The Rice XA21 Ectodomain Fused to the Arabidopsis EFR Cytoplasmic Domain Confers Resistance toXanthomonas oryzaepv.oryzae

Rice (Oryza sativa) plants expressing the XA21 cell surface receptor kinase are resistant toXanthomonas oryzaepv.oryzae(Xoo) infection. We previously demonstrated that expressing a chimeric protein containing the EFR (<underline>E</underline>LONGATION<underline>F</underline>ACTOR Tu<underline>R</underline>ECEPTOR) ectodomain and the XA21 endodomain (EFR:XA21) in rice does not confer robust resistance toXoo.

To test if the XA21 ectodomain is required forXooresistance, we produced transgenic rice lines expressing a chimeric protein consisting of the XA21 ectodomain and EFR endodomain (XA21:EFR) and inoculated these lines withXoo. We also tested if the XA21:EFR rice plants respond to a synthetic sulfated 21 amino acid derivative (RaxX21-sY) derived from the activator of XA21-mediated immunity, RaxX.

We found that five independently transformed XA21:EFR rice lines displayed resistance toXooas measured by lesion length analysis, and showed that five lines express markers of the XA21 defense response (generation of reactive oxygen species and defense response gene expression) after treatment with RaxX21-sY.

Our results indicate that expression of the XA21:EFR chimeric receptor in rice confers resistance toXoo. These results suggest that the endodomain of the EFR and XA21 immune receptors are interchangeable and the XA21 ectodomain is the key determinant conferring robust resistance toXoo.