Genome-wide identification of the PSK family and the association between SlPSK7 and auxin signaling in tomato

Background Phytosulfokine (PSK) is a tyrosine-disulfated pentapeptide with the core sequence YIYTQ that acts as a peptide hormone regulating plant growth, development, and stress responses by mediating intercellular communication. However, the PSK gene family has not been systematically characterized in tomato ( Solanum lycopersicum ). In this study, we performed a genome-wide identification of the PSK family in tomato and investigated the relationship between SlPSK7 and auxin signaling during fruit set. Results Eight SlPSK genes were identified in the tomato genome, and their conserved domains were predominantly located near the C-terminus. Phylogenetic analysis showed that PSK genes from evolutionarily related species clustered together, supporting the conserved roles of this family during plant evolution. Tissue-specific expression analysis revealed distinct expression patterns among SlPSK members, with SlPSK7 predominantly expressed in floral buds. Under exogenous treatments with the synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D), the ethylene biosynthesis inhibitor aminoethoxyvinylglycine (AVG), and the auxin transport inhibitor N-1-naphthylphthalamic acid (NPA), the expression pattern of SlPSK7 was highly consistent with that of auxin response factor (ARF) genes but opposite to that of IAA9 . In transient overexpression assays, elevated SlPSK7 expression at anthesis (0 d) was accompanied by increased transcript levels of ARF family genes, particularly ARF7 (up to 5.85-fold, P < 0.01); conversely, VIGS-mediated silencing of SlPSK7 significantly downregulated ARF family gene expression (to 0.34-fold, P < 0.05). Conclusions Eight PSK family members were identified in the tomato genome, and SlPSK7 was shown to be closely associated with ARF family genes. The expression pattern of SlPSK7 parallels that of the ARF family, the core positive regulators of auxin signaling, suggesting that SlPSK7 may act as a positive regulator of the auxin pathway. These findings provide a basis for further functional characterization of SlPSK7 in tomato fruit set regulation.