New Challenges for the Design of High Value Plant Products: Stabilization of Anthocyanins in Plant Vacuoles

Abstract

In the last decade plant biotechnologists and breeders have made several attempt to improve the antioxidant content of plant-derived food. Most efforts concentrated on increasing the synthesis of antioxidants, in particular anthocyanins, by inducing the transcription of genes encoding the synthesizing enzymes. We present here an overview of economically interesting plant species, both food crops and ornamentals, in which anthocyanin content was improved by traditional breeding or transgenesis. Old genetic studies in petunia and more recent biochemical work in brunfelsia, have shown that after synthesis and compartmentalization in the vacuole, anthocyanins need to be stabilized to preserve the color of the plant tissue over time. The final yield of antioxidant molecules is the result of the balance between synthesis and degradation. Therefore the understanding of the mechanism that determine molecule stabilization in the vacuolar lumen is the next step that needs to be taken to further improve the anthocyanin content in food. In several species a phenomenon known as fading is responsible for the disappearance of pigmentation which in some case can be nearly complete. We discuss the present knowledge about the genetic and biochemical factors involved in pigment preservation/destabilization in plant cells. The improvement of our understanding of the fading process will supply new tools for both biotechnological approaches and marker-assisted breeding.

Functionally Similar WRKY Proteins Regulate Vacuolar Acidification in Petunia and Hair Development in Arabidopsis

“…The AN1-AN11-PH4 triumvirate is also required for the stability of anthocyanins in the vacuole, as in certain genetic backgrounds ph4 and specific an1 alleles (formerly known as ph6 alleles) trigger the complete disappearance of anthocyanins and "fading" of the flower color after opening of the bud (de Vlaming et al, 1982(de Vlaming et al, , 1983Quattrocchio et al, 2006;Passeri et al, 2016). Fading is not triggered by the pH shift alone because it is not seen in ph5 and ph2 mutants but seems due to a different vacuolar defect caused by the misregulation of distinct AN1-AN11-PH4 target genes (Quattrocchio et al, 2006).…”

“…To date, seven such PH genes have been identified in petunia via mutants that all show a similar phenotype: petals with a bluish color and an increased pH of petal homogenates (de Vlaming et al, 1983;van Houwelingen et al, 1998). Molecular analysis revealed that ph6 mutants represent specific an1 alleles that lost the capacity to acidify the vacuole but can still drive anthocyanin biosynthesis (Spelt et al, 2002) and that PH5 and PH1 are direct target genes of the AN11-AN1-PH4 complex that encodes two interacting P-ATPase transmembrane transporters that reside in the tonoplast (Verweij et al, 2008;Faraco et al, 2014).The AN1-AN11-PH4 triumvirate is also required for the stability of anthocyanins in the vacuole, as in certain genetic backgrounds ph4 and specific an1 alleles (formerly known as ph6 alleles) trigger the complete disappearance of anthocyanins and "fading" of the flower color after opening of the bud (de Vlaming et al, 1982(de Vlaming et al, , 1983Quattrocchio et al, 2006;Passeri et al, 2016). Fading is not triggered by the pH shift alone because it is not seen in ph5 and ph2 mutants but seems due to a different vacuolar defect caused by the misregulation of distinct AN1-AN11-PH4 target genes (Quattrocchio et al, 2006).…”

Antioxidant Capacity of Anthocyanin Pigments

“…Anthocyanin identification is critical in adulteration and profiling [349,350] studies and in evaluating the quality of crude and processed food. The design of plant products with a high added value allows increasing the synthesis [351] of plant-derived food antioxidants and in particular anthocyanins. In an effort to expand the palette of natural organic colourants (colour additives of food and beverage products), the food industry has launched a search for new products, for example blue colourants [352,353].…”

Quantitative trait loci affecting intensity of violet flower colour in potato

“…Potato can also have pigmented tuber skin and flesh, sprouts, leaves and stems. Anthocyanins in potato tuber skin and flesh are not only serving as a genetic model but, just like in other anthocyanin-rich crops and vegetables, enhance the market value of the plant by increasing its visual appeal to the consumer, reducing softening, shrivelling and fungal rots (Passeri et al 2016). What is also important, plant products rich in anthocyanins are health-promoting components of the human diet.…”

“…What is also important, plant products rich in anthocyanins are health-promoting components of the human diet. The health benefits associated with anthocyanins intake include: prevention of the degenerative diseases and cancer, reduction of retinal damage, decreasing inflammatory markers expression, reduction of risk of type-2 diabetes and reduction of weight gain (Passeri et al 2016).…”