Plant pathogens secrete an arsenal of effector
proteins to impair host immunity. Some effectors possess enzymatic activities that can modify their host targets. Previously, we demonstrated that a Phytophthora sojae RXLR effector Avr3b acts as a
Nudix hydrolase when expressed in planta; and this enzymatic activity is required for full virulence of P. sojae strain P6497 in soybean (Glycine max). Interestingly, recombinant Avr3b produced by E. coli does not have the
hydrolase activity unless it was incubated with
plant protein extracts. Here, we report the activation of Avr3b by a prolyl-peptidyl
isomerase (
PPIase),
cyclophilin, in plant cells. Avr3b directly interacts with soybean
cyclophilin GmCYP1, which activates the
hydrolase activity of Avr3b in a
PPIase activity-dependent manner. Avr3b contains a putative
Glycine-
Proline (GP) motif; which is known to confer
cyclophilin-binding in other
protein substrates. Substitution of the
Proline (P132) in the putative GP motif impaired the interaction of Avr3b with GmCYP1; as a result, the mutant Avr3bP132A can no longer be activated by GmCYP1, and is also unable to promote Phytophthora
infection. Avr3b elicits hypersensitive response (HR) in soybean cultivars producing the resistance
protein Rps3b, but Avr3bP132A lost its ability to trigger HR. Furthermore, silencing of GmCYP1 rendered reduced cell death triggered by Avr3b, suggesting that GmCYP1-mediated Avr3b maturation is also required for Rps3b recognition. Finally,
cyclophilins of Nicotiana benthamiana can also interact with Avr3b and activate its enzymatic activity. Overall, our results demonstrate that
cyclophilin is a "helper" that activates the enzymatic activity of Avr3b after it is delivered into plant cells; as such,
cyclophilin is required for the avirulence and virulence functions of Avr3b.