Mutation of the rice XA21 predicted nuclear localization sequence does not affect resistance to Xanthomonas oryzae pv. oryzae

Abstract

Background. The rice receptor kinase XA21 confers robust resistance to the bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo). We previously reported that XA21 is cleaved in transgenic plants overexpressing XA21 with a GFP tag (Ubi-XA21-GFP) and that the released C-terminal domain is localized to the nucleus. XA21 carries a predicted nuclear localization sequence (NLS) that directs the C-terminal domain to the nucleus in transient assays, whereas alanine substitutions in the NLS disrupt the nuclear localization. Methods. To determine if the predicted NLS is required for XA21-mediated immunity in planta, we generated transgenic plants overexpressing an XA21 variant carrying the NLS with the same alanine substitutions (Ubi-XA21nls-GFP). Results. Ubi-XA21nls-GFP plants displayed slightly longer lesion lengths, higher Xoo bacterial populations after inoculation and lower levels of reactive oxygen species production compared with the Ubi-XA21-GFP control plants. However, the Ubi-XA21nls-GFP plants express lower levels of protein than that observed in Ubi-XA21-GFP. Discussion. These results demonstrate that the predicted NLS is not required for XA21-mediated immunity.