Cross-kingdom gene transfer as a driver of land plant evolution

Land plant evolution has been marked by bursts of novelty, often underpinned by extensive genomic innovation. A key mechanism driving these changes is horizontal gene transfer (HGT), the process by which genes move between species and even across taxonomic kingdoms. HGT can accelerate evolutionary change through the rapid introduction of new genes yet its importance in plant biology is only beginning to be understood. Here, we review the functional contributions of HGT during the origin and diversification of land plants. We discuss the occurrence of HGT throughout plant evolution and its impact on the origin of defining traits from cell walls to developmental programs. Beyond ancient contributions, HGT continues to drive the emergence of lineage-specific innovations. Recently acquired bacterial and fungal genes make complex functional contributions to processes including stress response, pathogen defence, and development across plant phylogeny. These observations suggest that HGT was, and continues to be, a major force shaping plant evolution, exemplifying the potential significance of HGT in eukaryotic biology more broadly.