In studies of shoot regeneration in Arabidopsis tissue culture, we found that expression of a subtilase gene AtSBT1.1 (At1g01900) correlated with conditions for efficient shoot regeneration (Lall et al., 2004) . It was not clear what the causal connection might be between efficient shoot regeneration and expression of a gene encoding a serine protease. Most subtilases are predicted to be secreted proteins (Rautengarten et al., 2005) , so we reasoned that AtSBT1.1 could be involved in processing of an extracellular growth factor or receptor related to shoot regeneration. Growth factors that might require the action of AtSBT1.1 include peptide hormones, such as AtPSKs. AtPSKs promote callus formation in tissue culture , and are synthesized as preproproteins with signal peptides that target them to the secretory pathway and with prosequences that are processed during maturation of the peptide hormone.