Fig. 1

Categorization of peptide hormone families in plants, based on their structures and biosynthetic pathways. Peptide families in plants can be categorized into four distinct groups based on their functions, structures, and biosynthetic pathways. Peptides that undergo post-translational modification (PTM) and exhibit cysteine-richness (Cys-rich) are secreted via the conventional protein secretion (CPS) pathway. In this pathway, preprotein precursors are initially processed in the endoplasmic reticulum (ER), subsequently transported to the Golgi apparatus, and finally secreted into the apoplast through the endomembrane system. In contrast to the CPS pathway, precursors derived from immunoreactive peptides (such as systemin and Pep) and small open reading frames (sORFs) or microRNAs are secreted via the unconventional protein secretion (UPS) pathway, which includes vacuoles, Exocyst-positive organelles (EXPOs), and multivesicular bodies (MVBs). Major references to classification schemes for these polypeptide families are presented, including ^aCPS and ^aUPS referenced from Wang X., 2018 (2018); ^bPTM and ^bCys-rich from Matsubayashi 2014 (2014); ^cImmunoreaction from Del Corpo D., 2024 (2024); ^dsORF/microRNA from Tavormina P., 2015 (2015); ^eCLE from Ohyama K., 2009 (2009); ^fRALF from Murphy E., 2014 (2014); ^gPEP from Huffaker A., 2006 (2006); ^hENOD40 from Campalans A., 2004 (2004)