Abstract
Plants respond to bacterial infection acutely with actin remodeling during plant immune responses. The mechanisms by which bacterial virulence factors (VFs) modulate plant actin polymerization remain enigmatic. Here, we show that plant-type-I formin serves as the molecular sensor for actin remodeling in response to two bacterial VFs: Xanthomonas campestris pv. campestris (Xcc) diffusible signal factor (DSF), and pathogen-associated molecular pattern (PAMP) flagellin in pattern-triggered immunity (PTI). Both VFs regulate actin assembly by tuning the clustering and nucleation activity of formin on the plasma membrane (PM) at the nano-sized scale. By being integrated within the cell-wall-PM-actin cytoskeleton (CW-PM-AC) continuum, the dynamic behavior and function of formins are highly dependent on each scaffold layer's composition within the CW-PM-AC continuum during both DSF and PTI signaling. Our results reveal a central mechanism for rapid actin remodeling during plant-bacteria interactions, in which bacterial signaling molecules fine tune plant formin nanoclustering in a host mechanical-structure-dependent manner.
Highlights
The actin cytoskeleton (AC) is dynamically remodeled during signal transduction to coordinate the changes of various cellular processes for development and defense mechanisms (Basu et al, 2008; Li and Staiger, 2018; Mostowy and Shenoy, 2015)
We found that pathogen-associated molecular pattern (PAMP) flagellin and QS diffusible signal factor (DSF)-mediated changes in formin clustering and activities are functionally coupled with the structural integrity of the mechanical scaffolds of the cell wall (CW)-PMAC continuum
Arabidopsis actin assembly is highly correlated with the nanoclustering of AtFH6 in response to bacterial DSF and PAMP signals To study the actin remodeling during plant immune signaling, we investigated Arabidopsis actin under the stimulations of two different types of virulence molecules: the PAMP flagellin and the Xanthomonas campestris pv. campestris (Xcc) quorum-sensing molecule DSF (Kakkar et al, 2015; Ryan et al, 2015)
Summary
The actin cytoskeleton (AC) is dynamically remodeled during signal transduction to coordinate the changes of various cellular processes for development and defense mechanisms (Basu et al, 2008; Li and Staiger, 2018; Mostowy and Shenoy, 2015). During host-pathogen interactions, plants perceive a diverse array of microbial virulence factors (VFs), such as the pathogen-associated molecular patterns (PAMPs), pathogensecreted extracellular molecules, and type-III effectors (Dodds and Rathjen, 2010). PAMPs trigger a series of plant pattern-triggered immune responses upon binding, which activates pattern recognition receptors (PRRs) on the cell surface. Several actin-binding proteins (ABPs) for different steps of actin tread milling are involved in actin remodeling during PTI signaling. The molecular mechanisms by which a host initiates rapid remodeling of the actin cytoskeleton during plant-bacteria interactions remain poorly understood. Questions, such as how actin nucleation is triggered or how the plant-defense scaffold—the cell wall (CW)-PM continuum—regulates the sensing of bacterial VFs by remodeling the cortical AC system, remain unaddressed
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