ER retrieval of Avr9 compromises its elicitor activity consistent with perception of Avr9 at the plasma membrane

The discovery of an endoplasmic reticulum (ER) retrieval motif at the C-terminus of the predicted Cf-9 resistance protein suggested that Cf-9 might function in the ER. To test whether Cf-9 could detect its cognate avirulence protein, Avr9, from the ER, variants of Avr9 with the ER retrieval motifs KDEL or HDEL were transiently expressed in Cf-9 tobacco. Cf-9-mediated necrosis in response to Avr9 tagged at the C-terminus with the KDEL motif was substantially delayed compared with unmodified Avr9. Interestingly, necrosis was completely abolished when Avr9 was tagged with the HDEL motif. Introduction of the S5A mutation, which destroys the sole N-linked glycosylation site of Avr9, ruled out the possibility that an ER-specific pattern of glycosylation prevented a response to the KDEL or HDEL variants of Avr9. These results suggest that Avr9 is perceived at the plasma membrane. They also suggest that HDEL is a more effective ER retrieval motif than KDEL in tobacco.