Integrated Metabolomic and Transcriptomic Profiles Provide Insights into the Mechanisms of Anthocyanin and Carotenoid Biosynthesis in Petals of Medicago sativa ssp. sativa and Medicago sativa ssp. falcata

Abstract

The petals of <i>Medicago sativa</i> ssp. <i>sativa</i> and <i>M</i>. <i>sativa</i> ssp. <i>falcata</i> are purple and yellow, respectively. Free hybridization between <i>M</i>. <i>sativa</i> ssp. <i>sativa</i> and <i>M</i>. <i>sativa</i> ssp. <i>falcata</i> has created hybrids with various flower colors in nature. Moreover, the flower colors of alfalfa are closely correlated with yield, nutritional quality, stress tolerance and other agronomic characteristics. To elucidate the underlying mechanisms of flower color formation in <i>M</i>. <i>sativa</i> ssp. <i>sativa</i> and <i>M</i>. <i>sativa</i> ssp. <i>falcata</i>, we conducted an integrative analysis of the transcriptome and metabolome of alfalfa with three different petal colors (purple, yellow and cream). The metabolic profiles suggested that anthocyanins and carotenoids are the crucial pigments in purple and yellow flowers, respectively. A quantitative exploration of the anthocyanin and carotenoid components indicated that the accumulations of cyanidin, delphinidin, peonidin, malvidin, pelargonidin and petunidin derivatives are significantly higher in purple flowers than in cream flowers. In addition, the content of carotenes (phytoene, α-carotene and β-carotene) and xanthophylls (α-cryptoxanthin, lutein, β-cryptoxanthin, zeaxanthin, antheraxanthin and violaxanthin derivatives) was markedly higher in yellow flowers than in cream flowers. Furthermore, we found that delphinidin-3,5-O-diglucoside and lutein were the predominant pigments accumulated in purple and yellow flowers, respectively. The transcriptomic results revealed that twenty-five upregulated structural genes (one <i>C4H</i>, three <i>4CL</i>, twelve <i>CHS</i>, two <i>CHI</i>, one <i>F3H</i>, one <i>F3'H</i>, one <i>F3'5'H</i> and four <i>DFR</i>) are involved in the accumulation of anthocyanins in purple flowers, and nine structural genes (two <i>PSY</i>, one <i>ZDS</i>, two <i>CRTISO</i>, two <i>BCH</i>, one <i>ZEP</i> and one <i>ECH</i>) exert an effect on the carotenoid biosynthesis pathway in yellow flowers. The findings of this study reveal the underlying mechanisms of anthocyanin and carotenoid biosynthesis in alfalfa with three classic flower colors.