Dear Editor,The general way to probe functions of a protein in vivo is to perturb its level and then observe subsequent phenotypic changes.In plants,modulation of protein level is mainly carried out at DNA or RNA level,which is indirect and thus affected by stability of the target protein.Thus,experimental approaches to perturb protein level directly are needed,but still limited in plants.In mammalian cells,a technique to modulate protein level directly has been developed.Engineered destabilizing domain (DD) of the human FKBP12 protein can confer instability to other proteins when fused to it.A small synthetic molecule ligand Shield 1 (Shld1) can bind DD and shield it from degradation.The level of DD fused protein can be controlled by adjusting Shld1 concentration (Banaszynski et a1.,2006).The DD-Shld1 system was further successfully applied in several other species,such as parasites Plasmodium falciparum (Armstrong and Goldberg,2007) and Toxoplasma gondii (Herm-G(o)tz et al.,2007).