Blue genome: chromosome-scale genome reveals the evolutionary and molecular basis of indigo biosynthesis in Strobilanthes cusia

Natural plant dyes have been developed and used across many traditional societies worldwide. The blue pigment indigo has seen widespread usage across South America, Egypt, Europe, India and China for thousands of years, mainly extracted from indigo-rich plants. The utilization and genetic engineering of indigo in industries and ethnobotanical studies on the effects of cultural selection on plant domestication are limited due to lack of relevant genetic and genomic information of dye plants. Strobilanthes cusia (Acanthaceae) is a typical indigo-rich plant important to diverse ethnic cultures in many regions of Asia. Here we present a chromosome-scale genome for S. cusia with a genome size of approximately 865 Mb. About 79% of the sequences were identified as repetitive sequences and 32 148 protein-coding genes were annotated. Metabolic analysis showed that the main indigoid pigments (indican, indigo and indirubin) were mainly synthesized in the leaves and stems of S. cusia. Transcriptomic analysis revealed that the expression level of genes encoding metabolic enzymes such as monooxygenase, uridine diphosphate-glycosyltransferase and beta-glucosidase were significantly changed in leaves and stems compared with root tissues, implying their participation in indigo biosynthesis. We found that several gene families involved in indigo biosynthesis had undergone an expansion in number, with functional differentiation likely facilitating indigo biosynthesis in S. cusia. This study provides insight into the physiological and molecular bases of indigo biosynthesis, as well as providing genomic data that provide the basis for further study of S. cusia cultivation by Asia's traditional textile producers.