The COP9 (constitutive photomorphogenesis 9) signalosome (CSN) is a protein 
complex involved in the ubiquitin proteasome system and a common host target of 
diverse pathogens in Arabidopsis. The known derubylation function of the COP9 
complex is carried out by subunit 5 encoded by AtCSN5A or AtCSN5B in 
Arabidopsis. A single CSN5-like gene (designated as TaCSN5) with three 
homeologues was identified on the long arms of wheat (Triticum aestivum L.) 
group 2 chromosomes. In this study, we identified and characterized the function 
of TaCSN5 in response to infection by the leaf rust pathogen. Down-regulation of 
all three TaCSN5 homeologues or mutations in the homeologues on chromosomes 2A 
or 2D resulted in significantly enhanced resistance to leaf rust. Enhanced leaf 
rust resistance corresponded to a seven-fold increase in PR1 
(pathogenesis-related gene 1) expression. Collectively, the data indicate that 
the wheat COP9 subunit 5-like gene acts as a negative regulator of wheat leaf 
rust resistance.