In our previous studies, we found a new Zeitlupe (ZTL) type F-box protein which is expressed at a higher level upon avirulent pathogen infection (Bozkurt et al., 2007). F-box proteins mark the proteins to be degraded through 26S proteasome system by ubiquitination. Since the information on the role of ubiquitin mediated proteolysis in disease responses is advancing rapidly, we sought to understand the way which F-box functions in resistance response as part of ubiquitin-proteasome pathway. Interestingly, in response to silencing of this F-box gene via BSMV mediated virus induced gene silencing (VIGS) method, barley plants lost resistance towards avirulent pathogen race. The Pallas-01 line having Mla1 R-gene showed hyphae formations when inoculated with avirulent powdery mildew race, Bgh103, after 4-fold silencing. This observation suggests that F-box protein functions as a positive regulator in powdery mildew disease mechanism and broadens the function of ZTL-type F-box proteins, previously known to have roles only in circadian clocks, flowering time control, and phytochrome pathway. (C) 2009 Elsevier Ltd. All rights reserved.