The endogenous sulfated pentapeptide phytosulfokine-alpha stimulates tracheary element differentiation of isolated mesophyll cells of zinnia

Dispersed zinnia (Zinnia elegans) mesophyll cells cannot differentiate into tracheary elements (TEs) at low cell density conditions even if auxin and cytokinin are present in the medium, indicating the involvement of intercel lular interactions during the initiation and/or subsequent progresses in TE differentiation. When zinnia cells were incubated at a low density (2.5 x 10(4) cells mL(-1)) in TE-inductive medium in the presence of various conce ntrations of phytosulfokine (PSK)-alpha, which was originally identified as an intercellular signal peptide involved in cell proliferation, TE differe ntiation was strongly stimulated in a dose-dependent fashion; more than 35% of the living cells differentiated into TEs by 5 d of culture in the prese nce of 10 nM PSK-alpha. Enzyme-linked immunosorbent assay and mass spectros copy confirmed that cultured zinnia cells produce nanomolar levels of PSKs under inductive conditions. These results suggest that PSK-alpha is a facto r responsible for TE differentiation of zinnia mesophyll cells.