Abstract
BackgroundPathogenic Gram-negative bacteria interact with their eukaryotic hosts by deploying the type III translocon to inject effector proteins into the cytosol of eukaryotic cells. The translocon compositions, the number and biochemical characteristics of type III translocators in animal-pathogenic bacteria have been well elucidated, but information is lacking for plant-pathogenic bacteria. With extensive studies on biological functions of the Hpa1 protein secreted by the type III secretion system in Xanthomonas oryzae pv. oryzae (Xoo), we show here that Hpa1 is a type III translocator based on measurements of two proteins categorized as transcription activator-like (TAL) effector.ResultsHpa1 was functionally associated with the TAL effector PthXo1 or AvrXa10 by genetic analysis of the wild-type Xoo strain and related mutants or recombinant strains. Inoculation experiments suggested that Hpa1 is required not only for the virulent role of PthXo1 in the susceptible rice variety Nipponbare, but also for the avirulent function of AvrXa10 on the resistant rice variety IRBB10. Hpa1 is unrelated to the secretion of PthXo1 and AvrXa10 out of bacterial cells. However, Hpa1 is critical for both TAL effectors to be translocated from bacterial cells into the cytosol of rice cells based on replicate experiments performed on the susceptible and resistant varieties, respectively. Hpa1-mediated translocation of PthXo1 is coincident with induced expression of rice SWEET11 gene, which is the regulatory target of PthXo1, resulting in the occurrence of the bacterial blight disease in the susceptible rice variety. By contrast, the immune hypersensitive response is induced in agreement with induced expression of rice Xa10 gene, which is the target of AvrXa10, only when AvrXa10 is translocated from bacteria into cells of the resistant rice variety. All the virulent or avirulent performances of the TAL effectors are nullified by directed mutation that removes the α-helix motif from the Hpa1 sequence.ConclusionsThe genetic and biochemical data demonstrate that Hap1 is a type III translocator at least for TAL effectors PthXo1 and AvrXa10. The effect of the directed mutation suggests that Hpa1 depends on its α-helical motif to fulfil the translocator function.
Highlights
Pathogenic Gram-negative bacteria interact with their eukaryotic hosts by deploying the type III translocon to inject effector proteins into the cytosol of eukaryotic cells
Hpa1 contributes to the virulent role of PthXo1 on rice variety Nipponbare In order to analyze the functional relationship between Hpa1 and PthXo1, we generated PXO99 mutants Δhpa1, ΔpthXo1, and Δhpa1ΔpthXo1 by sequence deletion and generated Δhpa1/hpa1, ΔpthXo1/pthXo1, and Δhpa1ΔpthXo1/hpa1/pthXo1 strains by genetic complementation (Fig. 1)
In leaf-top-clipping (Fig. 1a) and leaf-center-infiltrating (Fig. 2a) inoculation experiments performed on the susceptible rice variety Nipponbare, these bacteria displayed extensive variations in virulence levels
Summary
Pathogenic Gram-negative bacteria interact with their eukaryotic hosts by deploying the type III translocon to inject effector proteins into the cytosol of eukaryotic cells. Many effector proteins of Gram-negative bacteria, which are plant [1] or animal [2, 3] pathogens, are secreted by the type III secretion system (T3SS). Effectors must be translocated into the cytosol of eukaryotic cells in order to fulfill virulent or avirulent roles [1, 4]. All hydrophilic translocators so far identified in plantpathogenic bacteria are characteristic of harpins, which. Wang et al BMC Microbiology (2018) 18:105 belong to T3SS accessory proteins and contain a unitary hydrophilic domain or an additional enzymatic domain [5, 8,9,10,11,12]. Eukaryotic PM sensors, which are either lipids [2, 3, 15] or proteins [13, 14], are assumed to be indispensable for translocator recognition and translocon formation [2, 3, 5, 6]
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