Zhou Zhaoxi, Gao Hongmao, Ming Jianhong, Ding Zheli, Lin Xing'e, Zhan Rulin
Haikou Experimental Station, Chinese Academy of Tropical Agricultural Sciences (CATAS), Haikou, China.
BMC Genomics. 2020 Oct 22;21(1):734. doi: 10.1186/s12864-020-07133-5.
Elucidating the candidate genes and key metabolites responsible for pulp and peel coloration is essential for breeding pitaya fruit with new and improved appeal and high nutritional value. Here, we used transcriptome (RNA-Seq) and metabolome analysis (UPLC-MS/MS) to identify structural and regulatory genes and key metabolites associated with peel and pulp colors in three pitaya fruit types belonging to two different Hylocereus species.
Our combined transcriptome and metabolome analyses suggest that the main strategy for obtaining red color is to increase tyrosine content for downstream steps in the betalain pathway. The upregulation of CYP76ADs is proposed as the color-breaking step leading to red or colorless pulp under the regulation by WRKY44 transcription factor. Supported by the differential accumulation of anthocyanin metabolites in red pulped pitaya fruit, our results showed the regulation of anthocyanin biosynthesis pathway in addition to betalain biosynthesis. However, no color-breaking step for the development of anthocyanins in red pulp was observed and no biosynthesis of anthocyanins in white pulp was found. Together, we propose that red pitaya pulp color is under the strict regulation of CYP76ADs by WRKYs and the anthocyanin coexistence with betalains is unneglectable. We ruled out the possibility of yellow peel color formation due to anthocyanins because of no differential regulation of chalcone synthase genes between yellow and green and no detection of naringenin chalcone in the metabolome. Similarly, the no differential regulation of key genes in the carotenoid pathway controlling yellow pigments proposed that the carotenoid pathway is not involved in yellow peel color formation.
Together, our results propose several candidate genes and metabolites controlling a single horticultural attribute i.e. color formation for further functional characterization. This study presents useful genomic resources and information for breeding pitaya fruit with commercially attractive peel and pulp colors. These findings will greatly complement the existing knowledge on the biosynthesis of natural pigments for their applications in food and health industry.
阐明导致火龙果果皮和果肉着色的候选基因和关键代谢产物，对于培育具有新的改良外观和高营养价值的火龙果果实至关重要。在此，我们利用转录组（RNA测序）和代谢组分析（超高效液相色谱-串联质谱），在属于两个不同量天尺属物种的三种火龙果果实类型中，鉴定与果皮和果肉颜色相关的结构基因、调控基因和关键代谢产物。
我们的转录组和代谢组联合分析表明，获得红色的主要策略是增加酪氨酸含量，以用于甜菜素途径的下游步骤。在WRKY44转录因子的调控下，CYP76ADs的上调被认为是导致红色或无色果肉的颜色转变步骤。红色果肉火龙果果实中花青素代谢产物的差异积累支持了我们的结果，表明除了甜菜素生物合成外，花青素生物合成途径也受到调控。然而，未观察到红色果肉中花青素形成的颜色转变步骤，且在白色果肉中未发现花青素的生物合成。总之，我们提出红色火龙果果肉颜色受WRKYs对CYP76ADs的严格调控，且花青素与甜菜素共存不可忽视。由于黄色和绿色之间查尔酮合酶基因无差异调控，且在代谢组中未检测到柚皮素查尔酮，我们排除了花青素导致黄色果皮颜色形成的可能性。同样，控制黄色色素的类胡萝卜素途径中关键基因无差异调控，表明类胡萝卜素途径不参与黄色果皮颜色形成。
总之，我们的结果提出了几个控制单一园艺性状即颜色形成的候选基因和代谢产物，以供进一步功能表征。本研究为培育具有商业吸引力的果皮和果肉颜色的火龙果果实提供了有用的基因组资源和信息。这些发现将极大地补充现有关于天然色素生物合成的知识，以用于食品和健康产业。
