Abstract: As an important halophyte in the Yellow River Delta, the Amaranthaceae C₃ Suaeda salsa (L.) Pall. has attracted much attention for the “red carpet” landscape, and could be simply divided into red and green phenotypes according to the betacyanin content in the fleshy leaves. However, S. salsa has not been sequenced yet, which limited people's understanding of this species at the molecular level. We constructed a high-quality chromosome-scale reference genome by combining high-throughput sequencing, PacBio single molecule real-time sequencing, and Hi-C sequencing techniques with a genome size of 445.10?Mb and contigs N50 of 2.94?Mb. Through the annotation of the S. salsa genome, 298.76?Mb of the repetitive sequences and 23?965 protein-coding genes were identified, of which the proportion of long terminal repeats type in the repetitive sequences was the most abundant, about 50.74% of the S. salsa genome. Comparative genomics indicated that S. salsa underwent a whole-genome duplication event about 146.15 million years ago (Ma), and the estimated divergence time between S. salsa and Suaeda aralocaspica was about 16.9?Ma. A total of four betacyanins including betanidin, celosianin II, amaranthin and 6′-O-malonyl-celosianin II were identified and purified in both phenotypes, while two significantly up-regulated betacyanins (celosianin II and amaranthin) may be the main reason for the red color in red phenotype. In addition, we also performed transcriptomics and metabolomics in both phenotypes to explore the molecular mechanisms of pigment synthesis, and a series of structural genes and transcription factors concerning with betacyanin production were selected in S. salsa.

Key words: betalain, genome assembly, genome evolution, halophyte, Suaeda salsa