Ariel R. Vicente

Uniform ripening Encodes a Golden 2-like Transcription Factor Regulating Tomato Fruit Chloroplast Development

Pretty or Sweet The grocery-store tomato that looks beautiful but tastes like tart cardboard arises from selection processes favoring phenotypes that make commercial production more reliable. Significant in that selection process was a mutation that reduced the mottled color variations of unripe green tomatoes, leaving them a uniform, pale, green. Powell et al. (p. 1711) analyzed the molecular biology of the mutation. The uniform ripening mutation turns out to disable a transcription factor called Golden 2-like (GLK2). GLK2 expression increases the fruits photosynthetic capacity, resulting in higher sugar content. Controlling when tomatoes turn from green to red requires knocking out the gene that adds flavor. Modern tomato (Solanum lycopersicum) varieties are bred for uniform ripening (u) light green fruit phenotypes to facilitate harvests of evenly ripened fruit. U encodes a Golden 2-like (GLK) transcription factor, SlGLK2, which determines chlorophyll accumulation and distribution in developing fruit. In tomato, two GLKs—SlGLK1 and SlGLK2—are expressed in leaves, but only SlGLK2 is expressed in fruit. Expressing GLKs increased the chlorophyll content of fruit, whereas SlGLK2 suppression recapitulated the u mutant phenotype. GLK overexpression enhanced fruit photosynthesis gene expression and chloroplast development, leading to elevated carbohydrates and carotenoids in ripe fruit. SlGLK2 influences photosynthesis in developing fruit, contributing to mature fruit characteristics and suggesting that selection of u inadvertently compromised ripe fruit quality in exchange for desirable production traits.

The intersection between cell wall disassembly, ripening, and fruit susceptibility to Botrytis cinerea

Fruit ripening is characterized by processes that modify texture and flavor but also by a dramatic increase in susceptibility to necrotrophic pathogens, such as Botrytis cinerea. Disassembly of the major structural polysaccharides of the cell wall (CW) is a significant process associated with ripening and contributes to fruit softening. In tomato, polygalacturonase (PG) and expansin (Exp) are among the CW proteins that cooperatively participate in ripening-associated CW disassembly. To determine whether endogenous CW disassembly influences the ripening-regulated increase in necrotropic pathogen susceptibility, B. cinerea susceptibility was assessed in transgenic fruit with suppressed polygalacturonase (LePG) and expansin (LeExp1) expression. Suppression of either LePG or LeExp1 alone did not reduce susceptibility but simultaneous suppression of both dramatically reduced the susceptibility of ripening fruit to B. cinerea, as measured by fungal biomass accumulation and by macerating lesion development. These results demonstrate that altering endogenous plant CW disassembly during ripening influences the course of infection by B. cinerea, perhaps by changing the structure or the accessibility of CW substrates to pathogen CW-degrading enzymes. Recognition of the role of ripening-associated CW metabolism in postharvest pathogen susceptibility may be useful in the design and development of strategies to limit pathogen losses during fruit storage, handling, and distribution.

Strangers in the matrix: plant cell walls and pathogen susceptibility

Early in infection, pathogens encounter the outer wall of plant cells. Because pathogen hydrolases targeting the plant cell wall are well-known components of virulence, it has been assumed that wall disassembly by the plant itself also contributes to susceptibility, and now this has been established experimentally. Understanding how plant morphological and developmental remodeling and pathogen cell wall targeted virulence influence infections provides new perspectives about plant-pathogen interactions. The plant cell wall can be an effective physical barrier to pathogens, but also it is a matrix where many proteins involved in pathogen perception are delivered. By breaching the wall, a pathogen potentially reveals itself to the plant and activates responses, setting off events that might halt or limit its advance.

Quality of heat-treated strawberry fruit during refrigerated storage

Strawberries cv. Selva were heat-treated in an air oven (45 °C, 3 h) and then stored at 0 °C for 0, 7 or 14 days. Afterward, fruits were placed at 20 °C and monitored after 24, 48 or 96 h and the effect of heat treatment on the following parameters was recorded: weight loss, external color, anthocyanin content, firmness, titratable acidity, total and reducing sugars, fruit decay and count of colony forming units (CFUs) for bacteria and molds. Heat-treated fruits showed higher hue angle than controls, indicating the delay of red color development. The treatment diminished fruit lightness (L*), although the effect reverted during holding at 20 or at 0 °C. The application of the treatment caused an initial weight loss close to 2% but afterwards, heat-treated fruits showed lower weight loss rate at 20 °C. Heat-treated fruits had lower acidity than controls, but there was no difference in the content of total sugars between control and treated fruits. Heated fruits were slightly firmer at the end of the treatment, and they softened less than controls after 24 h at 20 °C. Heat-treated fruits remained firmer than controls after 7 days of cold storage, and the relative difference in softening persisted after 48 h at 20 °C. However, no difference in treated and control fruit firmness was observed after 14 days of storage at 0 °C and following 48 h at 20 °C. In the absence of storage, heat-treated fruits showed lower decay at 20 °C than controls. After 7 days at 0 °C followed by 72 h at 20 °C, the percentage of decayed fruits was lower in heat-treated than in control fruits. The treatment decreased the initial bacterial population, but did not modify the amount of mold initially present. After 7 days of cold storage, the CFU number for bacteria were lower in treated than in control fruits. This difference was still significant after 48 h at 20 °C. In the case of molds, heat-treated fruits that were stored for 7 or 14 days at 0 °C and then transferred to 20 °C for 48 h showed lower CFU value than controls.

Effect of short-term ozone treatments on tomato (Solanum lycopersicum L.) fruit quality and cell wall degradation.

We evaluated the effect of short-term gaseous ozone treatment (10 microL/L; 10 min) on tomato fruit quality and cell wall degradation. The treatments did not modify fruit color, sugar content, acidity, or antioxidant capacity but reduced fruit damage and weight loss and induced the accumulation of phenolic compounds. In addition, softening was delayed in ozone-treated fruit. Cell wall analysis showed that exposure to ozone decreased pectin but not hemicellulose solubilization. Polyuronide depolymerization was also reduced in ozone-treated fruit. While the treatments did not alter the activity of the pectin-degrading enzymes polygalacturonase (PG) and beta-Galactosidase (beta-Gal), a clear decrease in pectin methyl esterase (PME) was found. Results show that short-term ozone treatments might be useful to reduce fruit damage and excessive softening, two of the main factors limiting tomato postharvest life, without negatively affecting other quality attributes. The impact of the treatments on fruit softening might be associated with reduced disassembly (solubilization and depolymerization) of pectic polysaccharides.

Developmental and metabolic plasticity of white-skinned grape berries in response to Botrytis cinerea during noble rot

Noble rot causes major reprogramming of grape berry metabolism by activating stress responses and ripening processes, including pathways that are inactive or with limited flux in white-skinned berries. Noble rot results from exceptional infections of ripe grape (Vitis vinifera) berries by Botrytis cinerea. Unlike bunch rot, noble rot promotes favorable changes in grape berries and the accumulation of secondary metabolites that enhance wine grape composition. Noble rot-infected berries of cv Sémillon, a white-skinned variety, were collected over 3 years from a commercial vineyard at the same time that fruit were harvested for botrytized wine production. Using an integrated transcriptomics and metabolomics approach, we demonstrate that noble rot alters the metabolism of cv Sémillon berries by inducing biotic and abiotic stress responses as well as ripening processes. During noble rot, B. cinerea induced the expression of key regulators of ripening-associated pathways, some of which are distinctive to the normal ripening of red-skinned cultivars. Enhancement of phenylpropanoid metabolism, characterized by a restricted flux in white-skinned berries, was a common outcome of noble rot and red-skinned berry ripening. Transcript and metabolite analyses together with enzymatic assays determined that the biosynthesis of anthocyanins is a consistent hallmark of noble rot in cv Sémillon berries. The biosynthesis of terpenes and fatty acid aroma precursors also increased during noble rot. We finally characterized the impact of noble rot in botrytized wines. Altogether, the results of this work demonstrated that noble rot causes a major reprogramming of berry development and metabolism. This desirable interaction between a fruit and a fungus stimulates pathways otherwise inactive in white-skinned berries, leading to a greater accumulation of compounds involved in the unique flavor and aroma of botrytized wines.

Cell wall disassembly events in boysenberry (Rubus idaeus L. × Rubus ursinus Cham. & Schldl.) fruit development

Boysenberry fruit was harvested at five developmental stages, from green to purple, and changes in pectin and hemicellulose solubilisation and depolymerisation, polymer neutral sugar contents, and the activities of cell wall degrading enzymes were analysed. The high xylose to glucose ratio in the 4% KOH-soluble hemicellulose fraction suggests that xylans are abundant in the boysenberry cell wall. Although the cell wall changes associated with fruit development do not proceed in discrete stages and the cell wall disassembly is a consequence of highly regulated changes occurring in a continuum, the results suggest that the temporal changes in cell wall degradation in boysenberry account for at least three stages: an early stage (green to 75% red colour), associated with metabolism of cellulose and cross-linking glycans; an intermediate period (75 to 100% red colour), characterised by substantial pectin solubilisation without depolymerisation in which α-arabinofuranosidase increases markedly and 50% of the wall arabinose is lost; and a final stage (100% red colour to purple), characterised mainly by a reduction of pectic galactose content and a dramatic increase in pectin depolymerisation associated with higher polygalacturonase, pectin methylesterase, acetyl esterase and β-galactosidase activities. From a biotechnological perspective enzymes involved in pectin matrix disassembly seem to be the better candidates to affect boysenberry fruit late-softening by genetic intervention. A model for cell wall disassembly in boysenberry fruit is proposed.

Maintenance of fresh boysenberry fruit quality with UV-C light and heat treatments combined with low storage temperature

Summary Mature boysenberries (Rubus hybrid) were harvested, heat-treated (45°C for 1 or 3 h or 47°C for 1 h) or exposed to UV-C light (2.3, 4.6 or 9.2 kJ m–2), and stored at 20°C for 2 d. Fruit treated with 9.2 kJ m–2 or 45°C for 1 h showed less damaged drupelets per fruit and/or remained firmer than untreated fruit after 2 d. Those treatments were selected for further analyses. In another experiment, boysenberries were either UV-C (9.2 kJ m–2) or heat-treated (45°C for 1 h) and stored either at 20°C for 1 d or at 0°C for 4 d before transfer to 20°C for 1 d. Both UV-C and heat treatments reduced softening and/or fruit damage. Treated fruit had lower respiration rates and anthocyanin leakage than control fruit suggesting greater tissue integrity. Titratable acidity, pH, total sugar content and antioxidant activity in treated fruit showed fewer changes than in control fruit when stored at 20°C for 1 d. Results suggest that heat or UV-C treatment, alone or in combination with refrigerated storage, may be a useful non-chemical mean of maintaining boysenberry fruit quality and extending postharvest life.

Chapter 5 – Nutritional Quality of Fruits and Vegetables

Publisher Summary The nutritional value of fruits and vegetables depends on their composition, which shows a wide range of variation depending on the species, cultivar, and maturity stage. This chapter describes the general characteristics of the components of fruits and vegetables, related to their benefits as food sources. There are two types of acids, namely aliphatic (straight chain) and aromatic acids. The most abundant acids in fruits and vegetables are citric and malic (both aliphatic) acids. However, large amounts of tartaric acid occur in grapes. Malic acid is the major component in oranges and apples. The acid content of fruits and vegetables generally decreases during maturation. Aromatic organic acids occur in several fruits and vegetables, but in very low concentrations. Benzoic acid occurs in cranberries, quinic acid in bananas, and chlorogenic acid in potatoes. In general, vegetables are a richer source of minerals than fruits, but both vegetables and fruits are considered nutrient-dense foods in that they provide substantial amounts of micronutrients, such as minerals and vitamins, but relatively few calories. Minerals have both direct and indirect effects on human health. The direct effects of minerals focus on the consequences of their consumption on human nutrition, while the indirect effects refer to their incidence in fruit and vegetable quality and subsequent consumer acceptance. From a direct nutrition standpoint, potassium has the biggest presence in both fruits and vegetables, but nitrogen and calcium show major impacts on horticultural crop quality.

Use of UV-C Treatments to Maintain Quality and Extend the Shelf Life of Green Fresh-cut Bell Pepper (Capsicum annuum L.)

UNLABELLED The objective of this work was to select a Ultraviolet-C (UV-C) treatment for fresh-cut mature green bell pepper, and to evaluate the effect of its combination with refrigeration on quality maintenance. Bell pepper sticks were treated with 0, 3, 10, or 20 kJ/m² UV-C in the outer (O), inner (I), or both sides of the pericarp (I/O) and stored for 8 d at 10 °C. During the first 5 d of storage, all UV-C treatments reduced deterioration as compared to the control. The treatment with 20 kJ/m² I/O was the most effective to reduce deterioration, and was used for further evaluations. In a second group of experiments, mature green bell pepper sticks were treated with 20 kJ/m² I/O, stored at 5 °C for 7 or 12 d and assessed for physical and chemical analysis, and microbiological quality. UV-C-treated fruit showed lower exudates and shriveling than the control. UV exposure also reduced decay, tissue damage, and electrolyte leakage. After 12 d at 5 °C, UV-C irradiated peppers remained firmer and had higher resistance to deformation than the control. The UV-C treatments also reduced weight loss and pectin solubilization. UV-C exposure decreased the counts of mesophile bacteria and molds, and did not affect acidity or sugars. UV-C-treated fruit stored for 0 or 7 d at 5 °C did not show major differences in antioxidants from the control as measured against DPPH(•) or ABTS(•)⁺ radicals. Results suggest that UV-C exposure is useful to maintain quality of refrigerated fresh-cut green pepper. PRACTICAL APPLICATION Exposure to UV-C radiation before packing and refrigeration could be a useful nonchemical alternative to maintain quality and reduce postharvest losses in the fresh-cut industry.