RESUMEN
Allium crop breeding remains severely hindered due to the lack of high-quality reference genomes. Here we report high-quality chromosome-level genome assemblies for three key Allium crops (Welsh onion, garlic and onion), which are 11.17 Gb, 15.52 Gb and 15.78 Gb in size with the highest recorded contig N50 of 507.27 Mb, 109.82 Mb and 81.66 Mb, respectively. Beyond revealing the genome evolutionary process of Allium species, our pathogen infection experiments and comparative metabolomic and genomic analyses showed that genes encoding enzymes involved in the metabolic pathway of Allium-specific flavor compounds may have evolved from an ancient uncharacterized plant defense system widely existing in many plant lineages but extensively boosted in alliums. Using in situ hybridization and spatial RNA sequencing, we obtained an overview of cell-type categorization and gene expression changes associated with spongy mesophyll cell expansion during onion bulb formation, thus indicating the functional roles of bulb formation genes.