# Roles of C-Repeat Binding Factors-Dependent Signaling Pathway in Jasmonates-Mediated Improvement of Chilling Tolerance of Postharvest Horticultural Commodities

Authors: Libin Wang; Xiuxiu Sun; Weiqi Luo; Chunlu Qian
Journal: Journal of Food Quality
 (2018)

---

## Abstract

C-repeat binding factor- (CBF-) dependent signaling pathway is proposed to be a key responder to low temperature stress in plant. Jasmonates (JAs), the endogenous signal molecules in plant, participate in plant defense against (a)biotic stresses; however, the mechanism has not been fully clarified so far. With the progress made in JAs biopathway, signal transduction, and their relationship with CBF-dependent signaling pathway, our knowledge of the roles of the CBF-dependent signaling pathway in JAs-mediated improvement of chilling tolerance accumulates. In this review, we firstly briefly review the chilling injury (CI) characteristics of postharvest horticultural commodities, then introduce the biopathway and signal transduction of JAs, subsequently summarize the roles of the CBF-dependent signaling pathway under low temperature stress, and finally describe the linkage between JAs signal transduction and the CBF-dependent signaling pathway.


---

## Body

## 1. Introduction

Preservation, transportation, and marketing of postharvest horticultural commodities at low temperatures constitute the main strategy employed in food industries to retard quality deterioration [1]. However, upon exposure to low temperatures ranging from 0 to 15°C, a series of physiological and biochemical alternations is initiated in some horticultural commodities of tropical/subtropical origin, leading to the development of chilling injury (CI) and a great economical loss [2–5].During low temperature stress, cold signal is perceived by undefined sensor in membrane prior to the activation of several cold signaling pathways [6]. In the promoters of these genes, there arecis-elements termed C-repeat elements/dehydration-responsive elements (CRT/DRE), where C-repeat binding factors (CBFs) bind [7]. CBFs, which participate in transcriptional regulation of low temperature acclimation [6], are under the positive control of the upstream Inducer of CBF Expression (ICE), including ICE1 and ICE2 [8]. Activation of CBF-dependent signaling pathway, particularly the ICE1-CBF transcriptional cascade, is proposed to be a key response to low temperature stress in plant, enhancing their chilling resistance [6, 9]. Three CBFs, which are involved in low temperature stress, have been identified inArabidopsis, including AtCBF1, AtCBF2, and AtCBF3 [10]. Overexpression ofAtCBF2 results in the induction of 85cold-regulated (COR) genes inArabidopsis [11].Jasmonic acid and its derivatives, collectively referred to as jasmonates (JAs), are ubiquitous plant hormone [12]. Besides functioning in plant growth and developmental process, JAs play important roles in plant defense against a variety of (a)biotic stresses, such as pathogens [13], herbivory [14], mechanical wounding [15], low temperature [16], and high salinity [17].The roles of JAs in alleviating the CI of postharvest horticultural commodities have been the focus of research for many years. Upon exposure to (a)biotic stresses, the increase of endogenous JAs precedes the activation of JAs-mediated defense [18]; meanwhile, the application of exogenous JAs at proper concentrations could effectively ameliorate CI symptoms of postharvest horticultural commodities, such as papaya, avocado, banana, and tomato (Table 2), and this effect might be due to the activation of various physiological and biochemical responses [19]. However, this mechanism has not yet been fully clarified. With the aid of new technologies, our knowledge on JAs biosynthesis, signal transduction, and their relationship with CBF-dependent signaling pathway expands. In this review, we firstly briefly review the CI characteristic of postharvest horticultural commodities, then introduce the biopathway and signal transduction of JAs, subsequently summarize the roles of CBF-dependent signaling pathway during the low temperature stress, and finally describe the linkage between JAs signal transduction and the CBF-dependent signaling pathway. This review would provide a deeper understanding on the mechanism employed by prechilling JAs treatment to mitigate CI of postharvest horticultural commodities.


## 2. CI Characteristics of Postharvest Horticultural Commodities

### 2.1. CI Symptoms and Impact Factors

Exposure of horticultural commodities of tropical/subtropical origin to low temperature ranging from 0 to 15°C would initiate a series of physiological and biochemical alternations, leading to the development of various CI symptoms [2–4], which can be categorized into two groups: (1) developmental or metabolic symptoms with qualitative nature and (2) physiological symptoms, including pitting, discoloration, water soaking, internal breakdown, susceptibility to mechanical injury, and fungal attack [2]. Table 1 lists CI symptoms of several horticultural commodities along with their critical chilling temperatures [1].Table 1
Chilling injury (CI) symptoms of several horticultural commodities with their critical chilling temperatures [1].
Horticultural commodity Critical chilling temperature (°C) CI symptom when stored between 0°C and safe temperature Apple, certain cultivars 2-3 Internal browning, brown core, soggy breakdown, soft scald Asparagus 0–2 Dull, gray-green, limp tips Atemoya 4 Skin darkening, failure to ripen, pulp discoloration Avocado 4.5–13 Grayish-brown discoloration of flesh Bael 3 Brown spots on skin Banana, green or ripe 11.5–13 Dull color when ripened Bean (lima) 1–4.5 Rusty brown specks, spots, or areas Bean (snap) 7 Pitting and russeting Breadfruit 7–12 Abnormal ripening, dull brown discoloration Chayote 5–10 Dull brown discoloration, pitting, flesh darkening Cranberry 2 Rubbery texture, red flesh Cucumber 7 Pitting, water-soaked spots, decay Eggplant 7 Surface scald,Alternaria rot, blackening of seeds Ginger 7 Softening, tissue breakdown, decay Guava fruit 4.5 Pulp injury, decay Grapefruit 10 Scald, pitting, watery breakdown Jicama 13–18 Surface decay, discoloration Lemon 11–13 Pitting, membranous staining, red blotch Lime 7–9 Pitting, turning tan with time Lychee 3 Skin browning Mango fruit 10–13 Grayish scald-like discoloration of skin, uneven ripening Mangosteen 4–8 Hardening and browning of the cortex Melon Cantaloupe 2–5 Pitting, surface decay Honeydew 7–10 Reddish-tan discoloration, pitting, surface decay, failure to ripen Casaba 7–10 The same as above but no discoloration Crenshaw and Persian 7–10 The same as above but no discoloration Okra pod 7 Discoloration, water-soaked areas, pitting, decay Olive, fresh 7 Internal browning Orange 3 Pitting, brown stain Papaya 7 Pitting, failure to ripen, off-flavor, decay Passion fruit 10 Dark red discoloration on skin, loss of flavor, decay Pepper, sweet 7 Sheet pitting,Alternaria rot on pods and calyxes, darkening of seed Pineapple 7–10 Dull green when ripened Pomegranate 4.5 Pitting, external and internal browning Potato 3 Mahogany browning (Chippewa and Sebago), sweetening Pumpkin and hard-shell squash 10 Decay, especiallyAlternaria rot Rambutan 10 Darkening of exocarp Sweet potato 13 Decay, pitting, internal discolorations; hardcore when cooked Tamarillo 3-4 Surface pitting, discoloration Tomato Ripe 7–10 Water soaking and softening, decay Mature green 13 Poor color when ripe,Alternaria rot Water convolvulus 10 Darkening of leaves and stems Watermelon 4.5 Pitting, objectionable flavorTable 2
Summary of reports on prechilling jasmonates (JAs) treatment alleviating chilling injury (CI) of postharvest horticultural commodities. ABA, abscisic acid; ADC, arginine decarboxylase; ADP, adenosine diphosphate; AMP, adenosine monophosphate; AO, ascorbate oxidase; AOX, alternative oxidase; APX, ascorbate peroxidase; ARG, arginase; AsA, ascorbic acid; ATP, adenosine triphosphate; Ca+-ATPase, Ca2+-adenosine triphosphatase; CAT, catalase; CCO, cytochrome c oxidase; C2H4, ethylene; CO2, carbon dioxide; DHA, dehydroascorbate; DHAR, dehydroascorbate reductase; GAD, glutamate decarboxylase; GR, glutathione reductase; GSH, reduced glutathione; GSH-POD, glutathione peroxidase; GSSG, oxidized glutathione; GST, glutathione-S-transferase; H+-ATPase, H+-adenosine triphosphatase; H2O2, hydrogen peroxide; HSC, heat shock cognate; HSP, heat shock protein; LOX, lipoxygenase; MDA, malondialdehyde; MDHAR, monodehydroascorbate reductase; O2-∙, superoxide anion; OAT, ornithine aminotransferase; ODC, ornithine decarboxylase; PAL, phenylalanine ammonia-lyase; P5CS, pyrroline-5-carboxylate synthetase; PDH, proline dehydrogenase; PG, polygalacturonase; PME, pectin methyl esterase; POD, peroxidase; PPO, polyphenol oxidase; PR, pathogenesis-related-protein; SDH, succinic dehydrogenase; SOD, superoxide dismutase; TA, titratable acid; TSS, total soluble solid.
Species/cultivar Treatment Response Avocado Etinger [25] Immersion in 0.01 mM MeJA solution for 30 s (immersion) Reducing surface pitting and dark black stains in peel as well as discoloration in pulp Fuerte [25] Immersion in 0.0025 mM MeJA solution for 30 s Reducing surface pitting and dark black stains in peel as well as discoloration in pulp Hass [25] Immersion in 0.0025 mM MeJA solution for 30 s Reducing surface pitting and dark black stains in peel as well as discoloration in pulp Banana Carvendish [9] Immersion in 0.1 mM MeJA solution for 30 min Reducing pitting, brown patches, and electrolyte leakageIncreasing proline content and the values of lightness, chroma, and hue Upregulating the expression of MaMYC2a, MaMYC2b, MaCBF1, MaCBF2, MaCOR1, MaKIN2, MaRD2, and MaRD5 Grande Naine [26] Spray with 0.5 or 1 mM n-propyl dihydrojasmonate (PDJ) solution Reducing skin necrosis, browning, and membrane permeabilityIncreasing C2H4 evolution, CO2 production, and endogenous ABA content Grande Naine [27] Spray with 1 mM PDJ solution Reducing grayish peel browning and promoting fruit ripeningIncreasing C2H4 evolution, while decreasing chlorophyll fluorescence Upregulating the activities of SOD, CAT, and POD, while downregulating PPO activity Namwa [28] Immersion in 0.39 mM PDJ solution for 15 min Reducing skin browningIncreasing the contents of total phenolics and AsA in skin Upregulating SOD activity in skin Bell pepper Century [29] Fumigation with 0.1 mM MeJA vapor for 24 h Reducing surface pitting and blackened seedsEnhancing the expression of AOXs (both 1.5 kb and 3.5 kb) Maor [25] Immersion in 0.025 mM MeJA solution 30 s Reducing white surface pitting Grapefruit Marsh seedless [25] Immersion in 0.01 mM MeJA solution for 30 s Reducing rusty surface pitting Marsh Seedless [30] Immersion in 0.01 mM MeJA solution for 30 s Reducing decay and rusty surface pitting Guava fruit Unknown [31] Fumigation with 0.01 or 0.1 mM MeJA vapor for 8 h Reducing necrosis, black spots, and electrolyte leakageIncreasing sugar content Upregulating the activities of PAL and LOX Loquat fruit Fuyang [32] Fumigation with 0.01 mM MeJA vapor for 24 h Reducing the increase in firmness, loss of extractable juice, and internal browningIncreasing the contents of linoleic and linolenic acid as well as unsaturated/saturated fatty acid ratio, while decreasing O2-∙ production rate and the contents of H2O2, palmitic acid, stearic acid, and oleic acid Upregulating the activities of SOD, CAT, and APX, while downregulating LOX activity Fuyang [33] Fumigation with 0.01 mM MeJA vapor for 24 h Reducing the increase in firmness, internal browning, and loss of extractable juiceIncreasing the contents of water- and CDTA-soluble pectins, while decreasing the contents of lignin, alcohol insoluble residues, hemicellulose, cellulose, and Na2CO3-soluble pectin Upregulating PG activity, while downregulating the activities of PAL, POD, PPO, and PME Jiefangzhong [34] Fumigation with 0.01 mM MeJA vapor for 24 h Reducing the increases in firmness, internal browning index, weight loss, decay, loss of extractable juice, and electrolyte leakageIncreasing the contents of TSS and TA Jiefangzhong [35] Fumigation with 0.01 mM MeJA vapor for 24 h Reducing internal browningIncreasing the contents of AsA and GSH, while decreasing the contents of DHA and GSSG Upregulating the activities of MDHAR, DHAR, GR, APX, GPX, and GST, while downregulating AO activity Jiefangzhong [36] Fumigation with 0.01 mM MeJA vapor for 24 h Reducing the increase in firmness and loss of extractable juiceIncreasing the contents of proline and γ-aminobutyric acid Upregulating the activities of P5CS, OAT, and GAD, while downregulating PDH activity Mango fruit Choke anan [28] Immersion in 0.39 mM PDJ solution for 15 min Reduce skin browningIncreasing the contents of total phenolics and AsA in skin Upregulating SOD activity in skin Kent [37] Fumigation with 0.01 or 0.1 mM MeJA vapor for 24 h Reducing surface pitting and decay and improving ripeness and overall qualityIncreasing the contents of sucrose, total sugar, malic acid, shikimic acid, and total organic acid Kent and Tommy Atkins [38] Immersion in 0.1 mM MeJA solution for 2 min Reducing surface pitting, scalding and weight loss, and maintaining overall qualityIncreasing TSS content Nam Dok Mai number 4 [39] Immersion in 0.1 or 1 mM MeJA solution for 10 min Reducing skin browning and electrolyte leakageDecreasing MDA content in skin and pulp Tommy Atkins [40] Fumigation with 0.1 mM MeJA vapor for 24 h Reducing sheet pitting and electrolyte leakageIncreasing TSS content Unknown [41] Immersion in 0.39 mM PDJ solution for 15 min Reducing skin browning Okra pod Unknown [42] Fumigation with 0.1 or 1 mM MeJA vapor for 16 h Reducing electrolyte leakage and water soakingDecreasing MDA content Papaya Sunrise [43] Fumigation with 0.01 or 0.1 mM MeJA vapor for 16 h Reducing fungal decay, surface pitting, and loss of firmnessIncreasing the contents of malic acid, citric acid, and quinic acid Peach fruit Baifeng [44] Fumigation with 0.001 mM MeJA vapor for 12 h Reducing internal browning, flesh mealiness, and loss of extractable juiceIncreasing the contents of AsA, total phenolics, and TSS, while decreasing H2O2 content Upregulating the activities of PAL, SOD, CAT, APX, PME, and PG, while downregulating the activities of PPO and POD Baifeng [45] Fumigation with 0.001 mM MeJA vapor for 24 h Reducing internal browning and flesh mealinessIncreasing the contents of total acid, AsA, and total phenolics Upregulating the activities of SOD, PAL, PME, and PG, while downregulating the activities of PPO and POD Baifeng [46] Fumigation with 0.001 mM MeJA vapor for 24 h Reduce internal browning and electrolyte leakageIncreasing the contents of ATP, ADP, and energy charge, while decreasing the contents of AMP, MDA, O2-∙, and H2O2 Upregulating the activities of H+-ATPase, Ca2+-ATPase, SDH, and CCO Jiubao [47] Fumigation with 0.1 mM MeJA vapor for 12 h Reducing flesh browning, softening, and electrolyte leakageIncreasing TSS/TA ratio and the contents of Ca2+-precipitates, ester, carboxyl group, and TSS, while decreasing the contents of total phenolics and carboxylate group in cell wall Upregulating the activities of exo/endo-PG and POD, while downregulating the activities of PME and PPO Pineapple Pattavia [48] Immersion in 0.01, 0.1, and 1 mM MeJA solution for 5 min Reducing electrolyte leakage, surface browning of the pulp, and weight lossDecreasing total phenolics content Pomegranate Malas Save [49] Fumigation with 0.037, 0.074, or 0.110 mM MeJA vapor Reducing pitting, browning, and water loss Mollar de Elche [50] Fumigation with 0.01 or 0.1 mM MeJA vapor for 16 h Reducing dehydration, browning, pitting, softening, and electrolyte leakageIncreasing the contents of total phenolics and total anthocyanin Upregulating hydrophilic total antioxidant activity (H-TAA) Tomato Beefsteak [51] Fumigation with 0.01 mM MeJA vapor for 16 h Reducing surface pitting and decay (mainlyAlternariarot); Upregulating the expression of HSP17.4, HSP17.6, HSP17.7, HSC70-1, HSC70-2,and HSC70-3 Beefsteak [16] Fumigation with 0.01 mM MeJA vapor for 16 h Reducing surface pitting and decay (mainlyAlternariarot)Upregulating the expression of CAT1, PR-2b, PR-2a, and PR-3b Messina [52] Fumigation with 0.05 mM MeJA vapor for 21 h Reducing surface pittingIncreasing the contents of putrescine and proline Upregulating the activities of ARG, ADC, OAT, and ODC and the expression of LeARG1, LeARG2, LeADC, LeOAT, and LeODC Zucchini squash Elite [53] Immersion in 0.05 or 1.0 mM MeJA solution for 3 min Reducing surface pittingIncreasing contents of ABA, spermidine, and spermine Elite [54] Immersion in MeJA solution 1.0 mM for 3 min Reducing surface pitting, water-soaked areas, and decayIncreasing the contents of fructose, glucose, and malic acidMany factors can impact the chilling susceptibility of horticultural commodities, including the genotype, ripening stage, metabolic status, and other environmental conditions [20]. For example, green and breaker tomato fruits (<10% red coloration, USDA 1976) are more susceptible to low temperature stress than fruits at red-ripe stage [21]. A more severe CI phenomenon was observed in horticultural commodities subjected to lower environmental temperatures. Additionally, the duration of chilling exposure is positively correlated with CI index [22].However, it is the genetic makeup that determines to what degree a species or cultivar is sensitive or resistant to low temperature stress [23].Arabidopsis with higher expression level ofAtCBF3 demonstrates an enhanced chilling tolerance [24].


### 2.2. Physiological and Biochemical Alternations Resulting in the Development of Visual CI

The cell membrane is the primary cellular structure affected by low temperature stress [55]. The transition of cell membrane from a liquid-crystalline phase to a solid-gel structure during chilling stress causes the loss of membrane semipermeability, the dysfunction of membrane proteins, the increase of free cytosolic calcium, and the cessation of protoplasmic streaming, which is followed by the electrolyte leakage, energy supply reduction, and an increase in susceptibility to oxidative stress [55, 56] (Figure 1). If there is exposure to chilling stress for a longer time, further responses could be initiated along with the rupture of cell membrane, presenting as various visual CI symptoms such as pitting, water soaking, and decay [3, 4] (Figure 1).Figure 1
Possible physiological and biochemical mechanisms of chilling injury symptoms in postharvest horticultural commodities [2, 70].


## 2.1. CI Symptoms and Impact Factors

Exposure of horticultural commodities of tropical/subtropical origin to low temperature ranging from 0 to 15°C would initiate a series of physiological and biochemical alternations, leading to the development of various CI symptoms [2–4], which can be categorized into two groups: (1) developmental or metabolic symptoms with qualitative nature and (2) physiological symptoms, including pitting, discoloration, water soaking, internal breakdown, susceptibility to mechanical injury, and fungal attack [2]. Table 1 lists CI symptoms of several horticultural commodities along with their critical chilling temperatures [1].Table 1
Chilling injury (CI) symptoms of several horticultural commodities with their critical chilling temperatures [1].
Horticultural commodity Critical chilling temperature (°C) CI symptom when stored between 0°C and safe temperature Apple, certain cultivars 2-3 Internal browning, brown core, soggy breakdown, soft scald Asparagus 0–2 Dull, gray-green, limp tips Atemoya 4 Skin darkening, failure to ripen, pulp discoloration Avocado 4.5–13 Grayish-brown discoloration of flesh Bael 3 Brown spots on skin Banana, green or ripe 11.5–13 Dull color when ripened Bean (lima) 1–4.5 Rusty brown specks, spots, or areas Bean (snap) 7 Pitting and russeting Breadfruit 7–12 Abnormal ripening, dull brown discoloration Chayote 5–10 Dull brown discoloration, pitting, flesh darkening Cranberry 2 Rubbery texture, red flesh Cucumber 7 Pitting, water-soaked spots, decay Eggplant 7 Surface scald,Alternaria rot, blackening of seeds Ginger 7 Softening, tissue breakdown, decay Guava fruit 4.5 Pulp injury, decay Grapefruit 10 Scald, pitting, watery breakdown Jicama 13–18 Surface decay, discoloration Lemon 11–13 Pitting, membranous staining, red blotch Lime 7–9 Pitting, turning tan with time Lychee 3 Skin browning Mango fruit 10–13 Grayish scald-like discoloration of skin, uneven ripening Mangosteen 4–8 Hardening and browning of the cortex Melon Cantaloupe 2–5 Pitting, surface decay Honeydew 7–10 Reddish-tan discoloration, pitting, surface decay, failure to ripen Casaba 7–10 The same as above but no discoloration Crenshaw and Persian 7–10 The same as above but no discoloration Okra pod 7 Discoloration, water-soaked areas, pitting, decay Olive, fresh 7 Internal browning Orange 3 Pitting, brown stain Papaya 7 Pitting, failure to ripen, off-flavor, decay Passion fruit 10 Dark red discoloration on skin, loss of flavor, decay Pepper, sweet 7 Sheet pitting,Alternaria rot on pods and calyxes, darkening of seed Pineapple 7–10 Dull green when ripened Pomegranate 4.5 Pitting, external and internal browning Potato 3 Mahogany browning (Chippewa and Sebago), sweetening Pumpkin and hard-shell squash 10 Decay, especiallyAlternaria rot Rambutan 10 Darkening of exocarp Sweet potato 13 Decay, pitting, internal discolorations; hardcore when cooked Tamarillo 3-4 Surface pitting, discoloration Tomato Ripe 7–10 Water soaking and softening, decay Mature green 13 Poor color when ripe,Alternaria rot Water convolvulus 10 Darkening of leaves and stems Watermelon 4.5 Pitting, objectionable flavorTable 2
Summary of reports on prechilling jasmonates (JAs) treatment alleviating chilling injury (CI) of postharvest horticultural commodities. ABA, abscisic acid; ADC, arginine decarboxylase; ADP, adenosine diphosphate; AMP, adenosine monophosphate; AO, ascorbate oxidase; AOX, alternative oxidase; APX, ascorbate peroxidase; ARG, arginase; AsA, ascorbic acid; ATP, adenosine triphosphate; Ca+-ATPase, Ca2+-adenosine triphosphatase; CAT, catalase; CCO, cytochrome c oxidase; C2H4, ethylene; CO2, carbon dioxide; DHA, dehydroascorbate; DHAR, dehydroascorbate reductase; GAD, glutamate decarboxylase; GR, glutathione reductase; GSH, reduced glutathione; GSH-POD, glutathione peroxidase; GSSG, oxidized glutathione; GST, glutathione-S-transferase; H+-ATPase, H+-adenosine triphosphatase; H2O2, hydrogen peroxide; HSC, heat shock cognate; HSP, heat shock protein; LOX, lipoxygenase; MDA, malondialdehyde; MDHAR, monodehydroascorbate reductase; O2-∙, superoxide anion; OAT, ornithine aminotransferase; ODC, ornithine decarboxylase; PAL, phenylalanine ammonia-lyase; P5CS, pyrroline-5-carboxylate synthetase; PDH, proline dehydrogenase; PG, polygalacturonase; PME, pectin methyl esterase; POD, peroxidase; PPO, polyphenol oxidase; PR, pathogenesis-related-protein; SDH, succinic dehydrogenase; SOD, superoxide dismutase; TA, titratable acid; TSS, total soluble solid.
Species/cultivar Treatment Response Avocado Etinger [25] Immersion in 0.01 mM MeJA solution for 30 s (immersion) Reducing surface pitting and dark black stains in peel as well as discoloration in pulp Fuerte [25] Immersion in 0.0025 mM MeJA solution for 30 s Reducing surface pitting and dark black stains in peel as well as discoloration in pulp Hass [25] Immersion in 0.0025 mM MeJA solution for 30 s Reducing surface pitting and dark black stains in peel as well as discoloration in pulp Banana Carvendish [9] Immersion in 0.1 mM MeJA solution for 30 min Reducing pitting, brown patches, and electrolyte leakageIncreasing proline content and the values of lightness, chroma, and hue Upregulating the expression of MaMYC2a, MaMYC2b, MaCBF1, MaCBF2, MaCOR1, MaKIN2, MaRD2, and MaRD5 Grande Naine [26] Spray with 0.5 or 1 mM n-propyl dihydrojasmonate (PDJ) solution Reducing skin necrosis, browning, and membrane permeabilityIncreasing C2H4 evolution, CO2 production, and endogenous ABA content Grande Naine [27] Spray with 1 mM PDJ solution Reducing grayish peel browning and promoting fruit ripeningIncreasing C2H4 evolution, while decreasing chlorophyll fluorescence Upregulating the activities of SOD, CAT, and POD, while downregulating PPO activity Namwa [28] Immersion in 0.39 mM PDJ solution for 15 min Reducing skin browningIncreasing the contents of total phenolics and AsA in skin Upregulating SOD activity in skin Bell pepper Century [29] Fumigation with 0.1 mM MeJA vapor for 24 h Reducing surface pitting and blackened seedsEnhancing the expression of AOXs (both 1.5 kb and 3.5 kb) Maor [25] Immersion in 0.025 mM MeJA solution 30 s Reducing white surface pitting Grapefruit Marsh seedless [25] Immersion in 0.01 mM MeJA solution for 30 s Reducing rusty surface pitting Marsh Seedless [30] Immersion in 0.01 mM MeJA solution for 30 s Reducing decay and rusty surface pitting Guava fruit Unknown [31] Fumigation with 0.01 or 0.1 mM MeJA vapor for 8 h Reducing necrosis, black spots, and electrolyte leakageIncreasing sugar content Upregulating the activities of PAL and LOX Loquat fruit Fuyang [32] Fumigation with 0.01 mM MeJA vapor for 24 h Reducing the increase in firmness, loss of extractable juice, and internal browningIncreasing the contents of linoleic and linolenic acid as well as unsaturated/saturated fatty acid ratio, while decreasing O2-∙ production rate and the contents of H2O2, palmitic acid, stearic acid, and oleic acid Upregulating the activities of SOD, CAT, and APX, while downregulating LOX activity Fuyang [33] Fumigation with 0.01 mM MeJA vapor for 24 h Reducing the increase in firmness, internal browning, and loss of extractable juiceIncreasing the contents of water- and CDTA-soluble pectins, while decreasing the contents of lignin, alcohol insoluble residues, hemicellulose, cellulose, and Na2CO3-soluble pectin Upregulating PG activity, while downregulating the activities of PAL, POD, PPO, and PME Jiefangzhong [34] Fumigation with 0.01 mM MeJA vapor for 24 h Reducing the increases in firmness, internal browning index, weight loss, decay, loss of extractable juice, and electrolyte leakageIncreasing the contents of TSS and TA Jiefangzhong [35] Fumigation with 0.01 mM MeJA vapor for 24 h Reducing internal browningIncreasing the contents of AsA and GSH, while decreasing the contents of DHA and GSSG Upregulating the activities of MDHAR, DHAR, GR, APX, GPX, and GST, while downregulating AO activity Jiefangzhong [36] Fumigation with 0.01 mM MeJA vapor for 24 h Reducing the increase in firmness and loss of extractable juiceIncreasing the contents of proline and γ-aminobutyric acid Upregulating the activities of P5CS, OAT, and GAD, while downregulating PDH activity Mango fruit Choke anan [28] Immersion in 0.39 mM PDJ solution for 15 min Reduce skin browningIncreasing the contents of total phenolics and AsA in skin Upregulating SOD activity in skin Kent [37] Fumigation with 0.01 or 0.1 mM MeJA vapor for 24 h Reducing surface pitting and decay and improving ripeness and overall qualityIncreasing the contents of sucrose, total sugar, malic acid, shikimic acid, and total organic acid Kent and Tommy Atkins [38] Immersion in 0.1 mM MeJA solution for 2 min Reducing surface pitting, scalding and weight loss, and maintaining overall qualityIncreasing TSS content Nam Dok Mai number 4 [39] Immersion in 0.1 or 1 mM MeJA solution for 10 min Reducing skin browning and electrolyte leakageDecreasing MDA content in skin and pulp Tommy Atkins [40] Fumigation with 0.1 mM MeJA vapor for 24 h Reducing sheet pitting and electrolyte leakageIncreasing TSS content Unknown [41] Immersion in 0.39 mM PDJ solution for 15 min Reducing skin browning Okra pod Unknown [42] Fumigation with 0.1 or 1 mM MeJA vapor for 16 h Reducing electrolyte leakage and water soakingDecreasing MDA content Papaya Sunrise [43] Fumigation with 0.01 or 0.1 mM MeJA vapor for 16 h Reducing fungal decay, surface pitting, and loss of firmnessIncreasing the contents of malic acid, citric acid, and quinic acid Peach fruit Baifeng [44] Fumigation with 0.001 mM MeJA vapor for 12 h Reducing internal browning, flesh mealiness, and loss of extractable juiceIncreasing the contents of AsA, total phenolics, and TSS, while decreasing H2O2 content Upregulating the activities of PAL, SOD, CAT, APX, PME, and PG, while downregulating the activities of PPO and POD Baifeng [45] Fumigation with 0.001 mM MeJA vapor for 24 h Reducing internal browning and flesh mealinessIncreasing the contents of total acid, AsA, and total phenolics Upregulating the activities of SOD, PAL, PME, and PG, while downregulating the activities of PPO and POD Baifeng [46] Fumigation with 0.001 mM MeJA vapor for 24 h Reduce internal browning and electrolyte leakageIncreasing the contents of ATP, ADP, and energy charge, while decreasing the contents of AMP, MDA, O2-∙, and H2O2 Upregulating the activities of H+-ATPase, Ca2+-ATPase, SDH, and CCO Jiubao [47] Fumigation with 0.1 mM MeJA vapor for 12 h Reducing flesh browning, softening, and electrolyte leakageIncreasing TSS/TA ratio and the contents of Ca2+-precipitates, ester, carboxyl group, and TSS, while decreasing the contents of total phenolics and carboxylate group in cell wall Upregulating the activities of exo/endo-PG and POD, while downregulating the activities of PME and PPO Pineapple Pattavia [48] Immersion in 0.01, 0.1, and 1 mM MeJA solution for 5 min Reducing electrolyte leakage, surface browning of the pulp, and weight lossDecreasing total phenolics content Pomegranate Malas Save [49] Fumigation with 0.037, 0.074, or 0.110 mM MeJA vapor Reducing pitting, browning, and water loss Mollar de Elche [50] Fumigation with 0.01 or 0.1 mM MeJA vapor for 16 h Reducing dehydration, browning, pitting, softening, and electrolyte leakageIncreasing the contents of total phenolics and total anthocyanin Upregulating hydrophilic total antioxidant activity (H-TAA) Tomato Beefsteak [51] Fumigation with 0.01 mM MeJA vapor for 16 h Reducing surface pitting and decay (mainlyAlternariarot); Upregulating the expression of HSP17.4, HSP17.6, HSP17.7, HSC70-1, HSC70-2,and HSC70-3 Beefsteak [16] Fumigation with 0.01 mM MeJA vapor for 16 h Reducing surface pitting and decay (mainlyAlternariarot)Upregulating the expression of CAT1, PR-2b, PR-2a, and PR-3b Messina [52] Fumigation with 0.05 mM MeJA vapor for 21 h Reducing surface pittingIncreasing the contents of putrescine and proline Upregulating the activities of ARG, ADC, OAT, and ODC and the expression of LeARG1, LeARG2, LeADC, LeOAT, and LeODC Zucchini squash Elite [53] Immersion in 0.05 or 1.0 mM MeJA solution for 3 min Reducing surface pittingIncreasing contents of ABA, spermidine, and spermine Elite [54] Immersion in MeJA solution 1.0 mM for 3 min Reducing surface pitting, water-soaked areas, and decayIncreasing the contents of fructose, glucose, and malic acidMany factors can impact the chilling susceptibility of horticultural commodities, including the genotype, ripening stage, metabolic status, and other environmental conditions [20]. For example, green and breaker tomato fruits (<10% red coloration, USDA 1976) are more susceptible to low temperature stress than fruits at red-ripe stage [21]. A more severe CI phenomenon was observed in horticultural commodities subjected to lower environmental temperatures. Additionally, the duration of chilling exposure is positively correlated with CI index [22].However, it is the genetic makeup that determines to what degree a species or cultivar is sensitive or resistant to low temperature stress [23].Arabidopsis with higher expression level ofAtCBF3 demonstrates an enhanced chilling tolerance [24].


## 2.2. Physiological and Biochemical Alternations Resulting in the Development of Visual CI

The cell membrane is the primary cellular structure affected by low temperature stress [55]. The transition of cell membrane from a liquid-crystalline phase to a solid-gel structure during chilling stress causes the loss of membrane semipermeability, the dysfunction of membrane proteins, the increase of free cytosolic calcium, and the cessation of protoplasmic streaming, which is followed by the electrolyte leakage, energy supply reduction, and an increase in susceptibility to oxidative stress [55, 56] (Figure 1). If there is exposure to chilling stress for a longer time, further responses could be initiated along with the rupture of cell membrane, presenting as various visual CI symptoms such as pitting, water soaking, and decay [3, 4] (Figure 1).Figure 1
Possible physiological and biochemical mechanisms of chilling injury symptoms in postharvest horticultural commodities [2, 70].


## 3. Biopathway and Signal Transduction of JAs

### 3.1. JAs Biosynthetic Pathway

As shown in Figure2, JAs biosynthesis is initiated by a phospholipase-mediated release of α-linolenic acid in chloroplast, which is then oxidized by TomloxD into 13(S)-hydroperoxylinolenic acid (13-HPOT) [57, 58]. The latter is further oxidized and cyclized intocis-(+)-12-oxophytodienoic acid (cis-(+)-OPDA) by the actions of allene oxide synthase (AOS) and allene oxide cyclase (AOC) [59]. Subsequently,cis-(+)-OPDA is transported by COMATOSE1/PEROXIMAL1/PEROXISOME ABC TRANSPORTER (ABC CTS1/PXA1/PED3) or ion trapping from the chloroplast to the peroxisome [58], wherecis-(+)-OPDA is oxidized and then esterified before three rounds of β-oxidation with the generation of (+)-7-iso-JA [60]. The core enzymes for β-oxidation include acyl CoA oxidase (ACX), multifunctional protein (MFP), and 3-ketoacyl-CoA thiolase (KAT) [61]. Upon synthesis, the (+)-7-iso-JA is immediately epimerized to (−)-JA and further converted into various derivatives, such as methyl jasmonate (MeJA) and jasmonoyl-isoleucine (JA-Ile) [58, 62] (Figure 2). In plant, a small amount of JAs is derived from hexadecatrienoic acid [63].Figure 2
Biosynthetic pathway of jasmonates (JAs) [58, 71, 72].JAs biosynthesis in plant is regulated by various factors, like substrate availability, JASMONATE-ZIM-DOMAIN (JAZ) protein, Ca2+/mitogen-activated protein kinase (MAPK) cascades, and so on [19]. Among several branches known for the LOX pathway, the AOS and hydroperoxide lyase (HPL) branches are concurrent on the same substrate, 13-HPOT [19]. The HPL branch leads to the generation of green leaf volatiles (GLVs), which are defense compounds formed upon herbivore attack [64, 65]. Mutation ofOsHPL3 would reduce wound-induced GLV emission but increase JAs accumulation [66]. For more information relevant to the regulation of JAs biosynthesis, readers could refer to the review written by Wasternack and Hause [19].


### 3.2. JAs Signal Transduction

Recently, JA-Ile has been identified as the bioactive compound in JAs signaling [12]. Along with this discovery, other breakthroughs relevant to JAs signal transduction have been made, such as the identification of JAZ repressors [67, 68] and CORONATINE INSENSITIVE1-JAZ (COI1-JAZ) coreceptor [69].Figure3 summarizes the primary signal transduction process after its perception [58, 71, 72]: in the absence of JA-Ile, JAZ repressors suppress transcription factors (TFs) such as bHLHzip transcription factors (MYCs) by recruiting the corepressors TOPLESS (TPL) and TPL-related proteins via the adaptor protein, Novel Interactor of JAZ (NINJA), and thus suppress the expression of JAs-regulated genes. On the other hand, during plant development or (a)biotic stresses when JA-Ile levels are high, a F-box protein COI1, which is an integral part of the Skp-Cullin-F-box (SCF) complex, targets JAZ repressors by the aid of inositol pentakisphosphate 5 (IP5) for poly-ubiquitination; JAZ repressors are subsequently degraded by a 26S ubiquitin-proteasome system, thus relieving TFs blockage and activating the gene expression downstream [19, 73]. The initiation of transcription also needs the recruitment of general transcription factors (GTFs) and RNA polymerase II to TFs-bound promoter through the interaction of TFs and MEDIATOR25 (MED25) subunit of Mediator complex [71] (Figure 3).Figure 3
Signal transduction of jasmonates (JAs) [58, 71].


## 3.1. JAs Biosynthetic Pathway

As shown in Figure2, JAs biosynthesis is initiated by a phospholipase-mediated release of α-linolenic acid in chloroplast, which is then oxidized by TomloxD into 13(S)-hydroperoxylinolenic acid (13-HPOT) [57, 58]. The latter is further oxidized and cyclized intocis-(+)-12-oxophytodienoic acid (cis-(+)-OPDA) by the actions of allene oxide synthase (AOS) and allene oxide cyclase (AOC) [59]. Subsequently,cis-(+)-OPDA is transported by COMATOSE1/PEROXIMAL1/PEROXISOME ABC TRANSPORTER (ABC CTS1/PXA1/PED3) or ion trapping from the chloroplast to the peroxisome [58], wherecis-(+)-OPDA is oxidized and then esterified before three rounds of β-oxidation with the generation of (+)-7-iso-JA [60]. The core enzymes for β-oxidation include acyl CoA oxidase (ACX), multifunctional protein (MFP), and 3-ketoacyl-CoA thiolase (KAT) [61]. Upon synthesis, the (+)-7-iso-JA is immediately epimerized to (−)-JA and further converted into various derivatives, such as methyl jasmonate (MeJA) and jasmonoyl-isoleucine (JA-Ile) [58, 62] (Figure 2). In plant, a small amount of JAs is derived from hexadecatrienoic acid [63].Figure 2
Biosynthetic pathway of jasmonates (JAs) [58, 71, 72].JAs biosynthesis in plant is regulated by various factors, like substrate availability, JASMONATE-ZIM-DOMAIN (JAZ) protein, Ca2+/mitogen-activated protein kinase (MAPK) cascades, and so on [19]. Among several branches known for the LOX pathway, the AOS and hydroperoxide lyase (HPL) branches are concurrent on the same substrate, 13-HPOT [19]. The HPL branch leads to the generation of green leaf volatiles (GLVs), which are defense compounds formed upon herbivore attack [64, 65]. Mutation ofOsHPL3 would reduce wound-induced GLV emission but increase JAs accumulation [66]. For more information relevant to the regulation of JAs biosynthesis, readers could refer to the review written by Wasternack and Hause [19].


## 3.2. JAs Signal Transduction

Recently, JA-Ile has been identified as the bioactive compound in JAs signaling [12]. Along with this discovery, other breakthroughs relevant to JAs signal transduction have been made, such as the identification of JAZ repressors [67, 68] and CORONATINE INSENSITIVE1-JAZ (COI1-JAZ) coreceptor [69].Figure3 summarizes the primary signal transduction process after its perception [58, 71, 72]: in the absence of JA-Ile, JAZ repressors suppress transcription factors (TFs) such as bHLHzip transcription factors (MYCs) by recruiting the corepressors TOPLESS (TPL) and TPL-related proteins via the adaptor protein, Novel Interactor of JAZ (NINJA), and thus suppress the expression of JAs-regulated genes. On the other hand, during plant development or (a)biotic stresses when JA-Ile levels are high, a F-box protein COI1, which is an integral part of the Skp-Cullin-F-box (SCF) complex, targets JAZ repressors by the aid of inositol pentakisphosphate 5 (IP5) for poly-ubiquitination; JAZ repressors are subsequently degraded by a 26S ubiquitin-proteasome system, thus relieving TFs blockage and activating the gene expression downstream [19, 73]. The initiation of transcription also needs the recruitment of general transcription factors (GTFs) and RNA polymerase II to TFs-bound promoter through the interaction of TFs and MEDIATOR25 (MED25) subunit of Mediator complex [71] (Figure 3).Figure 3
Signal transduction of jasmonates (JAs) [58, 71].


## 4. Exogenous Application of JAs to Reduce CI of Postharvest Horticultural Commodities

Upon exposure to (a)biotic stresses, the increase of endogenous JAs precedes the activation of JAs-mediated defense [18]. A higher level of endogenous JA was observed in the skin of mangosteen during early storage at 7°C [41], which agreed with the finding in apple fruitlet [74]. These results imply a role of JAs in low temperature stress of postharvest horticultural commodities. Table 2 lists the reports on exogenous JAs application ameliorating CI symptoms of postharvest horticultural commodities, such as papaya, avocado, banana, tomato, bell pepper, grapefruit, guava, and loquat. The fumigation of “Sunrise” papaya with 0.01 or 0.1 mM MeJA vapors for 16 h significantly suppressed CI symptoms, such as fungal decay, surface pitting, and loss of firmness, after 32 d storage at 10°C + 4 d shelf life at 20°C [43].Based on the report of Aghdam and Bodbodak [75], the JAs-mediated chilling alleviation of postharvest horticultural commodities could be due to the physiological and biochemical responses as follows: (1) enhancement of membrane integrity through the increase of unsaturated fatty acids/saturated fatty acids (unSFA/SFA) ratio; (2) enhancement ofheat shock proteins (HSPs) gene expression; (3) enhancement of reactive oxygen intermediates- (ROIs-) scavenging capacity; (4) enhancement of arginine pathways, resulting in the accumulation of bioactive molecules with pivotal roles in chilling resistance; (5) enhancement of energy metabolism, leading to more adenosine triphosphate (ATP) accumulation; (6) alteration in phenylalanine ammonia-lyase (PAL) and polyphenol oxidase (PPO) activities (Table 2).Cultivar, ripening stage, and JAs dosage can impact the efficiency of JAs treatment [20, 25]. Ding et al. [16] found that a 0.1 mM MeJA fumigation of breaker “Beefsteak” tomato prior to 28 d storage at 5°C substantially inhibited the development of surface pitting andAlternaria decay; however, this concentration was not effective for pink fruit [76]. Although 0.037, 0.074, or 0.110 mM MeJA fumigation could effectively suppress pitting, browning, and water loss during low temperature storage of “Malas Save” pomegranate fruits, the optimal result was observed in fruits treated with the highest concentration [49].


## 5. Roles of CBF-Dependent Signaling Pathway during Low Temperature Stress

### 5.1. Activation of CBF-Dependent Signaling Pathway during Low Temperature Stress

Upon exposure to low temperature, plants immediately activate multiple signaling pathways to enhance chilling tolerance, which consists of transcriptional, posttranscriptional, translational, and posttranslational regulators of low temperature-induced expression of the functional genes [6]. In the promoters of these genes, there is a kind ofcis-elements termed C-repeat elements/dehydration-responsive elements (CRT/DRE), where CBFs can bind [7]. CBFs, which participate in transcriptional regulation of low temperature acclimation [6], are under the positive control of the upstream ICEs, including ICE1 and ICE2 [8]. Activation of CBF-dependent signaling pathway, especially the ICE1-CBF transcriptional cascade, is supposed to be a key responder to low temperature stress in plant, enhancing their chilling resistance [6, 9]. Chilling stress results in the induction ofOsICE1andOsICE2in rice(Oryza sativa) and subsequent upregulation ofOsCBF1, rice heat shock factor A3 (OsHsfA3), andrice trehalose 6-phosphate phosphatase 1 (OsTPP1) expression [80]. Overexpression ofSlICE1in tomato fruit initiates the expression ofSlCBF1,dehydrin Ci7 homolog (SlDRCi7), andpyrroline-5-carboxylate synthetase (SlP5CS) and enhance the accumulation of antioxidants, several amino acids, amines, and sugars, thus improving the chilling resistance of fruit [81, 82]. A similar result was observed in cucumber [83].InArabidopsis, three CBFs—namely, AtCBF1, AtCBF2, and AtCBF3—have been identified to be involved in low temperature stress [10]. Under normal condition, JAZ1 and JAZ4 inArabidopsis would interact with ICEs, thus suppressing CBF-dependent signaling pathway [84]. Upon low temperature acclimation when the biosynthesis of endogenous JAs is induced, ICE1 is released from the JAZs via a 26S proteasome-mediated degradation and then positively regulates the expression of CBFs [78]. Subsequently, CBFs bind to CRT/DRE inCORs and upregulate their expression, which could induce the physiological and biochemical responses and thus improve chilling tolerance of plant [6]. For the detailed information on the roles of CBF-dependent signaling pathway in chilling acclimation, readers could refer to the reviews on this topic [6, 85, 86].


### 5.2. Physiological and Biochemical Responses Induced by CBF-Dependent Signaling Pathway to Improve Chilling Tolerance

#### 5.2.1. Enhancing Arginine Pathways

Arginine is a precursor for the biosynthesis of bioactive molecules such as polyamines, proline, andγ-aminobutyric acid [36, 52] (Figure 4). Polyamines could stabilize the membrane based on its polycationic nature at physiological pH [87], proline acts as a protein-compatible hydrotrope or as a hydroxyl radical scavenger to regulate the NAD+/NADH ratio [88], while γ-aminobutyric acid is involved in osmotic regulation, ROS scavenge, and intracellular signal transduction [89].Figure 4
Arginine pathways which lead to the production of polyamines, proline, andγ-aminobutyric acid [52, 75]. Enzyme abbreviations are as follows: ADC, arginine decarboxylase; ARG, arginase; GAD, glutamate decarboxylase; NOS, NO synthase; ODC, ornithine decarboxylase; P5CR, pyrroline-5-carboxylate reductase; P5CS, pyrroline-5-carboxylate synthetase.As shown in Figure4, in plant arginase is metabolised by arginase (ARG), ornithine decarboxylase (ODC), and arginine decarboxylase (ADC) with ornithine and agmatine as intermetabolites [52]. Ornithine could also be converted into γ-aminobutyric acid by P5CS and glutamate decarboxylase (GAD) or into proline by pyrroline-5-carboxylate reductase (P5CR) (Figure 4) [36, 52].CBF-dependent signaling pathway plays a pivotal role in arginine metabolism. Tomato with upregulated expression ofSlICE1showed higher expression levels ofSlP5CS and higher concentrations of polyamines, proline, and γ-aminobutyric acid, thus improving the chilling tolerance of fruit [81, 82]. A similar phenomenon was observed by the overexpression ofAtCBF3/DREB1A inArabidopsis [24].


#### 5.2.2. Enhancing ROIs-Scavenging Capacity

ROIs are the partially reduced forms of O2 [90]. Chilling exposure could stimulate the production and overaccumulation of ROIs, which disrupts the cellular homeostasis and cause cell death [77, 91]. In plant, the major ROIs-scavenging pathways consist of (i) water-water cycle; (ii) ascorbate-glutathione cycle; (iii) glutathione-peroxidase (GPX) cycle; and (iv) catalase (CAT) [77] (Figure 5). Among these, the ascorbate-glutathione cycle plays a crucial role in regulating ROIs based on its wide distribution as well as the high affinity of ascorbate peroxidase (APX) for H2O.Figure 5
Reactive oxygen intermediates- (ROIs-) scavenging pathways in plant [77]. (a) Water-water cycle, (b) ascorbate-glutathione cycle, (c) glutathione-peroxidase (GPX) cycle, and (d) catalase (CAT). Enzyme abbreviations are as follows: APX, ascorbate peroxidase; AsA, ascorbic acid; SOD, superoxide dismutase; DHA, dehydroascorbate; DHAR, dehydroascorbate reductase; GR, glutathione reductase; GSH, glutathione; GSSG, oxidized glutathione; H2O2, hydrogen peroxide; MDA, monodehydroascorbate; MDAR, monodehydroascorbate reductase; PSI, photosystem I.ROIs-scavenging capacity in plant is correlated with CBF-dependent signaling pathway. Overexpression ofSlICE1 in tomatoes could enhance the accumulation of antioxidants, such as β-carotene, lycopene, and ascorbic acid, as well as antioxidant activity, thus improving the chilling tolerance of tomato [81, 82].


#### 5.2.3. Enhancing Energy Metabolism

Energy supply is also very important during the stress response as an ATP deficit would induce membrane lipid peroxidation and subsequently generate more free radicals, which would attack the cellular membrane [92, 93]. In plant, ATP is synthesized from glucose by a series of pathways, including glycolysis, Kreb’s cycle, electron transfer chain, and oxidative phosphorylation.CBF-dependent signaling pathway is supposed to be involved in energy metabolism. Overexpression ofSlICE1 in tomato enhanced the accumulation of metabolites relevant to ATP generation, such as glucose, fructose, glucose 6-phosphate, fructose 1,6-diphosphate, phosphoenolpyruvate, citric acid, and succinic acid, thus improving the chilling tolerance of tomato [81, 82]. Similarly, overexpression of theAtCBF3 could enhance the chilling tolerance ofArabidopsis, which was associated with higher abundances of glucose and fructose [24].


#### 5.2.4. Enhancing the Expression of HSPs

HSPs, a stress-responsive family of proteins with molecular weight between 15 and 115 kDa, are comprised of five subfamilies, including HSP70s, HSP60s, HSP90s, HSP100s, and small HSPs (sHSPs) [5]. They are widely distributed within cellular compartments and exert their protective role against stress based on molecular chaperone activity to sustain protein trafficking as well as maintain the integrity and function of the cell membrane [7, 94]. Furthermore, sHSPs can exert molecular chaperone activity independent of ATP [95].The expression levels ofHSPs genes are regulated by the CBF-dependent signaling pathway [96, 97]. Previously, Nakamura et al. [80] found that chilling stress caused the induction ofOsICE1andOsICE2in rice and subsequent upregulation ofOsCBF1, OsHsfA3, andOsTPP1 expression.


## 5.1. Activation of CBF-Dependent Signaling Pathway during Low Temperature Stress

Upon exposure to low temperature, plants immediately activate multiple signaling pathways to enhance chilling tolerance, which consists of transcriptional, posttranscriptional, translational, and posttranslational regulators of low temperature-induced expression of the functional genes [6]. In the promoters of these genes, there is a kind ofcis-elements termed C-repeat elements/dehydration-responsive elements (CRT/DRE), where CBFs can bind [7]. CBFs, which participate in transcriptional regulation of low temperature acclimation [6], are under the positive control of the upstream ICEs, including ICE1 and ICE2 [8]. Activation of CBF-dependent signaling pathway, especially the ICE1-CBF transcriptional cascade, is supposed to be a key responder to low temperature stress in plant, enhancing their chilling resistance [6, 9]. Chilling stress results in the induction ofOsICE1andOsICE2in rice(Oryza sativa) and subsequent upregulation ofOsCBF1, rice heat shock factor A3 (OsHsfA3), andrice trehalose 6-phosphate phosphatase 1 (OsTPP1) expression [80]. Overexpression ofSlICE1in tomato fruit initiates the expression ofSlCBF1,dehydrin Ci7 homolog (SlDRCi7), andpyrroline-5-carboxylate synthetase (SlP5CS) and enhance the accumulation of antioxidants, several amino acids, amines, and sugars, thus improving the chilling resistance of fruit [81, 82]. A similar result was observed in cucumber [83].InArabidopsis, three CBFs—namely, AtCBF1, AtCBF2, and AtCBF3—have been identified to be involved in low temperature stress [10]. Under normal condition, JAZ1 and JAZ4 inArabidopsis would interact with ICEs, thus suppressing CBF-dependent signaling pathway [84]. Upon low temperature acclimation when the biosynthesis of endogenous JAs is induced, ICE1 is released from the JAZs via a 26S proteasome-mediated degradation and then positively regulates the expression of CBFs [78]. Subsequently, CBFs bind to CRT/DRE inCORs and upregulate their expression, which could induce the physiological and biochemical responses and thus improve chilling tolerance of plant [6]. For the detailed information on the roles of CBF-dependent signaling pathway in chilling acclimation, readers could refer to the reviews on this topic [6, 85, 86].


## 5.2. Physiological and Biochemical Responses Induced by CBF-Dependent Signaling Pathway to Improve Chilling Tolerance

### 5.2.1. Enhancing Arginine Pathways

Arginine is a precursor for the biosynthesis of bioactive molecules such as polyamines, proline, andγ-aminobutyric acid [36, 52] (Figure 4). Polyamines could stabilize the membrane based on its polycationic nature at physiological pH [87], proline acts as a protein-compatible hydrotrope or as a hydroxyl radical scavenger to regulate the NAD+/NADH ratio [88], while γ-aminobutyric acid is involved in osmotic regulation, ROS scavenge, and intracellular signal transduction [89].Figure 4
Arginine pathways which lead to the production of polyamines, proline, andγ-aminobutyric acid [52, 75]. Enzyme abbreviations are as follows: ADC, arginine decarboxylase; ARG, arginase; GAD, glutamate decarboxylase; NOS, NO synthase; ODC, ornithine decarboxylase; P5CR, pyrroline-5-carboxylate reductase; P5CS, pyrroline-5-carboxylate synthetase.As shown in Figure4, in plant arginase is metabolised by arginase (ARG), ornithine decarboxylase (ODC), and arginine decarboxylase (ADC) with ornithine and agmatine as intermetabolites [52]. Ornithine could also be converted into γ-aminobutyric acid by P5CS and glutamate decarboxylase (GAD) or into proline by pyrroline-5-carboxylate reductase (P5CR) (Figure 4) [36, 52].CBF-dependent signaling pathway plays a pivotal role in arginine metabolism. Tomato with upregulated expression ofSlICE1showed higher expression levels ofSlP5CS and higher concentrations of polyamines, proline, and γ-aminobutyric acid, thus improving the chilling tolerance of fruit [81, 82]. A similar phenomenon was observed by the overexpression ofAtCBF3/DREB1A inArabidopsis [24].


### 5.2.2. Enhancing ROIs-Scavenging Capacity

ROIs are the partially reduced forms of O2 [90]. Chilling exposure could stimulate the production and overaccumulation of ROIs, which disrupts the cellular homeostasis and cause cell death [77, 91]. In plant, the major ROIs-scavenging pathways consist of (i) water-water cycle; (ii) ascorbate-glutathione cycle; (iii) glutathione-peroxidase (GPX) cycle; and (iv) catalase (CAT) [77] (Figure 5). Among these, the ascorbate-glutathione cycle plays a crucial role in regulating ROIs based on its wide distribution as well as the high affinity of ascorbate peroxidase (APX) for H2O.Figure 5
Reactive oxygen intermediates- (ROIs-) scavenging pathways in plant [77]. (a) Water-water cycle, (b) ascorbate-glutathione cycle, (c) glutathione-peroxidase (GPX) cycle, and (d) catalase (CAT). Enzyme abbreviations are as follows: APX, ascorbate peroxidase; AsA, ascorbic acid; SOD, superoxide dismutase; DHA, dehydroascorbate; DHAR, dehydroascorbate reductase; GR, glutathione reductase; GSH, glutathione; GSSG, oxidized glutathione; H2O2, hydrogen peroxide; MDA, monodehydroascorbate; MDAR, monodehydroascorbate reductase; PSI, photosystem I.ROIs-scavenging capacity in plant is correlated with CBF-dependent signaling pathway. Overexpression ofSlICE1 in tomatoes could enhance the accumulation of antioxidants, such as β-carotene, lycopene, and ascorbic acid, as well as antioxidant activity, thus improving the chilling tolerance of tomato [81, 82].


### 5.2.3. Enhancing Energy Metabolism

Energy supply is also very important during the stress response as an ATP deficit would induce membrane lipid peroxidation and subsequently generate more free radicals, which would attack the cellular membrane [92, 93]. In plant, ATP is synthesized from glucose by a series of pathways, including glycolysis, Kreb’s cycle, electron transfer chain, and oxidative phosphorylation.CBF-dependent signaling pathway is supposed to be involved in energy metabolism. Overexpression ofSlICE1 in tomato enhanced the accumulation of metabolites relevant to ATP generation, such as glucose, fructose, glucose 6-phosphate, fructose 1,6-diphosphate, phosphoenolpyruvate, citric acid, and succinic acid, thus improving the chilling tolerance of tomato [81, 82]. Similarly, overexpression of theAtCBF3 could enhance the chilling tolerance ofArabidopsis, which was associated with higher abundances of glucose and fructose [24].


### 5.2.4. Enhancing the Expression of HSPs

HSPs, a stress-responsive family of proteins with molecular weight between 15 and 115 kDa, are comprised of five subfamilies, including HSP70s, HSP60s, HSP90s, HSP100s, and small HSPs (sHSPs) [5]. They are widely distributed within cellular compartments and exert their protective role against stress based on molecular chaperone activity to sustain protein trafficking as well as maintain the integrity and function of the cell membrane [7, 94]. Furthermore, sHSPs can exert molecular chaperone activity independent of ATP [95].The expression levels ofHSPs genes are regulated by the CBF-dependent signaling pathway [96, 97]. Previously, Nakamura et al. [80] found that chilling stress caused the induction ofOsICE1andOsICE2in rice and subsequent upregulation ofOsCBF1, OsHsfA3, andOsTPP1 expression.


## 5.2.1. Enhancing Arginine Pathways

Arginine is a precursor for the biosynthesis of bioactive molecules such as polyamines, proline, andγ-aminobutyric acid [36, 52] (Figure 4). Polyamines could stabilize the membrane based on its polycationic nature at physiological pH [87], proline acts as a protein-compatible hydrotrope or as a hydroxyl radical scavenger to regulate the NAD+/NADH ratio [88], while γ-aminobutyric acid is involved in osmotic regulation, ROS scavenge, and intracellular signal transduction [89].Figure 4
Arginine pathways which lead to the production of polyamines, proline, andγ-aminobutyric acid [52, 75]. Enzyme abbreviations are as follows: ADC, arginine decarboxylase; ARG, arginase; GAD, glutamate decarboxylase; NOS, NO synthase; ODC, ornithine decarboxylase; P5CR, pyrroline-5-carboxylate reductase; P5CS, pyrroline-5-carboxylate synthetase.As shown in Figure4, in plant arginase is metabolised by arginase (ARG), ornithine decarboxylase (ODC), and arginine decarboxylase (ADC) with ornithine and agmatine as intermetabolites [52]. Ornithine could also be converted into γ-aminobutyric acid by P5CS and glutamate decarboxylase (GAD) or into proline by pyrroline-5-carboxylate reductase (P5CR) (Figure 4) [36, 52].CBF-dependent signaling pathway plays a pivotal role in arginine metabolism. Tomato with upregulated expression ofSlICE1showed higher expression levels ofSlP5CS and higher concentrations of polyamines, proline, and γ-aminobutyric acid, thus improving the chilling tolerance of fruit [81, 82]. A similar phenomenon was observed by the overexpression ofAtCBF3/DREB1A inArabidopsis [24].


## 5.2.2. Enhancing ROIs-Scavenging Capacity

ROIs are the partially reduced forms of O2 [90]. Chilling exposure could stimulate the production and overaccumulation of ROIs, which disrupts the cellular homeostasis and cause cell death [77, 91]. In plant, the major ROIs-scavenging pathways consist of (i) water-water cycle; (ii) ascorbate-glutathione cycle; (iii) glutathione-peroxidase (GPX) cycle; and (iv) catalase (CAT) [77] (Figure 5). Among these, the ascorbate-glutathione cycle plays a crucial role in regulating ROIs based on its wide distribution as well as the high affinity of ascorbate peroxidase (APX) for H2O.Figure 5
Reactive oxygen intermediates- (ROIs-) scavenging pathways in plant [77]. (a) Water-water cycle, (b) ascorbate-glutathione cycle, (c) glutathione-peroxidase (GPX) cycle, and (d) catalase (CAT). Enzyme abbreviations are as follows: APX, ascorbate peroxidase; AsA, ascorbic acid; SOD, superoxide dismutase; DHA, dehydroascorbate; DHAR, dehydroascorbate reductase; GR, glutathione reductase; GSH, glutathione; GSSG, oxidized glutathione; H2O2, hydrogen peroxide; MDA, monodehydroascorbate; MDAR, monodehydroascorbate reductase; PSI, photosystem I.ROIs-scavenging capacity in plant is correlated with CBF-dependent signaling pathway. Overexpression ofSlICE1 in tomatoes could enhance the accumulation of antioxidants, such as β-carotene, lycopene, and ascorbic acid, as well as antioxidant activity, thus improving the chilling tolerance of tomato [81, 82].


## 5.2.3. Enhancing Energy Metabolism

Energy supply is also very important during the stress response as an ATP deficit would induce membrane lipid peroxidation and subsequently generate more free radicals, which would attack the cellular membrane [92, 93]. In plant, ATP is synthesized from glucose by a series of pathways, including glycolysis, Kreb’s cycle, electron transfer chain, and oxidative phosphorylation.CBF-dependent signaling pathway is supposed to be involved in energy metabolism. Overexpression ofSlICE1 in tomato enhanced the accumulation of metabolites relevant to ATP generation, such as glucose, fructose, glucose 6-phosphate, fructose 1,6-diphosphate, phosphoenolpyruvate, citric acid, and succinic acid, thus improving the chilling tolerance of tomato [81, 82]. Similarly, overexpression of theAtCBF3 could enhance the chilling tolerance ofArabidopsis, which was associated with higher abundances of glucose and fructose [24].


## 5.2.4. Enhancing the Expression of HSPs

HSPs, a stress-responsive family of proteins with molecular weight between 15 and 115 kDa, are comprised of five subfamilies, including HSP70s, HSP60s, HSP90s, HSP100s, and small HSPs (sHSPs) [5]. They are widely distributed within cellular compartments and exert their protective role against stress based on molecular chaperone activity to sustain protein trafficking as well as maintain the integrity and function of the cell membrane [7, 94]. Furthermore, sHSPs can exert molecular chaperone activity independent of ATP [95].The expression levels ofHSPs genes are regulated by the CBF-dependent signaling pathway [96, 97]. Previously, Nakamura et al. [80] found that chilling stress caused the induction ofOsICE1andOsICE2in rice and subsequent upregulation ofOsCBF1, OsHsfA3, andOsTPP1 expression.


## 6. Linkage between JAs Signal Transduction and CBF-Dependent Signaling Pathway

Although several physiological and biochemical responses are observed after JAs treatment, the mechanism has not been clarified until recently. Activation of the CBF-dependent signaling pathway, especially the ICE1-CBF transcriptional cascade, is proposed to perform a large duty during JAs-induced chilling tolerance. Previously, Zhao et al. [9] observed that MYC2 was involved in JAs-induced chilling tolerance through physical interaction and functional coordination with ICE1, regulating the CBF-dependent signaling pathway. In association with the higher expression ofMaMYC2a andMaMYC2b after dipping “Carvendish” banana in 0.1 mM MeJA solution for 30 min was the enhanced expression ofMaCBF1,MaCBF2,and CBF downstream genes, includingCOR1,KIN2,RD2, andRD5 [9]. Since MYC2 is one of the TFs in JAs signal transduction pathway, these results imply a potential linkage between two signal transduction pathways. Furthermore, various physiological and biochemical responses induced by CBF-dependent signaling pathway were also observed in postharvest horticultural commodities after JAs treatment (see Sections 4 and 5).In combination with previous reports [6, 9, 78, 84], the primary signal transduction process after prechilling JAs treatment to activate CBF-dependent signaling pathway in order to improve the chilling resistance of postharvest horticultural commodities is illustrated in Figure 6: higher JAs content after prechilling JAs treatment triggers COI1-mediated degradation of JAZs, releasing MYC2 and ICE1 from repression. MYC2 and ICE1 then interact with each other to activate the expression of CBFs, which subsequently bind to the CRT/DRE box promoter sequence element to induce the expression of CBF downstream genes, such asCORs [79]. The induction ofCORsexpression needs SENSITIVE TO FREEZING 6 (SFR6) to recruit RNA polymerase II to the promoter [98]. Following this is the initiation of the gene expression downstream, which subsequently induces various physiological and biochemical responses, resulting in the improvement in the chilling tolerance of postharvest horticultural commodities.Figure 6
Prechilling jasmonates (JAs) treatment activates CBF-dependent signaling pathway and thus induce physiological and biochemical responses to enhance the chilling tolerance of postharvest horticultural commodities [9, 78, 79].


## 7. Conclusions and Perspectives

Activation of CBF-dependent signaling pathway, especially the ICE1-CBF transcriptional cascade, is proposed to be a key response to low temperature stress in plant, enhancing their chilling tolerance [6, 9].Prechilling JAs treatment shows a potential to alleviate the CI of postharvest horticultural commodities. However, our understanding of its mechanism is still rudimentary. In association with new discoveries in JAs biosynthesis, signal transduction, and the roles of the CBF-dependent signaling pathway during chilling stress, the linkage between JAs signal transduction and CBF-dependent signaling pathway is identified. This review summarizes the previous studies relevant to JAs and CBF-dependent signaling pathway and further proposed the primary signal transduction process after JAs treatment to activate CBF-dependent signaling pathway to enhance the chilling tolerance (Figure6). However, there are still some questions which need to be addressed, such as the roles of other MYCs (except for MYC2) in the activation of CBF-dependent signaling pathway.In addition, according to microarray and real-time analysis of wild-typeArabidopsis and itscoi1mutant, Hu et al. [84] observed that transcriptional levels of someCORs, which were not induced by CBF-dependent signaling pathway, were lower incoi1mutant upon low temperature exposure. These results suggest that some physiological and biochemical responses might result from CBF-independent signaling pathway [84]. Thus, further work is needed to clarify the roles of the physiological and biochemical responses induced by the CBF-independent signal pathway and their roles in JAs-mediated chilling resistance with the aid of genomics, proteomics, transcriptomics, and metabolomics, which will give us a more comprehensive understanding on the mechanism employed by exogenous JAs application to mitigate CI symptoms of postharvest horticultural commodities.



---

*Source: 8517018-2018-03-08.xml*