue which did not produce ethylene or convert methionine to ethylene. These observations suggest that the conversion of methionine into MTA and MTR is closely related to ethylene biosynthesis and provide indirect evidence that SAM may be an intermediate in the conversion of methionine to ethylene.When [(35)S]MTA was fed to climacteric or preclimacteric apple tissue, radioactivity was efficiently incorporated into MTR and methionine. However, when [(35)S]MTR was administered, radioactivity was efficiently incorporated into methionine but not MTA. This suggests that the sulfur of MTA is incorporated into methionine via MTR. A dual label experiment with [(35)S, Me-(3)H]MTA indicates that the CH(3)S group of MTA was transferred as a unit to form methionine.A scheme is presented for the production of ethylene from methionine, the first step being the activation of methionine by ATP to give SAM. SAM is fragmented to give ethylene, MTA, and other products. MTA is then hydrolyzed to MTR which donates its methylthio group to a four-carbon acceptor to reform methionine.

Carbon Dioxide and Ethylene Control of Spore Germination in Onoclea sensibilis L

Regulation of spore germination in the fern Onoclea sensibilis L. was investigated by applying CO(2) alone and in combination with ethylene. Sterile spores were sown aseptically on Knops solution in loosely capped culture tubes, enclosed individually in 2-liter chambers, and grown under continuous white light. When maintained in enclosed containers with the ethylene-absorbent mercuric perchlorate and with atmospheres enriched up to 2% CO(2) (v/v), spores germinated without any inhibition. Higher levels of applied CO(2) were progressively inhibitory. Inhibition by CO(2) was reversible. When CO(2) was permitted to escape and spores were exposed subsequently to ambient laboratory air, recovery from inhibition occurred within 48 hours. Also, inhibition by CO(2) was specific, since the same degree of inhibition resulted regardless of whether spores were treated with exogenous CO(2) for 48, 72, or 96 hours. The effect on germination of 1 mul/l added ethylene depended upon the amount of applied CO(2). When containers of KOH were enclosed and ambient CO(2) was absorbed, inhibition of germination by 1 mul/l exogenous ethylene was 90%. When CO(2) was applied in concentrations from 0.25 to 1.0% (v/v), CO(2) increasingly antagonized the inhibitory action of 1 mul/l added ethylene. Thus, photoinduced germination of spores was regulated by competitively interacting levels of CO(2) and ethylene.

Stem sensitivity and ethylene involvement in phototropism of mung bean

A system is described for the examination of phototropism in the epicotyl of a dicot seedling, mung bean (Phaseolus aureus Roxb.), under conditions approximating nature, including the use of intact, nonetiolated plants exposed to elevated, continuous, white, unilateral light. It is found that in this system perception of the phototropic stimulus by the leaves alone cannot account for the curvature, and that exposure of the stem is also necessary. The phototropic response was found to be strongly altered in nonintact plants. Hypobaric treatment indicates that ethylene may participate in phototropism, possibly by acting as an inhibitor of auxin transport.

The Site of Cellulose Synthesis: Cell Surface and Intracellular beta-1, 4-Glucan (Cellulose) Synthetase Activities in Relation to the Stage and Direction of Cell Growth

beta-1, 4-Glucan (cellulose) synthetase activity (UDP-glucose: beta-1, 4-glucan-glucosyl transferase) present at cell surfaces of growing regions of Pisum sativum epicotyl was assayed by supplying UDP-(14)C-glucose directly to thin slices of tissue. Initial rates of glucosyl transfer under these conditions approached the rates of cellulose deposition observed in vivo in intact tissue at various stages of growth. Normal tissue homogenization procedures destroyed the high surface activity, although a small amount of residual activity (3-10% of total) could be detected in particulate fractions. In homogenates from elongating tissue, the residual activity was almost entirely associated with Golgi membrane. In homogenates of tissue which had ceased elongating, whether because of normal maturation or treatment with ethylene (or high levels of auxin), the activity was present in Golgi plus a membrane fraction rich in smooth endoplasmic reticulum vesicles. It is suggested that cellulose synthetase activity associated with these two organelles represents intracellular enzyme in transit to specific sites of cellulose synthesis and microfibrillar orientation at the cell surface.

Concentration dependencies of some effects of ethylene on etiolated pea, peanut, bean, and cotton seedlings

The effects of a series of concentrations of ethylene (10, 20, 40, to 10,240 nl/l) on elongation, diameter, and geotropism of the stems and roots of etiolated seedlings of Pisum sativum L., Arachis hypogea L., Phaseolus vulgaris L., and Gossypium hirsutum L. were measured or observed. Of the 24 possible responses, 4 were unaffected at the concentrations used, 5 were affected slightly, and the remaining responses exhibited a 14-fold range of apparent half-maximum concentration dependencies (i.e. 95 nl/l for the effect on pea epicotyl geotropism to 1350 nl/l for the promotion of cotton hypocotyl diameter). Six or possibly eight of these responses appear to have the same concentration dependencies while the others fell in pairs or as individual responses. The data, if interpreted in a manner analogous to enzyme kinetics, are indicative of more than one primary mechanism for ethylene action in plants.

The site of cellulose synthesis. Hormone treatment alters the intracellular location of alkali-insoluble beta-1,4-glucan (cellulose) synthetase activities

Membrane preparations from growing regions of 8-day old Pisum sativum epicotyls contain multiple beta-1,4-glucan (cellulose) synthetase activities (UDP- or GDP-glucose: beta-1,4-glucan-glucosyl transferase), and the levels of some of these are influenced by treatments with the growth hormone, indoleacetic acid (IAA). When membranes from control epicotyl segments (zero time) are fractionated by isopycnic sedimentation in sucrose density gradients, all of the synthetase activities are associated mainly with Golgi membrane (density 1.55 g/cm3). After decapitation and treatment of epicotyls with IAA, synthetases also appear in a smooth vesicle fraction (density 1.11 g/cm3) which is rich in endoplasmic reticulum (ER) marker enzyme. Major fractions of these synthetases are not recovered in association with plasma membrane or washed cell walls. When [14-C]sucrose is supplied in vivo to segments +/- IAA, radioactive cellulose is deposited only in the wall. Cellulose or cellodextrin precursors do not accumulate in those membranes in which synthetase activities are recovered in vitro. In experiments where tissue slices containing intact cells are supplied with [14C]sugar nucleotide in vitro, alkali-insoluble beta-1,4-glucan is synthesized (presumably outside the protoplast) at rates which greatly exceeded (20-30 times) those obtained using isolated membrane preparations. Progressive disruption of cell structure results in increasing losses of this high activity. These results are consistent with the interpretation that Golgi and ER-associated synthetases are not themselves loci for cellulose synthesis in vivo, but represent enzymes in transit to sites of action at the wall:protoplast omterface. There they operate only if integrity of cellular organization is maintained.

Oxidative turnover of auxins in relation to the onset of ripening in bartlett pear

Pears (Pyrus communis var. Bartlett) kept in 100% O(2) showed an increase in the rate of softening, chlorophyll degradation, and ethylene evolution. The O(2) application could overcome, in part, the inhibition of ripening by 1 mm indoleacetic acid. Ripening of pears was also accelerated by the application of solutions containing indoleacetic acid-oxidation products, obtained by an overnight incubation of 0.1 and 1 mm indoleacetic acid with traces of H(2)O(2) and horseradish peroxidase. Although both treatments stimulated ethylene evolution, the promotion of ripening could not be attributed to an indirect ethylene effect. Indoleacetic acid oxidation products obtained in vivo by high O(2) tensions or in vitro by enzymatic degradation may function in the promotion of fruit ripening and the synthesis of ethylene.

Ethylene and the annona flower

The annona (Annona hybrida) flower is protogynous, and reaches its male stage about 26 hours after the beginning of the female stage. A rise in ethylene production was found to precede the male stage. Much more ethylene was produced by the reproductive organs than by the petals, with the anthers producing most of the ethylene. Ethylene treatments advanced the male stage, but not the female stage. Exposing flowers to hypobaric pressure postponed the onset of their male stage. Application of various growth substances causing ethylene production by the flower advanced the male stage.

Ethylene-induced Leaf Abscission Is Promoted by Gibberellic Acid

Gibberellic acid (GA(3)) promoted leaf abscission from cotton (Gossypium hirsutum L.) plants exposed to ethylene. With mature plants, only the rate of abscission was increased, but when vegetative plants were exposed to ethylene for 4 days or less, the amount of abscission was increased markedly. Promotion of abscission occurred at near saturating ethylene levels (10 mul/liter), over a wide range of GA(3) concentrations, and with both GA(3) and GA(7).GA(3) promoted abscission when Ethephon was substituted for ethylene and at locations not receiving direct application of GA(3). The magnitude of the abscission promotion by GA(3) was greater than that resulting from auxin transport inhibitors or abscisic acid. The characteristic inhibition of abscission by auxin occurred. The responses suggest that endogenous gibberellins may be involved in rapid abscission of apical leaves from vegetative cotton plants exposed to ethylene. Application of GA(3) may offer an additional option in agricultural manipulation of abscission and dehiscence.

Inhibition of in Vivo Conversion of Methionine to Ethylene by l-Canaline and 2,4-Dinitrophenol

l-Canaline, a potent inhibitor of pyridoxal phosphate-mediated reactions, markedly inhibited the conversion of methionine to ethylene and carbon dioxide by apple tissue. A 50% inhibition of methionine conversion into ethylene was obtained with 50 mum canaline and almost complete inhibition with 300 mum canaline. When 2,4-dinitrophenol, an oxidative phosphorylation uncoupler, was fed to apple tissue, it inhibited the conversion of radioactive methionine to ethylene by 50% at a concentration of 60 mum and by 90% at a concentration of 100 mum. Production of labeled carbon dioxide from acetate-1-(14)C was increased by 2,4-dinitrophenol, indicating that the inhibition of ethylene production was due to uncoupling of phosphorylation. Auxin-induced ethylene production by mungbean (Phaseolus mungo L.) hypocotyl sections was similarly inhibited by these inhibitors.These results support the proposal that pyridoxal phosphate is involved in the formation of ethylene from methionine, substantiate the requirement for ATP in ethylene production, and suggest that this ATP requirement occurs in the step (s) between methionine and ethylene. The biosynthetic mechanism probably involves activation of methionine by ATP followed by a pyridoxal phosphate-mediated gamma-elimination.

Red light enhancement of the phototropic response of etiolated pea stems

In the subapical third internode of 7-day-old etiolated pea seedlings, the magnitude of phototropic curvature in response to continuous unilateral blue illumination is increased when seedlings are pre-exposed to brief red light. The effect of red light on blue light-induced phototropism becomes manifest maximally 4 or more hours after red illumination, and closely parallels the promotive action of red light on the elongation of the subapical cells. Ethylene inhibits phototropic curvature by an inhibitory action on cell elongation without affecting the lateral transport of auxin. Pretreatment of seedlings with gibberellic acid causes increased phototropic curvature, but experiments using (14)C-gibberellic acid indicate that gibberellic acid itself is not laterally transported under phototropic stimuli. Neither red light nor gibberellic acid treatment has any promotive effect on blue light-induced lateral transport of (3)H-indoleacetic acid. Under conditions where phototropic curvature is increased by red light treatment, low concentrations of indoleacetic acid applied in lanolin paste to the apical cut end of the seedling cause an increased elongation response in subapical tissue. This could explain increased phototropic curvature caused by red light treatment.

The effect of ethylene on (1-14C)glycerol incorporation into phospholipids of etiolated pea stems

The effect of ethylene (10 p.p.m.) on the rate of incorporation of [1-(14)C]glycerol into phospholipids of etiolated pea stems was studied. After 2-3h treatment with ethylene, incorporation was decreased by 50%. It remained at this value for as long as ethylene was supplied (8h). Handling the plants also caused a temporary decrease in incorporation, which we attribute to the production of endogenous ;wound' ethylene. The percentage decrease in incorporation was the same in four major phospholipid fractions, i.e. phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol and phosphatidylinositol.

Initiation of Ripening in Bartlett Pear with an Antiauxin alpha(p-Chlorophenoxy)isobutyric Acid

A vacuum infiltration technique was used to apply an anti-auxin, alpha-(p-chlorophenoxy) isobutyric acid to mature green pears (Pyrus communis var. Bartlett). Application of alpha-(p-chlorophenoxy) isobutyric acid, at 0.02, 0.2, and 2.0 mm progressively accelerated the onset of chlorophyll degradation, softening, and CO(2) evolution. The action of alpha(p-chlorophenoxy) isobutyric acid is apparently independent of ethylene, since the auxin analogue depressed ethylene evolution and could overcome ethylene deficiency in fruit ripening under hypobaric conditions.The auxin analogue decreased the Michaelis constant of indoleacetic acid oxidase in vitro, suggesting that the antiauxin action of alpha-(p-chlorophenoxy) isobutyric acid is the acceleration in the breakdown of endogenous auxins in fruit and subsequently the initiation of ripening.