The molecular mechanism by which bacterial avirulence genes mediate re
cognition by resistant host plants has been enigmatic for more than a
decade. In this paper we provide evidence that the Xanthomonas campest
ris pv. vesicatoria avirulence protein AvrBs3 is recognized inside the
plant cell. Transient expression of avrBs3 in pepper leaves, using Ag
robacterium tumefaciens for gene delivery, results in hypersensitive c
ell death, specifically on plants carrying the resistance gene Bs3. In
addition, for its intracellular recognition, AvrBs3 requires nuclear
localization signals that are present in the C-terminal region of the
protein. We propose that AvrBs3 is translocated into plant cells via t
he Xanthomonas Hrp type III secretion system and that nuclear factors
are involved in AvrBs3 perception.