Six genes encoding PSKs (AtPSK1-6) have been identified in Arabidopsis. Each encodes a preproprotein precursor of approximately 80 residues, with the YIYTQ peptide near their C-termini . generated transgenic Arabidopsis plants (AtPSK4ox) over-expressing one of the AtPSKs, and found that root growth and callus formation were slightly enhanced in over-expression lines, but otherwise the seedlings were phenotypically indistinguishable from wild-type.