Functional interplay between two Xanthomonas oryzae pv. oryzae secretion systems in modulating virulence on rice

The type II (US) and type III (T3S) secretion systems are important for virulence of Xanthomonas oryzae pv. oryzae, causal agent of bacterial leaf blight of rice. The T3S of gramnegative bacterial plant pathogens has been shown to suppress host defense responses, including programmed cell death reactions, whereas the T2S is involved in secreting cell-wall-degrading enzymes. Here, we show that a T3S-deficient (T3S(-)) mutant of X. oryzae pv. oryzae can induce a basal plant defense response seen as callose deposition, immunize rice against subsequent X. oryzae pv. oryzae infection, and cause cell-death-associated nuclear fragmentation. A T2S(-)T3S(-) double mutant exhibited a substantial reduction in the ability to evoke these responses. We purified two major effectors of the X. oryzae pv. oryzae T2S and characterized them to be a cellulase (CIsA) and a putative cellobiosidase (CbsA). The purified CIsA, CbsA, and lipase/esterase (LipA; a previously identified T2S effector) proteins induced rice defense responses that were suppressible by X. oryzae pv. oryzae in a T3S-dependent manner. These defense responses also were inducible by the products of the action of these purified proteins on rice cell walls. We further show that a CbsA(-) mutant or a ClsA(-) LipA(-) double mutant are severely virulence deficient. These results indicate that the X. oryzae pv. oryzae T2S secretes important virulence factors, which induce innate rice defense responses that are suppressed by T3S effectors to enable successful infection.