Idit Ginzberg

Potato tuber skin is a protective corky tissue consisting of suberized phellem cells. Smooth-skinned varieties are characterized by a clean, shiny appearance compared to the darker hue of russeted potatoes. The rough skin of russeted cultivars is a desired, genetically inherited characteristic; however, unwanted russeting of smooth-skinned cultivars often occurs under suboptimal growth conditions. The involvement of epigenetic modifiers in regulating the smooth skin russeting disorder was tested. We used smooth-skin commercial cultivars with and without the russeting disorder and three lines from a breeding population segregating for russeting. Anatomically, the russet skin showed similar characteristics, whether the cause was environmentally triggered or genetically determined. The old outer layers of the corky phellem remain attached to the newly formed phellem layers instead of being sloughed off. Global DNA methylation analysis indicated a significant reduction in the percentage of 5-methylcytosine in mature vs. immature skin and russet vs. smooth skin. This was true for both the smooth-skin commercial cultivars and the russeted lines. The expression level of selected DNA methyltransferases was reduced in accordance. DNA demethylase expression did not change between the skin types and age. Hence, the reduced DNA methylation in mature and russet skin is more likely to be achieved through passive DNA demethylation and loss of methyltransferase activity.

The periderm is a corky tissue that replaces the epidermis when the latter is damaged, and is critical for preventing pathogen invasion and water loss. The periderm is formed through the meristematic activity of phellogen cells (cork cambium). The potato skin (phellem cells) composes the outer layers of the tuber periderm and is a model for studying cork development. Early in tuber development and following tuber expansion, the phellogen becomes active and produces the skin. New skin layers are continuously added by division of the phellogen cells until tuber maturation. Some physiological disorders of the potato tuber are related to abnormal development of the skin, including skinning injuries and russeting of smooth-skinned potatoes. Thus, characterizing the potato periderm contributes to modeling cork development in plants and helps to resolve critical agricultural problems. Here, we summarize the data available on potato periderm formation, highlighting tissue characteristics rather than the suberization processes.

The appearance of pomegranate (Punica granatum L.) fruit is highly important for its marketing. The primary concerns are obtaining sufficient red pigment accumulation and minimal cracking of the fruit skin (the outer red layer of the peel). We analyzed the skin transcriptome of pomegranate cv. Wonderful at distinct time points of fruit development to characterize the processes that occur in the skin during fruit ripening and which may reflect on processes in the whole fruit, such as the non-climacteric nature of pomegranate. The data suggested a ripening mechanism in pomegranate skin that differs from that in strawberry—the model plant for non-climacteric fruit where abscisic acid is the growth regulator that drives ripening—involving ethylene, polyamine, and jasmonic acid pathways. The biosynthetic pathways of important metabolites in pomegranate—hydrolyzable tannins and anthocyanins—were co-upregulated at the ripening stage, in line with the visual enhancement of red coloration. Interestingly, cuticle- and cell-wall-related genes that showed differential expression between the developmental stages were mainly upregulated in the skin of early fruit, with lower expression at mid-growth and ripening stages. Nevertheless, lignification may be involved in skin hardening in the mature fruit.

Pomegranate (Punica granatumL.) fruit is well known for its health-beneficial metabolites. The pomegranate peel consists of an inner thick spongy white tissue, and an outer smooth skin layer that accumulates anthocyanins in red cultivars when ripe. The skin is made up of epidermis cells covered by a cuticle, the latter being the first target of cracking and russeting. The present study focuses on the effect of Israel’s hot and dry climate on pomegranate growth, to elucidate the derived effects on fruit skin characteristics and its putative resistance to the building pressure from fruit expansion. Experiments were conducted for four years, in four orchards located in different regions of the country, each with a different typical microclimate. Fruit-growth parameters were followed using remote-sensing tools, microscopic study, and mineral analysis of the skin, followed by determination of the peel’s elastic modulus. Fruit expanded in two phases: a short rapid phase followed by a gradual phase with a sigmoidal growth-rate pattern. Extreme hot and dry climate during the period of maximal growth rate was associated with restricted growth and a high proportion of small-size fruit. Anatomical study indicated that the skin of mature pomegranate fruit is made up of epidermal cells that are relatively flat and spaced apart, and is expected to be less durable against internal pressure. In contrast, skin of early immature fruit has two layers of dense and rounded epidermis, and is expected to be more resistant to cracking. Tensile strength studies confirmed this trend—skin of mature fruit had a lower elastic modulus than young fruit. However, restrained growth due to extreme environmental cues may result in better resistance of the mature pomegranate fruit to cracking, and in better skin quality and appearance, albeit small fruits. On the other hand, temperate climate at the beginning of the growth period, which allows high growth rate and high daily shrinkage, leads to pomegranate skin disorders.

Pomegranate cv. ‘Wonderful’ fruit are susceptible to chilling injuries of the peel (CIp) when stored at 7 °C in modified-atmosphere bags for more than 3 months. The damage, manifested as superficial browning, is restricted to the fruit skin, i.e., the outer colored layer of the peel. To characterize possible causes of CIp development, fruit were collected at early harvest—when the premature fruit are poorly colored and susceptible to CIp development, and at late harvest—when mature fruit have fully red skin and less susceptibility to CIp. Skin samples were collected on day of harvest and at different time points during storage. Anatomical study of skin with CIp disorder showed a broken cuticle layer with underlying degenerated cells. A high total phenol content, which is associated with high antioxidant capacity, was not sufficient to prevent the development of CIp in the premature fruit. The concentration of punicalagin was the same for premature and mature skin at harvest and during storage, and therefore not associated with CIp development in the premature fruit skin. Furthermore, the expression of antioxidant-related genes CAT2, SOD and GR2 was similar for both premature and mature fruit skin. Poor pigmentation of the premature fruit skin and chilling-induced downregulation of key anthocyanin-biosynthesis genes were associated with CIp development. High total phenol concentration combined with high expression of the gene encoding PPO was also associated with CIp; however, high expression ratio of PAL to PPO was found in mature skin, and may be associated with reduced CIp disorder. The results presented suggest future possibilities for controlling the CIp phenomenon.

The root system of potato is made up of adventitious roots (AR) that form at the base of a sprout once it emerges from the mother tuber. By definition, AR originate from dormant preformed meristems, or from cells neighboring vascular tissues in stems or leaves. This may occur as part of the developmental program of the plant (e.g., potato), or when replacing the embryonic primary roots in response to stress conditions, such as flooding, nutrient deprivation, or wounding. AR formation is studied mainly in cereals and model plants, and less is known about its developmental program in root and tuber crops. In this review, we summarize the recent data on AR development in potato and relate this knowledge to what is known from model plants. For example, AR formation following stem cutting in potato follows a pattern of initiation, expression, and emergence phases that are known for other plants and involves auxin, the master regulator of AR induction and development. Molecular regulation of AR formation and the effect of environmental stresses are discussed. Understanding the origin and nature of AR systems in important crops will contribute to increased production and improve global food security.

Sweet potato (Ipomoea batatas) growth faces two critical problems: variability in storage root (SR) number and size among individual plants, and skinning injuries that render the SR susceptible to pathogen infections during storage. We hypothesized that application of potassium (K) fertilizer, an essential mineral for sweet potato, would contribute to increased yield, uniformity, and skin quality of SRs. Sweet potatoes were grown in sandy soil, which is poor in K, and in loess soil. The fertilizers potassium chloride (KCl) and polyhalite were applied before planting. Polyhalite is a hydrated sulfate of K, calcium, and magnesium that has been shown to improve potato skin appearance. Soil type was the major factor affecting SR yield—higher in sandy vs. loess soil. The K fertilizers did not affect yield in either soil type, or improve SR uniformity. However, the skin of the SRs from loess soil had more phellem layers and larger phellem cells following fertilization, mainly with KCl. Accordingly, the expression of suberin marker genes was significantly higher in mature vs. immature skin of SRs fertilized with KCl. Overall, soil type was the major factor affecting sweet potato yield, and addition of K positively affected skin morphology and related gene expression.

Skin cracking limits fruit quality and marketability. Suggested causes are environmental conditions, orchard management, and failure of the skin to resist surface tension due to fruit expansion. Fruit skin is made up of epidermis cells and cuticular matrix. Theoretical and experimental studies of skin mechanics, together with anatomical and molecular comparisons of cracking-susceptible vs. tolerant genotypes, suggest that increased cuticle thickness, high epidermal cell density and strong adhesion between neighboring cells are associated with cracking resistance. Calyx-end cracking disorder in apple is treated with a mixture of gibberellic acids 4 and 7 (GA_4 + 7) and the cytokinin 6-benzyladenine (BA) early in fruit development. The treatment not only significantly reduces cracking incidence in the orchard, it also provides information on the cellular and molecular factors determining fruit-skin resistance to growth strain. BA + GA_4 + 7 application results in an immediate increase in epidermal cell density that is maintained until fruit maturation. Moreover, the epidermal cells form clusters within the cuticular matrix, which may strengthen the cuticle by adding more cell-wall components and may enhance crack repair. Skin anatomical modifications are complemented by the expression of genes associated with epidermal cell patterning and cuticle formation. Gene-networking analysis supports the interaction between cell-wall synthesis, cuticle-formation, and GA-signaling gene clusters. Overall, data suggest that BA + GA_4 + 7 treatment does not modify developmental cues, but promotes or enhances the innate developmental program. This review presents data on BA- and GA_4 + 7-induced skin modifications that complement previously suggested models for cracking resistance in fruit. Knowledge gained on apple fruit skin traits may be applied to control cracking in other fruit as well.