Limited Tyrosine Utilization Explains Lower Betalain Contents in Yellow than in Red Table Beet Genotypes

Abstract

Betalains are tyrosine-derived pigments that consist of red-violet beta­cyanins and yellow beta­xanthins. These pigments are major sources of natural food dyes in the United States. Decades of table beet breeding efforts have increased betalain pigmentation, but yellow beta­xanthin accumulation has been lower than that of red beta­cyanins. To identify possible bottlenecks in betalain production, here we conducted comparative analyses of betalains and their precursor, tyrosine, in various beet genotypes. Consistent with previous studies, red beets had much higher betalain concentrations than yellow beets. Conversely, tyrosine levels were higher in yellow than in red genotypes in both table beet and Swiss chard. Interestingly, increased tyrosine levels were positively correlated with elevated betalain accumulation among red but not yellow genotypes, especially at a later developmental stage, suggesting that yellow beets are not efficiently converting tyrosine into betalain pigments. On the basis of these results, we hypothesize that further increase in tyrosine production will likely enhance beta­cyanin accumulation in red beets, whereas better utilization of the accumulated tyrosine will be required to further improve beta­xanthin production in yellow beets.

Relaxation of tyrosine pathway regulation underlies the evolution of betalain pigmentation in Caryophyllales

“…Tyr) may not always be enough to efficiently produce its downstream product, and ‘pushing’ the precursor supply may be also important. Indeed, in red beets, increased Tyr concentrations have a strong positive correlation with enhanced accumulation of betalains (Wang et al ., ), suggesting that elevated production of Tyr plays important role in overall production of betalains. Over 100‐fold increase in Tyr accumulation observed in N. benthamiana leaves expressing ADH α (Fig.…”

“…40 mg FW tissues were harvested, lyophilized, sent from the University of Cambridge (UK) to the University of Wisconsin‐Madison (USA), and analyzed exactly as described. Tyr and other amino acids were extracted and measured as described previously (Wang et al ., ) and quantified based on standard curves of authentic amino acid standards (Sigma‐Aldrich).…”

Imbalance of tyrosine by modulating TyrA arogenate dehydrogenases impacts growth and development of Arabidopsis thaliana

“…Amino acids and α‐tocopherol extraction, sample derivatization and GC‐MS (ISQ‐LT, ThermoScientific, Waltham, MA, USA) settings were performed as described by Wang et al . (), with some modifications. Briefly, 30–50 mg of fresh leaf tissue was ground in the presence of extraction buffer (v/v 2:1 of methanol and chloroform, 100 μ m norvaline, 100 μ m docosanol and 0.01% butylated hydroxytoluene).…”

The evolution of betalain biosynthesis in Caryophyllales

“…In fact, coexpression of BvCYP76AD1 and BvCYP76AD6 in tobacco has been shown to produce a combination of betaxanthins and betacyanins, resulting in an orange-pink coloration and suggesting that the ratio of expression of these enzymes may be responsible for the variety of hues observed in nature (Polturak et al, 2017). However, yellow beet cultivars containing a defective BvCYP76AD1 allele were also shown to display overall limited pigment production and increased tyrosine accumulation in planta, indicating that the lack of an active BvCYP76AD1 can substantially reduce the plant's capacity to oxidize tyrosine into L-DOPA (Wang et al, 2017). Although these tyrosine-hydroxylase-specific CYP76AD enzymes were initially identified in B. vulgaris, recent analyses have identified a homologue in M. jalapa, MjCYP76AD15 (Sunnadeniya et al, 2016;Polturak et al, 2018), which exhibits similar tyrosine hydroxylase activity.…”