Chemical nursing: phytosulfokine improves genetic transformation efficiency by promoting the proliferation of surviving cells on selective media.

The relative growth rate of plant cells in vitro is considerably affected by initial cell density. This troublesome effect has interfered with the establishment of efficient plant cell culture systems, especially when only a small number of cells are expected to survive, such as in the genetic transformation of cells under antibiotic selection. To improve the recovery of antibiotic-resistant cells, we examined the use of the peptide plant hormone phytosulfokine (PSK), which has been shown to promote cellular growth and development in vitro. The addition of PSK to selective media increased the recovery of transformed callus from Agrobacterium-infected carrot hypocotyl explants from 7% to 39%, which is more than a fivefold improvement over the control. Most calluses developed into normal plantlets with cotyledons and primary roots and, eventually, formed foliage leaves. Thus, chemical nursing using PSK shows promise as a tool for basic research in plant biology and biotechnological applications.