Promotes plant cell differentiation, organogenesis and somatic embryogenesis as well as cell proliferation. Expressed throughout the seedling. More abundant in fragments containing shoot or root apexes where cells proliferate vigorously. Sulfation is important for activity and for the binding to a putative membrane receptor. PSK-alpha is produced by endopeptidase digestion. PSK-beta is produced from PSK-alpha by exopeptidase digestion. The N-terminal tripeptide (YIY) is the active core. Belongs to the phytosulfokine family.