Fruit are rich in essential nutrients, yet most people do not consume the recommended amount of fresh produce to sustain healthy diets and reduce disease risks ERS — Food Availability and Consumption. To promote consumption, expand access, and reduce waste, fruit quality and shelf life need to be in- creased through breeding and biotechnology, alongside adequate harvest practices, transportation logistics, and post harvest treatments. Commercial fruit crops are primarily bred for high yield and extended shelf life to meet the expectations for mass production and global markets, however, recently, there has been a shift of focus toward developing new crop varieties that meet consumer demands for better flavor and nutrition. Balancing shelf life with consumer-based quality traits is perhaps the biggest challenge breeders and researchers face in the quest for better quality fruit, mainly because these attributes appear to have negative genetic correlations in many crops . Quality peaks when fruit reach their optimum ripeness. Many studies on fruit ripening of various plant species have emerged in the past decade, helping to identify genetic pathways and molecular regulators that can be manipulated for crop improvement . Moreover, biotechnology advances have provided access to high- quality genomic resources and tools, supporting breeding strategies, genetic modification, and gene editing in traditional and nontraditional fruit crops. Here, we review current knowledge of the genetics of fruit traits and argue that manipulating transcription factors is a promising approach to enhance fruit quality. We discuss how pleiotropic effects could potentially be avoided by targeting TFs that exclusively regulate specific pathways instead of master regulators. However, ripening master regulators may remain useful if their effects on gene expression can be fine-tuned . Similarly, plastic square flower bucket the timing and coordination of regulators need to be considered to achieve desired effects on fruit traits .
Finally, we consider the current climate surrounding consumer acceptance of genetically modified and gene-edited fruit.Fruit are used as food source for macronutrients and micronutrients . Macronutrients are essential to provide energy and maintain the body’s structure and functions. Vitamins are required for various bodily functions and can only be found in food sources such as fruit. Antioxidants inhibit cell damage caused by oxidative agents. Carotenoids , and phenolics , reviewed above, are all antioxidants, as well as vitamin C . Vitamin-D deficiency is a global health problem due to few dietary sources of this vitamin. Bio-fortification of vitamin D in tomatoes has recently become possible by engineering its biosynthesis from a pre-existing pathway . Owing to partial duplication of the pathway, a single enzyme could be knocked out with CRISPR–Cas9 to convert the precursor into vitamin D without an expense to other metabolites. This discovery has further implications for other Solanaceae plants. Ascorbic acid, an important antioxidant and nutrient for immune health and wound healing, has proven to be less easily bio-fortified into fruit because increasing biosynthesis also leads to activation of catabolic and recycling pathways . Post transcriptional regulation of an ascorbic acid biosynthesis enzyme from kiwifruit has been demonstrated using tobacco leaves . Removing the upstream open reading frame that repressed translation increased ascorbic acid concentration in the leaves. Function-specific TFs can increase ascorbic acid in fruit without negative impact on quality as demonstrated in tomato. SlHZ24, a bHLH TF, regulates ascorbic acid biosynthesis and catabolism genes, and its transient over expression has been reported to increase the accumulation of this vitamin . Other TFs, SlNL33 and SlNFYA10, have been found to regulate the pathway negatively, and silencing them also increased ascorbic acid level . Understanding regulatory pathways governing nutrients can also facilitate traditional breeding programs. For example, a genome-wide association study led to the discovery and validation that alleles in SlbHLH59 determine ascorbic acid content in tomato cultivars . A less specific effect on nutrient accumulation can be achieved with hormone applications. For example, ethylene application in kiwifruit, a climacteric fruit, increased ascorbic acid and other antioxidants during ripening . Ascorbic acid also increased after nitric oxide application in sweet pepper, a non-climacteric fruit .Fruit flavor is a critical quality trait for consumer acceptance. Flavor includes all sensations experienced when eating, consisting of taste, aroma, and texture . In fruit, taste is mainly defined by a balance between sweetness and acidity but can include bitterness and umami.
Fruit aroma comes from specific classes and combinations of volatile organic compounds . For example, the unique kiwifruit flavor is associated with esters, mainly ethyl butanoate and methyl butanoate . Modulation of function-specific transcriptional and post-transcriptional regulators offers an effective solution for flavor improvement. A bHLH TF in banana activates 11 starch-degrading genes expressed during fruit ripening and is a likely candidate for increasing sugar and sweetness . In strawberry, editing the uORF of a bZIP TF that controls sucrose biosynthesis led to its translational activation and higher sugar content in fruit . Combining datasets generated through genomic, transcriptomic, metabolomic, and consumer panel studies has proven to be an effective strategy for identifying flavor-related genes and TFs. In tomato, metabolite data, associated loci, and consumer panels were analyzed across hundreds of varieties to determine key genes contributing to flavor . Coupling metabolites relevant to flavor with transcriptomic analyses can produce gene networks and identify regulatory TFs involved in flavor pathways. This approach was used in kiwifruit, where the AcNAC4 TF regulating a key gene in ester biosynthesis was validated . Homologs of these NAC TFs have been implicated in ester formation in peach and apple , suggesting that their functions are conserved across diverse families of climacteric fruit . Flavor is impacted by post harvest handling. Chilling during transportation and storage can alter fruit flavor. Epigenetic factors have been shown to regulate the suppression of VOC biosynthesis in chilled tomatoes . Most climacteric fruit are picked unripe and later treated with ethylene to induce ripening. However, this practice has been associated with poor flavor development. A study in off-vine ripened tomatoes confirmed that the fruit presented reduced VOC emission and a low sugar/acid ratio, due to alterations in gene expression and decreased methylation of their promoters . Genomic resources can help anticipate consumer preference and assist in breeding to select fruit with enhanced flavor. A population genomic study revealed that distinct consumer preferences between eastern and western countries drove selection for peach cultivars with different acidity . Fruit VOC profile data can help predict consumer liking before performing sensory panels, allowing for a more efficient selection of high-flavor fruit, as seen in tomato and blueberry . Moreover, the tomato pan-genome helped identify a rare favorable allele selected against during domestication that could be incorporated back into new cultivars to improve fruit flavor .Fruit texture is associated with freshness, flavor, and shelf-life potential. Texture involves many attributes, such as firmness, juiciness, crispiness, and meltiness . Fruit softening is mainly attributed to the remodeling and degradation of the polysaccharides in the primary cell walls .
The cuticle layer, deposited on the CWs of epidermal tissue, also contributes to fruit firmness by preventing water loss and maintaining cell turgor pressure . Traditional breeding has focused on creating firmer fruit that withstand transportation and have longer shelf life. This has been accomplished in tomato by developing hybrid lines between elite varieties and non-ripening mutants such as ripening-inhibitor and non-ripening . These mutants have defects in TFs considered master regulators of many ripening processes,plastic plant pot including the induction of cell wall-degrading enzymes and changes in cuticle composition . Other efforts to improve firmness and shelf life in climacteric fruit have taken advantage of mutations affecting ethylene biosynthesis and perception, as this hormone is also known to regulate genes encoding CWDEs, among others . However, as already discussed, modulating master regulators or hormone pathways has numerous drawbacks to other quality traits, such as color and flavor. Thus, downstream TFs controlling specific CW enzymes, such as LOB TF in tomato, may prove to be better targets . Another possibility is to leverage the availability of natural or induced allelic variants in ripening master regulators to produce a range of fruit phenotypes. For example, spontaneous or equivalent delayed fruit deterioration and CRISPR–Cas9-generated mutations in the ripening regulator NOR can extend shelf life with minimal impact on other fruit attributes , compared with the canonical mutant nor . RNAi and gene editing approaches have been used to target ripening-specific CWDEs that influence fruit firmness, such as polygalacturonase , pectate lyase , and pectin methyl esterase . The first GM fruit product, the FLAVR SAVR™ tomato, was engineered with antisense RNA against SlPG, however, it did not show a phenotype for fruit firmness . In contrast, the CRISPR–Cas9 SlPL knockout in tomato significantly improved fruit firmness and shelf life . Similar observations were previously reported in strawberry using RNAi knockdowns of FvPL and FvPME. In addition to firmer fruit, tomato and strawberry SlPL mutants have reduced fruit susceptibility to fungal disease . Breeding strategies and molecular studies have also focused on ameliorating textural defects such as fruit mealiness caused by cold storage. Mealiness, considered the opposite of juiciness, occurs when neighboring cells lose adhesion and detach while remaining intact . In peach, quantitative trait loci associated with cold-tolerant varieties have been identified to support breeding for less mealy fruit . Beyond QTLs, understanding the genetic mechanisms behind the trait provides avenues for targeting breeding and genetic modifications. For instance, peach mealiness appears to be associated with increased DNA methylation, leading to the downregulation and hypermethylation of mealy-associated genes such as PpCYP82A3 . Finding molecular approaches to avoid the deposition of methyl groups in the promoters of key ripening genes in response to cold storage can serve as a potential solution to mealiness.The FLAVR SAVR™ tomato hit the market in the early 90s promising a product with longer shelf life. However, this GM fruit had high production costs and was not well accepted by consumers, which led to its removal from the marketplace . Since then, other bioengineered crops with improved plant disease resistance or production-related traits have become available worldwide . These products have not sparked much enthusiasm mainly because they were not generated considering consumer-based traits or due to public fear of GM organisms. Recently, two fruits bio-engineered for quality attributes were approved by both US and Canadian regulatory entities and are available to consumers. These are the Pinkglow™ pineapple and the Arctic™ apple. The latter was bioengineering to reduce oxidative browning in the cut fruit . These fruit are considered novelty items because they are less available in the marketplace and significantly more expensive than traditional cultivars. Both fruits are primarily sold online and offered in limited supplies. The Pinkglow™ pineapple costs nearly ten times more than a common yellow pineapple. Despite their limitations, these fruits were developed with the consumer in mind, which may entice the public more than previous GM products.In 2022, the USDA deregulated the purple tomato developed with the expression of two snapdragon TFs. Novel fruit colors and potential higher nutrition may draw consumers to a new emerging category of bio-engineered produce. These fruit will also need to meet high consumer expectations of flavor, affordability, and food safety to ensure their success. Gene editing techniques such as CRISPR–Cas9 enable the fine-tuning of quality traits in a variety of fruit crops and may be more well received than previous GM products. For example, Japan started selling the first Cas9-edited fruit in the world in 2021, a health-promoting γ-aminobutyric acid -enriched tomato . Gene-edited fruit without foreign DNA have a more straightforward regulatory path in the United States than GM products, increasing the speed to market and reducing costs associated with authorization. However, this is not the case in other parts of the world, such as the European Union. In conclusion, researchers are armed with knowledge on fruit ripening and tools to improve fruit quality and generate greater access to fresh, flavorful, and nutritious food. Demands for better-tasting, more sustainable fruit, are in reach . We sit on the edge of an era where gene-edited and bio-engineered commodities can become a new category in the market, if the consumer will allow it.Fleshy fruit gain most of their quality traits, such as color, texture, flavor, and nutritional value, as a result of physiological and biochemical changes associated with ripening.