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Nitrogen availability is one of the critical determinants of agricultural yield. Biological nitrogen fixation, such as legume–rhizobia symbiotic association, might function as a solution to fix nitrogen. Using phytosulfokine (PSK)-α sequences as a query, Yu et al., 2022 performed a comprehensive genome-wide search of legume species to identify PSK-δ, a divergent pentapeptide differing in single amino acid. Furthermore, PSK-δ exhibited nodule-specific expression with lower expression in the root, substantiating the nodule-specific temporal expression and suggesting its role in nodule development and nitrogen fixation. Additionally, in planta functional characterization in <i>Medicago truncatula</i> using overexpression and <i>Tnt1</i>-insertion mutant analysis indicated the role of PSK-δ in symbiotic nodulation. Interestingly, a similar phenotype of <i>MtPSKδ</i> mutant (<i>mtpskδ</i>) with that of wild-type control led to the hypothesis of its functional redundancy with PSK-α in nodule organogenesis. Further investigation regarding its position in the Nod-factor signaling pathway revealed the downstream function of PSK-δ in association with <i>MtENOD11</i> in regulating nodule formation.