En Garde! Inhibition of an Actin Depolymerizing Factor Activates Immune Responses during Plant–Microbe Interactions

Plant epidermal cells respond to microbes (both pathogenic and nonpathogenic) with a rapid increase in actin filament abundance (Crdenas et al., 1998; Takemoto and Hardham, 2004; Henty-Ridilla et al., 2013). This increase occurs when microbe-associated molecular patterns (MAMPs) bind to pattern recognition receptors (PRRs) on the cell surface and is a hallmark of the innate immune response (also known as pattern-triggered immunity [PTI]). Other features of PTI include fluxes in cytosolic ions and signaling phospholipids, increases in reactive oxygen species, activation of defense-related genes, and fortification of the cell wall.

Henty-Ridilla et al. (pages 340–352) set out to identify the molecular signals that link microbe perception with changes in the actin cytoskeleton and to examine the role of actin remodeling in PTI. Using the high spatiotemporal resolution of variable-angle fluorescence microscopy, the authors analyzed actin cytoskeleton dynamics in the epidermal cells of Arabidopsis thaliana hypocotyls expressing the actin reporter GFP-fABD2 (Staiger et al., 2009). Treatment with a bacterial MAMP, elf26, elicited changes in cytoskeletal dynamics within minutes; MAMP treatment reduced filament severing and promoted filament–filament annealing, leading to single filaments with increased average length and lifetime. These changes, which collectively resulted in increased actin filament abundance, relied on the presence of the cognate PRR, as mutants that lacked components of the PRR did not exhibit an increase in filament abundance following elf26 treatment.

Prompted by the observation that these MAMP-induced changes phenocopied an ACTIN DEPOLYMERIZING FACTOR4 knockout mutant (adf4), the authors hypothesized that ADF4 activity is inhibited or downregulated during PTI. To test this hypothesis, the authors analyzed the effect of elf26 and a fungal MAMP, chitin, on the actin cytoskeleton of the adf4 mutant. Whereas chitin treatment triggered an increase in actin abundance in the adf4 mutant, elf26 treatment did not. This finding suggests that ADF4 is required for innate immune signaling following bacterial, but not fungal, infection and that a distinct pathway underlies actin remodeling in cells exposed to fungi.

The authors then examined whether other PTI responses occurred downstream of ADF4 and actin remodeling. First, they examined callose deposition in the cell walls of seedlings subjected to elf26 or chitin. Whereas callose was normally deposited in wild-type plants treated with elf26 or chitin and in adf4 plants treated with chitin, adf4 plants did not increase callose deposition in response to elf26. Second, the authors monitored the transcription of defense-related genes following treatment with bacterial or fungal MAMPs. Target genes of the mitogen-activated protein kinase cascade were upregulated in both adf4 and wild-type seedlings after elf26 or chitin treatment, but those of the calcium-dependent protein kinase (CDPK) cascade were attenuated in adf4 seedlings. Therefore, ADF4 appears to function upstream of CDPK-dependent gene transcription during PTI.

Based on these findings, the authors propose a model (see figure) in which ADF4 is a target for inhibition by intermediates of PTI signaling following bacterial perception.

Model for the role of ADF4 during PTI. Inhibition of ADF4 activity underlies the increase in actin filament abundance in plant cells challenged with a bacterial MAMP, thus promoting cell wall fortification and CDPK-dependent gene expression during PTI. (Adapted from Henty-Ridilla et al. [2014], Figure 5H.)