Integrated Metabolomics and Transcriptomics Analyses Reveal Anthocyanin and Carotenoid Biosynthesis Involved in Color Development in Willow Bark

Abstract Background The rich yellow-orange to vividly deep red bark of willow (Salix spp.) branches have great ornamental and economic value. The mechanism of willow branch color has not been clarified yet. ResultsTo study the mechanisms regulating color development, we performed metabolomics and transcriptomics analyses using purple, green, and red willow barks. Seven anthocyanins were isolated; pelargonidin, petunidin 3-O-rutinoside, and cyanin chloride were the most abundant in the red bark and pelargonin chloride in the purple bark. Among the barks, green bark had the highest level of malvidin, but its level was not significantly higher than in the red bark; the purple bark had the highest amount of canthaxanthin, a carotenoid pigment. The integrated biosynthetic pathway of flavonoid, carotenoid, and chlorophyll was constructed for willow barks. Among the three barks, the expression of the structural genes ANS, ANR, and BZ1, involved in anthocyanin synthesis, was highest in the red bark, likely causing anthocyanin accumulation. The highest expression of CrtZ, involved in the carotenoid pathway, was observed in the purple bark, likely causing canthaxanthin accumulation. The high expression of DVR, POR, and CRD1 may be associated with the green pigment synthesis in chlorophyll biosynthetic pathway. Our research findings will facilitate genetic breeding to improve the color and luster of willows. ConclusionsIn conclusion, the purple bark is co-regulated by anthocyanins and carotenoids. The red bark is characterized by anthocyanin accumulation and chlorophyll degradation. The green pigment is regulated by maintaining of chlorophyll synthesis. The key regulation genes BZ1, CrtZ could be candidate genes regulating anthocyanin and canthaxanthin accumulation in red and purple barks. Our research findings facilitate the genetic breeding and cultivation of colorful willows with improved color and luster.