Horizontal transfer of plasmid-like extrachromosomal circular DNAs across graft junctions in Solanaceae

The transfer of genetic material between stocks and scions of grafted plants has been extensively studied; however, the nature and frequency of the transferred material remain elusive. Here, we report a grafting system involving woody goji as the stock and herbaceous tomato as the scion, which was developed using in vitro and in vivo approaches; the results confirmed horizontal transfer of multiple nuclear DNA fragments from donor goji cells to recipient tomato cells. Tomato tissues containing goji donor DNA fragments at or near the grafting junctions had a perennial-biased anatomical structure, from which roots or shoots were regenerated. Most of the fragments were plasmid-like extrachromosomal circular DNAs (eccDNAs) present in the regenerants derived from the cells and in their asexual offspring. Plants with transferred eccDNAs in regenerated roots or shoots (designated "Go-tomato") were grown perennially and showed excellent agronomic performance. The present study provides new insights into the replication, expression, and potential function of eccDNAs in the pleiotropic traits of Go-tomato. Mobile eccDNAs offer evidence of stock-to-scion horizontal DNA transfer beyond chromosomes and organelles, thereby contributing to the molecular understanding of graft-induced genetic variation, evolution, and breeding.