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

Ethylene, a Signaling Compound Involved in Seed Germination and Dormancy

The present review is focused on current findings on the involvement of ethylene in seed biology. The responsiveness of seeds to ethylene depends on the species and the dormancy status, improving concentrations ranging from 0.1 to 200 μL L-1. The signaling pathway of ethylene starts with its binding to five membrane-anchored receptors, which results in the deactivation of Constitutive Triple Response 1 (CTR1, a protein kinase) that does not exert its inhibitory effect on Ethylene Insensitive 2 (EIN2) by phosphorylating its cytosolic C-terminal domain. An analysis of germination in the presence of inhibitors of ethylene synthesis or action, and using seeds from mutant lines altered in terms of the genes involved in ethylene synthesis (acs) and the signaling pathway (etr1, ein2, ein4, ctr1 and erf1), demonstrates the involvement of ethylene in the regulation of seed dormancy. The promoting effect of ethylene is also regulated through crosstalk with abscisic acid (ABA) and gibberellins (GAs), essential hormones involved in seed germination and dormancy, and Reactive Oxygen Species (ROS). Using a mutant of the proteolytic N-degron pathway, Proteolysis (PRT6), the Ethylene Response Factors (ERFs) from group VII (HRE1, HRE2, RAP 2.2, RAP2.3 and RAP 2.12) have also been identified as being involved in seed insensitivity to ethylene. This review highlights the key roles of EIN2 and EIN3 in the ethylene signaling pathway and in interactions with different hormones and discusses the responsiveness of seeds to ethylene, depending on the species and the dormancy status.

Measurement of ethylene emission from Japanese red pine (Pinus densiflora) under field conditions in NOx-polluted areas

Emission of ethylene from the needles of Japanese red pine, Pinus densiflora, was measured in air-polluted areas in Hiroshima, Japan. We applied a suitable protocol to determine the rate of ethylene emission from the excised needles. The influence of excision of needles on ethylene emission was not detected during the first 4 h of incubation at 20 degrees C. Ethylene emissions were low in the unpolluted (clean) areas regardless of the altitude or season. The emission of stress ethylene increased with the atmospheric NO2 concentration, suggesting that atmospheric NOx or related substances induced the higher ethylene emission in the polluted areas (near urban and industrial areas). In all cases, 1-year-old needles emitted significantly larger amounts of ethylene than the current needles. Ethylene emission did not increase evenly in the polluted areas, but the frequency of trees emitting high ethylene increased. Therefore, threshold rates for the baseline ethylene emission were proposed.

Identification of two chilling-regulated 1-aminocyclopropane-1-carboxylate synthase genes from citrus (Citrus sinensis Osbeck) fruit

Diurnal change in the temperature below or above 12.5 degrees C hastens the degreening of citrus peel and elicits the phytohormone ethylene production in citrus fruit. Ethylene triggers the degradation of chlorophyll and synthesis of carotenoids in citrus peel. To investigate if ethylene is required for the degreening of citrus peel elicited by low temperatures, we studied the chilling-regulated gene expression of ACC synthase, one of the key enzymes catalyzing ethylene biosynthesis. We isolated and characterized a chilling-inducible 1-aminocyclopropane-1-carboxylate synthase (ACC synthase) gene, CS-ACS1, and a chilling-repressible gene, CS-ACS2, from citrus peel. The CS-ACS1 transcript 1.7 kb in length encodes a polypeptide of 483 amino acids (Mr 54,115, pI 6.63), whereas the CS-ACS2 transcript of 1.8 kb encodes a polypeptide of 477 amino acids (Mr 53,291, pI 6.72). Both genes showed a rapid but transient induction (within 2.4 h) of transcripts upon rewarming after the chilling (4 degrees C) treatment. After 24 h of incubation at room temperature, CS-ACS1 mRNA diminished to an undetectable level, whereas the CS-ACS2 mRNA regained its basal level of expression attained prior to the chilling treatment. Chilling-induced ethylene production and ACC accumulation were also observed upon rewarming. Both genes were also induced by the wound stress (excision). The protein synthesis inhibitor cycloheximide super-enhances the accumulation of both ACS transcripts at room temperature. Molecular analysis of the 3.3 kb genomic DNA of CS-ACS1 revealed that this gene consists of three introns and four exons. The intron 3 is exceptionally large ( 1.2 kb) and shares significant homology with mitochondrial DNA, supporting the intron-late theory.

Differential regulation of genes encoding 1-aminocyclopropane-1-carboxylate (ACC) synthase in etiolated pea seedlings: effects of indole-3-acetic acid, wounding, and ethylene

Treatment of 5- to 6-day-old etiolated pea (Pisum sativum L.) seedlings with indole-3-acetic acid (IAA) induced within 15 min an increase in the transcript levels of two genes encoding 1-aminocyclopropane-1-carboxylate (ACC) synthase, Ps-ACS1 and Ps-ACS2. Simultaneous treatment with ethylene inhibited this increase and also caused a decrease in ACC synthase enzyme activity as compared to that of seedlings treated with IAA alone. These results indicate that ethylene inhibits its own biosynthesis by decreasing ACC synthase transcript levels via a negative feedback loop. Wounding of pea stems had no effect on the expression of Ps-ACS1, but led within 10 min to an increase in the mRNA levels of Ps-ACS2. This increase was also inhibited by ethylene. The wound signal was transmitted over a distance of at least 4 cm through the stem with no delay in induction or response intensity. The rapid transmission of the wound response is consistent with the possibility that a hydraulic or electric signal is responsible for the spread of the wound response.

Ethylene Biosynthesis and Accumulation under Drained and Submerged Conditions (A Comparative Study of Two Rumex Species)

A model is presented of the regulation of ethylene biosynthesis in relation to submergence and flooding resistance. It is based on time-course measurements of ethylene production, ethylene accumulation, and concentrations of free and conjugated 1-aminocyclo-propane-1-carboxylic acid (ACC) in submerged and drained flooding-resistant Rumex palustris Sm. and flooding-sensitive Rumex acetosella L. plants. From these data, in vivo reaction rates of the final steps in the ethylene biosynthetic pathway were calculated. According to our model, submergence stimulates ACC formation and inhibits conversion of ACC to ethylene in both Rumex species, and as a result, ACC accumulates. This may explain the stimulated ACC conjugation observed in submerged plants. Although submergence inhibited ethylene production, physical entrapment increased endogenous ethylene concentrations in both flooding-resistant R. palustris and flooding-sensitive R. acetosella plants. However, R. palustris plants controlled their internal ethylene levels in the long term by a negative regulation of ACC synthase induced by ethylene. In flooding-sensitive R. acetosella plants, absence of negative regulation increased internal ethylene levels to more than 20 [mu]L L-1 after 6 d of submergence. This may accelerate the process of senescence and contribute to their low level of flooding resistance.

Effect of silver ions on ethylene biosynthesis by tomato fruit tissue

Mature-green tomato fruit (Lycopersicon esculentum Mill.) were treated asymmetrically with 2 millimolar silver thiosulfate (STS) through a cut portion of the peduncle while still attached to the plant. One-half of the fruit received silver and remained green while the other half ripened normally and was silver-free (less than 0.01 parts per billion). Harvested mature-green fruit were also treated with STS through the cut pedicel. Green tissue from silver-treated fruit had levels of 1-aminocyclopropane-1-carboxylic acid (ACC, the immediate ethylene precursor) slightly less or similar to that of turning or red-ripe tissue from the same fruit, and similar to that of mature-green tissue from control fruit. Ethylene production was higher in green tissue from silver-treated fruit than from either red tissue from the same fruit, or mature-green tissue from control fruit. By inhibiting ACC synthesis with aminoethoxyvinyl glycine, and by applying ACC +/- silver to excised disks of pericarp tissue from control or silver-treated tomatoes, we showed that short-term silver treatment did not affect the biological conversion of ACC to ethylene, while long-term treatment stimulated both the conversion of ACC to ethylene and the synthesis of ACC.

Ethylene-promoted malonylation of 1-aminocyclopropane-1-carboxylic acid participates in autoinhibition of ethylene synthesis in grapefruit flavedo discs

The increase in ethylene formation and in 1-aminocyclopropane-1-carboxylic acid (ACC) content in flavedo tissue of grapefruit (Citrus paradisi Macfad. cv. Ruby Red) in response to excision was markedly inhibited by exogenous ethylene. Ethylene treatment inhibited the synthesis of ACC, but increased the tissue's capability to malonylate ACC to N-malonyl-ACC, resulting in further reduction in the endogenous ACC content. The development of extractable ACC-malonyl-transferase activity in the tissue was markedly promoted by treatment with exogenous ethylene. These results indicate that the autoinhibition of ethylene production in this tissue results not only from suppression of ACC synthesis, but also from promotion of ACC malonylation; both processes reduce the availability of ACC for ethylene synthesis.

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

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

Ethylene as an effector of wound-induced resistance to cellulase in oat leaves

Peeling the abaxial epidermis from oat leaves (Avena sativa var. Victory) induces the formation of wound ethylene and the development of resistance to cellulolytic digestion of mesophyll cell walls. Ethylene release begins between 1 and 2 hours after peeling in the light or dark. Aminoethoxyvinylglycine (AVG, 0.1 millimolar), CoCl(2) (1.0 millimolar), propyl gallate (PG, 1.0 millimolar) or aminooxyacetic acid (AOA, 1.0 millimolar) inhibits, whereas AgNO(3) stimulates wound ethylene formation. Incubation on inhibitors of ethylene biosynthesis (AVG, CoCl(2), PG, AOA) or action (AgNO(3), hypobaric pressure or the trapping of ethylene with HgClO(4)) also prevents the development of wound-induced resistance to enzymic cell wall digestion. 1-Aminocyclopropane-1-carboxylic acid (ACC, 1.0 millimolar) reverses AVG (0.1 millimolar) inhibition of the development of resistance. Exogenous ethylene partially induces the development of resistance in unwounded oat leaves.These results suggest that peeling of oat leaves induces ethylene biosynthesis, which in turn effects changes in the mesophyll cells resulting in the development of resistance to cellulolytic digestion.

Effects of exogenous ethylene on ethylene production in citrus leaf tissue

Exogenous ethylene stimulated ethylene production in intact citrus (Citrus sinensis L. Osbeck cv. "Washington Navel") leaves and leaf discs following a 24-hour exposure. Studies with leaf discs showed that ethylene production decreased when ethylene was removed by aeration. The extent of stimulation was dependent upon the concentration of exogenous ethylene (1-10 microliters per liter). Silver ion blocked the autocatalytic effect of ethylene at concentrations of 0.5 millimolar and lower, but increased ethylene production at higher concentrations. The stimulating effect of ethylene resulted from the enhancement of both 1-aminocyclopropane-1-carboxylic acid (ACC) formation and the conversion of ACC to ethylene. Whereas autocatalysis was evident following 24 hours incubation, autoinhibition of wound- and mannitol-induced ethylene production was observed during the first 24-hour incubation. Ethylene treatment during this period resulted in a marked decrease in ACC levels and ethylene production rates. Furthermore, in leaf discs treated for 24 hours with ethylene, ethylene production rates increased greatly during the first 2 hours after removal of exogenous ethylene by aeration. This increase was eliminated if the discs were transferred to propylene instead of air, indicating that the autocatalytic effect of ethylene is counteracted by its autoinhibitory effect. It is suggested that autocatalysis involves increased synthesis of ACC synthase and the enzyme responsible for the conversion of ACC to ethylene, whereas autoinhibition involves suppression of the activity of these two enzymes.

Autoinhibition of Ethylene Production in Citrus Peel Discs : SUPPRESSION OF 1-AMINOCYCLOPROPANE-1-CARBOXYLIC ACID SYNTHESIS

Wound ethylene formation induced in flavede tissue of citrus fruit (Citrus paradisi MacFad. cv. Ruby Red) by slicing was almost completely inhibited by exogenous ethylene. The inhibition lasted for at least 6 hours after removal of exogenous ethylene and was then gradually relieved. The extent of inhibition was dependent upon the concentration of ethylene (1 to 10 microliters/liter) and the duration of treatment. The increase in wound ethylene production in control discs was paralleled by an increase in 1-aminocyclopropane-1-carboxylic acid (AAC) content, whereas in ethylene-treated discs there was little increase in ACC content. Application of ACC completely restored ethylene production in ethylene-pretreated discs, indicating that the conversion of ACC to ethylene is not impaired by the presence of ethylene. Thus, autoinhibition of ethylene synthesis was exerted by reducing the availability of ACC. Ethylene treatment resulted in a decrease in extractable ACC synthase activity, but this decrease was too small to account for the marked inhibition of ACC formation. The data indicate that autoinhibition of ethylene production in citrus flavede discs results from suppression of ACC formation through repression of the synthesis of ACC synthase and inhibition of its activity.

Stress-induced Ethylene Production in the Ethylene-requiring Tomato Mutant Diageotropica

Ethylene synthesis in vegetative tissues is thought to be controlled by indoleacetic acid (IAA). However, ethylene synthesis in the diageotropica (dgt) mutant of tomato (Lycopersicon esculentum Mill.) was much less sensitive to IAA than in the normal variety (VFN8). Yet, mechanical wounding stimulated ethylene production by the mutant. The dgt tomato provides an opportunity to study the regulation of stress ethylene independent of IAA effects. Waterlogging (i.e. anaerobic stress) stimulated production of the ethylene precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), in the roots. The ACC was transported to the shoot where it was converted to ethylene. The dgt mutant efficiently utilized ACC for ethylene synthesis under aerobic conditions. The results confirm that the genetic lesion in dgt is located at a step prior to the formation of ACC. Furthermore, induction of ethylene synthesis by anaerobic or mechanical stresses in this mutant is independent of IAA action.

Studies of Rapidly Induced Wound Ethylene Synthesis by Excised Sections of Etiolated Pisum sativum L., cv. Alaska: IV. Requirement of a Water-soluble, Heat-stable Factor

The rate of wound ethylene synthesis was reduced by more than 85% when 9-millimeter subapical sections of etiolated 7-day-old Pisum sativum L., cv. Alaska seedlings were incubated in water during the 26-minute induction period prior to wound ethylene synthesis, but the rate of synthesis was unaffected if sections were incubated in water during the actual synthesis of wound ethylene. The characteristic timing of the wound response was unaffected by either treatment. The ability of various chemical solutions and aqueous plant extracts to alter the rate of wound ethylene synthesis was studied by first incubating subapical pea stem sections in solutions under anaerobic conditions (anaerobiosis delays the induction and synthesis of wound ethylene; Plant Physiol 61: 675-679), and then measuring wound ethylene synthesis after the tissue was transferred to air. Solutions of several reported precursors of ethylene synthesis, such as methionine, homoserine, or propanal, did not reverse the water-caused reduction of wound ethylene synthesis. A water-soluble, heat-stable factor in extracts from pea seedlings, and solutions of 23 nanomolar triacontanol, 10 micromolar kinetin, or 10 micromolar benzyladenine prevented the reduction of wound ethylene synthesis, but were ineffective if administered after an initial 15-minute anaerobic water incubation. This suggested that the active solutions may have only prevented the loss of some ephemeral, though necessary factor, rather than actually containing the substrate or inducer of wound ethylene synthesis. Attempts to isolate and characterize the active fraction from aqueous tissue extracts were unsuccessful. Free radical quenchers, inhibitors of protein synthesis, and rhizobitoxine, an inhibitor of ethylene synthesis from methionine, all reduced wound ethylene synthesis when administered in solutions which previously had maintained wound ethylene synthesis.

Rapidly Induced Wound Ethylene from Excised Segments of Etiolated Pisum sativum L., cv. Alaska: III. Induction and Transmission of the Response

Increased ethylene synthesis was rapidly induced throughout the apical meristematic region of etiolated seedlings of Pisum sativum L., cv. Alaska by cuts made 1 centimeter from the apical hook. The wound signal was transmitted at about 2 millimeters per minute. Accumulation of substance(s) at the cut surfaces of excised sections, as the result of interrupted translocation, did not initiate or significantly contribute to wound-induced ethylene synthesis, nor was the cut surface the site of enhanced ethylene synthesis. Cutting subapical sections into shorter pieces showed that cells less than 2 millimeters from a cut surface produced about 30% less ethylene than cells greater than 2 millimeters from a cut surface.