- 30.7F: Abscisic Acid, Ethylene, and Nontraditional Hormones
- Ethylene Role in Plant Growth, Development and Senescence: Interaction with Other Phytohormones
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- F: Abscisic Acid, Ethylene, and Nontraditional Hormones - Biology LibreTexts
The overproduction of cytokinins in petunia flowers transformed with P -S AG IPT has been reported to delay corolla senescence and decrease sensitivity to ethylene Chang et al. An increase in ethylene, in petunia flowers exogenously treated with cytokinin, was found during senescence, and the lack of a negative effect can be explained considering the expression of the ethylene receptors was down-regulated by treatment with BA Trivellini et al.
Similarly, the application of thidiazuron, a cytokinin-like compound, enhanced ethylene production but simultaneously extended vase life by inhibiting leaf yellowing in cut stock flowers Ferrante et al.
30.7F: Abscisic Acid, Ethylene, and Nontraditional Hormones
These results suggest that despite the enhanced ethylene production, flowers that accumulated cytokinins showed an increased flower longevity. In contrast, exogenous cytokinins delayed senescence, suggesting they might play a role in the regulation of the time of senescence Van Doorn et al. The HD—Zip I transcription factors are unique to plants and have been reported to be involved in various plant development responses, including flower senescence Xu et al.
Moreover, the silencing of the key regulatory enzyme in the GA biosynthetic pathway, RhGA20ox1 accelerated the senescence in rose petals. Another recent study suggests that a reduction in the bioactive GA content enhances the ethylene-mediated flower senescence Yin et al. In this study, the overexpression of a basic helix-loop-helix bHLH transcription factor, PhFBH4 , increased the abundance of transcripts of ethylene biosynthesis genes and also increased ethylene production. Another study reported that the transcriptome changes associated with delayed flower senescence on transgenic petunia by inducing the expression of etr , down-regulated genes involved in gibberellin biosynthesis, response to gibberellins stimulus, and ethylene biosynthesis, at different time points Wang H.
Similarly to the ethylene, ABA accumulation accelerates the senescence of cut flowers and flowering potted plants Ferrante et al. In rose, ABA was reported to increase the sensitivity of flowers to ethylene, as the gene expression of some ethylene receptors increased after exogenous ABA treatment Muller et al. The over-expression of PhHD-Zip accelerated petunia flower senescence and this condition is another example highlighting the interaction of different hormones Chang et al.
These results suggest that PhHD-Zip plays an important role in regulating petunia flower senescence. Moreover, a transcriptome study reported that several genes involved in ABA biosynthesis, catabolism, and signaling pathways were induced by exogenous cytokinins BA treatment Trivellini et al. In the experiment reported by Chang et al. These results suggest that in addition to the ethylene pathway, the cytokinins seem to be strongly involved in the regulation of ABA biosynthesis and its degradation in flower tissues, thus ABA plays a primary role in petunia flower senescence.
The fruit is the development of the ovary after the fertilization and protects the seeds until complete maturation. The seeds represent the germ plasm of the plants and are responsible for the dissemination of the species. From an ecological point of view, fruits during the unripe stage represent an organ that must be protected from insects or frugivores.
A fruit must be unattractive and its green color allows the camouflage itself with leaves. The ripening of fruits is a unique coordination of various biochemical and developmental pathways regulated by ethylene, which affects color, texture, nutritional quality and aroma of fruits Barry and Giovannoni, During ripening in climacteric fruits, the ethylene regulates firmness and color changes involving chlorophyll reduction, increase in carotenoids or anthocyanins, sugars, and biosynthesis of volatile organic compounds VOCs. Ethylene is tightly correlated with the VOCs biosynthesis, which increases in ripe fruit and enhances the attraction of frugivores.
It has been found that transgenic apples expressing antisense genes for ACS or ACO produced lower VOCs and in particular, the strongest reduction was observed in the esters, which were 3—4 fold lower compared with WT Dandekar et al. The exogenous application of ethylene reconverted the VOCs evolution. This result indicates that ethylene inhibits the key steps of volatile biosynthesis. The study with the application of 1-MCP or AVG demonstrated that ethylene regulates VOCs biosynthesis directly through the pathway of volatile biosynthesis and indirectly through the ethylene perception.
In fact, apricots Prunus armeniaca treated with ethylene biosynthesis inhibitor, such as AVG, strongly reduced the VOCs biosynthesis, while the 1-MCP, an ethylene action inhibitor, enhanced the evolution of aldehydes Valdes et al. A Schematic and simplified ethylene and VOCs biosynthesis during fruit development. VOCs biosynthesis derive from different pathways such as phenylpropanoids, fatty acid, and carotenoids degradation.
B The main enzymes involved in cell wall degradation during fruit ripening and senescence. The action of these enzymes induces loss of firmness and softening.
Ethylene Role in Plant Growth, Development and Senescence: Interaction with Other Phytohormones
In climacteric fruits, ethylene biosynthesis increases and shows a peak corresponding to respiration pattern, while in non-climacteric fruits the ethylene declines with fruit ripening and senescence. The tomato has been used as a model plant for studying the role of ethylene in fruit ripening. The transition from unripe to ripe fruit induces several biochemical changes that involve ethylene biosynthesis and perception. Unripe fruits produce a low amount of ethylene and are insensitive to exogenous ethylene. Hence, ethylene treatments do not induce the fruit ripening system 1.
At the beginning of ripening, ethylene production increases and induces an increase of autocatalytic biosynthesis. These fruits, in this development stage, if exposed to exogenous ethylene show a burst of ethylene production and ripen faster system 2. Fruits are classified in system 1 when they produce a low amount of ethylene and tissues are insensitive to exogenous ethylene Alexander and Grierson, The delay of ethylene increase is the most common strategy used in post-harvest for prolonging the storage and increasing the shelf life.
The inhibition of ethylene biosynthesis or action usually leads to an extension of shelf life of the climacteric fruits. Ethylene regulates fruit ripening by affecting the ACS and ACO genes and the fruit specific polygalacturonase, involved in the depolymerization of cell wall pectin during ripening Smith et al. It affects pectin methylesterase PME , which provides accessibility to pectin by polygalacturonase and phytoene synthase responsible for the pigmentation of many fruits and flowers Koch and Nevins, ; Fray and Grierson, Cloned mRNAs that accumulate in the unripe tomato fruits exposed to exogenous ethylene were investigated through blot hybridization experiment.
The expression of cloned genes was developmentally regulated by the ethylene during fruit ripening, with more mRNAs produced by these genes in ripe fruits than in unripe fruits and the increase in mRNA was repressed by norbornadiene, an ethylene action inhibitor Lincoln et al. Gene expression analysis of Never-ripe Nr and additional tomato receptor homologs indicated that Nr and LeETR4 transcripts were most abundant in the ripen fruit tissues Zhou et al.
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Alba et al. The mutation of the ethylene receptor Nr , which reduces ethylene sensitivity and inhibits ripening, also influenced fruit morphology, seed number, ascorbate accumulation, carotenoid biosynthesis, ethylene evolution, and the expression of many genes during fruit maturation, indicating that ethylene governed multiple aspects of development both prior and during fruit ripening in tomato Alba et al. In tomato, the E8 gene plays a role in the negative regulation of ethylene biosynthesis through repression of ethylene signal transduction.
The expression of the gene increased during ripening and its antisense repression resulted in an increased ethylene evolution but delayed ripening Penarrubia et al. The relationship between ethylene and auxin in the fruit development has been studied. Auxins are involved in fruit development and inhibit ripening Brady, The exogenous application of auxins in different fruits delayed the senescence such as observed in Bartlett pears Pyrus communis ; Frenkel and Dyck, , banana Musa acuminate ; Purgatto et al.
The application of auxin lowered the ethylene production in sliced apples Malus domestica , if applied at pre-climacteric phase, while enhancing its biosynthesis at the climacteric stage Lieberman et al. There exists a crosstalk between auxin and ethylene; and Bleecker and Kende pointed out that auxins can stimulate the biosynthesis of more climacteric ethylene through its inductive action on the expression of the key enzyme ACS Abel and Theologis, Ethylene and auxins are tightly related during fruit senescence.
The free auxin increases during senescence and stimulates ethylene biosynthesis. Further studies are required to understand the ethylene sensitivity changes after 1-MCP treatment. The nature and transcriptional response of CTG led to discovering a rise in free auxin in the 1-MCP treated fruits. The exogenous application of cytokinins or compounds with cytokinins-like activity increased the sugar content of fruits and induced earlier ripening.
Recent studies have shown that CPPU delayed the ethylene increase during fruit ripening and also delayed central placenta softening Ainalidou et al. In avocado, the application of isopentenyl adenosine increased the ethylene and fruit ripening Bower and Cutting, The studies regarding the role of cytokinins in the plant senescence are available in the literature, but the relationship between cytokinins and ethylene during fruit ripening and senescence has not yet completely been elucidated and needs further investigations.
In tomato fruit, ABA biosynthesis occurs via carotenoids degradation pathways and the key enzyme is the 9- cis -epoxycarotenoid dioxygenase NCED. The ABA content increases following the biosynthesis of carotenoids during ripening. These changes are associated with ripening and also with ethylene production. The exogenous application of ABA increases ethylene biosynthesis Mou et al. These results suggest that ABA can be a trigger for ethylene production and influence fruit ripening Zhang et al.
In banana fruit, ABA stimulates ripening independently from the ethylene. ABA application increases all hydrolases, which can enhance the softening, with exception to the polygalacturonase activity Lohani et al. Interestingly, these authors provide new insights into the regulatory mechanism underlying tomato fruit development and ripening with the ethylene involved in the downstream signal transduction of ABA and sucrose, as a negative regulator of ASR gene expression, which influenced the expression of several cell wall and ripening-related genes leading to fruit softening.
The relationship of other phytohormones such as ABA and GA with ethylene during fruit senescence needs to be elucidated.
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The loss of firmness or softening of fruits is a very important quality parameter. The softening is due to cell wall degradation induced from several enzymes that are synergistically activated. Almost all these enzymes are encoded by multi-genes family, which regulates the spatial-temporal activation of these enzymes. Ethylene plays a crucial role in regulating these genes and enzymes during ripening and senescence. The cell wall degradation is facilitated by expansins that are proteins, which are involved in the enlargement of cell matrix.
This phenomenon occurs during cell wall growth and disruption. The action of these enzymes has been found to be tightly associated with the fruit ripening and senescence Civello et al. The expansins are tightly dependent on pH. The transcription of these enzymes is carried out by gene families, which have been isolated and characterized in several plant species.
Different isoforms can provide the expansins action during plant growth and fruit senescence, linking the development stage with the activation of specific isoforms. The inhibition of ethylene biosynthesis also reduced and inhibited the EXP1 gene expression Rose et al.
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The activation of the expansin EXP1 has also been shown in other climacteric fruits such as banana Trivedi and Nath, Pectin methylesterase is an enzyme activated before fruit ripening and catalyzes the de-esterification of pectin, by removing the methyl group C-6 of galacturonic acid and allows the polygalacturonase action.
The PME has an important role during fruit senescence and cell wall degradation with loss of firmness. This enzyme is stimulated by ethylene and inhibited by ethylene inhibitors such as 1-MCP El-Sharkawy et al. This enzyme is activated after the action of PME and is also induced by ethylene.
F: Abscisic Acid, Ethylene, and Nontraditional Hormones - Biology LibreTexts
In antisense ACC synthase tomato, the exposure to ethylene rapidly increased transcript accumulation of the PG. The gene expression of PG was directly correlated with ethylene concentrations used Sitrit and Bennett, Bananas treated with ethylene increased the activity of this enzyme, while the use of 1-MCP reduced its activity Lohani et al. Analogous results were observed in mango treated with ethylene for inducing ripening or treated with 1-MCP for delaying ripening Chourasia et al.
The cell wall degrading enzymes is sequentially activated during ripening and senescence. Ethylene is one key regulator of these enzymes at transcriptional and post-transcriptional level Figures 2A,B. It may be summarized that ethylene plays a key role in plant growth and development. The action of ethylene in the growth and development may not be isolated.
It triggers the network of signaling pathways and influences through the interaction with other phytohormones regulation of several processes. The understanding of the crosstalk between ethylene and other phytohormones in regulating growth and senescence could provide a promising strategy to manipulate the content of these hormones through molecular techniques in order to get specific plant responses.
During plant life, the transition from vegetative to reproductive stages and senescence is largely influenced by ethylene and its interplay with other plant hormones. This networking not only influences the ethylene concentration but also tissues sensitivity.
go There are few studies focusing on the molecular changes in plant tissues after the combined treatments of ethylene with other plant hormones. These studies should be extended to different organs and development stages to deeply understand the intricate network affecting relevant agronomic traits such as yield, longevity, and appearance morphology.
The discovery of new synergistic or antagonist relationships among ethylene and other hormones can have great potential to support cell division and differentiation processes during plant development, to enhance crop yield by delaying aging and prolong shelf-life of flowers and maintain the quality of climacteric fruits. Moreover, the equilibrium between the ethylene biosynthesis and its perception influences the crop adaptability and performance under different stress conditions.