The proteolytic processing of PSK4 appears to be highly regulated in the tissue culture system -in part by up-regulation of AtSBT1.1 gene expression. Among the 56 subtilases encoded in the Arabidopsis genome, AtSBT1.1 appears to be solely responsible for the release of PSK4 from its precursor in roots, because pAtPSK4-myc is not cleaved in AtSBT1.1 knock-out lines. PSKs are growth factors that stimulate callus formation in culture Sakagami, 1996, 2006) . Thus, up-regulation of AtSBT1.1 and the release of AtPSK4 from its precursor protein may be key factors in promoting the growth of cells and callus formation in tissue culture.