To date, B. cinerea biocontrol products are mostly Bacillus subtilis-based, but their use in commercial strawberry production is limited because of their insufficient applicability in the field or supply chain . Nevertheless, there is social and scientific interest in using biocontrol against B. cinerea as an alternative to chemical pesticides. Isolates of Colletotrichum gloeosporioides, Epicoccum purpurascens, Gliocladum roseum, Penicillium sp., Trichoderma sp. have displayed high efficiency in controlling B. cinerea and were reported to reduce grey mould incidence on strawberry stamens by 79%–93% and on fruit by 48%–76% . Interestingly, in some experiments, the efficiency of biocontrol by these organisms exceeded the efficacy of control via the fungicide captan. Similar results were obtained for other microbes, such as the yeasts A. pullulans and Candida intermedia , the filamentous ascomycete Ulocladium atrum , or the bacterium Bacillus amyloliquefaciens . Biocontrol via microbes can work via different modes of action, including competition for nutrients, secretion of antibiotic compounds and induction of host defence mechanisms like the up-regulation of chitinase and peroxidase activity . Because biocontrol of B. cinerea relies on a variety of mechanisms, the most significant effects are observed when different organisms are applied in combination . As alternative to applying living microbes, use of extracts or volatiles derived from biocontrol microbes has been suggested . Use of non-synthetic antifungal substances, like phenol-rich olive oil mill wastewater,growing blackberries in containers has also been reported to control B. cinerea growth in vitro and on strawberries . However, these approaches are not implemented on a commercial scale due to high costs compared to the conventional B. cinerea control.
It is common practice to handpick strawberries and place them into clamshells in the field, in order to reduce wounding and bruising of the fruit. Rapid and constant cooling of strawberries at temperatures below 2.5 ºC is another critical strategy to reduce or inhibit reactivation of B. cinerea quiescent infections . Often, strawberries are also stored in modified atmospheres, which are generally low in oxygen and high in carbon dioxide to slow down metabolic processes, senescence and fungal decay . Relative humidity during storage is usually kept around 85%–90% to prevent dehydration of fruit, but limit fungal growth . Novel post harvest treatments of strawberries have been suggested to prevent B. cinerea infections during storage. Examples are edible fruit coatings of chitosan, silk fibroin or methylcellulose that prevent water loss and can include antifungal compounds . MeJA treatment to induce fruit defence mechanisms , ultraviolet and visual light treatment , enrichment of storage atmosphere with chlorine or ozone , and soft mechanical stimulation have also been tested as alternative treatments. Most of these approaches are still in an experimental stage or not yet adaptable to commercial settings.Significant phenotypic variation of incidence or severity of grey mould has been reported; however, F. x ananassa genotypes appear to be universally susceptible and complete resistance has not been observed . Substantial genotypic variation has not been documented and the heritability of resistance to B. cinerea is unknown. Mild phenotypic differences in fruit resistance levels reported in various post harvest studies indicate that genetic variation for resistance may be limited and that its heritability is low. A contributing factor is the intrinsic characteristics of the pathogen, its broad host range, diverse ways of infection and necrotrophic lifestyle, which explain the absence of a gene-for-gene resistance of strawberry to B. cinerea .
Therefore, breeding for escape and tolerance, which includes physiological and biochemical traits, is a more practical option . While limited in scale and scope, earlier studies strongly suggest that the incidence and progression of B. cinerea infections differ between cultivars with soft fruit and those with firm fruit . Hence, previously reported differences amongst cultivars could be the result of the pleiotropic effects of selection for increased fruit firmness and shelf life and the associated developmental and ripening changes, as opposed to direct genetic gains in innate resistance to the pathogen. As discussed, fruit firmness is an important trait associated with resistance to B. cinerea . The strawberry germplasm displays natural variation for fruit firmness and developing cultivars with firmer fruit is an important aim in breeding programmes . Changes in flower morphology could also enhance tolerance to B. cinerea. In strawberry, most B. cinerea infections in fruit appear to originate from primary infections of flowers or secondary infections caused by direct contact with infected flower parts . It was reported that removal of stamen and petals result in lower grey mould incidence . Faster abscission of flower parts, especially petals, has the potential to aid the escape of strawberries from B. cinerea infections . Similarly, plants with pistillate flowers have a lower grey mould incidence in fruit . B. cinerea growth inhibition in stamens is reported to vary within the strawberry germplasm, potentially due to differences in their biochemical composition . Similarly, antifungal compounds in fruit can prevent or limit B. cinerea infections. Several reports indicate that anthocyanin accumulation contributes to tolerance of strawberries to B. cinerea . Anthocyanins do not just improve tolerance to grey mould but also provide health benefits . Breeding for higher anthocyanin content in strawberries is possible and facilitated by existing variation in the germplasm . Inducing anthocyanin accumulation in flowers could also help to limit flower infections. As breeding for higher B. cinerea tolerance will be tedious and likely will not result in complete resistance, complementary approaches should be considered.
Currently, no genetically modified strawberry cultivars are commercially grown; however, several reports show great potential to improve tolerance to grey mould via trans- or cis-genesis. For example, the expression of chitinases or PGIPs from other organisms in strawberries can prevent or slow down fungal infections . Another potential transgenic approach is to increase fruit firmness by altering the expression or activity of pectin degrading enzymes, such as PL or PG . The existing natural variation of PL expression levels and activity in the cultivated strawberry germplasm could be used for cisgenic approaches. Increasing phenolic levels in strawberries by genetic modifications can also be explored as the transcription factor MYB10 was identified as a regulator of anthocyanin levels in strawberries ; Medina-Puche et al., 2014). Transgenic plants with ectopic over expression of MYB10 show elevated anthocyanin levels throughout the entire plant ; however, the resistance of these plants against B. cinerea have not been tested. In summary, these novel breeding approaches should be supported by integrative management strategies including horticultural and agronomic practices, and potentially biocontrol, to achieve maximum control of the disease.Plant gene editing may be the greatest innovation in plant breeding since the Green Revolution. It has already been used to make discoveries in plant biology and has a profound potential to create new crops with desirable characteristics. There are already exciting developments,square pot which show that gene editing may be able to live up to expectations and can be used to produce novel plant phenotypes that would improve agricultural production. Most authorities estimate that food production will have to double in the next 50 years to keep pace with population growth. The focus on global food security, however, is usually on starch-rich cereals and ignores or underestimates the vital importance of horticultural crops. These perishable commodities are often nutrient-dense with bioactive phytochemicals, the consumption of which is needed for a healthy and thriving population. However, an uncomfortable fact is that in addition to losses that may result from disease, drought, extremes of temperature, and other environmental stresses experienced in the field, an additional 25–40%—an average of 33%—of all fruit and vegetables produced globally are never eaten after harvest. This estimate still does not illustrate the extreme losses that can occur in some developing countries, which may be as high as 75%. Current worldwide horticultural crop production is insufficient to meet human nutritional requirements, making post harvest loss and waste all the more unsustainable. Only recently has the need to reduce the loss of horticultural crops after harvest been given the attention it deserves. Although the causes of post harvest loss and waste are complicated, we suggest that technology-assisted breeding for new and improved fruit, vegetables, and ornamentals, compatible with supply chain constraints but delivered at peak quality to the consumer, could be an important part of the solution over the long-term. In this review, we examine the potential for gene editing to make a measurable and robust impact on post harvest waste and loss. Rather than a technical or critical assessment of methodologies or research areas, we focus on connecting the bio-physiology of post harvest produce, the needs of the produce industry, and the wealth of existing molecular research, to suggest a holistic yet straightforward approach to crop improvement. The main focus of the review is the discussion of genes that could influence the quality and shelf-life of produce. First, we examine the steps that are taken to extend shelf-life in the produce supply chain, and the impact of supply chain management on consumer-desired quality traits.
Then we briefly review the CRISPR–Cas9 method to emphasize the flexibility, ease, and power with which traits can be modified. Finally, we take a critical look at remaining barriers which must be overcome to make gene editing for post harvest traits technically and economically viable. This review serves both as an introduction to post harvest and gene editing and as a resource for researchers attempting to utilize the latter for the former.Post harvest waste and post harvest loss are sometimes used interchangeably, but this is incorrect. Post harvest loss is unintentional. It describes the incidental losses that result from events occurring from farm-to-table, such as physical damage, internal bruising, premature spoiling, and insect damage, among others. Produce loss is also described as quantitative because it is measurable. This does not imply that data is easily available, only that it can be assessed. Post harvest waste, in contrast, is intentional. It describes when produce is discarded because it does not meet buyer expectations, even though it is edible. Produce may be rejected by growers, distributors, processing companies, retailers, and consumers for failing to meet desired or established preferences. Produce waste is described as qualitative because it is difficult to measure and assess. Still, in the US, it is estimated that 7% of post harvest losses of fruit and vegetables occur on the farm, while more than twice that, i.e., 17% and 18% are wasted in consumer-facing businesses and in homes, respectively. Produce post harvest loss and waste threatens environmental sustainability, and is especially catastrophic when viewed in the light of the twin challenges of global climate change and increasing population growth. PLW means inefficient use of financial investments in horticulture and more critically, non-renewable natural resources. Technological measures to curb PLW, such as maintaining a cold-chain and use of plastic packaging, additionally have energy and carbon costs. Improving the shelf-life and quality attributes of post harvest crops by genetic modification or smart breeding could be among many solutions to lessen the severity of these problems.Produce must be kept alive from farm to table; however, the biological nature of horticultural produce is often in congruent with modern commercial supply chain operations. Produce and ornamentals are high in water content, and often metabolically active, which makes them highly perishable. This becomes a challenge given the number of food miles fruit, vegetables, and ornamentals can travel in the global supply chain . Modern post harvest supply chains may be separated spatially by thousands of miles, and temporally, by several months. Produce trucked and shipped from the field is often treated: cooled, washed, sorted, dipped, sprayed, or held at desirable temperatures and modified atmospheres to preserve “health”. The majority of produce from mid to large-scale operations may move through a byzantine system of processors, distributors, and trucking and shipping entities. Maintaining an unbroken cold-chain, adequate packing, and shipping are essential to preserving quality and shelf-life. . Produce, even after harvest, respires , transpires water, and, for the “climacteric fruits”, can emit high levels of ethylene, which can be accelerated at high temperatures. Optimizing storage and handling conditions requires managing these biological processes , which may differ for each produce-type or variety, and from how the preharvest environment influences biology at harvest and thereafter.