Introduction
The postharvest longevity of cut roses varies by physiological and morphological factors, which are determined by the interaction between variety and preharvest growth conditions (In et al. 2007). Cut roses often end their vase life at the early stage of flower development and this results from complicated interrelations between various physiological process in the floral organs, leaves and stems. The brief vase life of cut roses is mostly attributed to early occurrence of negative water balance, which occurs when water loss exceeds the absorption of the stems, and damage by hormone ethylene. Previously, it has been concluded that ethylene is amongst the most important factors that cause premature of flowers, acceleration of flower opening, wilting and abscission of petals and leaves, discoloration of petals and leaves, and bending of peduncles (In et al. 2017;Macnish et al. 2010). In addition, cut rose cultivars often show different response to both exogenous and endogenous ethylene.
Ethylene plays an essential role in regulation of flower senescence via changing a sequence of molecular signaling during the flower development. In multiple flower species, ethylene perception by ethylene receptors is indispensable for the onset and the sustenance of the ethylene-dependent senescence program. When ethylene is perceived by its receptors, ethylene signal is sent through physiological and biochemical events that regulate transcriptional changes in ethylene signaling genes, and thereby leads to ethylene responses and senescence in flowers (Binder 2020). In plants, ethylene synthesized under the metabolic regulation that highly controlled during the flower development and senescence. First, S-adenosyl-L-methionine (SAM) is converted to the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase (ACS). Subsequently, ACC is converted to e thylene by A CC o xidase ( ACO) (Yang and Hoffman 1984). During flower development, the sensitivity and response to ethylene are mostly mediated by the variation in transcript levels of ACS1, ACO1, and CTR1, which is the immediate downstream component of ethylene receptors (In et al. 2013).
The negative effects of ethylene have significantly reduced the postharvest quality and competitiveness of cut rose flowers in the wholesale and retail markets. However, there is currently no effective technology for the control and removal of ethylene in rose flowers. Based on the current literatures, in this review, we discussed the impacts of ethylene on the longevity, flower senescence, postharvest quality, and relationship between ethylene sensitivity and gene expression as well as the use of ethylene antagonists in rose flowers. Understanding the ethylene role and flower responses to the hormone in roses will help us to develop the effective method for controlling ethylene and improving the postharvest quality of cut flowers.
Effect of Ethylene on Postharvest Quality of Cut Roses
Ethylene can either be produced in response to stresses and by senescing endogenous ethylene within plant organs or exogenous ethylene in the environment. The postharvest quality of cut rose is often influenced by ethylene during transport and handling to wholesale markets or retailer shops where the air is often contaminated with ethylene (Müller et al. 1998;Müller et al. 2000). Additionally, the unfavorable conditions such as darkness, shaking, or high temperature during transport, can decrease the vase life of cut rose flowers due to the production of endogenous ethylene in response to the stress conditions. However, the concentration of internal ethylene production in cut roses in response to the stresses was highly cultivar specific (Nell and Noordegraaf 1991).
Many senescence symptoms such as petal wilting, color change, failure of flower opening, or petal abscission were strongly accelerated by ethylene in potted rose flowers (Müller et al. 1998). In cut roses, exogenous ethylene has many negative effects such as accelerated abscission in leaves and petals, premature wilting, fresh weight loss, flower opening, leaf yellowing, and stimulation in autocatalytic ethylene production (Müller et al. 2001;Serek et al. 1967;Tan et al. 2006;Xue et al. 2008). Exogenous ethylene decreased longevity of miniature rose and cut rose cultivars (Müller et al. 1998;Müller et al. 2000;Müller et al. 2001;Serek et al. 1996). Treatment with ethylene for 48 h promoted, inhibited, or showed no effect on flower opening of 27 rose cultivars (Reid et al. 1989). Treatment with ethylene at different doses (1, 10, and 100 μL・L-1) significantly decreased the vase life of cut rose flowers, with 100 μL・L-1 exhibiting the greatest effects (Chamani et al. 2005). Exposure to ethylene (1 μL・L-1) for 2 4 h significantly decreased vase life of 27 rose cultivars by accelerating petal abscission and wilting, and suppressed the flower opening of 17 rose cultivars (Macnish et al. 2010). It has also been indicated that treatment with 10 μL・ L-1 ethylene for 20 h reduced the longevity in 21 cut rose cultivars (about 63.6% of the tested cultivars) (In et al. 2017). Among these, the vase life of 17 rose cultivars was significantly decreased by 2–5 days, compared with non-treated control flowers. Cut rose flowers generally end their vase life by early petal wilting, discoloration, bending of peduncles (bent neck), petal abscission, and leaf yellowing and abscission (Fig. 1A). Ethylene exposure greatly increased petal wilting to more than 40% in both ethylene-sensitive and –insensitive rose cultivars (Fig. 1B, C). Ethylene exposure also can often cause abnormal flower shapes (bull bud) during vase life. Compared with the non-treated control flowers, 20 cultivars exhibited a greater than 20% reduction in maximum flower diameter after ethylene treatment (In et al. 2017).
Ethylene Production and Sensitivity in Roses
Roses have been classified as ethylene-sensitive flowers, but the nature of the ethylene sensitivity change in rose flowers has not been well studied. Ethylene sensitivity and production in roses are highly fluctuating among cultivars (Müller et al. 1998;Müller et al. 2001). The rise of internal ethylene production in rose flowers was up-regulated by ethylene biosynthesis genes RhACSs and RhACO1 (Müller et al. 2001). Almost rose cultivars exhibit a climacteric rise in ethylene production during flower senescence, although the longevity is fluctuating among cultivars (Macnish et al. 2010;Müller et al. 2001). Some rose cultivars show a climacteric rise in endogenous ethylene production during flower senescence, while other cultivars exhibit a medium or very low ethylene production. The longevity of cut rose flowers in absence of exogenous ethylene can be depended on a function of endogenous ethylene production. For instant, the short vase life of flowers in cultivar ‘Bronze’ is related to a climacteric peak in ethylene production, similar to other climacteric flower species. Whereas, the long vase life of ‘Charming Parade’ rose is related to a very low endogenous ethylene production by the flowers (Müller et al. 1998). However, there is not always the direct interrelationship between the ethylene sensitivity and longevity in rose flowers. Although the ‘Charming Parade’ rose flowers exhibited the long vase life under the ethylene-free condition, this cultivar was highly sensitive to exogenous ethylene (Müller et al. 1998). The rose cultivar ‘Vanilla Kordana’ showed a climacteric rise in ethylene production during senescence process, although its vase life was long (Müller et al. 1998).
In roses, ethylene treatment led to enhance ethylene production of flowers, relating to high expression levels of RhACO1 and RhACSs (In et al. 2017;Müller et al. 2001). Recently, the relationship between ethylene sensitivity and vase life of cut flowers has been also studied in 33 rose cultivars (In et al. 2017). The vase life of roses varies approximately 280% among the cultivars (Table 1). Based on the vase life change subsequent to ethylene exposure (10 μL・L-1), rose cultivars were classified as ethylene-sensitive (SENS; vase life decreased by 2-5 d) and ethylene-insensitive (INSENS; vase life decreased by 0-1.9 d) groups (Table 1). Interestingly, the ethylene sensitivity of the rose cultivars is not significantly related to the vase life span of cut flowers. After exposure to ethylene, the vase life of SENS flowers was mainly terminated by the petal wilting (Fig. 1), which is a common senescence symptom that observed in typical ethylene-sensitive floral organs. Ethylene also induced the mRNA levels of ethylene biosynthesis genes (RhACS2, RhACS4, and RhACO1) in SENS flowers. The vase life reduction of SENS flowers was strongly related to the mRNA level of RhACO1 in petals (Fig. 2A), but that of INSENS flowers was closely correlated with water relations rather than the increase in the gene expression (Fig. 2B).
Expression and Role of Ethylene-related Genes in Roses
In roses, transcripts of five ethylene receptors, RhETR1, RhETR2, RhETR3, RhETR4 and RhETR5, have been identified. RhETR1, RhETR2 and RhETR4 expose high amino acid identity with Arabidopsis and other receptors with three membrane-spanning domains, while RhETR3 has high identity to receptors with four membrane-spanning domains (Müller et al. 2000). The differences in the receptor levels were strongly related with the distinct in ethylene sensitivity and longevity among rose cultivars (Müller et al. 2002). In ethylene-sensitive cultivars, the expression of RhETR3 was increased in senescent flowers and exhibited a shorter vase life. Whereas, ethylene insensitive cultivars presented low expression levels of the receptor isoforms throughout development and exhibited a longer vase life (Müller et al. 2002). Previous results showed that exogenous ethylene can also regulate rose bud opening. Flower opening in roses is regulated by ethylene through either stimulating the process or repressing it depending on the cultivar. The different response of flower opening to ethylene exposure between the cultivars is explained by the difference in the mRNA levels of ethylene receptor and signaling genes RhETR1, RhETR3, and RhCTRs (Ma et al. 2006). In addition, role of ethylene biosynthesis genes has been explored in rose flowers. RhACS1 gene plays an important role in the flower development, petal senescence and wounding response (In et al. 2017;Ma et al. 2005), RhACS2 and RhACS4 are mainly related to the flower senescence, and RhACS3 is related to ethylene dependency, wounding inhibition, flower development and opening (In et al. 2017;Ma et al. 2005). Interestingly, the expression of ethylene biosynthesis and receptor genes is also related to the expansion of the rose petals during rehydration and dehydration (Liu et al. 2013). Among five ethylene biosynthesis genes (RhACS1–RhACS5), the mRNA levels of RhACS1 and RhACS2 were strongly induced by both rehydration and dehydration in sepals and gynoecia (Liu et al. 2013). Whereas, among the five ethylene receptor genes (RhETR1–RhETR5), the expression of RhETR3 was dramatically induced by rehydration and dehydration in the petals (Liu et al. 2013).
In recent years, the expression of ethylene-related genes has been also well characterized in all floral organs of cut rose flowers (Al-Salem and Serek. 2017;Ha et al. 2019a;Liu et al. 2013;Tan et al. 2006;Xue et al. 2008). In cut roses, after ethylene exposure, the rapid and consequential increase in ethylene production in the gynoecia was related to increased expression of RhACS2 and RhACS3. The transcript levels of RhETR1 and RhETR3 were enhanced by ethylene, and RhETR3 has proven to be expressed in an organ-specific manner (Tan et al. 2006;Xue et al. 2008). The differences in the expression of ethylene biosynthesis and ethylene signal transduction genes at different stages of flower development and in various floral organs were also investigated in the two miniature rose cultivars ‘Vanilla’ and ‘Lavender’, which showed low and high ethylene sensitivity. The expression of ethylene receptor genes (RhETR1, RhETR2, and RhETR3), signaling genes (RhCTR1, RhCTR2, RhEIN3, and RhEIL), and ethylene biosynthesis genes (RhACS1 and RhACS2) varied between cultivars and floral organ tissues. Moreover, the expression of ethylene related genes was not correlated with the ethylene sensitivity of the miniature rose cultivars (Al-Salem and Serek 2017). The transcript levels of ethylene-related genes were also monitored in various floral organs (leaves, receptacles, sepals, stigmas, stamens, pedicels, and petals) of ethylene-sensitive and ethylene-insensitive cut rose cultivars (Ha et al. 2019a). Different from the miniature rose cultivars, the expression of ethylene related genes in all floral organs was significantly higher in ethylene-sensitive than ethylene-insensitive cut rose cultivars. The expression of ethylene biosynthesis genes in floral organs in response to ethylene was also different depending on the ethylene sensitivity level of the rose cultivars (Ha et al. 2019a). Exposure to exogenous ethylene for 20 h increased the accumulation of RhACS1 – RhACS4 and RhACO1 transcripts in the leaves and petals of ethylene-sensitive cultivar and in stigmas and stamens of ethylene-insensitive cultivar, resulting in increased the expression of ethylene receptor genes (RhETR1-RhETR5) in the floral organs (Ha et al. 2019a).
In cut ‘Lovely Lydia’ roses, the expression levels of ethylene biosynthesis, receptor, and signaling genes extremely varied among seasons (In and Lim 2018). The expression of RhACO1, RhACS2, and RhACS4 in cut flowers was low in spring and summer and significantly increased in autumn and winter. The mRNA levels of RhETR4 and RhETR5 were also low in summer flowers and markedly increased in winter flowers. The expression pattern of the downstream target RhCTR1 was similar to the expression of RhETR4 and RhETR5, while the seasonal change in the expression of RhCTR2 was insignificant. Whereas, the expression of RhEIN3-2 and RhEIN3-3 was highest in summer flowers (In and Lim 2018). Especially, the initial mRNA levels of RhETR4 and RhEIN3-2 are important factors that determining potential vase life of cut roses. The mRNA levels of these two genes were affected by growth conditions, such as low temperature and high relative humidity (RH). RH conditions during growth modified the initial mRNA levels of the ethylene responsive genes, thereby affecting the potential vase life of cut flowers (Fig. 3).
Inhibition of Ethylene Biosynthesis and Action in Roses
The negative effects of ethylene on plants can be alleviated by ethylene removal technologies and ethylene action/biosynthesis inhibitors. Inhibiting components of the ethylene biosynthesis pathway can interrupt ethylene synthesis in many ornamental plant species. Aminoxyacetic acid (AOA), α-aminoisobutyric acid (AIB), and aminoetoxyvinylglycine (AVG) are common chemicals that are used for inhibiting ethylene production in plants. AIB suppresses ACC oxidase activity, while AVG and AOA inhibit the conversion of SAM to ACC (Broun and Mayak 1981;Capitani et al. 2002;Serek and Andersen 1993). Recently, selenium ( Se) is also c onsidered a s an e thylene biosynthesis inhibitor and works effectively in many cut flower species (Costa et al. 2020). Se extended vase life of cut flowers by down-regulating ethylene synthesis via blocking ACC synthase activity (Costa et al. 2020). Inhibitors of ACC oxidase and ACC synthase effectively prevent only ethylene biosynthesis and not ethylene action.
The suppression of ethylene action can be attained by using of antagonist compounds that bind to the ethylene receptors, therefore blocking downstream signaling of ethylene. Among the ethylene antagonists, 1-methyl cyclopropane (1-MCP), silver thiosultphate (STS), diazocyclopentadiene (DACP), and 2,5-norbornadience (2,5-NBD) are commonly applied in plants (Çelikel and Reid 2002;Serek et al. 1994;Serek et al. 1995;Wang and Woodson 1989). Compared with other antagonist, 1-MCP is the most common compound that is used to control ethylene action during postharvest handling of vegetables, fruits, and cut flowers (Sisler and Serek 2006). 1-MCP can work effectively at low concentrations and protect flowers for several days against ethylene. The effective concentration of 1-MCP depends on different plant organs, plant species, treatment methods, and treatment duration. Pre-treatments with 1-MCP at various doses (5–100 nL・L-1) for 6 h showed different effectiveness in miniature rose flowers (Müller et al. 2000). The application of 1-MCP (0.9 μL・L-1, for 16 h) as sachet o r fumigation e ffectively r educed t he deleterious effects of exogenous ethylene in cut rose flowers (Macnish et al. 2010). Cut roses were singly pretreated with 10 μL・L-1 1-MCP by spray significantly suppressed ethylene production during storage, improved postharvest quality, and extended vase life of cut flowers by six days comparing with control flowers (Huang et al. 2017). The effectiveness of single treatment with 1-MCP also depends on cultivar differences. For example, in miniature potted rose cultivars, 1-MCP treatment significantly decreased endogenous ethylene production in ethylene-sensitive cultivars, whereas it was not effective in ethylene-insensitive cultivars (Müller et al. 2000).
It has been shown that 1-MCP bind competitively to ethylene receptors with a much higher affinity for the receptors compared to that of ethylene (Sisler et al. 1996;Hall et al. 2000;Binder et al. 2004;Sisler and Serek 2006). The development of new ethylene receptors during flower development recovers tissue sensitivity to ethylene and the regaining of the sensitivity can be suppressed by multiple applications of 1-MCP (Feng et al. 2004;In et al. 2013). Therefore, the simultaneous inhibition of ethylene biosynthesis and binding is more sufficient to protect cut flowers against ethylene than the single application of ethylene biosynthesis or action inhibitors. The simultaneous treatment of AIB and 1-MCP or AVG and 1-MCP effectively delayed senescence symptoms and extended vase life of cut rose flowers for both ethylene-sensitive and -insensitive cultivars (Fig. 4A) (Ha et al. 2019b, 2020). These combined treatments significantly suppressed the mRNA levels of ethylene biosynthesis genes (RhACS2 and RhACO1) (Fig. 4B) (Ha e t al. 2019b, 2020) , the ethylene-induced increase of ethylene receptor genes (RhETR1-RhETR5), the reduction of RhCTR1-2, and the induction of RhEIN3s transcripts in petals of the both ethylene-sensitive and -insensitive rose cultivars (Ha et al. 2020).
Ethylene and Scent in Roses
Recently, the relationship between ethylene, longevity, and scent level emission has been studied in cut rose flowers. Almost studies indicated that the volatile emission in roses is not regulated by ethylene. Volatile emission patterns during vase period were not similar to the production of endogenous ethylene in rose petals (Borda et al. 2007;Borda et al. 2011). Ethylene treatment exhibited differential effects among rose cultivars and was not related to the fragrance level of the flowers. For instant, fragrant ‘Lovely Dream’ and ‘Erin’ flowers were less sensitive to exogenous ethylene, while other fragrant flower ‘Osiana’ had a high ethylene sensitivity with petal abscission after 24 h of ethylene treatment (Borda et al. 2007). Moreover, the fragrance production in these rose cultivars was not clearly influenced by ethylene (Borda et al. 2007;Borda et al. 2011). Overall, the volatile emission level in roses is not regulated by exogenous or endogenous ethylene and occurs independently of flower senescence.
Conclusion
Herein, we have reviewed the effects of ethylene on flower quality, expression of ethylene-related genes during flower development and senescence, and also discussed the ethylene sensitivity as well as application of ethylene inhibitors in cut rose flowers. Ethylene sensitivity of cut roses varies by the genetic characteristics at harvest, and thus the intensity of ethylene damage on senescence and postharvest life of flowers is variable depending on the interaction of the variety and the preharvest factors. Simultaneous application of ethylene synthesis and binding antagonists can effectively suppress the flower responses to ethylene, and thereby improve postharvest quality in multiple rose cultivars, which have significant variation in the ethylene sensitivity.