Three Proteins (Hpa2, HrpF and XopN) Are Concomitant Type III Translocators in Bacterial Blight Pathogen of Rice

Plant PI4P is required for bacteria to translocate type-3 effectors

Type-3 effectors (T3E) of phytopathogenic Gram-negative bacteria fulfill a virulent role, causing disease, or an avirulent role, inducing immunity, following their translocation into plant cells. This study aimed to validate the hypothesis that bacterial T3E translocation requires lipidic compounds in plant cell membranes. Based on genetic, molecular, and biochemical assays, we determined that phosphatidylinositol 4-phosphate (PI4P) associated with plant cell membranes is essential for the translocation of T3E by bacterial pathogens. Replicate experimental data revealed that PI4P cooperates with the type-3 translocase HrpF to facilitate the translocation of effectors TAL and Xop from Xanthomonas oryzae and Hop from Pseudomonas syringae into the cells of Oryza sativa and Nicotiana benthamiana, respectively. Genetic and molecular analyses confirmed that, once translocated into plant cells, the distinct effectors induce disease or immunity. Combined genetic and pharmacological analyses revealed that when PI4P content is suppressed via genetic or pharmacological measures, the T3 effector translocation is considerably suppressed, resulting in serious inhibition of bacterial infection. Overall, these findings demonstrate that cooperative functioning of HrpF-PI4P is conserved in bacterial effectors and plants.

Functional modulation of an aquaporin to intensify photosynthesis and abrogate bacterial virulence in rice

Plant aquaporins are a recently noted biological resource with a great potential to improve crop growth and defense traits. Here, we report the functional modulation of the rice (Oryza sativa) aquaporin OsPIP1;3 to enhance rice photosynthesis and grain production and to control bacterial blight and leaf streak, the most devastating worldwide bacterial diseases in the crop. We characterize OsPIP1;3 as a physiologically relevant CO₂ -transporting facilitator, which supports 30% of rice photosynthesis on average. This role is nullified by interaction of OsPIP1;3 with the bacterial protein Hpa1, an essential component of the Type III translocon that supports translocation of the bacterial Type III effectors PthXo1 and TALi into rice cells to induce leaf blight and streak, respectively. Hpa1 binding shifts OsPIP1;3 from CO₂ transport to effector translocation, aggravates bacterial virulence, and blocks rice photosynthesis. On the contrary, the external application of isolated Hpa1 to rice plants effectively prevents OsPIP1;3 from interaction with Hpa1 secreted by the bacteria that are infecting the plants. Blockage of the OsPIP1;3-Hpa1 interaction reverts OsPIP1;3 from effector translocation to CO₂ transport, abrogates bacterial virulence, and meanwhile induces defense responses in rice. These beneficial effects can combine to enhance photosynthesis by 29-30%, reduce bacterial disease by 58-75%, and increase grain yield by 11-34% in different rice varieties investigated in small-scale field trials conducted during the past years. Our results suggest that crop productivity and immunity can be coordinated by modulating the physiological and pathological functions of a single aquaporin to break the growth-defense tradeoff barrier.