Living xylem cells encode a large number of conserved gene families responsible for vascular sap peptides

Plant long-distance signaling peptides travel through the vascular system to coordinate development and respond to environmental cues, yet their precursor genes and expression origins remain elusive. We characterized 4,804 sap peptide precursor genes in Populus trichocarpa using an integrated approach that combined liquid chromatography tandem mass spectrometry (LC-MS/MS) peptidomics, transcriptomics, and comparative genomics. This study expands the known precursor families from approximately 50 to thousands, the majority of which are conserved across angiosperms. Transcriptome analysis across xylem developmental stages revealed that living xylem cells, typically viewed as precursors to non-living structures, predominantly express these specifically at transitions between primary and secondary growth stages, indicating an active role in plant-wide signaling coordination. The precursor genes show conservation at the transcriptome level and are under strong purifying selection. Our findings provide a comprehensive overview of the gene families encoding sap peptides, redefining xylem as an active participant in plant communication and adaptation.