The IDA cell separation pathway connects developmental and defense responses

The abscission of floral organs and emergence of lateral roots in Arabidopsis is regulated by the peptide ligand INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) and the receptor protein kinases HAESA (HAE), HAESA-LIKE 2 (HSL2) and members of the SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK) family. These cell separation processes lead to induction of defense-associated genes to protect against pathogen invasion. However, the molecular coordination between abscission and immunity has not been thoroughly explored. Here we show that IDA induces a receptor-dependent release of cytosolic calcium and an extracellular release of reactive oxygen species which are signatures of defense responses. IDA promotes heteromerization between HSL2 and RECEPTOR LIKE KINASE 7 (RLK7), a receptor that enhances immunity and pathogen responses, and utilizes this novel signaling module to regulate the expression of defense-associated genes. We propose a molecular mechanism by which IDA drives specific immune response in cells destined for separation to guard them from pathogen attack.