ABSTRACT

Brassinosteroids are natural polyhydroxylated steroidal plant growth regulators or phyto-hormones. These are ubiquitous in plant kingdom and influence a wide variety of molecular, physiological and biochemical responses of plants. Brassinosteroids have also been applied and their possible role has been investigated on postharvest physiology of various horticultural crops. Brassinosteroids regulate ripening of different non-climacteric and climacteric fruits and influence colour metabolism. They inhibit activities of peroxidase and polyphenol oxidase enzymes and delay enzymatic browning. Exogenous application of brassinosteroids inhibits cell wall degradation and delays softening of fruits. In addition, their application regulates sugar and energy metabolism in different fruit and vegetable crops. They suppress lipoxygenase and phospholipase D enzyme activities and conserve higher unsaturated fatty acid contents, suppress electrolyte leakage, inhibit lipid peroxidation and maintain higher membrane integrity eventually leading to suppressed chilling injury during postharvest storage. These alleviate oxidative stress and prolong storage life potential of various horticultural crops. So, the present review summarizes various roles and mechanism of action of brassinosteroids in extending postharvest life and maintaining quality of different horticultural crops.

INTRODUCTION

The demand of fruits, vegetables and flowers has much increased in the recent years all over the world. Fresh fruits and vegetables are now important part of healthy human diet whereas flowers are used at various occasions. It is not possible to provide fresh fruits and vegetables to all consumers. So, different fruits and vegetables need to be stored at low temperature to extend their storage life so that these can be shifted to distant local and foreign markets. Storage of fruits and vegetables under low temperature conditions generally extends the postharvest life but at the same time it also leads to certain undesirable physiological disorders especially chilling injury and postharvest decay (Luengwilai and Beckles, 2013; Aghdam and Bodbodak, 2014; Patel et al., 2016; Islam et al., 2018). So, some eco-friendly and suitable chemical treatments are required for the management of various postharvest issues of horticultural crops.

Brassinosteroids are natural steroidal plant growth regulators or phyto-hormones. These were first discovered from the pollen of rape plants. It has been reported that brassinosteroids have significant growth enhancing effect on various crop plants (Mitchell et al., 1970; Vardhini, 2017). There are different types of brassinosteroids (Fig. 1). Among these, brassinazole, 24-epibrassinolide (24-EBR), brassinolide (BL) and 28-homobrassniolide (28-HBL) are so far the major and more oftenly used brassinosteroids at various levels for modulation of preharvest and postharvest aspects of different horticultural crops (Aghdam et al., 2016; Nawaz et al., 2017).

Figure 1: Chemical structures of some important brassinosteroids (adopted from Vardhini et al., 2017; Baghel et al., 2019).

Brassinosteroids play an imperative role in reducing various abiotic and biotic stresses at various levels (Nawaz et al., 2017). These have also been found effective in maintaining various quality related attributes of different horticultural crops (Aghdam et al., 2016). It was noted that treatment of Satsuma mandarins with EBR resulted in reduced decay and oxidative stress (Zhu et al., 2015a). In the same way, EBR application was effective in reducing chilling injury of

eggplant (Gao et al., 2015) and peach fruits (Gao et al., 2016). In addition, EBR application also reduced enzymatic browning in lotus roots (Gao et al., 2017). Moreover, 28-HBL delayed skin browning in cold stored medlar fruits (Ekinci et al., 2019). Application of EBR delayed senescence of kiwifruits at ambient conditions (Lu et al., 2019). Postharvest EBR application also delayed degradation of chlorophyll in lime fruits (Tavallali, 2018) and leaf yellowing in broccoli florets (Cai et al., 2019). The EBR application extended vase life of daylilies by delaying yellowing of flowers (Yao et al., 2017). So, it is evident from the literature that brassinosteroids regulate various aspects of different horticultural crops. So, the present review is focused on different physiological roles of brassinosteroids and their possible modes of action in different horticultural crops.

APPLICATION METHODS OF BRASSINOSTEROIDS

Efficacy of any postharvest treatment generally depends on the method of application. Different brassinosteroids application methods have been reported in literature including aqueous dipping and spray treatments depending upon the produce. In most of the cases, immersion and/or dipping application method has been used (Li et al., 2012; Zhu et al., 2015a). In contrast, spray application was used in daylilies (Yao et al., 2017) and broccoli florets (Cai et al., 2019). Vacuum infiltration has also been used (Tavallali, 2018). However, it is worth to mention that none of the study investigated vacuum infiltration, dipping and spray application methods simultaneously on the same crop in same experiment. In case of fresh cut produce, prolonged dipping treatments of brassinosteroids may leads to excessive uptake so in this case dipping time should be reduced to minimal in order to avoid any toxic effects.

Crosstalk of Brassinosteroids With Other Phytohormones

Brassinosteroids crosstalk with various other hormones to co-ordinate certain physiological cascades in either synergistic or antagonistic manner. Exogenous brassinosteroids application up-regulated expression of 1-aminocyclopropane-1-carboxylate synthase (ACS) gene that is needed for ethylene biosynthesis (Muday et al., 2012). Moreover, brassinosteroids act during post-transcriptionally and enhances ACS proteins such as ACS5, ACS6 and ACS9 stability by inhibiting its ubiquitous mediated 26S proteasomes. Consequently, in response to different exogenous and/or endogenous signals, ACS is controlled continuously by brassinosteroids to adjust the biosynthesis of ethylene under various conditions (Hansen et al., 2009). Increased exogenous and endogenous brassinosteroids concentrations may lead to rapid ripening and accelerated senescence due to increased ethylene production. So, these may also regulate senescence. On contrary, abscisic acid (ABA) application can counteract brassinosteroids induced changes and delay the onset of senescence (Zhao et al., 1990). Exogenous brassinosteroids applications may also influence endogenous levels of other hormones. It was noted that gibberellic acid, indole-3-acetic acid, zeatin riboside and ABA decreased initially till day-12 and then all of these hormones increased up to day-24 in EBR applied daylily flowers. Overall, the concentrations of indole-3-acetic acid, gibberellic acid, zeatin riboside and ABA were significantly higher in EBR treated daylily flower buds (Table 1) (Yao et al., 2017). Although brassinosteroids crosstalk with other hormones however investigation of this aspect in postharvest studies under their exogenous applications is still limited and need further detailed research.

Table 1: Effects of different brassinosteroids on postharvest physiology and quality of horticultural crops.

BRASSINOSTEROIDS AND POSTHARVEST PHYSIOLOGY OF HORTICULTURAL CROPS

Brassinosteroids and ripening

Brassinosteroids play critical roles in ripening of different fruits. Exogenous application of EBR and HBL resulted in elevated lycopene concentration and enhanced degradation of chlorophyll contents in tomato. The enhanced ripening of tomato was associated with increased biosynthesis of ethylene that eventually accelerated ripening of tomato (Vardhini and Rao, 2002). Exogenous brassinosteroids application induces their increased endogenous production due to up-regulation of GhDWF4 gene that ultimately accelerated ripening of tomato fruits (Ye et al., 2015). In another study, it was noted that exogenous application enhanced their endogenous content and up-regulated FaBRI1 gene expression that subsequently led to accelerated development of red colouration and ripening of strawberry fruits (Chai et al., 2013). In another work, it was observed that exogenous EBR treatment up-regulated expression of FaBRI1 receptor and signalling pathway related with FaBRZ1 and FaBIN2 genes at pink stage. In addition, EBR application also affected phenylpropanoid pathway leading to enhanced accumulation of anthocyanins in strawberry fruits (Ayub et al., 2018). Increased endogenous concentration of brassinosteroids was found to be responsible for ripening of grape berries. It was noted that brassinosteroids biosynthesis encoding gene such as DWARF1 and enzyme i.e. Brassinosteroid-6-Oxidase were up-regulated and played a major role in ripening of grape berries (Symons et al., 2006). Application of EBR also resulted in increased ethylene production and respiration rate concomitant with enhanced softening and rapid colour development in ‘Kensington Pride’ mango fruits during ambient conditions (Table 1) (Zaharah and Singh, 2012; Zaharah et al., 2012). Exposure of tomato fruits to 3 µmol L^-1 brassinolide increased expression of lycopene synthesis related genes including LeGLK2 and LePSY1 and stimulated ripening process at higher rate than control (Zhu et al., 2015b).

Brassinosteroids play critical roles in ripening of different fruits. Exogenous application of EBR and HBL resulted in elevated lycopene concentration and enhanced degradation of chlorophyll contents in tomato. The enhanced ripening of tomato was associated with increased biosynthesis of ethylene that eventually accelerated ripening of tomato (Vardhini and Rao, 2002). Exogenous brassinosteroids application induces their increased endogenous production due to up-regulation of GhDWF4 gene that ultimately accelerated ripening of tomato fruits (Ye et al., 2015). In another study, it was noted that exogenous application enhanced their endogenous content and up-regulated FaBRI1 gene expression that subsequently led to accelerated development of red colouration and ripening of strawberry fruits (Chai et al., 2013). In another work, it was observed that exogenous EBR treatment up-regulated expression of FaBRI1 receptor and signalling pathway related with FaBRZ1 and FaBIN2 genes at pink stage. In addition, EBR application also affected phenylpropanoid pathway leading to enhanced accumulation of anthocyanins in strawberry fruits (Ayub et al., 2018). Increased endogenous concentration of brassinosteroids was found to be responsible for ripening of grape berries. It was noted that brassinosteroids biosynthesis encoding gene such as DWARF1 and enzyme i.e. Brassinosteroid-6-Oxidase were up-regulated and played a major role in ripening of grape berries (Symons et al., 2006). Application of EBR also resulted in increased ethylene production and respiration rate concomitant with enhanced softening and rapid colour development in ‘Kensington Pride’ mango fruits during ambient conditions (Table 1) (Zaharah and Singh, 2012; Zaharah et al., 2012). Exposure of tomato fruits to 3 µmol L^-1 brassinolide increased expression of lycopene synthesis related genes including LeGLK2 and LePSY1 and stimulated ripening process at higher rate than control (Zhu et al., 2015b).

Brassinosteroids and chilling injury

Chilling injury (CI) is one of the major postharvest physiological disorders affecting wide variety of horticultural crops during cold storage. CI generally depends upon maturity stage and cultivar. CI leads to quality degradation and limits postharvest storage life. CI also negatively affects integrity of cell membranes (Wang et al., 2012; Gao et al., 2015; Liu et al., 2016b; Li et al., 2018). Brassinosteroids play an important role in chilling tolerance of fruit and vegetable crops. It was observed that higher brassinosteroids concentration led to cold tolerance of cucumber (Xia et al., 2011). Application of 15 µmol L^-1 brassinolide alleviated CI (indicated by reduced calyx discoloration and surface pitting) in green bell pepper fruits at 3 °C storage (Wang et al., 2012). Treatment of eggplant with 10 µmol L^-1 EBR reduced electrolyte leakage that in turn reduced the symptoms of CI at 1 °C conditions (Gao et al., 2015). Postharvest 0.5 µmol L^-1 brassinolide application to bamboo shoots enhanced activities of ornithine-δ- aminotransferase (OAT) and Δ1-pyrroline-5-carboxylate synthetase (P5CS) and suppressed proline dehydrogenase (PDH) activity, which eventually resulted in CI alleviation during storage at 1 °C (Table 1) (Liu et al., 2016b). Application of 1.5 mg L^-1 brassinosteroids led to substantially higher alleviation of CI in Washington Navel oranges during storage at 3 °C (Ghorbani and Pakkish, 2014). In the same way, 1 mg L^-1 application of brassinolide in combination with hot water treatment significantly reduced the symptoms of CI due to lower oxidative damage in lime fruits at 5 °C (Rezakhani and Pakkish, 2017). Recently, Li et al. (2018) reported that 40 µmol L^-1 EBR application suppressed electrolyte leakage and increased chlorophyll fluorescence (Fv/Fm) in banana fruits. Moreover, 50 different proteins related with energy biosynthesis were up-regulated and up-regulation of methionine de novo biosynthesis-related proteins was also enhanced; thus, CI was considerably reduced in EBR exposed banana fruits at 8 °C temperature (Table 1).

Brassinosteroids and colour metabolism

Colour is an important characteristic of horticultural crops. The characteristic colour is responsible for purchasing of certain crops in the markets. Loss of colour ultimately reduces visual quality and market potential of the commodity. Spray application of 0.5 mg L^-1 solution of EBR delayed chlorophyll loss and reduced yellowing with substantially higher good quality flowers of daylilies (Table 1) (Yao et al., 2017). Vacuum infiltration of 10 µmol L^-1 brassinolide reduced chlorophyll degradation and showed conserved green colour due to inhibited yellowing in ‘Tahiti’ and ‘Persian lime’ fruits. The same treatment also showed higher hue angle and reduced chroma values in both cultivars of lime (Tavallali, 2018). In the same way, 2 µmol L^-1 spray application of EBR reduced yellowing, showed higher chlorophyll contents and exhibited higher chlorophyll fluorescence (Table 1). The reduced chlorophyll degradation and higher chlorophyll fluorescence eventually resulted in significantly reduced yellowing in broccoli florets (Cai et al., 2019).

Brassinosteroids and postharvest browning

Postharvest browning is one of the major undesirable disorders that occur in different fruits and vegetables (Ali et al., 2016; Ali et al., 2019). Browning not only reduces visual quality of the produce but also leads to decreased market potential and purchase decision of consumers. It has been observed that brassinosteroids have anti-browning property. The 10 µmol L^-1 EBR treatment reduced the increase in PPO and POD activities and delayed pulp browning for 15 days at 1 °C in eggplant (Table 1) (Gao et al., 2015). Application of 3 µL BL treatment markedly reduced browning of white button mushroom for 16 days during storage at 4 °C (Ding et al., 2016). In the same way, 80 nmol L^-1 EBR treatment suppressed quinones production and reduced PPO and POD activities along with reduced phenols oxidation that in turn efficiently inhibited enzymatic browning in root slices of lotus (Gao et al., 2017). Similarly, 5 µL HBL exposure of medlar fruits significantly reduced skin browning for 60 days at 0 °C (Ekinci et al., 2019).

Brassinosteroids and ethylene biosynthesis/respiration rate

Ethylene and respiration are very critical as their increased level generally leads to short storage/shelf life and prompt senescence of produce (Golden et al., 2014; Razzaq et al., 2014). So, in order to reduce climacteric peak of ethylene and respiration, some suitable treatments are required. However, information about the effects of brassinosteroids on ethylene production and respiration rate is contradictory. EBR treatment significantly increased respiration rate and ethylene production in ‘Kensington Pride’ mango fruits during ambient storage (Zaharah and Singh et al., 2012; Zaharah et al., 2012). Postharvest exposure of 3 µmol L^-1 brassinolide increased activities of ethylene biosynthesis related genes such as LeACS2, LeACS4, LeACO4 and LeACO1 and significantly enhanced ethylene production in tomato fruits. In addition, it also markedly increased respiration rate (Table 1) (Zhu et al., 2015b). In contrast, dipping of green asparagus spears in 10 µmol L^-1 brassinolide solution substantially reduced its respiration rate (Wu and Yang et al., 2016). Spray treatment of 0.5 mg L^-1 concentrated EBR showed decreased respiration rate of daylily flower buds (Yao et al., 2017). EBR application (3 µmol L^-1) led to higher expression of ACC synthase related genes such as BoACO3 and BoACS4 that subsequently accelerated ethylene biosynthesis in broccoli florets (Table 1) (Cai et al., 2019). It is clear from the aforementioned results that reports of brassinosteroids regarding their influences on ethylene production and respiration rate are contradictory. So, further comprehensive research is still needed to study the impact of brassinosteroids on crop by crop basis.

Brassinosteroids and lipid peroxidation

Membrane lipids are important because these contribute in membrane integrity. The conservation of higher levels of unsaturated fatty acids (USFA) such as linolenic acid and linoleic are very important. Reduction in proportion of USFA in membranes eventually leads to increased cell membranes fluidity and lipid peroxidation (Li et al., 2012; Aghdam et al., 2016). Lipoxygenase (LOX) and phospholipase D (PLD) enzymes cause irreversible damage to membranes and increased lipid peroxidation eventually leading to CI during postharvest storage at low temperature conditions (Mao et al., 2007; Aghdam et al., 2016). Brassinosteroids have been found appropriate to suppress lipid peroxidation and membrane permeability. Postharvest exposure of tomato fruits to brassinosteroids suppressed LOX and PLD enzymes activities and maintained significantly higher membrane integrity due to reduced lipid peroxidation (Table 1) (Aghdam and Mohammadkhani, 2014). In the same way, brassinosteroids application reduced electrolyte leakage and inhibited lipid peroxidation in green bell peppers (Wang et al., 2012; Aghdam and Mohammadkhani, 2014), eggplant (Gao et al., 2015), peach (Gao et al., 2016), mushroom (Ding et al., 2016), lotus root slices (Ding et al., 2017) and daylily flower buds (Yao et al., 2017).

Brassinosteroids and antioxidant activities

Antioxidant enzymes such as catalase (CAT), ascorbate peroxidase (APX), peroxidase (POD), superoxide dismutase (SOD) and glutathione reductase (GR) are important to alleviate the detrimental effects of oxidative stress during postharvest storage (Wang et al., 2012; Ali et al., 2018). It has been reported that brassinosteroids induce higher activities of antioxidant enzymes as observed in 15 µmol L^-1 brassinolide treated green bell peppers (Wang et al., 2012). Similarly, 0.4 or 0.8 mg L^-1 EBR application induced higher activities of CAT, SOD, POD and PAL enzymes (Table 1). The higher PAL activity also caused increased biosynthesis of phenolics in table grapes during postharvest storage (Liu et al., 2016a). The similar response was observed in green asparagus where combined treatment of brassinolide and carboxymethyl chitosan coating significantly enhanced CAT, POD, SOD and PAL enzymes activities (Wu and Yang, 2016). Higher activities of CAT, SOD and APX enzymes were also noted in 3 µmol L^-1 brassinolide treated white button mushroom (Ding et al., 2016). Similarly, postharvest treatment with 0.5 mg L^-1 EBR showed higher activities of APX, SOD, CAT and POD in daylily that eventually resulted in reduced accumulation of H₂O₂ and O₂^•˗ contents and showed extended vase life (Yao et al., 2017).

Brassinosteroids and sugar metabolism

Sugar metabolism contributes in senescence regulation of certain crops (Rolland et al., 2002). The concentration of fructose and glucose increases rapidly during senescence along with significant reduction in starch content concomitant with senesce-like indications such as yellowing. This phenomenon has been observed in certain fruit crops. A rapid increase in fructose and glucose contents and reduction in starch concentration was observed in kiwifruits during ripening (Zhang et al., 2004). Postharvest application of brassinosteroids has been found effective in delaying the increase in sugars concentration. Treatment of kiwifruit with 5 µmol L^-1 EBR delayed degradation of starch and activities of neutral invertase, acid invertase, sucrose synthase, sucrose phosphate synthase, fructokinase and hexokinase enzymes. The suppressed activities of the said enzymes subsequently led to reduced increase in glucose, sucrose and fructose contents (Table 1) (Lu et al., 2019). So, rapid increase in sugars may be delayed with the exogenous application of brassinosteroids and its possible effects should be investigated on other horticultural crops as well.

Brassinosteroids and energy regulation

Sufficient energy availability is essential to sustain normal metabolism and to delay onset of senescence during postharvest life of horticultural crops. Provision of sufficient extracellular ATP also delays senescence and alleviates oxidative stress during postharvest storage of horticultural crops (Aghdam et al., 2018). Sufficient energy supply is also critical for maintenance of cell membrane integrity after harvest. It has been observed that higher membrane integrity correlates with sufficient ATP availability and energy charge (Jin et al., 2013). Application of 0.5 µmol L^-1 BL led to increased activities of H⁺-ATPase, Ca⁺²-ATPase, cytochrome C oxidase (CCO) and succinate dehydrogenase (SDH) in bamboo shoots (Table 1) (Liu et al., 2016b). Moreover, it also increased adenosine monophosphate (AMP) content and reduced the decrease of ATP, adenosine diphosphate (ADP) and energy charge during storage period of 42 days. It was also observed that higher energy availability and energy charge enhanced membrane integrity and resulted in CI tolerance of bamboo shoots (Liu et al., 2016b). Although only BL was studied in relation to energy regulation, but it was highly effective in conserving higher energy. So, other brassinosteroids such as brassinazole, EBR and HBL should also be studied in this regard on various other horticultural crops.

Brassinosteroids and postharvest softening

Rapid softening is a major postharvest constraint that leads to short storage life potential and shelf life of climacteric fruits. Different enzymes have been found correlated with disassembly of cell walls and softening (Brummell and Harpster, 2001). Degradation of cell walls is catalysed by numerous enzymes including pectin esterase (PE), polygalacturonase (PG), β-galactosidase (β-gal), endo-1,4-β-glucanase (EGase) and pectate lyase (PL) (Razzzaq et al., 2013; He et al., 2018). So, reduction in activities of these enzymes is critical for managing fruit softening. Brassinosteroids have been found suitable in inhibiting activities of cell wall degrading enzymes. Postharvest treatment with 10 µmol L^-1 EBR and 5 µmol L^-1 brassinazole significantly suppressed expression of DkPL1, DkEGase1, DkPE2 and DkPG1 genes that in turn reduced the activities of EGase, PL, PG and β-gal (Table 1). The same treatments also resulted in higher cellulose, pectin content and acid-soluble pectin in persimmon fruits. Reduced activities of EGase, PL, PG and β-gal and higher conservation of different pectin contents eventually resulted in significantly suppressed softening of persimmon fruits during ambient storage (He et al., 2018). So, brassinosteroids have good potential of softening reduction and should be investigated in other climacteric fruits.

Brassinosteroids and postharvest decay

Fresh vegetables and fruits are perishable having living cells that during ripening continue to respire which provide energy and increase susceptibility to decay at later stages during postharvest storage. Postharvest decay has been found associated with loss of quality and quantity of the produce resulting from bacterial and fungal pathogens. However, fungal diseases are more crucial as these lead to significant economic losses of the commodities (Aghdam et al., 2016). Postharvest decay of fruits and vegetables occurs due to latent infections taking place in the field or through wounds occurring during harvest and handling during supply chains (Hussain et al., 2015). The control of postharvest decay of fruits and vegetables is critical to reduce losses of the produce. Postharvest 5 µmol L^-1 brassinosteroids treatment increased phenylalanine ammonia lyase (PAL) and polyphenol oxidase (PPO) activities that significantly suppressed Penicillium expansum induced blue mould rot in jujube fruits (Table 2) (Zhu et al., 2010). Similarly, application of EBR (0.4 or 0.8 mg L^-1) increased PAL and peroxidase (POD) enzymes activities that subsequently suppressed Botrytis cinerea induced grey mould disease of table grapes during postharvest storage (Table 2) (Liu et al., 2016a).

Table 2: Effects of different brassinosteroids on postharvest diseases of horticultural crops.

CONCLUSION AND FUTURE PERSPECTIVES

In conclusion, brassinosteroids regulate various important physiological aspects of horticultural crops. Literature on alleviation of chilling injury and inhibition of enzymatic browning, softening, regulation of energy and sugars metabolism is consistent. Brassinosteroids application also induces PPO and PAL enzymes activities and reduces decay during postharvest storage. However, effect of brassinosteroids on ethylene biosynthesis and respiration rate is contradictory. Some literature reported that brassinosteroids application induces ethylene production and accelerates ripening but other showed that brassinosteroids suppress ethylene biosynthesis and delays ripening. So, a further comprehensive research work is still required on crop by crop basis. It is also suggested to study the effects of brassinosteroids on aroma volatiles and phenolics metabolism. Proteomic based study in response to application of brassinosteroids should also be carried out. Brassinosteroids crosstalk with other hormones but this aspect is generally overlooked in postharvest storage studies. Concentrations and application methods of brassinosteroids still need to be optimized.

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