Receptor-like serine/threonine-kinase acting on substrates that controls floral organ abscission. Regulated by the 'INFLORESCENCE DEFICIENT IN ABSCISSION' (IDA) family of ligands. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Expression is restricted to the abscission zone. Expression increases shortly before the onset of abscission. No visible phenotype; due to the redundancy with RLK5/HAE. Hae and hsl2 double mutants have a strong abscission defect. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.