Importance of color
Undoubtedly, one of the factors that most influences consumers when they are considering a purchase is the appearance of the product. And one of the most important factors influencing the decision is the colour of the product. In vegetable products for fresh consumption, colour is a fundamental criterion when it comes to making a choice. This is why companies are trying to offer new products in a wider range of colours.
In the case of peppers for fresh consumption, varieties have traditionally been marketed with green (fruits that have not yet reached physiological maturity), red and yellow colours. But the range of existing colours in the germplasm of pepper is much wider than the one present in the great majority of the commercial varieties that reach the general public and that, as I have already mentioned, is restricted to red, yellow and in some cases orange.
Genetic control
Over time, different models of genetic control of the colour of ripe pepper fruit have been presented. Depending on the germplasm used to analyse the studies of the inheritance of the skin colour of the pepper fruit, this has been explained on the basis of one gene, two genes or three genes.
The most widely accepted genetic model is based on the regulation of pepper colour expression on the basis of three independent genes. This model was proposed in 1985 by Hurtado-Hernandez & Smith (although it is based on an earlier work by Kormos & Kormos), where from crosses between pepper lines with white, red and yellow ripening fruit they deduced the interaction of 3 different genes that gave rise to 8 different phenotypes. The genes were named Y/y, C1/c1 and C2/c2. The allelic combinations gave rise to the following phenotypes:
- Y_,C1_,C2_: Red
- Y_,c1c1,C2_: Light red
- Y_,C1_,c2c2: Orange
- Y_,c1c1,c2c2: Pale orange
- yy,C1_,C2_: Orange yellow
- yy,C1_,c2c2_: Pale orange yellow
- yy,c1c1,C2_:Lemon yellow
- yy,c1c1,c2c2: White
Subsequently, in 1998, the Y/y gene was found to encode the CCS (Capsanthin-capsorubin synthase) enzyme responsible for the conversion of antheraxanthin to capsanthin and violaxanthin to capsorubin, the compounds that give the red colour to ripe pepper fruit. It was also established that the Y/y gene is located in linkage group 3 of the pepper genetic map. On the other hand, it has been found that not all genotypes that do not encode red pigments have the same defect with respect to the wild gene, which means that there are several non-coding alleles of the Y gene in the pepper germplasm.
In 2001 it was established that the C2/c2 gel encoded for the PSY (Phytoene synthase) enzyme, with the recessive allele leading to a 1/10 reduction in pigment accumulation. This gene has been placed in linkage group 7 of pepper.
The last of the genes for which its activity was known was C1/c1, which in 2020 was published to coincide with the PRR2 gene that regulates plastidic development in pepper fruits.
In the diagram of the synthesis of the pigments present in the pepper, it can be seen that one of the genes (Y/y) encodes the CCS enzyme, which is at the end of the whole chain, while the gene (C2/c2), which gives rise to the PSY enzyme, is located almost at the beginning of the whole synthesis process. In contrast, the gene (C1/c1) is involved in the formation of cell organelles where the whole synthesis process described in the previous scheme takes place, and not in the synthesis chain itself.
The previous model of 3 genes explains the colouring of the ripe fruit in red, yellow, orange and white, some of them in different intensity of colour tone that gives rise to the 8 possible phenotypes. But there are more colours described apart from those described, such as chocolate and green when the fruit is ripe.
In a study carried out by Paul G. Smith in 1950, it was found that the chocolate brown colour of some varieties was due to the presence of a recessive gene that prevented the degradation of chlorophyll during the physiological ripening process of the fruit and that when present together with red pigments gave rise to chocolate brown colouring. This gene was named cl (chlorophyll retainer).
It was also found that when crossing a chocolate-coloured variety with another variety with yellow fruit colour, in F2 a new phenotype was found which was green when ripe, due to the presence of the clcl gene and the allelic combination which confers the yellow colour on the fruit, given that the chlorophyll does not degrade and masks the yellow pigment, the fruit remained with an olive green colour when ripe. This cl gene is included in the group of genes called stay green (sgr) which inhibit the degradation of chlorophyll also present in other species such as tomato with the green flesh (gf) gene. In fact, it has been established that cl corresponds to the sequence of the CaSGR gene.
Irrespective of the colour in the ripe state of pepper fruits, they also show a diversity of colours in the unripe state, ranging from different shades of green, light yellow, as well as violet or black. In early work, two genes controlling the immature green colour of the fruit were described. As well as two other genes: Anthocyaning (A) and modifier of A (MoA) that controlled the presence of purple colour and its intensity.
Although much is known about the inheritance of the colour of pepper fruits, there are still unknown aspects, such as the interaction of some of the genes described and more information on the genetic control of the colouring of immature fruits, as well as the interaction of both colour control systems in both ripening stages. And undoubtedly the interaction with the environmental conditions, which produces, depending on the production environment, different colour intensities for the same genotype over time and the aspects related to the kinetics of the colour change.
Interest of fruit color in the genetic improvement of pepper
Undoubtedly, the colour of the fruit both in the unripe and ripe state is an important trait in the genetic improvement of new pepper varieties, since the consumer shows a great interest in novelties related to the presentation of the product, and therefore the colour and intensity of the colour is a trait to which more and more attention is being paid.
Some breeding companies are developing varietal series where they offer the same variety with different fruit colours, an example of this activity is the case of Sweet Palermo peppers of the company Rijk Zwaan, or the Tribelli snack peppers of the company Enza Zaden.
Several companies also offer a special range of peppers in different colours, including chocolate, purple and black.
References
Humberto Hurtado-Hernandez, Paul G. Smith, Inheritance of mature fruit color in Capsicum annuum L., Journal of Heredity, Volume 76, Issue 3, May 1985, Pages 211–213. Link
Lefebvre, V., Kuntz, M., Camara, B. et al. The capsanthin-capsorubin synthase gene: a candidate gene for the y locus controlling the red fruit colour in pepper. Plant Mol Biol 36, 785–789 (1998). Link
Huh, J., Kang, B., Nahm, S. et al. A candidate gene approach identified phytoene synthase as the locus for mature fruit color in red pepper (Capsicum spp.). Theor Appl Genet 102, 524–530 (2001). Link
Jeong H-B, Jang S-J, Kang M-Y, Kim S, Kwon J-K and Kang B-C (2020) Candidate Gene Analysis Reveals That the Fruit Color Locus C1 Corresponds to PRR2 in Pepper (Capsicum frutescens). Front. Plant Sci. 11:399. Link
Ya-Qin Lang, Satoshi Yanagawa, Tsuneo Sasanuma, Tetsuo Sasakuma, Orange Fruit Color in Capsicum due to Deletion of Capsanthin-capsorubin Synthesis Gene, Breeding Science, 2004, Volume 54, Issue 1, Pages 33-39, Released March 18, 2004, Online ISSN 1347-3735, Print ISSN 1344-7610. Link
Paul G. Smith, Inheritance Of Brown And Green Mature Fruit Color In Peppers, Journal of Heredity, Volume 41, Issue 5, May 1950, Pages 138–140. Link
Borovsky, Y., Paran, I. Chlorophyll breakdown during pepper fruit ripening in the chlorophyll retainer mutation is impaired at the homolog of the senescence-inducible stay-green gene. Theor Appl Genet 117, 235–240 (2008). Link
Gordon J. Lightbourn, Robert J. Griesbach, Janet A. Novotny, Beverly A. Clevidence, David D. Rao, John R. Stommel, Effects of Anthocyanin and Carotenoid Combinations on Foliage and Immature Fruit Color of Capsicum annuum L., Journal of Heredity, Volume 99, Issue 2, March/April 2008, Pages 105–111. Link
Rödiger, A., Agne, B., Dobritzsch, D., Helm, S., Müller, F., Pötzsch, N. and Baginsky, S. (2021), Chromoplast differentiation in bell pepper (Capsicum annuum) fruits. Plant J, 105: 1431-1442. Link
