Identification and Expression Profiling of the PSK Gene Family in Sorghum Under Salt Stress

Soil salinization has intensified globally due to climate warming and unsustainable agricultural practices such as improper irrigation and fertilization, leading to a continuous expansion of saline-alkali land area. Salt stress severely reduces crop yield, posing a major obstacle to agricultural modernization. Phytosulfokine ( PSK ) is a class of peptide hormones encoded by genes and post-translationally processed into mature polypeptides. It plays crucial roles in plant growth, development, and stress responses. The mature PSK peptide contains two sulfonated tyrosine residues, which are essential for its biological activity. Sorghum, originating from Africa, is a stress-resilient coarse grain crop characterized by drought tolerance, salt tolerance, and adaptability to poor soils, making it an ideal model for studying crop responses to salt stress. In this study, seven PSK genes were identified from the sorghum genome, distributed across five chromosomes, with considerable variation in amino acid length among members. The sorghum PSK gene family can be divided into two subfamilies. All members contain 2–3 introns, a feature distinct from many other small peptide genes. A conserved C-terminal pentapeptide sequence (DYIYTQ) was identified in all members, which is critical for PSK function. Promoter analysis revealed that the PSK1 promoter harbors numerous stress- and hormone-responsive cis-elements. Transcriptome expression analysis indicated that PSK1 is highly expressed in roots, stems, and leaves, suggesting its important role in sorghum. RT-qPCR analysis showed that the expression of all PSK members was downregulated under salt stress over time. This study provides foundational insights into the characteristics of the PSK gene family and its response to salt stress in sorghum, facilitating further research on the role of PSK in salt tolerance mechanisms.