Comparative Transcriptomic Analysis Reveals the Regulatory Role of PSK-δ in Trigonelline Biosynthesis in Trigonella foenum-graecum

Trigonelline is a bioactive pyridine alkaloid in Trigonella foenum-graecum, which is a legume species, and is known for its hypoglycemic and hypolipidemic activities. Phytosulfokine-δ (PSK-δ), a recently identified legume-specific phytosulfokine peptide, has been implicated in the regulation of nodulation and root development. However, whether PSK-δ participates in the regulation of secondary metabolite biosynthesis remains unclear. In this study, exogenous PSK-δ treatment significantly promoted trigonelline accumulation in T. foenum-graecum seedlings, leading to a 93% increase in trigonelline content after 5 days relative to the scrambled-pentapeptide-treated control, whereas a slight downward trend was observed at 15 days. Comparative transcriptomic analyses between PSK-δ-treated seedlings and the corresponding controls at 5 and 15 days identified 15 candidate genes associated with trigonelline biosynthesis and PSK signaling, whose expression patterns were consistent with trigonelline accumulation dynamics. Co-expression network analysis between these 15 candidate genes and 831 transcription factor-encoding genes identified candidate transcription factors potentially involved in the coordinated regulation of trigonelline biosynthesis and PSK signaling. These findings suggest that PSK-δ may serve as a potential peptide-based biostimulant for regulating alkaloid accumulation in medicinal legume cultivation and provide insights into peptide hormone-mediated regulation of plant metabolism.