Anthocyanins are flavonoid pigments conferring red, blue and purple colors to plant tissues. Because they are visible to the naked eye, these pigments are a model for genetics, molecular biology and cell biology. Consequently, both structural and regulatory genes of the biosynthetic pathway are identified in a plethora of species (Figure 1A). A complex of highly conserved WD40, bHLH and MYB proteins (MBW complex) activates the transcription of structural genes encoding enzymes of the anthocyanin pathway (Koes et al., 2005; Jaakola, 2013). In all species analyzed, the WD40 is expressed ubiquitously, whereas expression of bHLH and MYB factors is confined to pigmented tissues.

The MYB component of the MBW complex that activate the (pro)anthocyanin pathway is able to activate transcription of its bHLH partner and is therefore consider a “master regulator” as it can, alone, induce activation of the pathway (Spelt et al.,2000; Nesi et al.,2001;Kiferle et al.,2015).After synthesis, anthocyanins are transported to the vacuolar lumen where they are stored. This process is studied by several groups (Francisco et al.,2013; Chanoca et al.,2015;Hu et al.,2016) but it is still not fully understood in spite of the substantial role it might play in the final anthocyanin content in plant tissues.
Plant products rich in anthocyanin like berries, eggplant, grape, and red cabbage, are part of the human diet. Several studies reported that anthocyanin-intake prevents the onset and development of degenerative diseases.
The presence of anthocyanin in plant tissues positively affects their market value in addition by increasing the aesthetical appeal and by reducing softening, shriveling, rotting and fungal infection (Zhang et al.,2015c). Furthermore color novelty is a major driving force in the ornamentals and cut flower industry.
Increased anthocyanin content is, for all mentioned reasons, an obvious goal for crop breeding and biotechnology. Therefore combinations of classical and molecular methods, have been used to generate new varieties with enhanced anthocyanin content as well as different colors and pigmentation patterns.
Till now, research in ornamental and food crops aimed to alter genes controlling anthocyanin synthesis, since it was taken for granted that the end products are stable once they are deposited in the vacuole.
Here we review the state of the art in improving anthocyanin production in plant tissues and report recent insights into the (in)stability of anthocyanins in vacuoles, suggesting that the understanding of the mechanism behind anthocyanin stabilization in planta is required for breeding and biotechnology to take the next step toward plant varieties with increased economical and nutraceutical value.

STUDYING FLOWER PIGMENTATION TAUGHT US HOW TO COLOR OUR FOOD

Much of the current knowledge on anthocyanin chemistry and genetics originates from studies on flower pigmentation in model species. Some of the results have been applied to generate new varieties of cut flowers and ornamental flowering plants with novel colors and pigmentation patterns.
The substrate specificity of the enzymes of the anthocyanin pathway determines the final pattern of chemical decorations and thereby the pigment color (Provenzano et al.,2014;Rinaldo et al.,2015). Together with the understanding of the biosynthetic pathway regulation (Koes et al.,2005;Jaakola,2013), this knowledge was applied to enhance the nutraceutical value and the appeal of several economically relevant plant products.

The dynamics of metabolic flows affects channeling of precursors toward anthocyanin production (Zvi et al.,2012; Sheehan et al.,2015;Zhang et al.,2015b) and this should be considered when designing strategies to generate genotypes with new colors or enhanced anthocyanin content.
Flower pigmentation patterns originate from differential expression of the structural genes in different cells. While irregular patterns are mostly due to transposon insertions in structural and/or regulatory genes (Figure 1C;Lister et al., 1993;Spelt et al.,2000;Itoh et al.,2002), flecks, sector veins and coloration of different flower parts are due to differential expression of genes encoding for MYB proteins of the MBW transcription complex regulating the anthocyanin pathway.

Much of the current knowledge on anthocyanin chemistry and genetics originates from studies on flower pigmentation in model species. Some of the results have been applied to generate new

varieties of cut flowers and ornamental flowering plants with novel colors and pigmentation patterns.
The substrate specificity of the enzymes of the anthocyanin pathway determines the final pattern of chemical decorations and thereby the pigment color (Provenzano et al.,2014;Rinaldo et al.,2015). Together with the understanding of the biosynthetic pathway regulation (Koes et al.,2005;Jaakola,2013), this knowledge was applied to enhance the nutraceutical value and the appeal of several economically relevant plant products.

The dynamics of metabolic flows affects channeling of precursors toward anthocyanin production (Zvi et al.,2012; Sheehan et al.,2015;Zhang et al.,2015b) and this should be considered when designing strategies to generate genotypes with new colors or enhanced anthocyanin content.
Flower pigmentation patterns originate from differential expression of the structural genes in different cells. While irregular patterns are mostly due to transposon insertions in structural and/or regulatory genes (Figure 1C;Lister et al., 1993;Spelt et al.,2000;Itoh et al.,2002), flecks, sector veins and coloration of different flower parts are due to differential expression of genes encoding for MYB proteins of the MBW transcription complex regulating the anthocyanin pathway.

The picture of anthocyanin synthesis and regulation gained from studies in flowers was confirmed in several crops where homolog MBW complexes regulate pigment accumulation in different plant parts.
Modern crops are the result of a domestication process that, for most species, went on for the last 10.000 years. Selection resulted sometimes in the loss of pigmentation in some plant parts. Pigmentation in tomato fruits, for example, was probably a trait indirectly counter-selected by breeding as the fruits of several closely related wild Solanum species are colored. The introgression in domesticated tomato of two loci, Aft (Anthocyanin fruit) and atv (atroviolacea) from wild Solanum, results in the accumulation of anthocyanins in the epidermis and the pericarp of the fruit (Povero et al.,2011), indicating that it is possible to restore fruit pigmentation by adding few genes.

CONCLUSION
Anthocyanin-rich plants produced by traditional breeding or biotechnology, could contribute to human health reducing the incidence of major diseases (Martin et al.,2011), while new flower colors and patterns (Yoshida et al.,2009;Tanaka and Brugliera,2013;Zhao and Tao,2015) are interesting for the ornamental market. Success was booked in producing plants with enhanced anthocyanin synthesis by increasing the expression of MYB factors that activate transcription of structural anthocyanin genes. However, degradation also contributes to the final anthocyanin yield in plant products making the understanding of this phenomenon important for future strategies of crop improvement.
Studies in brunfelsia provide insight into the biochemistry of anthocyanin degradation (Zipor et al.,2015).
It is unclear whether a certain degree of anthocyanin degradation, is functional to the plant. So far only speculations are possible. Anthocyanins protect tissues from free radicals and in some species accumulate in seedlings where they shield the photosynthetic machinery from light. Their degradation later in development probably improves photosynthesis (Gould et al.,2002b). In brunfelsia, anthocyanin degradation in flowers is accompanied by release of fragrant volatiles and both processes could be signals for pollinators (Zipor et al., 2015). However, no evidence is available for correlations between the two phenomena. Reactive oxygen species (ROS) formed in aging flowers or maturing fruits from photo- oxidation, photorespiration, and Mehler reaction, could induce anthocyanin degradation and this might protect other cellular components from damages (Hernández et al.,2009). Moreover anthocyanins inhibit Fenton hydroxyl radical generation by scavenging superoxide and hydrogen peroxide (Gould et al., 2002a,2010). A better characterization of the genes/factors involved in color fading will answer to the many questions we presented here and open the possibility to ‘design’ plant cells with stable vacuolar content. Mutants makes it possible to approach the characterization of the FADING locus and of the MBW target genes involved in anthocyanin stabilization. Considering that anthocyanins are not stable outside the vacuole (Mueller et al.,2000), the MBW complex could control vacuolar physiology and mutants might have vacuolar defect resulting in anthocyanin leakage. Factors involved in both fading and its prevention could function in totally unrelated pathways. Their participation in massive anthocyanin degradation might be a peculiarity of rare genotypes that amplify a moderate pigment loss normally occurring after vacuolar accumulation.
Genetic analyses in species, like petunia, where well-defined mutants affecting this phenomenon are available (de Vlaming et al.,1982,1983;Quattrocchio et al.,2006) open the way
to identify the genes that determine anthocyanin trun-over in vivo, to assess whether complete disappearance of color is an “accident” originating from human selection during crop domestication, and to gain tools to improve stabilization of anthocyanin (and possibly also other products) in the vacuolar lumen.

 

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