Mobile gene silencing inArabidopsisis regulated by hydrogen peroxide

In plants and nematodes, RNAi can spread from cells from which it is initiated to other cells in the organism. The underlying mechanism controlling the mobility of RNAi signals is not known, especially in the case of plants. A genetic screen designed to recover plants impaired in the movement but not the production or effectiveness of the RNAi signal identified RCI3 , which encodes a hydrogen peroxide (H 2 O 2 )-producing type III peroxidase, as a key regulator of silencing mobility in Arabidopsis thaliana . Silencing initiated in the roots of rci3 plants failed to spread into leaf tissue or floral tissue. Application of exogenous H 2 O 2 reinstated the spread in rci3 plants and accelerated it in wild-type plants. The addition of catalase or MnO 2 , which breaks down H 2 O 2 , slowed the spread of silencing in wild-type plants. We propose that endogenous H 2 O 2 , under the control of peroxidases, regulates the spread of gene silencing by altering plasmodesmata permeability through remodelling of local cell wall structure, and may play a role in regulating systemic viral defence.