XopR T3SS-effector of Xanthomonas oryzae pv. oryzae suppresses cell death-mediated plant defense response during bacterial blight development in rice.

Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial blight disease that limits the rice production globally. The bacterium secretes effector proteins directly into plant cells through a type III secretion system (T3SS). Here, we examined the role of a conserved XopR T3SS-effector in the suppression of host basal defense response. Phylogenetic and sequence analysis showed that XopR is well conserved within Xoo strains but shares varying degree of similarity among the other Xanthomonas species. The expression of XopR was shown to be regulated by hrpX, a key regulator of hrp cluster. For functional analysis we employed two mutant strains of Xoo, one lacks xopR gene and other lacks hrpX gene (making the strain defective in T3SS). Programmed cell death (PCD) events was examined both in rice and tobacco leaves through trypan blue staining method. In XopR expressing tobacco leaves the PCD induction was compromised. We observed higher PCD on rice leaves inoculated with Xoo mutants lacking either xopR or functional T3SS as compared to wild type. Contrary, when xopR gene was complemented in mutated strain the PCD was suppressed which clearly suggests that XopR acts as suppressor of the PCD mediated defense response. The EYFP::XopR fusion protein was shown to be localized to the plasma membrane of Nicotiana benthamiana and onion epidermal cells. Altogether our study leads to the understanding that XopR T3SS-effector is essential for Xoo to suppress PCD, primarily to support the in planta colonization of Xoo during blight pathogenesis.