Under sustained low-temperature stress, tissue acclimation fails, leading to exhaustion and the onset of severe PCI. Preharvest factors, including genotype, environmental variables, and agronomic practices, all interact to influence PCI severity. PCI is more severe in tissues harvested before reaching horticultural maturity , as the developmental pathways are incomplete and will be largely disrupted by chilling and rewarming. Although PCI is a significant problem, determining the PLW that can be ascribed to PCI is challenging because of the difficulty in identifying when it occurs. Most damage appears in retail outlets or in consumers’ homes, which is hard to monitor. Further, symptoms are internal in many species, for example, pineapple, nectarines, and so on, and some abnormalities in texture and flavor are only detected when the fruit or vegetable is eaten. PCI symptoms are also misdiagnosed. For example, PCI increases susceptibility to pathogens,plastic pots for planting which is often mistaken as the primary cause of loss, and poor-quality produce due to PCI may be attributed to early harvest or poor varietal selection. Given the above factors, quantifying economic losses due to PCI is also difficult.
The global trade of fresh fruits and vegetables was worth 115 billion USD in 2018. More than half of the 50 highest-traded global commodities are PCI-susceptible, and PLW globally is estimated at ∼33%. If PCI reaches even 0.5% of PLW, it could cost USD $144 M. Additional costs related to 1) shipping at temperatures higher than the commonly used 4°C, and 2) the complex logistics that factor in harvest date and storage life into transportation, are not included into this estimate, but they collectively reduce flexibility for growers and distributors. Periodically, the apparent benefits of extending shelf-life by storing produce at inappropriate temperatures may outweigh the negative impact on quality: if fruit are stored at non-PCI-inducing temperatures for the equivalent time, they may spoil and will be rejected, whereas fruit with invisible PCI symptoms are salable. This incentivizes refrigerating sensitive produce, which may be profitable in the short term, but leads to long-term consumer distrust in produce quality and value. The abnormalities associated with PCI that lead to consumer dissatisfaction, waste, and loss can be linked to specific cellular dysfunctions . Mealy texture, surface pitting, and fungal susceptibility are due to reduced pectin solubilization and depolymerization, and micro-fractures in the cell-wall network. Tissue browning is initiated when organelles lose their structural integrity in chilled tissues. Chilling leads to membrane disassociation that releases polyphenol oxidase, peroxidase, and their phenolic substrates into the cytosol where they react to form quinones.
Polymerization of quinone creates the brown pigments in chilling-injured tissues. Cold storage disrupts the finely-tuned ripening program that is modulated by the interplay of hormones, transcriptional factors , and epigenetic marks often, with negative consequences for fruit quality.Upon rewarming, increases in respiration and ethylene production are initiated, and visible chilling-injury symptoms develop, the magnitude of which is inversely proportional to the storage temperature. Even mild chilling injury in red tomato fruit triggers epigenetic changes in ripening TFs such as RIN, NOR, and CNR that down regulate the production of key volatiles responsible for hedonistic values. However, this is not always so, ‘abnormal chilling injury behavior’ occurs at milder storage temperatures and has been reported in peach and nectarine, plum, persimmon, and papaya. The mechanisms underlying ‘abnormal chilling injury’ are unknown, but the enhancement of sugar and energy metabolism may be relevant. Many economically important commodities, for example, zucchini, cucumber, and bell pepper, are harvested before reaching physiological maturity, and will thus have different cold-stress responses to those commonly studied such as tomato and banana, which are harvested when mature. Commodities harvested at immaturity typically have higher respiratory and deterioration rates, greater water loss because of incomplete cuticle development, and inefficient reactive oxygen species scavenging systems, which will influence their PCI response. In potato, chilling leads to cold-induced sweetening — starch breakdown and sugar accumulation, where the latter serves as protective compatible solutes. When these ‘sugared’ tubers are fried, baked, or roasted, they turn black as acrylamide forms, which is visually unappealing, bitter in taste, and harmful to human health.
The shelf-life of leafy greens is relatively short, given their limited energy reserves and high transpiration rate. Fresh basil is a popular herb with high export value, but chilling causes changes in leaf photosynthetic parameters and stimulates ethylene biosynthesis, which accelerates senescence. Loss of membrane permeability, suppression of the protective antioxidant system, tissue browning, and Botrytis attack, all lead to premature spoilage. Reducing the severity of the negative traits of PCI that lead to waste and loss could be achieved by inducing allelic diversity at single or multiple genes that directly regulate relevant pathways . Integration of multi-omics data of cold-injured tissue compared with non-chilled tissues, has helped identify gene targets that influence PCI . In tomato, two important discoveries were made from genes identified using functional genomics: over expressing the TF SlGRAS4 reduced fruit-surface pitting, and promoted a more uniform color due to increased β– carotene content after chilling. Likewise, over expression of SlCYP90B3, a key brass inosteroid biosynthetic gene, improved the antioxidant response of fruits during cold storage, reducing PCI. These genes coordinated multiple pathways to improve PCI tolerance . Candidate genes for improving PCI tolerance have also been found by applying physical and chemical treatments that alleviate symptoms, and by studying the associated changes in the signal-transduction pathway. This is an active area of research where the literature is expanding rapidly. For example, physical treatments such as dipping in hot water before chilling mitigated fruit PCI, and have been associated with the upregulation of heat-shock proteins in banana and in mango, among others. The MaAPY gene family and the ATP receptor MaDORN1.19 are likely to be important for maintaining ATP homeostasis under chilling, after exogenous application of ATP or GTP to banana fruit. TFs MabHLH060 and MabHLH183, which are associated with reduced cold-induced membrane rigidity, were induced by Ethrel®, an ethylene releasing agent, which also reduced banana fruit PCI visual symptoms. Melatonin reduced peel browning in bananas, by inducing miR528 expression, which in turn down regulated several ‘browning genes’, i.e., MaPPO1, MaPPO2, and MaPPO3.
In tomato fruit, melatonin improved surface pitting, increased the expression of FAD3 and 7, and reduced the expression of phospholipase Dand lipoxygenasegenes, which helped to maintain membrane integrity under cold stress. New insights into regulatory networks governing PCI can be gained through-omics profiling as shown in several examples. Treating peach fruit with methyljasmonate delayed internal browning , maintained fruit texture and aroma volatiles,plant pot drainage and the accompanying transcriptomic and methylomic changes were revealed. In bell pepper, MeJA reduced surface pitting, shriveling, discoloration, and seed browning, and differences in the transcriptome, proteome and metabolome compared with untreated fruit were detected. Other studies combined cold storage with additional factors such as varying carbon dioxide, or fruit harvested at different times, and identified differentially expressed transcripts by RNASeq. These differential transcripts, proteins, and metabolites may help to identify gene networks and their regulators for genetic engineering. Gene targets for minimizing CIS in potato have been identified. CIS has been extensively studied compared with PCI in fruit, because potato is a staple for one billion people, and in some production areas, tubers are stored at low temperature for up to eight months. During tuber storage, there are cycles of synthesis and degradation of both sucrose and starch, but at temperatures between 4 and 10°C, the degradative fluxes are activated, so that reducing sugars accumulate . This change in metabolism occurs via the upregulation of genes encoding the beta-amylase, glucan water dikinase, sucrose phosphate synthase, and invertase enzymes. Attempts to alleviate CIS by modulating the activity of core enzymes of carbohydrate metabolic pathways have been made, although the role of each enzyme isoform is still to be elucidated. Recently identified non-metabolic genes that regulate CIS , for example, A CBF/DREB transcription factor and ring finger protein 1, are good targets for developing new germplasm. There is an acute need for a greater foundational understanding of PCI. Several advances have been made in model species, where regulatory elements of the cold signal-transduction pathway response have been identified and functionally verified. Integrating the discrete ‘snapshot’ studies discussed previously into full models across tissues, developmental stages, and conditions, is the next step for developing functional biotechnological solutions. PCI is often assessed in a single tissue sampled from a defined region. Not only is valuable information about the spatial evolution of the process lost, but events occurring in all the tissues that are consumed are not captured. A few studies have addressed this gap and serve as a guideline for future work. In tomato, tissue specific development of PCI was detected, even though pericarp is usually the only tissue traditionally studied. In pineapple, scanning electron microscopy and histochemical staining of fruit revealed that IB starts at the phloem and diffuses throughout fruit tissues from the core.
A breakthrough was made when a high resolution spatiotemporal transcriptome atlas in tomato was developed, which showed that ripening is not homogeneous. Because cold interferes with fruit ripening, some PCI symptoms would be expected to occur heterogeneously. Looking holistically at the chilling response across cells and tissues in harvested organs would uncover additional regulatory features of PCI. Uncontrolled and physiologically abnormal expression of genes through genetic engineering may severely disrupt the multiple finely-balanced gene-regulatory networks, resulting in deleterious phenotypes, especially if constitutively expressed in tissues where they do not normally occur. Regulated promoter systems to direct tissue gene expression in a highly controllable manner, with spatial and temporal precision, may be useful to study and design long-term solutions to PCI. Sequential changes in gene expression by promoter engineering are also a promising approach. Precise editing of chilling-associated cis-regulatory elements and differentially methylated regions due to chilling by Clustered Regularly Inter spaced Short Palindromic Repeats could promote ripening under chilling conditions and alleviate PCI . Achieving this precision is an immensely challenging prospect that likely depends on attaining the previously described holistic knowledge of chilling response. There are many longstanding challenges related to PCI that remain largely to be tackled, and which limit progress. The nature of research appears fragmented, and many species are studied with a substantial focus on symptom alleviation through exogenous treatments rather than development of endogenous/innate solutions. The importance of PCI is reflected in the number of papers published in Google Scholar using the search term ‘Postharvest chilling injury’, which has increased 7- fold over the past 20 years . This directly points to the urgency of developing biotechnological approaches to address PCI, and the immediacy with which solutions are needed. Environmental and management factors, both pre- and post harvest, influence whether a tissue will manifest PCI after cold storage. Time of year, time of day of harvest, and the growing environment are all documented to influence the trait. Experiments are by necessity, multi-factorial, and rely on large harvests of fruit, tubers, and so on, which limit the number of experiments that can be set up. In addition, plants must reach advanced developmental stages to obtain fruit or tubers, and this is followed by weeks of post harvest cold storage, limiting the number of experiments that can be performed in a year. This is exacerbated in perennial crops that fruit annually and may be biennial, offering fewer opportunities for experimentation. Replication of experiments may not be economically feasible, therefore, at a minimum, a thorough reporting of these factors should be included in PCI studies.There is a limit to which the data so derived can be translated to PCI. The growing plant may use avoidance, escape, and tolerance to cope with cold, while in contrast, options for surviving anthropogenic cold stress in stored tissues with limited nutrients are few. Cold responsive genes in Arabidopsis rosettes may have conserved functions in leafy greens, but genes and pathways from model crops, for example, cereals, will have limited relevance to the consumed tissues in horticultural crops. Therefore, post harvest studies must redescribe the behavior of these pathways under the conditions of each experiment, which is laborious and expensive or work with tempered assumptions about them.