Chromosome Level Genome Assembly Reveals Cassia Tora May Be an Ancient Species in Leguminosae

Background: Cassia tora L. is an annual leguminous plant. Its seeds had wide utility in herbal medicine in Asian, but is usually regard as a potent, invasive weed which even helps farmers eliminate other parasitic weed species. However, compared with other important legume crops, C. tora is far from fully developed and the genetic basis is greatly lacking. A reference genome sequence will be great valuable resource for its genome evolution, genetic breeding and development.Results: Here, we de novo assembled a chromosome-scale genome for C. tora by combining PacBio sequencing technology with chromatin interaction mapping, resulting in 621-Mb genome with a contig N50 of 2.5 Mb, of which 77.44% was ordered and oriented on 13 pseudochromosomes. The genome contained 32,361 protein-coding genes with a repetitive DNA content of approximately 58.15%. Gypsy-type LTRs constituted the largest subfamily. LTR insertion events seldom occurred in this genome over the past 10 million years. Comparative genomic analyses showed that C. tora diverged ~95 million years ago (MYA), which revealed it has the most distant genetic relationship with 11 other legumes. Compared with other legume crops, Cassia tora is maybe an ancient species in Leguminosae.Conclusions: The high-quality reference genome sequence reported here furnishes unprecedented insights into genome dynamics and provides an important basis for future research on legume genome evolution.