Plants deploy cell-surface and intracellular leucine rich-repeat domain (LRR) immune receptors to detect pathogens 1 . LRR receptor kinases and LRR receptor proteins at the plasma membrane recognize microorganism-derived molecules to elicit pattern-triggered immunity (PTI), whereas nucleotide-binding LRR proteins detect microbial effectors inside cells to confer effector-triggered immunity (ETI). Although PTI and ETI are initiated in different host cell compartments, they rely on the transcriptional activation of similar sets of genes 2 , suggesting pathway convergence upstream of nuclear events. Here we report that PTI triggered by the Arabidopsis LRR receptor protein RLP23 requires signalling-competent dimers of the lipase-like proteins EDS1 and PAD4, and of ADR1 family helper nucleotide-binding LRRs, which are all components of ETI. The cell-surface LRR receptor kinase SOBIR1 links RLP23 with EDS1, PAD4 and ADR1 proteins, suggesting the formation of supramolecular complexes containing PTI receptors and transducers at the inner side of the plasma membrane. We detected similar evolutionary patterns in LRR receptor protein and nucleotide-binding LRR genes across Arabidopsis accessions; overall higher levels of variation in LRR receptor proteins than in LRR receptor kinases are consistent with distinct roles of these two receptor families in plant immunity. We propose that the EDS1-PAD4-ADR1 node is a convergence point for defence signalling cascades, activated by both surface-resident and intracellular LRR receptors, in conferring pathogen immunity.Arabidopsis thaliana (hereafter Arabidopsis) cell-surface LRR receptor kinases (LRR-RKs) and LRR receptor protein (LRR-RP)-SOBIR1 complexes recruit the co-receptor BAK1 and signal through receptor-like cytoplasmic kinases (RLCKs) to elicit PTI 3 . Intracellular coiled-coil (CC)-nucleotide-binding LRR (NLR) or TOLL-INTERLEUKIN 1 RECEP-TOR (TIR)-NLR receptors 4 require ADR1-type and NRG1-type helper NLRs (hNLRs) and the lipase-like EDS1 family proteins EDS1, PAD4 and SAG101 to confer ETI 5,6 . While the defence outputs for PTI and ETI are qualitatively similar 2 , where and how pathways activated in different cell compartments converge remain unclear. Effective plant defence relies on mutual potentiation of PTI and ETI pathways 7,8 , suggesting mechanistic links between these two tiers of the plant immune system. RLCKs PBL30 and PBL31 mediate PTIThe Arabidopsis class VII RLCK (RLCK-VII) BIK1 promotes LRR-RK-mediated PTI but is a negative regulator of LRR-RP-mediated PTI 9 . To identify RLCK-VII members with positive roles in LRR-RP-dependent PTI, we screened an Arabidopsis RLCK-VII transfer DNA mutant library 10 for ethylene production elicited by fungal pg13(At) 11 , oomycete nlp20 and bacterial eMax (which are recognized by RLP42, RLP23 and RLP1, respectively) 3 (Extended Data Fig. 1a). A pbl31 mutant was defective in response to these elicitors compared with wild-type plants (Columbia-0 (Col-0)) (Extended Data Fig. 1a). PBL31 belongs to RLCK-VII subfamily 7, together ...

SignificanceCell-fate determination and cellular behavior in plants rely mainly on positional information and intercellular communication. A plethora of cues are perceived by surface receptors and integrated into an adequate cellular output. Here, we show that the small receptor-like protein RLP44 acts as an intermediary to connect the receptors for two well-known signaling molecules, brassinosteroid and phytosulfokine, to control cell fate in the root vasculature. Furthermore, we show that the brassinosteroid receptor has functions that are independent from the responses to its hormone ligands and reveal that phytosulfokine signaling promotes procambial cell identity. These results provide a mechanistic framework for the integration of multiple signaling pathways at the plasma membrane by shifting associations of receptors in multiprotein complexes.

Plants depend on various cell surface receptors to integrate extracellular signals with developmental programs. One of the beststudied receptors is BRASSINOSTEROID INSENSITIVE 1 (BRI1) in Arabidopsis (Arabidopsis thaliana). Upon binding of its hormone ligands, BRI1 forms a complex with a shape-complementary coreceptor and initiates a signal transduction cascade, which leads to a variety of responses. At the macroscopic level, brassinosteroid (BR) biosynthetic and receptor mutants have similar growth defects, which initially led to the assumption that the signaling pathways were largely linear. However, recent evidence suggests that BR signaling is interconnected with several other pathways through various mechanisms. We recently described that feedback from the cell wall is integrated at the level of the receptor complex through interaction with RECEPTOR-LIKE PROTEIN 44 (RLP44). Moreover, BRI1 is required for another function of RLP44: the control of procambial cell fate. Here, we report a BRI1 mutant, bri1 cnu4 , which differentially affects canonical BR signaling and RLP44 function in the vasculature. Although BR signaling is only mildly impaired, bri1 cnu4 mutants show ectopic xylem in place of procambium. Mechanistically, this is explained by an increased association between RLP44 and the mutated BRI1 protein, which prevents the former from acting in vascular cell fate maintenance. Consistent with this, the mild BR response phenotype of bri1 cnu4 is a recessive trait, whereas the RLP44-mediated xylem phenotype is semidominant. Our results highlight the complexity of plant plasma membrane receptor function and provide a tool to dissect BR signaling-related roles of BRI1 from its noncanonical functions. Plant cells perceive a multitude of extracellular signals through a battery of plasma membrane-bound receptors that are crucial for the integration of environmental and developmental signals. The response to the growth-regulatory brassinosteroid (BR) phytohormones is mediated by one of the best-characterized plant signaling pathways (Singh and Savaldi-Goldstein, 2015) initiated by a receptor complex containing the Leu-rich repeat receptor-like kinase BRASSINOSTE-ROID INSENSITIVE 1 (BRI1; Li and Chory, 1997) and its coreceptors of the SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) family (Ma et al., 2016; Hohmann et al., 2017). Binding of the brassinosteroid ligand mediates hetero-dimerization of BRI1 and SERK family members such as BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1; Li et al., 2002; Nam and Li, 2002), which in turn triggers extensive autoand transphosphorylation of the intracellular BAK1 and BRI1 kinase domains (Hohmann et al., 2017). The activated kinases recruit and activate downstream BR signaling components, which eventually leads to vast changes in gene expression mediated by BR signalingregulated transcription factors such as BRASSINA-ZOLE-RESISTANT 1 (BZR1; Wang et al., 2002) and BRI1-EMS-SUPPRESSOR 1 (BES1; Yin et al., 2002). Among the transcriptional targets of these transcription factors, cell...

Brassinosteroids (BR) are involved in the control of several developmental processes ranging from root elongation to senescence and adaptation to environmental cues. Thus, BR perception and signaling have to be precisely regulated. One regulator is BRI1‐associated kinase 1 (BAK1)‐interacting receptor‐like kinase 3 (BIR3). In the absence of BR, BIR3 forms complexes with BR insensitive 1 (BRI1) and BAK1. However, the biophysical and energetic requirements for complex formation in the absence of the ligand have yet to be determined. Using computational modeling, we simulated the potential complexes between the cytoplasmic domains of BAK1, BRI1 and BIR3. Our calculations and experimental data confirm the interaction of BIR3 with BAK1 and BRI1, with the BAK1 BIR3 interaction clearly favored. Furthermore, we demonstrate that BIR3 and BRI1 share the same interaction site with BAK1. This suggests a competition between BIR3 and BRI1 for binding to BAK1, which results in preferential binding of BIR3 to BAK1 in the absence of the ligand thereby preventing the active participation of BAK1 in BR signaling. Our model also suggests that BAK1 and BRI1 can interact even while BAK1 is in complex with BIR3 at an additional binding site of BAK1 that does not allow active BR signaling.

Brassinosteroids (BR) are key hormonal regulators of plant development. However, whereas the individual components of BR perception and signaling are well characterized experimentally, the question of how they can act and whether they are sufficient to carry out the critical function of cellular elongation remains open. Here, we combined computational modeling with quantitative cell physiology to understand the dynamics of the plasma membrane (PM)-localized BR response pathway during the initiation of cellular responses in the epidermis of the Arabidopsis root tip that are be linked to cell elongation. The model, consisting of ordinary differential equations, comprises the BR induced hyperpolarization of the PM, the acidification of the apoplast and subsequent cell wall swelling. We demonstrate that the competence of the root epidermal cells for the BR response predominantly depends on the amount and activity of H+-ATPases in the PM. The model further predicts that an influx of cations is required to compensate for the shift of positive charges caused by the apoplastic acidification. A potassium channel was subsequently identified and experimentally characterized, fulfilling this function. Thus, we established the landscape of components and parameters for physiological processes potentially linked to cell elongation, a central process in plant development.