Phytosulfokine‐α regulates auxin‐dependent somatic embryogenesis through calcium signaling in hybrid sweetgum

SUMMARY Phytosulfokine‐α (PSK‐α) is a key endogenous pentapeptide that regulates plant growth and development and is known to increase the efficiency of somatic embryogenesis (SE) in various tree species. However, its molecular regulatory mechanisms remain poorly characterized. Exogenous application of PSK‐α significantly increased the SE efficiency of hybrid sweetgum ( Liquidambar styraciflua × L. formosana ) embryogenic callus (EC). Phylogenetic analysis revealed high conservation of genes encoding PSK‐α precursors among dicots, suggesting that PSK is a conserved signaling molecule that modulates growth and development in hybrid sweetgum. Consistent with these observations, we found that PSK receptors (PSKR) localize to the plasma membrane, where they recognize their ligand PSK‐α and initiate downstream signaling. Further studies revealed that PSK‐α application triggered a transient increase in intracellular calcium ion (Ca 2+ ) concentration and upregulated the expression of genes encoding calmodulin (CaM) in the EC. Protein interaction assays confirmed that LsfCaM6 interacts with LsfPSKR1. Loss‐of‐function studies demonstrated that silencing LsfCaM genes ( LsfCaM1/5/6 ) significantly inhibited the embryogenic capacity of the EC. Protein interaction assays confirmed that LsfCaM6 interacts with LsfYUCCA (LsfYUC), which promotes auxin (IAA) accumulation and thus mediates PSK‐α‐regulated SE. In summary, this study reveals a molecular framework in which the plant peptide PSK‐α initiates auxin‐dependent SE through Ca 2+ signaling in trees, providing valuable insights for further research on plant peptides in regeneration biology.