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With the deepening of genetic research on skin color traits of cucumber commercial fruits, genes controlling skin color traits have been gradually excavated and located. At present, the discovered molecular markers include polymorphic markers ME9EM1-309 and ME9EM14-425, which are 6cM and 8.3cM distant from the green gene of cucumber peel, respectively[32]. Molecular markers E43M61 and E34M59, SSR23517 and SSR23141 linked to the w gene of cucumber white skin trait[33]. The markers SNP2145087 and SNP3286953 were related to lgp[6]. In addition, there are molecular markers SSR10 and SSR37 associated with the skin color uniform(u) gene[34]. What’s exciting are genes related to light green skin, white skin, yellow-green skin and stripe of young fruit have been mainly cloned (Table 1).
Table 1. Genes that control peel color in cucumber were identified in recent years.
Pericarp color character Regulatory gene (ID) Function Reference NCBI reference sequence Yellow green CsMYB36 (Csa2G352940) Interacts with genes related to chlorophyll synthesis Hao et al. 2018[7] XP_004143035.1 White CsAPRR2/W (Csa3G904140) Chlorophyll content and chloroplast number decreased Tang et al. 2018[35] XP_011652667.1 Light green CsYcf54 (Csa6G133820) An important component of cyclase in chlorophyll synthesis Lun et al. 2016[4] XP_004139926.1 Bright green CsARC5 (Csa7G051430) Chloroplast division Zhou et al. 2015[5] XP_004136854.1 Stripe CsIST (Csa1G005490) The number and volume of chloroplasts decreased Song et al. 2020[26] XP_004138283.1 Glossiness CsDULL (Csa5G577350) The regulation of cutin and wax
biosynthesis/transportationZhai et al. 2022[3] NP_001295848.1 Although progress has been made in determination of major gene-controlled cucumber immature fruit peel color, several issues appeared during those processes. For instance, the genetic distance between the molecular markers developed by the genetic population constructed with different parents and the target gene was inconsistent[35]. Even if the same experimental materials were used, due to the difference in the size of the genetic population, the obtained markers and the localization interval of the target genes are different[36, 37], or using the same materials but different mapping methods to obtain the immature cucumber peel color candidate genes which are located not even on the same chromosome[4, 5]. These results indicate that the genetic localization and gene mining of commercial cucumber skin color traits may be restricted by many factors such as population size, marker selection, genetic background and environmental conditions. Therefore, although some genes that regulate the skin color of cucumber fruits have been preliminarily mapped by previous studies (Table 1), due to the relative complexed color traits, the location interval of many important genes is still large, and the target genes are still difficult to be defined, for example, u, which further affects the in-depth research on the function and regulation mechanism of skin color genes in commercial fruits.
Since the 1930s, researchers have noted the diversity of cucumber skin color and the genetic characteristics of those phenotype. Nevertheless, there were still two viewpoints on the genetic rules of skin color genes of commercial cucumber fruits: the one is favored quality traits controlled by a single gene, and the other is favored quantitative traits controlled by major effect genes and modification genes. The possible reasons for the disunity of the above research conclusions are as follows: (1) the investigation results most rely on simple visual results, which tends to lead to the poor accuracy of the standard classification of skin color difference (Fig. 1); (2) In recent years, with the help of a chromatic meter and other means to the determination of pigment content after peeling and cutting fruit skins, it is easy to add pulp to the determination results meaning that pigment content cannot fully represent the color of fruit skins[10−12]; (3) The genetic background and investigation environment, fruit development stages of test materials of separated research groups are different, which may lead to inconsistent conclusions. Although the results differ due to the different genetic background, testing method and growing environment, the skin color character of commercial cucumber fruit should be a relatively simple quantitative genetic character. At present, it has been established physiologically that the commercial color of cucumber is caused by the difference of pigment composition, content and chloroplast number[38]. Some candidate genes controlling fruit color have been successfully mined from the genetic level, but functional verification of gene editing has not been completed (Table 1).
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Among the existing Cucurbitaceae with more than 800 species known worldwide, cucumber has the smallest genome and the most studied gene function so far[19]. Cucumber research most focused on the biological basis of cucumber fruit development. Cucumber varieties grown and used around the world have rich skin color characteristics. These variation characters facilitate research into the biological basis of fruit color. Cucumber fruit skin color development is a complex biological process regulated by genotypes, transcription factors, endogenous/exogenous hormones, and enzymes. This paper discussed the physiological characteristics, gene mining, and regulatory mechanism of cucumber peel color.
As a new model plant for fruit development, cucumber still has a certain gap with traditional model plants such as Arabidopsis and rice in gene function research. Research reveals that while there have been several investigations into the genetic basis of cucumber peel color features, the majority of them have focused on placement rather than molecular regulation or particular functional analysis. Network analysis of the metabolome and transcriptome has been applied as a powerful tool to investigate the novel genes and pathways in plants, so it is also effective for exploring the novel pathway involved in the formation of yellow peel in cucumber[56, 57]. According to current research, most research on cucumber pericarp color traits focuses on the young pericarp, with little focus on the mature pericarp color traits, particularly the molecular genetic effect. To investigate the genetic mechanism and gene effect of cucumber mature pericarp color is the foundation for future molecular genetic improvement, breeding, and utilization of cucumber pericarp color. Therefore, the next research direction should focus on the exploration of regulation mechanism, further supplement and improve the genetic law of cucumber peel color and provide a technical basis for creating new germplasm to meet market demand.
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About this article
Cite this article
Cui Y, Li S, Dong Y, Wu H, Gao Y, et al. 2023. Genetic regulation and molecular mechanism of immature cucumber peel color: A review. Vegetable Research 3:9 doi: 10.48130/VR-2023-0009
Genetic regulation and molecular mechanism of immature cucumber peel color: A review
- Received: 09 September 2022
- Accepted: 11 January 2023
- Published online: 09 March 2023
Abstract: Peel color is an essential exterior fruit quality attribute determining the commodity sale of fresh market cucumber. The color phenotype of cucumber peel are traits which are not only controlled by genetics but also affected by the environment. However, understanding the molecular mechanism of skin color formation in immature cucumbers is still in its infancy. In addition, the latest systematic review on research are also lacking. In this review, we first analyzed the physiological factors that influence pericarp color change. The research progress of cucumber peel color was then reviewed at the genetic and molecular levels, and the existing problems in the research were also mentioned. Meanwhile, this paper presents new insights on fruit skin color in order to provide a reference for the fruit development of cucumber and other Cucurbitaceae crops.
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Key words:
- Cucumber /
- Gene /
- Molecular mechanism /
- Peel color