Russeting in Apple Is Initiated After Exposure to Moisture Ends-I. Histological Evidence

Imaging plant cell walls using fluorescent stains: The beauty is in the details

Plants continuously face various environmental stressors throughout their lifetime. To be able to grow and adapt in different environments, they developed specialized tissues that allowed them to maintain a protected yet interconnected body. These tissues undergo specific primary and secondary cell wall modifications that are essential to ensure normal plant growth, adaptation and successful land colonization. The composition of cell walls can vary among different plant species, organs and tissues. The ability to remodel their cell walls is fundamental for plants to be able to cope with multiple biotic and abiotic stressors. A better understanding of the changes taking place in plant cell walls may help identify and develop new strategies as well as tools to enhance plants' survival under environmental stresses or prevent pathogen attack. Since the invention of microscopy, numerous imaging techniques have been developed to determine the composition and dynamics of plant cell walls during normal growth and in response to environmental stimuli. In this review, we discuss the main advances in imaging plant cell walls, with a particular focus on fluorescent stains for different cell wall components and their compatibility with tissue clearing techniques. Lay Description: Plants are continuously subjected to various environmental stresses during their lifespan. They evolved specialized tissues that thrive in different environments, enabling them to maintain a protected yet interconnected body. Such tissues undergo distinct primary and secondary cell wall alterations essential to normal plant growth, their adaptability and successful land colonization. Cell wall composition may differ among various plant species, organs and even tissues. To deal with various biotic and abiotic stresses, plants must have the capacity to remodel their cell walls. Gaining insight into changes that take place in plant cell walls will help identify and create novel tools and strategies to improve plants' ability to withstand environmental challenges. Multiple imaging techniques have been developed since the introduction of microscopy to analyse the composition and dynamics of plant cell walls during growth and in response to environmental changes. Advancements in plant tissue cleaning procedures and their compatibility with cell wall stains have significantly enhanced our ability to perform high-resolution cell wall imaging. At the same time, several factors influence the effectiveness of cleaning and staining plant specimens, as well as the time necessary for the process, including the specimen's size, thickness, tissue complexity and the presence of autofluorescence. In this review, we will discuss the major advances in imaging plant cell walls, with a particular emphasis on fluorescent stains for diverse cell wall components and their compatibility with tissue clearing techniques. We hope that this review will assist readers in selecting the most appropriate stain or combination of stains to highlight specific cell wall components of interest.

Factors influencing fruit cracking: an environmental and agronomic perspective

Fruit cracking, a widespread physiological disorder affecting various fruit crops and vegetables, has profound implications for fruit quality and marketability. This mini review delves into the multifaceted factors contributing to fruit cracking and emphasizes the pivotal roles of environmental and agronomic factors in its occurrence. Environmental variables such as temperature, relative humidity, and light exposure are explored as determinants factors influencing fruit cracking susceptibility. Furthermore, the significance of mineral nutrition and plant growth regulators in mitigating fruit cracking risk is elucidated, being calcium deficiency identified as a prominent variable in various fruit species. In recent years, precision farming and monitoring systems have emerged as valuable tools for managing environmental factors and optimizing fruit production. By meticulously tracking parameters such as temperature, humidity, soil moisture, and fruit skin temperature, growers can make informed decisions to prevent or alleviate fruit cracking. In conclusion, effective prevention of fruit cracking necessitates a comprehensive approach that encompasses both environmental and agronomic factors.

Time course of changes in the transcriptome during russet induction in apple fruit

BACKGROUND

Russeting is a major problem in many fruit crops. Russeting is caused by environmental factors such as wounding or moisture exposure of the fruit surface. Despite extensive research, the molecular sequence that triggers russet initiation remains unclear. Here, we present high-resolution transcriptomic data by controlled russet induction at very early stages of fruit development. During Phase I, a patch of the fruit surface is exposed to surface moisture. For Phase II, moisture exposure is terminated, and the formerly exposed surface remains dry. We targeted differentially expressed transcripts as soon as 24 h after russet induction.

RESULTS

During moisture exposure (Phase I) of 'Pinova' apple, transcripts associated with the cell cycle, cell wall, and cuticle synthesis (SHN3) decrease, while those related to abiotic stress increase. NAC35 and MYB17 were the earliest induced genes during Phase I. They are therefore linked to the initial processes of cuticle microcracking. After moisture removal (Phase II), the expression of genes related to meristematic activity increased (WOX4 within 24 h, MYB84 within 48 h). Genes related to lignin synthesis (MYB52) and suberin synthesis (MYB93, WRKY56) were upregulated within 3 d after moisture removal. WOX4 and AP2B3 are the earliest differentially expressed genes induced in Phase II. They are therefore linked to early events in periderm formation. The expression profiles were consistent between two different seasons and mirrored differences in russet susceptibility in a comparison of cultivars. Furthermore, expression profiles during Phase II of moisture induction were largely identical to those following wounding.

CONCLUSIONS

The combination of a unique controlled russet induction technique with high-resolution transcriptomic data allowed for the very first time to analyse the formation of cuticular microcracks and periderm in apple fruit immediately after the onset of triggering factors. This data provides valuable insights into the spatial-temporal dynamics of russeting, including the synthesis of cuticles, dedifferentiation of cells, and impregnation of cell walls with suberin and lignin.

Apple fruit periderms (russeting) induced by wounding or by moisture have the same histologies, chemistries and gene expressions

Russeting is a cosmetic defect of some fruit skins. Russeting (botanically: induction of periderm formation) can result from various environmental factors including wounding and surface moisture. The objective was to compare periderms resulting from wounding with those from exposure to moisture in developing apple fruit. Wounding or moisture exposure both resulted in cuticular microcracking. Cross-sections revealed suberized hypodermal cell walls by 4 d, and the start of periderm formation by 8 d after wounding or moisture treatment. The expression of selected target genes was similar in wound and moisture induced periderms. Transcription factors involved in the regulation of suberin (MYB93) and lignin (MYB42) synthesis, genes involved in the synthesis (CYP86B1) and the transport (ABCG20) of suberin monomers and two uncharacterized transcription factors (NAC038 and NAC058) were all upregulated in induced periderm samples. Genes involved in cutin (GPAT6, SHN3) and wax synthesis (KCS10, WSD1, CER6) and transport of cutin monomers and wax components (ABCG11) were all downregulated. Levels of typical suberin monomers (ω-hydroxy-C₂₀, -C₂₂ and -C₂₄ acids) and total suberin were high in the periderms, but low in the cuticle. Periderms were induced only when wounding occurred during early fruit development (32 and 66 days after full bloom (DAFB)) but not later (93 DAFB). Wound and moisture induced periderms are very similar morphologically, histologically, compositionally and molecularly.

Solving the regulation puzzle of periderm development using advances in fruit skin

Periderm protects enlarged organs of most dicots and gymnosperms as a barrier to water loss and disease invasion during their secondary growth. Its development undergoes a complex process with genetically controlled and environmental stress-induced characters. Different development of periderm makes the full and partial russet of fruit skin, which diverges in inheritance with qualitative and quantitative characters, respectively, in pear pome. In addition to its specific genetics, fruit periderm has similar development and structure as that of stem and other organs, making it an appropriate material for periderm research. Recently, progress in histochemical as well as transcriptome and proteome analyses, and quantitative trait locus (QTL) mapping have revealed the regulatory molecular mechanism in the periderm based on the identification of switch genes. In this review, we concentrate on the periderm development, propose the conservation of periderm regulation between fruit and other plant organs based on their morphological and molecular characteristics, and summarize a regulatory network with the elicitors and repressors for the tissue development. Spontaneous programmed-cell death (PCD) or environmental stress produces the original signal that triggers the development of periderm. Spatio-temporal specific PCD produced by PyPPCD1 gene and its homologs can play a key role in the coordinated regulation of cell death related tissue development.

Russeting of Fruits: Etiology and Management

The skin of a fruit protects the vulnerable, nutrient-rich flesh and seed(s) within from the hostile environment. It is also responsible for the fruit’s appearance. In many fruitcrop species, russeting compromises fruit appearance and thus commercial value. Here, we review the literature on fruit russeting, focusing on the factors and mechanisms that induce it and on the management and breeding strategies that may reduce it. Compared with a primary fruit skin, which is usually distinctively colored and shiny, a secondary fruit skin is reddish-brown, dull and slightly rough to the touch (i.e., russeted). This secondary skin (periderm) comprises phellem cells with suberized cell walls, a phellogen and a phelloderm. Russeted (secondary) fruit skins have similar mechanical properties to non-russeted (primary) ones but are more plastic. However, russeted fruit skins are more permeable to water vapor, so russeted fruits suffer higher postharvest water loss, reduced shine, increased shrivel and reduced packed weight (most fruit is sold per kg). Orchard factors that induce russeting include expansion-growth-induced strain, surface wetness, mechanical damage, freezing temperatures, some pests and diseases and some agrochemicals. All these probably act via an increased incidence of cuticular microcracking as a result of local concentrations of mechanical stress. Microcracking impairs the cuticle’s barrier properties. Potential triggers of russeting (the development of a periderm), consequent on cuticular microcracking, include locally high concentrations of O2, lower concentrations of CO2 and more negative water potentials. Horticulturists sometimes spray gibberellins, cytokinins or boron to reduce russeting. Bagging fruit (to exclude surface moisture) is also reportedly effective. From a breeding perspective, genotypes having small and more uniform-sized epidermal cells are judged less likely to be susceptible to russeting.

Refinement of Nonantibiotic Spray Programs for Fire Blight Control in Organic Pome Fruit

Fire blight-susceptible, certified organic pome fruit is produced on 9,000 ha in the Pacific Northwest region of the United States with acreage continuing to expand despite a 2014 prohibition on antibiotics as allowable materials for infection suppression. Nonantibiotic practices for fire blight pathogen suppression mirror conventional management, but the full-bloom-to-petal-fall period when antibiotics are typically sprayed for fire blight control continues to receive research scrutiny owing to drawbacks and weaknesses of alternative materials. As solitary treatments, effective nonantibiotic materials (e.g., a yeast biocontrol, soluble coppers, and potassium aluminum sulfate) raise the risk of a crop-value-reducing, phytotoxic response termed "fruit russeting." Conversely, materials with less russeting risk (e.g., Bacillus-based biorationals) are less effective for fire blight control. Spray programs using a sequence of materials applied from midbloom to petal fall have the potential to provide high levels of protection with reduced russeting risk. In orchard trials, the effects of nonantibiotic spray programs on the epiphytic population size of Erwinia amylovora in flowers, yeast biocontrol population size, floral pH, infection suppression, and fruit russeting revealed strategies for sequencing sprays of nonantibiotic materials. The yeast biocontrol, Blossom Protect (Aureobasidium pullulans), sprayed at 70% bloom, was an important contributor to fire blight pathogen suppression as was the soluble copper material, Previsto, when applied at full bloom. Choice of material for the petal-fall spray timing was important to fruit russeting risk but apparently less important to overall infection incidence. Consequently, treatment programs of Blossom Protect at 70% bloom, a soluble copper at full bloom, and a Bacillus-based biorational at petal fall, best balance the quality of infection suppression with the risk of fruit russeting.

Direct Evidence for a Radial Gradient in Age of the Apple Fruit Cuticle

The pattern of cuticle deposition plays an important role in managing strain buildup in fruit cuticles. Cuticular strain is the primary trigger for numerous fruit-surface disorders in many fruit crop species. Recent evidence indicates a strain gradient may exist within the apple fruit cuticle. The outer layers of the cuticle are more strained and thus more susceptible to microcracking than the inner layers. A radial gradient in cuticle age is the most likely explanation. Our study aimed to establish whether (or not) deposition of new cutin in a developing apple fruit occurs on the inner surface of the cuticle, i.e., immediately abutting the outward-facing epidermal cell wall. Developing apples were fed with ¹³C oleic acid through the skin. Following a 14-d period for incorporation, the fruit was harvested and the cuticular membranes (CMs) isolated enzymatically. The CMs were then ablated to varying extents from the inner or the outer surfaces, using a cold atmospheric pressure plasma (CAPP). Afterwards, the ablated CMs were dewaxed and the ¹³C contents were determined by mass spectrometry. The incorporation of ¹³C in the cutin fraction was higher than in the wax fraction. The ¹³C content was highest in non-ablated, dewaxed CM (DCM) and decreased as ablation depth from the inner surface increased. There was no change in ¹³C content when ablation was carried out from the outer surface. As fruit development proceeded, more ¹³C label was found towards the middle of the DCM. These results offered direct evidence for deposition of cutin being on the inner surface of the cuticle, resulting in a radial gradient in cuticular age-the most recent deposition (youngest) being on the inner cuticle surface (abutting the epidermal cell wall) and the earliest deposition (oldest) being on the outer surface (abutting the atmosphere).

Low cuticle deposition rate in 'Apple' mango increases elastic strain, weakens the cuticle and increases russet

Russeting compromises appearance and downgrades the market value of many fruitcrops, including of the mango cv. ‘Apple’. The objective was to identify the mechanistic basis of ‘Apple’ mango’s high susceptibility to russeting. We focused on fruit growth, cuticle deposition, stress/strain relaxation analysis and the mechanical properties of the cuticle. The non-susceptible mango cv. ‘Tommy Atkins’ served for comparison. Compared with ‘Tommy Atkins’, fruit of ‘Apple’ had a lower mass, a smaller surface area and a lower growth rate. There were little differences between the epidermal and hypodermal cells of ‘Apple’ and ‘Tommy Atkins’ including cell size, cell orientation and cell number. Lenticel density decreased during development, being lower in ‘Apple’ than in ‘Tommy Atkins’. The mean lenticel area increased during development but was consistently greater in ‘Apple’ than in ‘Tommy Atkins’. The deposition rate of the cuticular membrane was initially rapid but later slowed till it matched the area expansion rate, thereafter mass per unit area was effectively constant. The cuticle of ‘Apple’ is thinner than that of ‘Tommy Atkins’. Cumulative strain increased sigmoidally with fruit growth. Strains released stepwise on excision and isolation (ε_exc+iso), and on wax extraction (ε_extr) were higher in ‘Apple’ than in ‘Tommy Atkins’. Membrane stiffness increased during development being consistently lower in ‘Apple’ than in ‘Tommy Atkins’. Membrane fracture force (F_max) was low and constant in developing ‘Apple’ but increased in ‘Tommy Atkin’. Membrane strain at fracture (ε_max) decreased linearly during development but was lower in ‘Apple’ than in ‘Tommy Atkins’. Frequency of membrane failure associated with lenticels increased during development and was consistently higher in ‘Apple’ than in ‘Tommy Atkins’. The lower rate of cuticular deposition, the higher strain releases on excision, isolation and wax extraction and the weaker cuticle account for the high russet susceptibility of ‘Apple’ mango.

Variations in the Community Structure of Fungal Microbiota Associated with Apple Fruit Shaped by Fruit Bagging-Based Practice

The various fungal communities that adhere to apple fruit are influenced by agricultural practices. However, the effects of fruit bagging-based management practice on the fungal microbiota are still unknown, and little is known about the fungal communities of bagged apple fruit. We conducted a study using apple fruit grown in a conventionally managed orchard where pesticide use is an indispensable practice. Fungal communities were collected from the calyx-end and peel tissues of bagged and unbagged fruit and characterized using barcode-type next-generation sequencing. Fruit bagging had a stronger effect on fungal richness, abundance, and diversity of the fungal microbiota in comparison to non-bagging. In addition, bagging also impacted the compositional variation of the fungal communities inhabiting each fruit part. We observed that fruit bagging had a tendency to maintain ecological equilibrium since Ascomycota and Basidiomycota were more distributed in bagged fruit than in unbagged fruit. These fungal communities consist of beneficial fungi rather than potentially harmful fungi. Approximately 50 dominant taxa were detected in bagged fruit, for example, beneficial genera such as Articulospora, Bullera, Cryptococcus, Dioszegia, Erythrobasidium, and Sporobolomyces, as well as pathogenic genera such as Aureobasidium and Taphrina. These results suggested that fruit bagging could significantly increase fungal richness and promote healthy fungal communities, especially the harmless fungal communities, which might be helpful for protecting fruit from the effects of pathogens. This study provides a foundation for understanding the impacts of bagging-based practice on the associated fungal microbiota.

Peridermal fruit skin formation in Actinidia sp. (kiwifruit) is associated with genetic loci controlling russeting and cuticle formation

BACKGROUND

The skin (exocarp) of fleshy fruit is hugely diverse across species. Most fruit types have a live epidermal skin covered by a layer of cuticle made up of cutin while a few create an outermost layer of dead cells (peridermal layer).

RESULTS

In this study we undertook crosses between epidermal and peridermal skinned kiwifruit, and showed that epidermal skin is a semi-dominant trait. Furthermore, backcrossing these epidermal skinned hybrids to a peridermal skinned fruit created a diverse range of phenotypes ranging from epidermal skinned fruit, through fruit with varying degrees of patches of periderm (russeting), to fruit with a complete periderm. Quantitative trait locus (QTL) analysis of this population suggested that periderm formation was associated with four loci. These QTLs were aligned either to ones associated with russet formation on chromosome 19 and 24, or cuticle integrity and coverage located on chromosomes 3, 11 and 24.

CONCLUSION

From the segregation of skin type and QTL analysis, it appears that skin development in kiwifruit is controlled by two competing factors, cuticle strength and propensity to russet. A strong cuticle will inhibit russeting while a strong propensity to russet can create a continuous dead skinned periderm.

Russeting in Apple is Initiated after Exposure to Moisture Ends: Molecular and Biochemical Evidence

Exposure of the fruit surface to moisture during early development is causal in russeting of apple (Malus × domestica Borkh.). Moisture exposure results in formation of microcracks and decreased cuticle thickness. Periderm differentiation begins in the hypodermis, but only after discontinuation of moisture exposure. Expressions of selected genes involved in cutin, wax and suberin synthesis were quantified, as were the wax, cutin and suberin compositions. Experiments were conducted in two phases. In Phase I (31 days after full bloom) the fruit surface was exposed to moisture for 6 or 12 d. Phase II was after moisture exposure had been discontinued. Unexposed areas on the same fruit served as unexposed controls. During Phase I, cutin and wax synthesis genes were down-regulated only in the moisture-exposed patches. During Phase II, suberin synthesis genes were up-regulated only in the moisture-exposed patches. The expressions of cutin and wax genes in the moisture-exposed patches increased slightly during Phase II, but the levels of expression were much lower than in the control patches. Amounts and compositions of cutin, wax and suberin were consistent with the gene expressions. Thus, moisture-induced russet is a two-step process: moisture exposure reduces cutin and wax synthesis, moisture removal triggers suberin synthesis.