Genetic factors identified in short-distance movement of the silencing signal in the SUC- PDS system . (a) The photobleaching phenotype seen in the leaves of JAP plants. (b) The SUC-PDS transgene (an inverted-repeat construct using the phloem-specific promoter from the SUC2 gene to target the PHYTOENE DESATURASE 3 (PDS) endogenous mRNAs) is expressed in companion cells and produces a silencing signal that is dependent on the POL IV pathway (NRPD1-RDR2-CLSY) as well as DCL4 and presumably DCL1 and AGO1. (c) The silencing signal moves to the mesophyll cells and produces photobleaching (a). AGO4 and DCL3 antagonize the spread of the silencing signal and probably act in the cells that produce the silencing signal. Where the POL IV pathway functions and whether DCL4 and AGO4/DCL3 are required in the cells that receive a silencing signal remains unknown. The circle indicates the nucleus and the red square corresponds to plasmodesmata. Aspects of this process that remain unknown are indicated by question marks.