Selenomethionine as a dormancy-breaking agent in seeds of Stylosanthes humilis

Physiological dormancy of scarified seeds of Townsville stylo (Stylosanthes humilis H.B.K.) was released by seleno-L-methionine (SeM), but not by L-methionine. This regulating effect was impaired by inhibitors of ethylene biosynthesis and action; in the first case SeM action was restored by 2-chloroethylphosphonic acid (CEPA) and 1-aminocyclopropane-1-carboxylic acid (ACC). The Se-aminoacid proved to be toxic in a time-dependent manner to seedling growth, inhibiting primarily the hypocotyl expansion. This toxicity is suggested to trigger ethylene biosynthesis, which would promote germination of dormant seeds.     

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.     

Growth responses of Stylosanthes humilis (Fabaceae) populations to saline stress

The growth responses to salinity of seedlings of six populations of Stylosanthes humilis from three ecogeographic regions of Northeast Brazil, characterized by wet or semi-arid climate, were analyzed following 28 days in solution cultures at concentrations of 0, 40, 80 and 120 mM NaCl. Root and shoot mass, shoot length and characters of foliar damage (number of leaves with chlorosis and necrosis) of the populations were more affected by increasing NaCl than numbers of leaves and branches and root length. Although S. humilis has been considered a salt sensitive legume, significant differences in salt tolerance between populations were found. The estimated concentrations which reduced shoot dry mass by 50 and 25% varied between populations from 84 to 108 and from 49 to 83 mM NaCl, respectively. Salt tolerance in S. humilis during the initial growth stage was lower than the reported one for germination. With one exception, populations from semi-arid climate with saline soils showed higher salt tolerance than those from non saline soils. The results suggest that salt tolerance in these populations is mainly associated with the occurrence of salinity in the soil of their provenance.     

Ameliorative effect of brassinosteroid and ethylene on germination of cucumber seeds in the presence of sodium chloride

Brassinosteroids are a class of plant polyhydroxysteroids with a diverse of functions in plant growth and development, while ethylene is a gaseous hormone involved in regulation of numerous physiological processes. To evaluate the roles of BR and ethylene in seed germination under conditions of salt stress, effects of 24-Epibrassinolide (EBR) and 1-aminocyclopropane-1-carboxylic acid (ACC) on seed germination of cucumber (Cucumis sativus) seeds in the presence of 250 mM NaCl were investigated. Seed germination was significantly inhibited by the presence of NaCl in the incubation medium, and the inhibitory effect was significantly alleviated by addition of EBR and ACC to the incubation medium containing NaCl. There was an increase in ethylene evolution during seed germination and this increase was suppressed by salt stress. The reduction in ethylene evolution from imbibed seeds by salt stress was attenuated by EBR. Salt stress inhibited ACC oxidase (ACO) activity and EBR reversed the salt stress-induced decrease in ACO activity. Salt stress reduced expression of gene encoding ACO (CsACO2), and EBR reversed the salt stress-induced down-regulation of CsACO2. The alleviative effect of EBR on seed germination in the presence of NaCl was diminished by antagonist of ethylene synthesis, aminoethoxyvinylglycine. These results indicate that both ethylene and BR are likely to be associated with suppression of seed germination under salt stress and that the mitigating effect of BR on salt stress-induced inhibition of seed germination may occur through its interaction with ethylene synthesis.     

Breaking dormancy of <Emphasis Type="Italic">Stylosanthes humilis</Emphasis> seeds with low pH solutions

An acidic condition (low pH) of the germination media promoted dormancy breakage of scarified seeds of Townsville stylo (Stylosanthes humilis H.B.K.), an annual tropical forage legume, whether produced by either an unbuffered (HCl-KOH) or a buffered (phthalate, McIlvaine) medium. Except for aminooxyacetic acid, all ethylene biosynthesis inhibitors tested and supplied with the low pH solutions decreased germination to variable extents. Low pH-stimulated dormant seeds produced ethylene 4-fold as much than untreated seeds. Production of ethylene by seeds treated with high pH solutions, which did not affect their dormant state, was also very low.     

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.     

Comparison of the role of gibberellins and ethylene in response to submergence of two lowland rice cultivars, Senia and Bomba

We examined the gibberellin (GA) and ethylene regulation of submergence-induced elongation in seedlings of the submergence-tolerant lowland rice (Oryza sativa L.) cvs Senia and Bomba. Elongation was enhanced after germination to facilitate water escape and reach air. We found that submergence-induced elongation depends on GA because it was counteracted by paclobutrazol (an inhibitor of GA biosynthesis), an effect that was negated by GA₃. Moreover, in the cv Senia, submergence increased the content of active GA₁ and its immediate precursors (GA₅₃, GA₁₉ and GA₂₀) by enhancing expression of several GA biosynthesis genes (OsGA20ox1 and -2, and OsGA3ox2), but not by decreasing expression of several OsGA2ox (GA inactivating genes). Senia seedlings, in contrast to Bomba seedlings, did not elongate in response to ethylene or 1-aminocyclopropane-1-carboxylic-acid (ACC; an ethylene precursor) application, and submergence-induced elongation was not reduced in the presence of 1-methylcyclopropene (1-MCP; an ethylene perception inhibitor). Ethylene emanation was similar in Senia seedlings grown in air and in submerged-grown seedlings following de-submergence, while it increased in Bomba. The expression of ethylene biosynthesis genes (OsACS1, -2 and -3, and OsACO1) was not affected in Senia, but expression of OsACS5 was rapidly enhanced in Bomba upon submergence. Our results support the conclusion that submergence elongation enhancement of lowland rice is due to alteration of GA metabolism leading to an increase in active GA (GA₁) content. Interestingly, in the cv Senia, in contrast to cv Bomba, this was triggered through an ethylene-independent mechanism.     

Ethylene antagonizes the inhibition of germination in Arabidopsis induced by salinity by modulating the concentration of hydrogen peroxide

The capacity of plants to achieve successful germination and early seedling establishment under high salinity is crucial for tolerance of plants to salt. The gaseous hormone ethylene has been implicated in modulating salt tolerance, but the detailed role of how ethylene modulates the response of early seedling establishment to salt is unclear. To better understand the role of the ethylene signal transduction pathway during germination and seedling establishment, an ethylene insensitive mutation (ein2-5) and an ethylene sensitive mutation (ctr1-1) of Arabidopsis were analyzed under saline conditions and compared with the wild type plant (Col-0) as control. High salinity (>100?mM NaCl) inhibited and delayed germination. These effects were more severe in the ethylene insensitive mutants (ein2-5) and less severe in the constitutive ethylene sensitive plants (ctr1-1) compared with Col-0 plants. Addition of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) or inhibitors of ethylene action implied that ethylene was essential for early seedling establishment under normal and saline conditions. Salt stress increased the endogenous concentration of hydrogen peroxide (H₂O₂) in germinating seeds and ACC reduced its concentration. Our results suggest that ethylene promotes germination under salinity by modulating the endogenous concentration of H₂O₂ in germinating seeds. These findings demonstrate that ethylene is involved in regulating germination as an initiator of the process rather than consequence, and that ethylene promotes germination by modulating the endogenous concentration of H₂O₂ in germinating seeds under salinity.     

Phylogenetic relationships in the genusStylosanthes (Leguminosae) based upon chloroplast DNA variation

A detailed analysis of chloroplast DNA restriction fragment length variation was undertaken to reconstruct the maternal phylogeny of 18 taxa from both sections of the papilionoid tropical forage legume genusStylosanthes. Data were analysed by means of the computer program PAUP, using an heuristic search with Wagner parsimony. The resulting cladogram dividedStylosanthes into four separate clades, which comprised: (i) theS. guianensis complex and related species (i.e.S. gracilis, S. grandifolia andS. montevidensis); (ii)S. hispida, tetraploidS. hamata s. l.,S. sympodialis, S. humilis, S. leiocarpa, S. angustifolia and certain accesions ofS. scabra; (iii)S. calcicola, S. viscosa, diploidS. hamata s. str., andS. fruticosa, plus accessions ofS. scabra, S. capitata and one accession ofS. grandifolia; and (iv)S. macrocephala and other accessions ofS. capitata not included within clade 3. Results are generally congruent with previously established interspecific relationships and, moreover, enabled identification of putative maternal progenitors for four tetraploid taxa:S. humilis was identified as a likely maternal parent of bothS. sympodialis andS. hamata s. l.,S. viscosa as a maternal parent ofS. scabra, andS. macrocephala as a maternal parent ofS. capitata.     

Assessment of Genetic Diversity and Identification of Informative Molecular Markers for Germplasm Characterization in Caribbean Stylo (Stylosanthes hamata)

Caribbean stylo (Stylosanthes hamata) is a tropical fodder and cover crop. Along with four other Stylosanthes species (S. scabra, S. humilis, S. viscosa, S. guianensis), it was introduced in India. It became well adapted in certain parts of the country and has been recommended for the improvement of range and degraded lands. A collection of 63 S. hamata accessions was fingerprinted with RAPID, ISSR and STS markers. Though the mean discriminating power of these marker systems ranges from 0.65 to 0.71, high values of marker index (2.91), resolving power (14.92) and effective number of patterns per assay unit (50.65) makes ISSR as a better marker system in comparison to other two markers used in this study. Thirteen RAPD and eleven STS primers could differentiate a maximum of 42 and 17 accessions, respectively, whereas two ISSR primers produced distinct fingerprints of all the S. hamata accessions. Mean genetic similarities of accession ranged from 0.83 (ISSR) to 0.91 (RAPD). Two RAPD, two STS and four ISSR primers generated a set of 12 diagnostic markers which could be useful for germplasm characterization and management.     

The effect of plant growth regulators, nitric oxide, nitrate, nitrite and light on the germination of dimorphic seeds of Suaeda salsa under saline conditions

Suaeda salsa, a leaf succulent shrub in the family Chenopodiaceae, is one of the most important halophytes in China. Suaeda salsa produces dimorphic seeds (soft brown seeds and hard black seeds). Seeds of S. salsa were collected from the coastal salt flats near Huanghua City, China. Experiments were conducted to determine the salinity-alleviating effect of plant growth regulators, nitric oxide, nitrate, nitrite and light on the germination of dimorphic seeds of S. salsa. Brown seeds had a higher germination rate than black seeds in all experiments. Black seeds were more sensitive to salt in the absence of light in comparison to brown seeds. Brown seeds absorbed water more quickly in comparison to black seeds and were found to be more tolerant of salt stress. Our results showed that 1-aminocyclopropane-1-carboxylate (ACC, the immediate precursor of ethylene), nitrite, GA₄ and BA improved seed germination in the presence of salt. However, nitrate, GA₁, GA₃ failed to alleviate salt stress. ABA inhibited seed germination and seedling growth. Possible mechanisms involved in the alleviation of salt stress in S. salsa seeds and the ecological adaptation of the seeds to the environment are discussed.     

外源水杨酸对盐胁迫下大黄种子萌发及幼苗生长的影响

该研究以掌叶大黄、唐古特大黄和药用大黄种子为材料,采用双层滤纸培养法,设置系列浓度NaCl (0、100、150、200、250 mmol/L) 胁迫试验,以及系列浓度水杨酸(SA)溶液(0、50、100、150、200、250 mg/L)拌种和浸种后盐胁迫实验,测定3种大黄种子萌发及幼苗生长指标,揭示外源水杨酸对盐胁迫下大黄种子萌发及幼苗生长的影响。结果显示：(1)随NaCl浓度增大3种大黄种子的发芽率均呈直线下降趋势,且子叶、胚轴、根和苗等生长均受到强烈抑制。(2)在拌种条件下, 200 mmol/L NaCl胁迫下掌叶大黄苗长在200 mg/L SA处理下受到显著促进; 200 mmol/L NaCl浓度盐胁迫下唐古特大黄种子发芽率在250 mg/L SA处理下受到显著抑制;100 mmol/L NaCl胁迫下药用大黄种子发芽势在200 mg/L SA处理下受到显著抑制,其发芽率在150 mg/L SA处理下得到显著抑制,其苗长在250 mg/L SA处理下受到显著抑制。(3)在浸种条件下, 200 mmol/L NaCl胁迫下掌叶大黄种子发芽率在50 mg/L SA处理下显著提高,其幼苗根长和苗长的生长在250 mg/L SA处理受到显著促进;200 mmol/L NaCl胁迫下唐古特大黄种子的发芽势在200 mg/L SA处理下得到显著促进,其幼苗根和苗的生长在50 mg/L SA处理下得到显著促进;100 mmol/L NaCl 胁迫下药用大黄根和苗的生长在100 mg/L SA处理下均得到显著促进。研究表明,3种大黄种子和幼苗对盐胁迫的响应趋势一致,但对不同浓度SA拌种和浸种的响应有较大差异。     

Seed germination of the halophyteSuaeda japonica under salt stress

Suaeda japonica Makino belonging to the family Chenopodiaceae, is a halophyte and grows at the shore of Ariake sea in Japan. This plant presumably possesses high salt resistant nature, thus, we examined the mechanisms of seed germination under salt stress. The seeds maintained 80% germination rates on the medium containing 0.7 M NaCl. Germination rates varied depending on salt type; the germination rates under NaCl or KCI exhibited relatively lower values than ones under sodium gluconate or potassium gluconate. This different responses for salts seemed to be as a result of the presence of Cl⁻ ions. Although very high levels of betaine (compatible solute), were kept in the seedlings grown under no salt stress, the contents gradually increased as concentration of NaCl increased. Betaine is a factor present in plants that works to alleviate the effects of excessive soil salts. It is synthesized in leaves from betaine aldehyde, and this process is catabolized by betaine aldehyde dehydrogenase (BADH). When the seedlings were cultivated on the medium without NaCl, relatively high level of BADH activity was found. The activity increased 5-fold in the seedlings grown under 0.5 M NaCl stress. Increases in betaine content and BADH activity were found during seed germination. InS. japonica, the salt stress promoted BADH activity, subsequently endogenous betaine contents were increased, and increased betaine seemed to secure seed germination under salt stress.     

盐胁迫条件下提高内源乙烯对拟南芥幼苗生长和根尖活性氧水平的影响

以模式植物拟南芥（Arabidopsis thaliana）为材料,研究了内源乙烯对幼苗耐盐性的影响。研究结果表明,在施加了浓度为100 mmol·L^-1的NaCl胁迫的基质环境中,野生型拟南芥幼苗的根长和根重都显著减小。在施加外源乙烯利后不仅能够缓解盐胁迫对幼苗根伸长生长的抑制作用,而且能够缓解盐胁迫对幼苗根增重生长的抑制作用。施加外源ACC则只能缓解盐胁迫对幼苗根增重生长的抑制作用,而不能缓解盐胁迫对根的伸长生长的抑制。此外,100 mmol·L^-1 NaCl的胁迫条件下,拟南芥幼苗根尖中ROS水平明显升高,而施加了乙烯利和ACC处理下,幼苗根尖ROS的水平在NaCl胁迫下并没有明显的升高,说明内源乙烯可以调控植物体内的ROS维持在正常的水平,使植物体免受氧化损伤,从而提高了幼苗耐盐性。     

Ethylene promotes ethylene biosynthesis during pea seed germination by positive feedback regulation of 1-aminocyclo-propane-1-carboxylic acid oxidase

 Increased ethylene evolution accompanies seed germination of many species including Pisum sativum L., but only a little is known about the regulation of the ethylene biosynthetic pathway in different seed tissues. Biosynthesis of the direct ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), the expression of ACC oxidase (ACO), and ethylene production were investigated in the cotyledons and embryonic axis of germinating pea seeds. An early onset and sequential induction of ACC biosynthesis, accumulation of Ps-ACO1 mRNA and of ACO activity, and ethylene production were localized almost exclusively in the embryonic axis. Maximal levels of ACC, Ps-ACO1 mRNA, ACO enzyme activity and ethylene evolution were found when radicle emergence was just complete. Treatment of germinating seeds with ethylene alone or in combination with the inhibitor of ethylene action 2,5-norbornadiene showed that endogenous ethylene regulates its own biosynthesis through a positive feedback loop that enhances ACO expression. Accumulation of Ps-ACO1 mRNA and of ACO enzyme activity in the embryonic axis during the late phase of germination required ethylene, whereas Ps-ACS1 mRNA levels and overall ACC contents were not induced by ethylene treatment. Ethylene did not induce ACO in the embryonic axis during the early phase of germination. Ethylene-independent signalling pathways regulate the spatial and temporal pattern of ethylene biosynthesis, whereas the ethylene signalling pathway regulates high-level ACO expression in the embryonic axis, and thereby enhances ethylene evolution during seed germination. Received: 28 September 1999 / Accepted: 27 December 1999     

Isolation of ACC deaminase-producing habitat-adapted symbiotic bacteria associated with halophyte Limonium sinense (Girard) Kuntze and evaluating their plant growth-promoting activity under salt stress

Background and aims

Many plant growth-promoting endophytes (PGPE) possessing 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity can reduce the level of stress ethylene and assist their host plants cope with various biotic and abiotic stresses. However, information about the endophytic bacteria colonizing in the coastal halophytes is still very scarce. This study aims at isolating efficient ACC deaminase-producing plant growth-promoting (PGP) bacterial strains from the inner tissues of a traditional Chinese folk medicine Limonium sinense (Girard) Kuntze, a halophyte which has high economic and medicinal values grown in the coastal saline soils. Their PGP activity and effects on host seed germination and seedling growth under salinity stress were also evaluated.

Methods

A total of 126 isolates were obtained from the surface sterilized roots, stems and leaves of L. sinense (Girard) Kuntze. They were initially selected for their ability to produce ACC deaminase as well as other PGP properties such as production of indole-3-acetic acid (IAA), N₂-fixation, and phosphate-solubilizing activities and subsequently identified by the 16S rRNA gene sequencing. For selected strains, seed germination, seedling growth, and flavonoids production in axenically growth L. sinense (Girard) Kuntze seedlings at different NaCl concentrations (0–500 mM) were quantified.

Results

Thirteen isolates possessing ACC deaminase activity were obtained. The 16S rRNA gene sequencing analysis showed them to belong to eight genera: Bacillus, Pseudomonas, Klebsiella, Serratia, Arthrobacter, Streptomyces, Isoptericola, and Microbacterium. Inoculation with four of the selected ACC deaminase-producing strains not only stimulated the growth of the host plant but also influenced the flavonoids accumulation. All four strains could colonize and can be re-isolated from the host plant interior tissues.

Conclusions

These results demonstrate that ACC deaminase-producing habitat-adapted symbiotic bacteria isolated from halophyte could enhance plant growth under saline stress conditions and the PGPE strains could be appropriate as bioinoculants to enhance soil fertility and protect the plants against salt stress.     

油菜素内酯浸种对盐胁迫番茄种子萌发的影响及其生理机制

为揭示油菜素甾醇类化合物提高作物耐盐的效应和机理,研究了10^-11、10^-10、10^-9、10^-8、10^-7、10^-6、10^-5 mol/L 2,4-表油菜素内酯（EBL）浸种处理对0、50、100、150、175 mmol/L NaCl胁迫7 d的番茄种子萌发、生长、溶质积累、抗氧化代谢的影响。结果显示：NaCl浓度越高的盐胁迫下,10^-9 mol/L EBL浸种可体现出越显著的促进番茄种子萌发的效应;在所有处理下,EBL浸种浓度过高,即10^-6、10^-5 mol/L EBL,均表现出对种子萌发的抑制效应。盐胁迫下种子萌发后,一定浓度的EBL浸种可表现出明显的增加种子胚根和下胚轴长,提高萌发种子鲜重和种子活力指数,其中10^-9 mol/L EBL浸种处理促进效果最适;EBL浸种浓度过高,则表现出抑制效应。150 mmol/L NaCl胁迫或非盐胁迫下,10^-9 mol/L EBL浸种均可降低萌发种子体内的O₂^·-、H₂O₂、丙二醛（MDA）和脯氨酸（Pro）含量;盐胁迫下,10^-9 mol/L EBL浸种可显著提高萌发种子可溶性糖（SS）和可溶性蛋白（SP）的含量。150 mmol/L NaCl胁迫或非盐胁迫下,10^-9 mol/L EBL处理可不同程度促进番茄种苗超氧化物歧化酶（SOD）和过氧化物酶（POD）活性的上升。综上所述,盐胁迫下,一定浓度范围内的EBL浸种可明显促进番茄种子萌发或成苗,其中以10^-9 mol/L EBL浸种的效果最好,主要是因为EBL施用可积极促进番茄种子萌发中物质转化,SS和SP等溶质积累增多,增强其渗透调节能力;同时SOD和POD酶活增强,缓解盐胁迫导致番茄种子萌发中的次生氧化胁迫。     

Identification and Metabolism of 1-(Malonylamino)cyclopropane-1-carboxylic Acid in Germinating Peanut Seeds

Peanut seeds (Arachis hypogea L. Yue-you 551) contain 50 to 100 nanomoles per gram conjugated 1-aminocyclopropanecarboxylic acid (ACC). Based on paper chromatography, paper electrophoresis, and gas chromatography-mass spectrometry, it was verified that the major ACC conjugate was N-malonyl-ACC (MACC). Germinating peanut seeds converted [2-¹⁴C]ACC to ethylene 70 times more efficiently than N-malonyl-[2-¹⁴C]ACC; when ACC was administered, most of it was metabolized to MACC. Germinating peanut seeds produced ethylene and converted l-[3,4-¹⁴C]methionine to ethylene; this ethylene biosynthesis was inhibited by aminoethoxyvinylglycine. These data indicate that MACC occurs in peanut seeds but does not serve as the source of ethylene during germination; ethylene is, however, synthesized from methionine via ACC.