Abstract
Agrobacterium tumefaciens is a specialized plant pathogen that causes crown gall disease and is commonly used for Agrobacterium-mediated transformation. As a pathogen, Agrobacterium triggers plant immunity, which affects transformation. However, the signaling components and pathways in plant immunity to Agrobacterium remain elusive. We demonstrate that two Arabidopsis mitogen-activated protein kinase kinases (MAPKKs) MKK4/MKK5 and their downstream mitogen-activated protein kinases (MAPKs) MPK3/MPK6 play major roles in both Agrobacterium-triggered immunity and Agrobacterium-mediated transformation. Agrobacteria induce MPK3/MPK6 activity and the expression of plant defense response genes at a very early stage. This process is dependent on the MKK4/MKK5 function. The loss of the function of MKK4 and MKK5 or their downstream MPK3 and MPK6 abolishes plant immunity to agrobacteria and increases transformation frequency, whereas the activation of MKK4 and MKK5 enhances plant immunity and represses transformation. Global transcriptome analysis indicates that agrobacteria induce various plant defense pathways, including reactive oxygen species (ROS) production, ethylene (ET), and salicylic acid- (SA-) mediated defense responses, and that MKK4/MKK5 is essential for the induction of these pathways. The activation of MKK4 and MKK5 promotes ROS production and cell death during agrobacteria infection. Based on these results, we propose that the MKK4/5-MPK3/6 cascade is an essential signaling pathway regulating Agrobacterium-mediated transformation through the modulation of Agrobacterium-triggered plant immunity.
Highlights
Agrobacterium tumefaciens is a plant pathogen that causes crown gall disease in a wide range of species (Smith and Townsend, 1907)
These results demonstrate that MPK3/MPK6 activity and defense-responsive gene expression resulted from agrobacteria infection
These results indicate that MPK3/MPK6 activity and defense-responsive gene expression induced by agrobacteria are highly dependent on the MKK4 and MKK5 function, and MKK4 and MKK5 are essential to Agrobacterium-triggered immunity
Summary
Agrobacterium tumefaciens is a plant pathogen that causes crown gall disease in a wide range of species (Smith and Townsend, 1907) This pathogen has the ability to mobilize and integrate a transfer DNA (T-DNA) segment from its tumor-inducing (Ti) plasmid to the host cell genome, leading to the formation of tumorous growths (Escobar and Dandekar, 2003). Throughout the interaction between Agrobacterium and hosts, Agrobacterium infection triggers the innate immune response of the host, which is characterized by the induction of defense gene expression (Ditt et al, 2001, 2006; Veena et al, 2003; Duan et al, 2018) This response is elicited by the perception of pathogen-associated molecular patterns (PAMPs) by specific plant receptors. The detailed mechanism and signaling pathway of plant immunity to Agrobacterium remain unclear
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