Phosphorylation-dependent routing of RLP44 towards brassinosteroid or phytosulfokine signalling

Sulfated peptides and their receptors: Key regulators of plant development and stress adaptation

Four distinct types of sulfated peptides have been identified in Arabidopsis thaliana. These peptides play crucial roles in regulating plant development and stress adaptation. Recent studies have revealed that Xanthomonas and Meloidogyne can secrete plant-like sulfated peptides, exploiting the plant sulfated peptide signaling pathway to suppress plant immunity. Over the past three decades, receptors for these four types of sulfated peptides have been identified, all of which belong to the leucine-rich repeat receptor-like protein kinase subfamily. A number of regulatory proteins have been demonstrated to play important roles in their corresponding signal transduction pathways. In this review, we comprehensively summarize the discoveries of sulfated peptides and their receptors, mainly in Arabidopsis thaliana. We also discuss their known biological functions in plant development and stress adaptation. Finally, we put forward a number of questions for reference in future studies.

OneFlowTraX: a user-friendly software for super-resolution analysis of single-molecule dynamics and nanoscale organization

Super-resolution microscopy (SRM) approaches revolutionize cell biology by providing insights into the nanoscale organization and dynamics of macromolecular assemblies and single molecules in living cells. A major hurdle limiting SRM democratization is post-acquisition data analysis which is often complex and time-consuming. Here, we present OneFlowTraX, a user-friendly and open-source software dedicated to the analysis of single-molecule localization microscopy (SMLM) approaches such as single-particle tracking photoactivated localization microscopy (sptPALM). Through an intuitive graphical user interface, OneFlowTraX provides an automated all-in-one solution for single-molecule localization, tracking, as well as mobility and clustering analyses. OneFlowTraX allows the extraction of diffusion and clustering parameters of millions of molecules in a few minutes. Finally, OneFlowTraX greatly simplifies data management following the FAIR (Findable, Accessible, Interoperable, Reusable) principles. We provide a detailed step-by-step manual and guidelines to assess the quality of single-molecule analyses. Applying different fluorophores including mEos3.2, PA-GFP, and PATagRFP, we exemplarily used OneFlowTraX to analyze the dynamics of plant plasma membrane-localized proteins including an aquaporin, the brassinosteroid receptor Brassinosteroid Insensitive 1 (BRI1) and the Receptor-Like Protein 44 (RLP44).

The cell surface is the place to be for brassinosteroid perception and responses

Adjusting the microenvironment around the cell surface is critical to responding to external cues or endogenous signals and to maintaining cell activities. In plant cells, the plasma membrane is covered by the cell wall and scaffolded with cytoskeletal networks, which altogether compose the cell surface. It has long been known that these structures mutually interact, but the mechanisms that integrate the whole system are still obscure. Here we spotlight the brassinosteroid (BR) plant hormone receptor BRASSINOSTEROID INSENSITIVE1 (BRI1) since it represents an outstanding model for understanding cell surface signalling and regulation. We summarize how BRI1 activity and dynamics are controlled by plasma membrane components and their associated factors to fine-tune signalling. The downstream signals, in turn, manipulate cell surface structures by transcriptional and post-translational mechanisms. Moreover, the changes in these architectures impact BR signalling, resulting in a feedback loop formation. This Review discusses how BRI1 and BR signalling function as central hubs to integrate cell surface regulation.

An F-box protein attenuates fungal xylanase-triggered immunity by destabilizing LRR-RLP NbEIX2 in a SOBIR1-dependent manner

Receptor-like proteins (RLPs) lacking the cytoplasmic kinase domain play crucial roles in plant growth, development and immunity. However, what remains largely elusive is whether RLP protein levels are fine-tuned by E3 ubiquitin ligases, which are employed by receptor-like kinases for signaling attenuation. Nicotiana benthamiana NbEIX2 is a leucine-rich repeat RLP (LRR-RLP) that mediates fungal xylanase-triggered immunity. Here we show that NbEIX2 associates with an F-box protein NbPFB1, which promotes NbEIX2 degradation likely by forming an SCF E3 ubiquitin ligase complex, and negatively regulates NbEIX2-mediated immune responses. NbEIX2 undergoes ubiquitination and proteasomal degradation in planta. Interestingly, NbEIX2 without its cytoplasmic tail is still associated with and destabilized by NbPFB1. In addition, NbPFB1 also associates with and destabilizes NbSOBIR1, a co-receptor of LRR-RLPs, and fails to promote NbEIX2 degradation in the sobir1 mutant. Our findings reveal a distinct model of NbEIX2 degradation, in which an F-box protein destabilizes NbEIX2 indirectly in a SOBIR1-dependent manner.

Three-Fluorophore FRET Enables the Analysis of Ternary Protein Association in Living Plant Cells

Protein-protein interaction studies provide valuable insights into cellular signaling. Brassinosteroid (BR) signaling is initiated by the hormone-binding receptor Brassinosteroid Insensitive 1 (BRI1) and its co-receptor BRI1 Associated Kinase 1 (BAK1). BRI1 and BAK1 were shown to interact independently with the Receptor-Like Protein 44 (RLP44), which is implicated in BRI1/BAK1-dependent cell wall integrity perception. To demonstrate the proposed complex formation of BRI1, BAK1 and RLP44, we established three-fluorophore intensity-based spectral Förster resonance energy transfer (FRET) and FRET-fluorescence lifetime imaging microscopy (FLIM) for living plant cells. Our evidence indicates that RLP44, BRI1 and BAK1 form a ternary complex in a distinct plasma membrane nanodomain. In contrast, although the immune receptor Flagellin Sensing 2 (FLS2) also forms a heteromer with BAK1, the FLS2/BAK1 complexes are localized to other nanodomains. In conclusion, both three-fluorophore FRET approaches provide a feasible basis for studying the in vivo interaction and sub-compartmentalization of proteins in great detail.