Promotes plant cell differentiation, organogenesis and somatic embryogenesis as well as cell proliferation. Expression is regulated by a signal transduction pathway activated by auxin and cytokinin. Sulfation is important for activity and for the binding to a putative membrane receptor. Deletion of the sulfate groups of Tyr-67 and Tyr-69 resulted in compounds with respectively 0.6% and 4% of the activity. 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.