Ethylene can trigger the senescence process, especially in the sensitive species. The ethylene biosynthesis is higher during the first stage of leaf formation and declines until it reaches maturity when the leaf is completely expanded, then it increases again during the early step of the senescence initiation. The ACC content only increases in senescing leaves and shows the same pattern of ethylene production Hunter et al.
At the molecular level, it has been shown that different genes of the same family encode for the enzymes of ethylene biosynthesis that are activated during leaf development and their expression is timely regulated Hunter et al. The biosynthesis occurs in any part of the plant and at any stage of leaf development. Consequently, the biological responses depend on the tissue sensitivity. The plant responses to ethylene vary considerably between and within species and are modulated by differential hormonal sensitivity.
The visual symptoms of leaf senescence are represented by chlorophyll degradation and the leaf abscission Lewington et al. At the molecular level, ethylene has been shown to be involved in the organized cell dismantling and the activation of nutrients recycling from senescing leaves to the other organs. Leaf cells undergo a sequential and organized dismantling process which includes nucleic acid reduction, protein degradation, and turnover reduction, Lutts et al. Leaf senescence is activated at the mature stage of leaf development when leaves are fully expanded.
During leaf senescence, three different stages can be identified: initiation, organization of degradation, and death processes. The most common visible symptom of leaf senescence is the yellowing caused by the chlorophyll degradation and its impaired biosynthesis. The initial step of chlorophyll breakdown is catalyzed by chlorophyllase that convert chlorophyll a and b to chlorophyllide and phytol Matile et al. Chlorophyll loss increases after ethylene exposure in different cut flowers such as stock Matthiola incana and chrysanthemum Dendranthema grandiflora; Reyes-Arribas et al.
Tobacco leaves treated for 24 h showed higher chlorophyll degradation but did not anticipate the increase in ethylene biosynthesis and respiration, typically of the climacteric trend Aharoni and Lieberman, The effect of ethylene was found tightly associated with leaf age as demonstrated in old Arabidopsis mutants and also depended on the length of the treatment Jing et al.
Another effect induced by ethylene on leaf senescence is the abscission or induction of necrosis. Leaf abscission is a coordinated process, which involves several structural changes in the cells located in the abscission zone. It is a seasonal process that normally occurs in deciduous plants Taylor and Whitelaw, , where ethylene and auxins also have a crucial role. Interaction between Ethylene and Other Hormones during Leaf Senescence Plant hormones can repress or enhance leaf senescence in plants or after harvest.
This section gives an insight into the interaction of ethylene with other hormones and provides responses during leaf senescence. Ethylene and Auxin Leaf senescence is affected by auxin content and ethylene biosynthesis Ferrante and Francini, In particular, leaf abscission is under the control of auxin and ethylene.
Burg suggested that ethylene caused leaf abscission in vivo by inhibiting auxin synthesis and transport or enhancing auxin degradation, thus, lowering diffusible auxin level.
In the abscission zone, ethylene and auxin act antagonistically and auxin concentrations were associated with tissue sensitivity to ethylene. The equilibrium between ethylene and auxin is crucial for the regulation of leaf abscission. During leaf senescence, the auxin concentration declined and tissue sensitivity to ethylene increased as well as ethylene biosynthesis Brown, Using a transcriptome approach, 1, transcription factors TFs were found to be differentially regulated in the soybean leaf abscission.
Among these, TFs were differentially expressed in the abscission zone Kim et al. Ethylene and auxin were strongly regulated by these transcription factors. However, the exogenous applications of these hormones also regulated the expression of these genes delaying or anticipating the leaf senescence and abscission.
Riov and Goren suggested that ethylene inhibited auxin transport in the veinal tissues and reduced the amount of auxin transported from the leaf blade to the abscission zone in orange Citrus sinensis , necklace poplar Populus deltoids , and eucalyptus Eucalyptus camaldulensis.
La Rue showed that the removal of leaf blade induced abscission, but the application of auxin to the site of removal resulted in the inhibition of abscission. Ethylene has been shown to play an antagonistic role to auxins in the abscission of various organs. Abscission was delayed in the ethylene-insensitive Arabidopsis mutants ein2 and etr Patterson and Bleecker, , while ethylene application hastened abscission in various organs and species. Ethylene induced the expression of a polygalacturonase which is required for cell separation in tomato petioles Hong et al.
This suggested the antagonistic effects of auxin and ethylene in the abscission. Ethylene and Cytokinins Cytokinins can suppress leaf senescence leading to greater retention of chlorophyll known as Richmond and Lang demonstrated.
The effect of cytokinins on leaf senescence was demonstrated by the autoregulation of cytokinins biosynthesis during senescence using an isophentenyl transferase IPT gene under the regulation of senescence-associated gene 12 SAG12 promoter Gan and Amasino, This promoter has been widely used to activate genes expression during senescence.
The SAG12 gene encodes for a cysteine protease that was activated during senescence independently from the trigger events. Therefore, the SAG12 promoter can have great application in the agricultural science and the postharvest sector. Deletion studies on the SAG12 promoter demonstrated that young and mature leaves contained factors that exhibited differential binding to the senescence responsive promoter element Noh and Amasino, This strategy was effective in delaying leaf senescence in several crops such as alfalfa Medicago sativa; Calderini et al.
The senescence delay reduced ethylene biosynthesis in the transformed plants. The exogenous application of cytokinins in potted and cut flowers delayed the leaf yellowing and decreased ethylene biosynthesis.
The 6-benzyladenine BA applied as pulse treatments successfully delayed leaf yellowing in cut goldenrod Solidago canadensis; Philosoph-Hadas et al. The effect of BA treatment on the ethylene is due to the inhibition of leaf senescence that leads to lower ethylene biosynthesis. Ethylene and Gibberellins Gibberellins are considered as leaf senescence inhibitors and are able to avoid or delay leaf yellowing.
Gibberellins are commonly used as postharvest treatments in several cut flowers to prevent the leaf yellowing Ferrante et al.
The reduction of functional gibberellins content or the conjugation of them with glucose inactivation induced leaf yellowing in several sensitive species. The exogenous applications are able to delay senescence and reduce ethylene biosynthesis.
In cut stock flowers, the gibberellin 3 GA3 applications did not enhance the ethylene biosynthesis, but strongly increased ethylene production, combining with thidiazuron TDZ Ferrante et al. However, leaf yellowing was not affected by the ethylene production. This showed that the tissues were insensitive to ethylene because the leaves probably were not ready to senesce. However, further research should be taken into consideration to reveal the exact role of both the hormones in leaf senescence.
Ethylene and Abscisic Acid Abscisic acid is considered a leaf senescence inducer and its exogenous applications lead to leaf senescence in mature leaves of different crops. The saul1 mutant Senescence-Associated E3 Ubiquitin Ligase 1 naturally exhibited an accelerated leaf senescence phenotype with an increase of the ABA level, providing genetic evidence of the ABA signaling role during leaf senescence Raab et al.
The exact timing of flowering can be controlled by the plant-environment interaction and endogenous developmental competence of plants to flower, which allows the transition from the vegetative phase to a reproductive phase Lin et al. Changes in the levels of ethylene influence the genetic circuits that integrate different signals for the regulation of flowering timing.
In Arabidopsis, through the growth comparison of ethylene-related mutants, eto1, etr1, ein and ein, with the wild-type WT , the regulatory role of ethylene in the transition from vegetative to reproductive growth in Arabidopsis was discovered Ogawara et al.
The ethylene-overproducing mutant eto1, produces an excessive amount of ethylene Guzman and Ecker, by affecting the post-transcriptional regulation of a key enzyme of ethylene biosynthesis, the 1-aminocyclopropanecarboxylic acid synthase ACS Woeste et al.
These perturbations in the ethylene signaling may flow large or less amount of ethylene signal respectively, into the hormonal pathway leading to an early- or late-flowering phenotype compared to WT Ogawara et al. However, the effects of ethylene in the regulation of flower transition appear complex. In addition, contrasting roles of ethylene have been noticed in rice Oryza sativa. Ethylene promotes a reproductive transition in rice through the activity of its receptor protein OsETR2 Wuriyanghan et al.
Conversely, flowering time was delayed in Osctr2 loss-of-function and 35S:OsCTR2 transgenic lines, indicating that ethylene represses the floral transition in rice Wang Q. These evidences suggest that ethylene signaling delays flowering in both rice and Arabidopsis. On the other hand, exogenous ethylene, or ethephon, has been widely used to induce flowering of Bromeliads, such as Ananas comosus and Aechmea fasciata, as well as early sprouting, early flowering and formation of more flowers per inflorescence in dormant corms of common triteleia Triteleia laxa; Han et al.
Furthermore, an inhibitor of ethylene biosynthesis, amino vinylglycine AVG , can delay the natural flowering of pineapple Kuan et al. LeACS2 through Arg resulted in complete inactivation of the enzyme, while deletion of 46—52 amino acids from the C-terminus resulted in an enzyme that had nine times higher affinity for the substrate S-AdoMet than the wild-type enzyme. The individual ACS proteins have sequence variation in the C-terminal regions that influence the stability of the corresponding protein through post-translational modification.
Based on distinct consensus sequences present in the C-termini, the ACS isoforms can be grouped into three types types 1—3 Yoshida et al. Whether or not this classification reflects different cellular functions of the ACS proteins is as yet unknown, but each subgroup seems to have common post-translational modifications. Joo et al. They proposed that phosphorylation of ACS6 introduced negative charges to the C-terminus of the protein, which reduced the turnover by the degradation machinery and therefore enhanced the stability of ACS proteins.
In Arabidopsis, the dominant ethylene-overproducing mutants eto2 and eto3 were shown to have mutations in the C-termini of the type 2 isoforms, ACS5 and ACS9, respectively Chae et al.
The ETO1 protein directly interacts with and inhibits the enzyme activity of full-length ACS5 but not of a truncated form of the enzyme, resulting in a marked accumulation of ACS5 protein and ethylene production Chae et al. The enzyme had defied early attempts at purification, probably because it was wrongly assumed to be a membrane-bound enzyme.
A systematic search identified a candidate tomato cDNA called TOM13, which was induced by wounding, and during senescence and ripening, which are all situations when ethylene synthesis increases Smith et al. Work by Holdsworth et al. Hamilton et al. The predicted structure of the protein indicated that it was likely to be soluble, rather than membrane bound, and also indicated the nature of cofactors it might require, thus giving vital clues that led to the first purification of the enzyme John, ACO is a member of a superfamily of non-haem iron oxygenases and oxidases, most of which utilize Fe II as a cofactor and 2-oxoglutarate 2OG as a co-substrate John et al.
No significant differences in optimum pH 6. The side chains of Arg and Arg are proposed to be involved in binding bicarbonate, which leads to the activation of the ACO enzyme Zhang et al. The bicarbonate-dependent two-electron oxidation of ACC occurs concomitantly with the reduction of dioxygen and oxidation of a reducing agent, probably ascorbate, to produce ethylene, CO2, cyanide, and two water molecules Bassan et al. In the absence of bicarbonate, ACO undergoes rapid inactivation as it is unable to oxidize ACC to ethylene efficiently.
ACO is encoded by a multigene family in all plant species studied. The Arabidopsis genome encodes five ACO genes. Expression analysis reveals that the ACO genes display a high degree of differential expression in tissues at various stages of the life cycle. For example, tomato ACO1 is the main gene required during fruit ripening Blume and Grierson, , and ACO1, 2, and 3 transcripts accumulate during the senescence of leaves, fruit, and flowers Barry et al.
Four ACO genes are expressed during flower development, with each showing a temporally and spatially distinct pattern of accumulation Barry et al. ACO1 is predominantly expressed in the petals and the stigma and style, ACO2 expression is mainly restricted to tissues associated with the anther cone, whereas ACO3 transcripts accumulate in all of the floral organs examined apart from the sepals Barry et al.
The levels of ACO transcripts have been shown to be regulated by ethylene itself and other phytohormones. Chae et al. The auxin-induced OsACO2 expression was partially inhibited by ethylene, while ethylene induction of OsACO3 transcription was completely blocked by auxin, indicating that the two genes are regulated by complex hormonal networks in a gene-specific manner.
Furthermore, post-translational modification was also suggested as they found that okadaic acid, a potent inhibitor of protein phosphatase, effectively suppressed the IAA induction of OsACO2 expression. It has been suggested that the product of the E8 reaction participates in feedback regulation of ethylene biosynthesis during fruit ripening, but the mechanism remains to be elucidated. Homeotic proteins transcriptionally regulate ACS and ACO genes Transcriptional regulation of both ACS and ACO in response to various developmental and environmental factors, including floral organ development, ripening, senescence, and stresses such as wounding, pathogens, ozone, and UV-B, has been observed in all plant species studied Fig.
Identification of the corresponding transcriptional regulators, however, has proved difficult despite the presence of numerous putative cis-elements in the promoter sequences of ACO and ACS genes. For example, the LeACO1 promoter sequences contain ethylene-responsive elements EREs , core motifs of transcription factors, such as GT-1 and Dof1 that have been shown to be important for light-regulated genes, and putative homeobox protein-binding sites Z Lin and D Grierson, unpublished data.
Nevertheless, two recent publications have reported that ethylene biosynthetic genes are transcriptionally regulated by homeotic proteins Ito et al. These hormones overlap signal transduction pathways or gene expression profiles by rapid induction or by preventing the degradation of transcriptional regulators [ 2 — 5 ].
Among all of the described phytohormones, ethylene, a naturally occurring triple response growth regulator shoot elongation, stem thickening and horizontal growth habit in seedlings, has been studied since ancient times [ 6 ]. Ethylene is also involved in leaf abscission, fruit ripening and senescence [ 6 , 7 ] as well as seed germination, growth of adventitious roots under flooding conditions, epinasty stimulation, inhibition of shoot growth and stomatal closing and flowering [ 8 , 9 ].
Moreover, it is involved in a wide variety of stresses, including wounding, pathogen attack, flooding, drought, hypoxia, and temperature shifts [ 10 , 11 ]. MTA is recycled through a series of Yang cycle reactions back to methionine [ 14 ].
Furthermore, the ACSs can be regulated by putative endogenous signal receptors i. This process activates kinase protein signaling, which culminates in the stabilization of type II ACSs. Furthermore, MPK kinases are able to phosphorylate the C-terminal of type I ACSs, which preserve and stabilize their degradation via the 26S proteasome pathway, thereby increasing the production of ethylene and inducing other ethylene-dependent signaling pathways [ 21 ].
The enzyme directly responsible for the ethylene biosynthesis is 1-aminociclopropaneacid carboxylic oxidase ACO or EFE - ethylene forming enzyme; EC 1. Several reports have suggested that the ACC metabolite could combine with other organic molecules. Different studies have demonstrated that the ACC N-malonyzation pathway in various plant tissues is involved in the regulation of ethylene production, wherein the conjugate 1-malonyl-ACC MACC is formed by 1-aminocyclopropaneacid carboxylic acid-N-malonyltransferase, an enzyme that has been purified from plant protein extracts but without reference to its respective gene [ 23 , 24 ].
The ACD gene was first identified in A. The classic routes of ethylene intracellular signal transduction, initially described in A. Both receptors and CTR1 function as negative regulators of the signal transduction pathway in the absence of ethylene.
Thus, in the absence of ethylene, the phosphorylated EIN2 C-terminal domain is ubiquitinated and then degraded by the 26S proteasome [ 30 ]. Thus, these factors that accumulate in the nucleus interact with target gene promoters and trigger different ethylene responses [ 33 ]. Ethylene signal transduction triggers substantial changes in the gene expression of plant cells. Promoter region analyses of the genes induced by ethylene led to the identification of cis-acting elements as well as the trans-acting protein EREBP ethylene responsive element binding protein family, which interacts with DNA and ERFs ethylene response factors [ 35 — 37 ].
The mechanism underlying environmental stress tolerance has been extensively studied in model plants in attempts to determine its impact on agriculture [ 40 ].Interaction between Ethylene and Other Hormones during Flower Development The present section gives an insight into the interaction of ethylene with other hormones during flower development. Among these, TFs were differentially expressed in the abscission zone Kim et al. Thus, the combination of these data indicated the involvement of ethylene biosynthesis and signaling in soybean responses to water stress. Using an RNase protection assay, Barry et al enhanced ethylene levels. Chemical inhibitors of ethylene action are also useful for enzyme activity of full-length ACS5 but not of a truncated form of the enzyme, resulting in a marked accumulation of ACS5 protein and ethylene production Chae et. In Arabidopsis, these responses were at least in part The present section gives an insight into the interaction et al.
LeACS2 through Arg resulted in complete inactivation of the enzyme, while deletion of 46—52 amino acids from the C-terminus resulted in an enzyme that had nine times higher affinity for the substrate S-AdoMet than the wild-type enzyme. In tomato Solanum lycopersicum, syn. Ethylene governs the development of leaves, flowers, and fruits. In silver vase A. Interaction between Ethylene and Other Hormones during Leaf Growth and Development The following section highlights the interaction of ethylene with other hormones and plant responses during leaf growth and development. The ACD gene was first identified in A.
The response of leaf growth and development to ethylene depends on concentration and species involved in the study Fiorani et al. Soybean ethylene biosynthesis model Based on the model for ethylene biosynthesis in A.
However, the exogenous applications of these hormones also regulated the expression of these genes delaying or anticipating the leaf senescence and abscission. In contrast, ethrel ethylene source treatment decreased flower development due to the decreased endogenous level of auxin, while GA treatment significantly increased it Makwana and Robin,
The bicarbonate-dependent two-electron oxidation of ACC occurs concomitantly with the reduction of dioxygen and oxidation of a reducing agent, probably ascorbate, to produce ethylene, CO2, cyanide, and two water molecules Bassan et al. The Arabidopsis genome encodes five ACO genes. ACC synthase , ACC oxidase , ethylene biosynthesis , ethylene cross-talk , ethylene signalling , flower development , fruit ripening , sex determination , ubiquitin-mediated degradation Introduction In this review, original, or widely accepted, gene names have been used, and where synonyms exist this is indicated.
Several studies in model plants have evaluated the importance of this hormone in crosstalk signaling with different metabolic pathways, in addition to responses to biotic stresses. Table 1 Ethylene biosynthesis and signal transduction gene summary in different plants Full size table The putative soybean proteins that participate in the metabolic pathways involved in ethylene biosynthesis and signaling mediated by this molecule are highly conserved, with domains that have already been described for their homologs in model organisms. The study of Khan on mustard suggested that there exists a correlation between ethylene and growth of plants following the defoliation of mature leaves. Changes in ethylene level, its perception, and the hormonal crosstalk directly or indirectly regulate the lifespan of plants.
These data allowed for the inference of the first accurate in silico models for soybean ethylene biosynthesis and signaling, which facilitated a better understanding of the molecular mechanisms involved in this important phytohormone. Riov and Goren suggested that ethylene inhibited auxin transport in the veinal tissues and reduced the amount of auxin transported from the leaf blade to the abscission zone in orange Citrus sinensis , necklace poplar Populus deltoids , and eucalyptus Eucalyptus camaldulensis. Under water deficit, ethylene production was paralleled by an increase and subsequent decrease in ACC, suggesting that water stress induced the de novo synthesis of ACC synthase, which is the rate-controlling enzyme along the pathway of ethylene biosynthesis. Direct evidence in support of the role of methionine as an ethylene precursor in vivo was reported by Lieberman et al. In a classical study, Burg and Burg reported that auxin-induced flowering in pineapple by stimulating ethylene formation. The bicarbonate-dependent two-electron oxidation of ACC occurs concomitantly with the reduction of dioxygen and oxidation of a reducing agent, probably ascorbate, to produce ethylene, CO2, cyanide, and two water molecules Bassan et al.