The COP9 (Constitutive photomorphogenesis 9) signalosome (CSN) is a highly conserved protein complex that influences several signaling and developmental processes. The COP9 signalosome consists of eight subunits, among which two subunits, CSN5 and CSN6, contain an Mpr1/Pad1 N-terminal (MPN) domain and the remaining six subunits contain a proteasome, COP9 signalosome, and initiation factor 3 (PCI) domain. In plants, each MPN subunit is encoded by two genes, which is not the case in other organisms. This review aims to provide in-depth knowledge of each COP9 signalosome subunit, concentrating on genetic analysis of both partial and complete loss-of-function mutants. At the beginning of this review, the role of COP9 signalosome in the hormonal signaling and defense is discussed, whereas later sections deal in detail with the available partial loss-of-function, hypomorphic mutants of each subunit. All available hypomorphic mutants are compared based on their growth response and deneddylation activity.
Keywords: COP9 signalosome; CSN subunit; abiotic stress; biotic stress; hormonal signaling; hypomorphic mutants.