Serine protease. Has a substrate preference for the hydrophobic residues Phe and Ala and the basic residue Asp in the P1 position, and for Asp, Leu or Ala in the P1' position (By similarity). Interferes with CLAVATA 3 (CLV3) signaling, but does not cleave CLV3. Expressed in the vasculature of roots and leaves, stomata, sepals, stigma, anthers and siliques. No visible phenotype under normal growth conditions. Plants over-expressing SBT5.4 show a clavata-like phenotype with fasciated inflorescence stems and compounded terminal buds. This phenotype is abolished by mutating Ser-567 to Ala which completely disrupts the catalytic center of the protease (PubMed:19588163). Belongs to the peptidase S8 family. Truncated C-terminus.