Abstract
Agrobacterium tumefaciens uses the type IV secretion system, which consists of VirB1-B11 and VirD4 proteins, to deliver effectors into plant cells. The effectors manipulate plant proteins to assist in T-DNA transfer, integration, and expression in plant cells. The Arabidopsis reticulon-like (RTNLB) proteins are located in the endoplasmic reticulum and are involved in endomembrane trafficking in plant cells. The rtnlb4 mutants were recalcitrant to A. tumefaciens infection, but overexpression of RTNLB4 in transgenic plants resulted in hypersusceptibility to A. tumefaciens transformation, which suggests the involvement of RTNLB4 in A. tumefaciens infection. The expression of defense-related genes, including FRK1, PR1, WRKY22, and WRKY29, were less induced in RTNLB4 overexpression (O/E) transgenic plants after A. tumefaciens elf18 peptide treatment. Pretreatment with elf18 peptide decreased Agrobacterium-mediated transient expression efficiency more in wild-type seedlings than RTNLB4 O/E transgenic plants, which suggests that the induced defense responses in RTNLB4 O/E transgenic plants might be affected after bacterial elicitor treatments. Similarly, A. tumefaciens VirB2 peptide pretreatment reduced transient T-DNA expression in wild-type seedlings to a greater extent than in RTNLB4 O/E transgenic seedlings. Furthermore, the VirB2 peptides induced FRK1, WRKY22, and WRKY29 gene expression in wild-type seedlings but not efr-1 and bak1 mutants. The induced defense-related gene expression was lower in RTNLB4 O/E transgenic plants than wild-type seedlings after VirB2 peptide treatment. These data suggest that RTNLB4 may participate in elf18 and VirB2 peptide-induced defense responses and may therefore affect the A. tumefaciens infection process.
Highlights
The typical type IV secretion system (T4SS)-containing phytopathogenic bacterium Agrobacterium tumefaciens is well known for its ability to transfer DNA into plant cells
The PAMP-triggered immune response includes activation of MAPK signaling cascades, transcriptional reprogramming mediated by WRKY transcription factors, reactive oxygen species (ROS) burst, and other defense responses
We observed that when RTNLB4 level was abnormal in plants, the elf18-induced plant defense responses were alleviated, so RTNLB4 may be involved in the elf18-derived immune response
Summary
The typical type IV secretion system (T4SS)-containing phytopathogenic bacterium Agrobacterium tumefaciens is well known for its ability to transfer DNA into plant cells. FLS2 forms a complex with BRASSINOSTEROID INSENSITIVE 1-associated receptor kinase 1 (BAK1), activating the downstream mitogen-activated protein kinases MPK3 and MPK6, which induces several regulatory factors, WRKY22, WRKY29, FLG22-INDUCED RECEPTOR-LIKE KINASE 1 (FRK1), other defense-related genes, and reactive oxygen species (ROS) production [26,27,28,29,30,31,32] Another well-known PRR is the Arabidopsis elongation factor-thermo unstable (EF-Tu) receptor (EFR) that activates the plant immune response by recognizing the EF-Tu of A. tumefaciens [33]. The two peptides induced relatively higher expression of several defense-related genes, including FRK1, WRKY22, WRKY29, MPK3, and MPK6, in wild-type plants than in RTNLB4 O/E transgenic plants. RTNLB4 may have important roles in A. tumefaciens elf and VirB2 peptide-induced plant defense responses
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