Plasmodesmata targeting and intercellular trafficking of Tomato spotted wilt tospovirus movement protein NSm is independent of its function in HR induction

The movement protein NSm of Tomato spotted wilt tospovirus (TSWV) plays pivotal roles in viral intercellular trafficking. Recently, the TSWV NSm was also identified as an avirulence (Avr) determinant during the Sw-5b-mediated hypersensitive response (HR). However, whether the cell-to-cell movement of NSm is coupled to its function in HR induction remains obscure. Here, we showed that the NSm mutants defective in targeting plasmodesmata and cell-to-cell movement were still capable of inducing Sw-5b-mediated HR. In addition, introduction of a single amino-acid substitution, C118Y or T120N, identified previously from TSWV resistance-breaking isolates, into the movement-defective NSm mutants resulted in the failure of HR induction. Collectively, our results showed that the intercellular trafficking of NSm is uncoupled from its function in HR induction. These findings shed light on the evolutionary mechanism of R-Avr recognition and may be used to explain why this uncoupled phenomenon can be observed in many different viruses.