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.