Phytosulfokine-α requires auxin to stimulate carrot non-embryogenic cell proliferation

Phytosulfokine-α (PSK-α), a peptidyl plant growth factor that was originally isolated from conditioned medium derived from asparagus mesophyll cell culture, stimulates cell proliferation in some plants in suspension culture. However, the physiological relationship between phytohormones and this growth factor in plants is not well understood. Using carrot (Daucus carota L.) non-embryogenic cell culture, we investigated the stimulatory effects of PSK-α on the cell proliferation induced by different combinations and concentrations of auxin [2,4-dichlorophenoxyacetic acid (2,4-D), indole-3-acetic acid (IAA), or naphthaleneacetic acid (NAA)] and PSK-α. Cell proliferation was not stimulated by PSK-α at high auxin concentrations, but was promoted at low auxin concentrations. There was no proliferation in the absence of auxin. These results indicate that PSK-α stimulates cell proliferation when cell division activity is low, and that auxin is required for this process. We also evaluated PSK-α production using medium derived from non-embryogenic cell culture with high or low concentrations of auxin (2,4-D, IAA or NAA) in a competition ELISA system. More PSK-α was produced at high auxin concentrations than at low auxin concentrations. These results suggest that auxin participates in PSK-α production.