**4. Role of ethylene in postharvest physiology**

Ethylene is a gaseous hormone which can be produced by almost all parts of higher plants. It is a colourless, odourless gas with solubility in water 20 mg/lit at 200 C and 250 mh/lit at 00 C [11]. The meristematic and nodal regions of plants play active part in ethylene biosynthesis. However, the production of ethylene gets accelerated during leaf abscission, senescence and ripening processes. Wounding in plant and physiological stresses like chilling, drought, and diseases also enhances ethylene production in plants [11]. The biosynthesis of ethylene initially begins with amino acid methionine (Met) which gets converted to S-Adenosyl methionine (SAM) in presence of enzyme Adomet synthase. The enzyme ACC synthase converts SAM to 1-Aminocyclopropane-1-carboxylic acid (ACC) which further gets converted ethylene by enzyme ACC oxidase which can be described as below (**Figure 3**).

Ethylene is considered as multifunctional phytohormone that controls both growth and senescence in plants [12]. It regulates the development of leaves, flowers and fruits and also promotes senescence depending upon the level of ethylene applied to plants [13–15]. It is known to regulate wide range of responses in plants namely viz., seed germination, cell expansion, cell differentiation, flowering, abscission, senescence etc. Some important physiological role of ethylene is described below (**Figure 4**).


### **Figure 4.**

*Physiological role of ethylene in plants.*

### **4.1 Ethylene as a fruit ripening hormone**

The term fruit ripening involves changes in texture of fruit including softening due to enzymatic breakdown of cell wall, starch hydrolysis, accumulation of sugar and absence of phenolic compounds in fruits that makes the same ready to eat. Ethylene has been identified as ripening hormone since past long years and increase in concentration of ethylene in such fruits accelerates ripening phenomenon. The fruits can be categorized into two major classes on the basis of ethylene production i.e., climacteric and non-climacteric. Climacteric fruits are those which shows sudden characteristics respiratory rise when ripen in response to ethylene. As much treatment with ethylene causes fruit to produce additional amount of ethylene, referred to as autocatalytic. On the other hand, fruits which do not shows the rise in respiration rate upon treatment with ethylene called as non-climacteric fruits.


The respiratory climacteric and ethylene production are two most important and decisive parameters in ripening process. Change in ripening patterns determines the fact that whether fruits has ripen naturally or artificially upon ethylene exposure. Although artificial ripening of climacteric fruits enhances ripening process but also results in spoilage of products as well deteriorating market quality and demand [16]. The elevation in rates of respiration enhances fruit ripening and diminishes the postharvest life of both climacteric as well as non-climacteric fruits. In climacteric fruits, ethylene accelerates the time without modifying magnitude in order to achieve maximum respiration rates while in non-climacteric, since it lacks autocatalytic activities once ethylene is removed the respiration process slows down and respiration rates progresses in concentration dependent manner [17]. Furthermore, it is matter of fact that climacteric respiration does not always associated with increased ethylene responses instead depends on fruit species. Although the biochemical responses on

#### *Role of Post-Harvest Physiology in Evolution of Transgenic Crops DOI: http://dx.doi.org/10.5772/intechopen.94694*

the same are not fully understood yet but the several physiological and molecular studies enlighten that ethylene is a primary factor which is responsible for increased respiration rates and it can be thus concluded that climacteric respiration is an ethylene regulated event [18]. Ethylene on the other hand, does not primarily involved in ripening phenomenon in non-climacteric fruits. But it is noteworthy that despite of lack of climacteric ethylene, the ripening responses of non-climacteric fruits are responsive to ethylene. Thus, ethylene is a crucial regulator of ripening process in climacteric fruits and also plays an important role in regulating the same in non-climacteric fruits as well. The presence of ethylene is not always desirable in entertaining shelf life of postharvest products. However, the extent of damage depends on ethylene concentration, length of exposure time and temperature of the product. Therefore, in order to achieve maximum postharvest benefits of produce the controlled use of ethylene is necessary in order to prevent damage and excessive ripening. Exposure of postharvest products to ethylene accelerates the rate of ageing and senescence (**Table 1**).

Ethylene biosynthesis in fruits occurs through Yang cycle. The biosynthesis of ethylene is governed by several multi genic families of ACS and ACO enzymes. The expression of ACS and ACO genes are controlled by several environmental & hormonal factors and it consists of positive and negative feedback regulation. As mentioned, fruits are divided into climacteric and non-climacteric on the basis of ripening phenomenon. In climacteric fruits like tomato normal fruit ripening involves ethylene burst. In climacteric plants, two systems of ethylene regulation are identified namely system I and system II. System I is ethylene auto-inhibitory, operates during vegetative growth and stimulates the synthesis of basal ethylene levels, detected in all tissues including non-climacteric fruits. System II remains functional during ripening of climacteric fruit and senescence phenomenon when ethylene synthesis is autocatalytic [19]. Tomato is a model plant to study ethylene biosynthesis and its perception in plants. Several ACS genes are also identified in other plants including apple, melon, pear, banana, citrus, papaya etc. In tomato, ripening results in change in fruit color from green to red, degradation of chlorophyll and accumulation of carotenoids. In tomato, 8 genes have been identified which includes LeACS1A, LeACS1B, LeACS2-7. LeACS2 and LeACS4 are greatly expressed during ripening process whereas LeACS1A and LeACS6 are expressed before onset of ripening. Further studies on mutants revealed that only LeACS6 is ethylene regulated while rest is unaffected. The genes LeACS1A and LeACS6 plays major part in ethylene production in SystemI whereas transition phase includes enhanced


#### **Table 1.**

*Effect of ethylene on postharvest quality.*

expression of LeACS1A and LeACS6 is also induced. The positive feedback regulation of gene LeACS2 maintains System II phase which gets started during transition phase. Similarly, in banana MaACS1 gene is related to ripening phenomenon as its transcript and ACC content enhances during ripening. In *Actinidia chinensis*, the levels of ACS mRNA is upregulated during climacteric ethylene production but ACS itself does not gets affected by exogenous ethylene [20]. Although ACS genes are transcriptionally regulated but post-translation regulation has also been reported. ACC oxidase (ACO) is another crucial enzyme which plays important role in ethylene biosynthesis. The level of ACO is enhanced in pre climacteric fruit before rise in ACS enzyme activity. ACO gene transcripts have been studied in various fruits like tomato, kiwi, pear, apple, banana etc. which regulated temporal and spatial expression. In tomato, so far 3 ACO genes have been identified namely LeACO1 (expressed in ripening fruit), LeACO2 (anther), LeACO3 mRNA (floral organs) with weak expression in fruit at breaker stage. In banana, MaACO1 mRNA is up regulated with onset of ripening but decreases during late ripening stage. In melon, Cm-ACO1 gene is highly expressed during ethylene production but Cm-ACO3 is induced in flowers only. In *A. chinensis*, exposure to ethylene induces up regulation of genes for ACO and Adometsynthetase enzyme and also before respiratory climacteric rise in ethylene biosynthesis.
