Members of the XB3 Family from Diverse Plant Species Induce Programmed Cell Death in Nicotiana benthamiana

Programmed cell death has been associated with plant immunity and senescence. The receptor kinase XA21 confers resistance to bacterial blight disease of rice (Oryza sativa) caused by Xanthomonas oryzae pv. oryzae (Xoo). Here we show that the XA21 binding protein 3 (XB3) is capable of inducing cell death when overexpressed in Nicotiana benthamiana. XB3 is a RING finger-containing E3 ubiquitin ligase that has been positively implicated in XA21-mediated resistance. Mutation abolishing the XB3 E3 activity also eliminates its ability to induce cell death. Phylogenetic analysis of XB3-related sequences suggests a family of proteins (XB3 family) with members from diverse plant species. We further demonstrate that members of the XB3 family from rice, Arabidopsis and citrus all trigger a similar cell death response in Nicotiana benthamiana, suggesting an evolutionarily conserved role for these proteins in regulating programmed cell death in the plant kingdom.