Connecting the dots: Membrane nanodomains mediate clustering of actin-nucleator Type I formins in Arabidopsis immune responses

Plants perceive pathogen attack and defend themselves with spatiotemporally coordinated immune activations. Pattern- or microbe-associated molecular patterns (PAMPs or MAMPs) such as flg22 or elf26 are recognized, respectively, by FLS2 and EFR receptors on the plasma membrane (PM), triggering ROS production, calcium influx, and MAPK cascades that lead to the expression of defense-related genes (Boller and Felix, 2009). It remains enigmatic how these signals are relayed through the scaffolding continuum of the plant cell wall (CW), the PM, and the actin cytoskeleton (AC) at the cell surface. Type I formins are integral components of the CW-PM-AC continuum with their extracellular CW-binding regions, transmembrane domains, and actin-binding domains. The plant AC undergoes rapid and differential reorganization during pattern-triggered immunity (PTI) and effector-triggered immunity, and Type I formins recently were shown to activate actin assembly by clustering at the PM upon PAMP elicitation and effector subversion (Ma et al., 2021; Sun et al., 2021). But it is unknown how formins are guided to form clusters at the PM and initiate actin assembly during PTI signaling.

In this issue, using black rot bacteria Xanthomonas campestris pv. campestris (Xcc) and flg22, Zhiming Ma and colleagues (Ma et al., 2022) show that PTI induces progressively increased formin nanoclustering that drives actin polymerization (see Figure). The PM is characterized by highly ordered lipid nanodomains. Pharmacological studies with methyl-β-cyclodextrin (MβCD), a commonly used sterol-depleting and nanodomain-disrupting agent, indicated that formin clustering is dependent on PM nanodomain assembly. To identify key factors of PM nanodomains that drive formin clustering, the authors focused on remorin, one of the best-characterized nanodomain-residing and nanodomain-assembly proteins. Following treatment with Xcc and flg22, fluorescently labeled remorin AtREM1.2 and AtREM1.3 in Arabidopsis showed progressively increased nanoclustering—almost identical to that of formins. The time-dependent condensation of formins was attenuated in the rem1.2 1.3c double mutant during PTI signaling. Thus, PAMP-triggered formin nanoclustering appears to be dependent on remorin. Moreover, mutation of remorins, disruption of ACs, and inhibition of formins by drugs resulted in reduced callose deposition or increased bacterial susceptibility, suggesting the interconnected functions of remorin, formin nanoclustering, and actin polymerization in plant PTI (Ma et al., 2022).

Figure.

Proposed model of actin reorganization mediated by remorin–formin nanoclustering during innate plant immunity responses. Type I formins are integrated within the CW-PM-AC continuum. Perception of PAMPs triggers local high-order assembly of remorins through IDR-mediated self-oligomerization and, thus, the recruitment and gradual condensation of formins, and resultant actin polymerization in a time-dependent manner. Reprinted from Ma et al. (2022), Figure 5.

In a second report in this issue from the same group, Tuan Mihn Tran and coauthors (Tran et al., 2022) reveal PM nanodomain-dependent perception of bacterial outer membrane vesicles (OMVs), as well as OMV-induced plant immune responses. Xcc OMVs were found to be integrated into the Arabidopsis PM and spatially aligned with membrane nanodomains, inducing increased PM lipid order, clustering of remorin proteins, and enhanced nanodomain formation. Disruption of nanodomains by MβCD or by genetic mutations reduced OMV integration to the PM, suggesting feedback regulation between PM nanodomains and OMV integration. Coarse-grained molecular dynamic simulations also indicated interactions between PM and OMVs, predicting that sterol in the PM regulates the integration of OMVs and that saturated lipids from OMVs may increase the lipid order in the PM. Computational modeling combined with cell biology, genetics, and biomechanics approaches thus provide new insights for how the biochemical composition and biophysical properties of the PM mediate plant immunity responses (Tran et al., 2022).