Scanning for New BRI1 Mutations via TILLING Analysis

Mechanistic Insights into the Function of 14-3-3 Proteins as Negative Regulators of Brassinosteroid Signaling in Arabidopsis

Brassinosteroids (BRs) are vital plant steroid hormones sensed at the cell surface by a membrane signaling complex comprising the receptor kinase BRI1 and a SERK family co-receptor kinase. Activation of this complex lead to dissociation of the inhibitor protein BKI1 from the receptor and to differential phosphorylation of BZR1/BES1 transcription factors by the glycogen synthase kinase 3 protein BIN2. Many phosphoproteins of the BR signaling pathway, including BRI1, SERKs, BKI1 and BZR1/BES1 can associate with 14-3-3 proteins. In this study, we use quantitative ligand binding assays to define the minimal 14-3-3 binding sites in the N-terminal lobe of the BRI1 kinase domain, in BKI1, and in BZR1 from Arabidopsis thaliana. All three motifs require to be phosphorylated to specifically bind 14-3-3s with mid- to low-micromolar affinity. BR signaling components display minimal isoform preference within the 14-3-3 non-ε subgroup. 14-3-3λ and 14-3-3 ω isoform complex crystal structures reveal that BKI1 and BZR1 bind as canonical type II 14-3-3 linear motifs. Disruption of key amino acids in the phosphopeptide binding site through mutation impairs the interaction of 14-3-3λ with all three linear motifs. Notably, quadruple loss-of-function mutants from the non-ε group exhibit gain-of-function BR signaling phenotypes, suggesting a role for 14-3-3 proteins as overall negative regulators of the BR pathway. Collectively, our work provides further mechanistic and genetic evidence for the regulatory role of 14-3-3 proteins at various stages of the BR signaling cascade.

Molecular Lesions in BRI1 and Its Orthologs in the Plant Kingdom

Brassinosteroids (BRs) are an essential group of plant hormones regulating numerous aspects of plant growth, development, and stress responses. BRI1, along with its co-receptor BAK1, are involved in brassinosteroid sensing and early events in the BR signal transduction cascade. Mutational analysis of a particular gene is a powerful strategy for investigating its biochemical role. Molecular genetic studies, predominantly in Arabidopsis thaliana, but progressively in numerous other plants, have identified many mutants of the BRI1 gene and its orthologs to gain insight into its structure and function. So far, the plant kingdom has identified up to 40 bri1 alleles in Arabidopsis and up to 30 bri1 orthologs in different plants. These alleles exhibit phenotypes that are identical in terms of development and growth. Here, we have summarized bri1 alleles in Arabidopsis and its orthologs present in various plants including monocots and dicots. We have discussed the possible mechanism responsible for the specific allele. Finally, we have briefly debated the importance of these alleles in the research field and the agronomically valuable traits they offer to improve plant varieties.

R-SNARE protein YKT61 mediates root apical meristem cell division via BRASSINOSTEROID-INSENSITIVE1 recycling

Root growth is sustained by cell division and differentiation of the root apical meristem (RAM), in which brassinosteroid (BR) signaling mediated via the dynamic targeting of BRASSINOSTEROID-INSENSITIVE1 (BRI1) plays complex roles. BRI1 is constitutively secreted to the plasma membrane (PM), internalized, and recycled or delivered into vacuoles, whose PM abundance is critical for BR signaling. Vesicle-target membrane fusion is regulated by heterotetrameric SNARE complexes. SNARE proteins have been implicated in BRI1 targeting, but how SNAREs affect RAM development is unclear. We report that Arabidopsis (Arabidopsis thaliana) YKT61, an atypical R-SNARE protein, is critical for BR-controlled RAM development through the dynamic targeting of BRI1. Functional loss of YKT61 is lethal for both male and female gametophytes. By using weak mutant alleles of YKT61, ykt61-partially complemented (ykt61-pc), we show that YKT61 knockdown results in a reduction of RAM length due to reduced cell division, similar to that in bri1-116. YKT61 physically interacts with BRI1 and is critical for the dynamic recycling of BRI1 to the PM. We further determine that YKT61 is critical for the dynamic biogenesis of vacuoles, for the maintenance of Golgi morphology, and for endocytosis, which may have a broad effect on development. Endomembrane compartments connected via vesicular machinery, such as SNAREs, influence nuclear-controlled cellular activities such as division and differentiation by affecting the dynamic targeting of membrane proteins, supporting a retro-signaling pathway from the endomembrane system to the nucleus.

Comprehensive Analysis of BR Receptor Expression under Hormone Treatment in the Rubber Tree (Hevea brasiliensis Muell. Arg.)

Brassinosteroids (BRs) are important for plant growth and development, with BRI1 and BAK1 kinases playing an important role in BR signal transduction. Latex from rubber trees is crucial for industry, medicine and defense use. Therefore, it is beneficial to characterize and analyze HbBRI1 and HbBAK1 genes to improve the quality of the resources obtained from Hevea brasiliensis (rubber tree). Based on bioinformatics predictions and rubber tree database, five HbBRI1s with four HbBAK1s were identified and named HbBRI1~HbBRL3 and HbBAK1a~HbBAK1d, respectively, which were clustered in two groups. HbBRI1 genes, except for HbBRL3, exclusively contain introns, which is convenient for responding to external factors, whereas HbBAK1b/c/d contain 10 introns and 11 exons, and HbBAK1a contains eight introns. Multiple sequence analysis showed that HbBRI1s include typical domains of the BRI1 kinase, indicating that HbBRI1s belong to BRI1. HbBAK1s that possess LRR and STK_BAK1_like domains illustrate that HbBAK1s belong to the BAK1 kinase. BRI1 and BAK1 play an important role in regulating plant hormone signal transduction. Analysis of the cis-element of all HbBRI1 and HbBAK1 genes identified hormone response, light regulation and abiotic stress elements in the promoters of HbBRI1s and HbBAK1s. The results of tissue expression patterns indicate that HbBRL1/2/3/4 and HbBAK1a/b/c are highly expressed in the flower, especially HbBRL2-1. The expression of HbBRL3 is extremely high in the stem, and the expression of HbBAK1d is extremely high in the root. Expression profiles with different hormones show that HbBRI1 and HbBAK1 genes are extremely induced by different hormone stimulates. These results provide theoretical foundations for further research on the functions of BR receptors, especially in response to hormone signals in the rubber tree.

Brassinosteroid signals cooperate with katanin-mediated microtubule severing to control stamen filament elongation

Proper stamen filament elongation is essential for pollination and plant reproduction. Plant hormones are extensively involved in every stage of stamen development; however, the cellular mechanisms by which phytohormone signals couple with microtubule dynamics to control filament elongation remain unclear. Here, we screened a series of Arabidopsis thaliana mutants showing different microtubule defects and revealed that only those unable to sever microtubules, lue1 and ktn80.1234, displayed differential floral organ elongation with less elongated stamen filaments. Prompted by short stamen filaments and severe decrease in KTN1 and KTN80s expression in qui-2 lacking five BZR1-family transcription factors (BFTFs), we investigated the crosstalk between microtubule severing and brassinosteroid (BR) signaling. The BFTFs transcriptionally activate katanin-encoding genes, and the microtubule-severing frequency was severely reduced in qui-2. Taken together, our findings reveal how BRs can regulate cytoskeletal dynamics to coordinate the proper development of reproductive organs.

Expression of Potato StDRO1 in Arabidopsis Alters Root Architecture and Drought Tolerance

Potato (Solanum tuberosum L) is the third important crop for providing calories to a large human population, and is considered sensitive to moderately sensitive to drought stress conditions. The development of drought-tolerant, elite varieties of potato is a challenging task, which can be achieved through molecular breeding. Recently, the DEEPER ROOTING 1 (DRO1) gene has been identified in rice, which influences plant root system and regulates grain yield under drought stress conditions. The potato StDRO1 protein is mainly localized in the plasma membrane of tobacco leaf cells, and overexpression analysis of StDRO1 in Arabidopsis resulted in an increased lateral root number, but decreased lateral root angle, lateral branch angle, and silique angle. Additionally, the drought treatment analysis indicated that StDRO1 regulated drought tolerance and rescued the defective root architecture and drought-tolerant phenotypes of Atdro1, an Arabidopsis AtDRO1 null mutant. Furthermore, StDRO1 expression was significantly higher in the drought-tolerant potato cultivar "Unica" compared to the drought-sensitive cultivar "Atlantic." The transcriptional response of StDRO1 under drought stress occurred significantly earlier in Unica than in Atlantic. Collectively, the outcome of the present investigation elucidated the role of DRO1 function in the alternation of root architecture, which potentially acts as a key gene in the development of a drought stress-tolerant cultivar. Furthermore, these findings will provide the theoretical basis for molecular breeding of drought-tolerant potato cultivars for the farming community.

Engineering Chimeras by Fusing Plant Receptor-like Kinase EMS1 and BRI1 Reveals the Two Receptors' Structural Specificity and Molecular Mechanisms

Brassinosteriods (BRs) are plant hormones essential for plant growth and development. The receptor-like kinase (RLK) BRI1 perceives BRs to initiate a well-known transduction pathway which finally activate the transcription factors BZR1/BES1 specifically regulating BR-mediated gene expression. The RLK EMS1 governs tapetum formation via the same signaling pathway shared with BRI1. BRI1 and EMS1 have a common signal output, but the gene structural specificity and the molecular response remain unclear. In this study, we identified that the transmembrane (TM), intracellular juxtamembrane (iJM), kinase, and leucin-rich repeats 1-13 (LRR1-13) domains of EMS1 could replace the corresponding BRI1 domain to maintain the BR receptor function, whereas the extracellular juxtamembrane (eJM) and LRR1-14 domains could not, indicating that the LRR14-EJM domain conferred functional specificity to BRI1. We compared the kinase domains of EMS1 and BRI1, and found that EMS1’s kinase activity was weaker than BRI1’s. Further investigation of the specific phosphorylation sites in BRI1 and EMS1 revealed that the Y1052 site in the kinase domain was essential for the BRI1 biological function, but the corresponding site in EMS1 showed no effect on the biological function of EMS1, suggesting a site regulation difference in the two receptors. Furthermore, we showed that EMS1 shared the substrate BSKs with BRI1. Our study provides insight into the structural specificity and molecular mechanism of BRI1 and EMS1, as well as the origin and divergence of BR receptors.

Brassinosteroids synthesised by CYP85A/A1 but not CYP85A2 function via a BRI1-like receptor but not via BRI1 in Picea abies

Brassinosteroids (BRs) are essential plant hormones. In angiosperms, brassinolide and castasterone, the first and second most active BRs, respectively, are synthesised by CYP85A2 and CYP85A/A1, respectively. BRs in angiosperms function through an essential receptor, BR Insensitive 1 (BRI1). In addition, some angiosperms also have non-essential BRI1-like 1/3 (BRL1/3). In conifers, BRs promote seed germination under drought stress; however, how BRs function in gymnosperms is unknown. In this study, we performed functional complementation of BR biosynthesis and receptor genes from Picea abies with respective Arabidopsis mutants. We found that P. abies possessed functional PaCYP85A and PaBRL1 but not PaCYP85A2 or PaBRI1, and this results in weak BR signaling, and both PaCYP85A and PaBRL1 were abundantly expressed. However, neither BR treatment of P. abies seedlings nor expression of PaBRL1 in the Arabidopsis Atbri1 mutant promoted plant height, despite the fact that BR-responsive genes were activated. Importantly, chimeric AtBRI1 replaced with the BR-binding domain of PaBRL1 complemented the Atbri1 phenotypes. Furthermore, PaBRL1 had less kinase activity than BRI1 in vitro. Overall, P. abies had weak but still active BR signaling, explaining aspects of its slow growth and high stress tolerance. Our study sheds light on the functional and evolutionary significance of distinct BR signaling that is independent of BRI1 and brassinolide.

Identification and characterization of the LRR repeats in plant LRR-RLKs

Background

Leucine-rich-repeat receptor-like kinases (LRR-RLKs) play central roles in sensing various signals to regulate plant development and environmental responses. The extracellular domains (ECDs) of plant LRR-RLKs contain LRR motifs, consisting of highly conserved residues and variable residues, and are responsible for ligand perception as a receptor or co-receptor. However, there are few comprehensive studies on the ECDs of LRR-RLKs due to the difficulty in effectively identifying the divergent LRR repeats.

Results

In the current study, an efficient LRR motif prediction program, the “Phyto-LRR prediction” program, was developed based on the position-specific scoring matrix algorithm (PSSM) with some optimizations. This program was trained by 16-residue plant-specific LRR-highly conserved segments (HCS) from LRR-RLKs of 17 represented land plant species and a database containing more than 55,000 predicted LRRs based on this program was constructed. Both the prediction tool and database are freely available at http://phytolrr.com/ for website usage and at http://github.com/phytolrr for local usage. The LRR-RLKs were classified into 18 subgroups (SGs) according to the maximum-likelihood phylogenetic analysis of kinase domains (KDs) of the sequences. Based on the database and the SGs, the characteristics of the LRR motifs in the ECDs of the LRR-RLKs were examined, such as the arrangement of the LRRs, the solvent accessibility, the variable residues, and the N-glycosylation sites, revealing a comprehensive profile of the plant LRR-RLK ectodomains.

Conclusion

The “Phyto-LRR prediction” program is effective in predicting the LRR segments in plant LRR-RLKs, which, together with the database, will facilitate the exploration of plant LRR-RLKs functions. Based on the database, comprehensive sequential characteristics of the plant LRR-RLK ectodomains were profiled and analyzed.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12860-021-00344-y.

Kinase Function of Brassinosteroid Receptor Specified by Two Allosterically Regulated Subdomains

Plants acquire the ability to adapt to the environment using transmembrane receptor-like kinases (RLKs) to sense the challenges from their surroundings and respond appropriately. RLKs perceive a variety of ligands through their variable extracellular domains (ECDs) that activate the highly conserved intracellular kinase domains (KDs) to control distinct biological functions through a well-developed downstream signaling cascade. A new study has emerged that brassinosteroid-insensitive 1 (BRI1) family and excess microsporocytes 1 (EMS1) but not GASSHO1 (GSO1) and other RLKs control distinct biological functions through the same signaling pathway, raising a question how the signaling pathway represented by BRI1 is specified. Here, we confirm that BRI1-KD is not functionally replaceable by GSO1-KD since the chimeric BRI1-GSO1 cannot rescue bri1 mutants. We then identify two subdomains S1 and S2. BRI1 with its S1 and S2 substituted by that of GSO1 cannot rescue bri1 mutants. Conversely, chimeric BRI1-GSO1 with its S1 and S2 substituted by that of BRI1 can rescue bri1 mutants, suggesting that S1 and S2 are the sufficient requirements to specify the signaling function of BRI1. Consequently, all the other subdomains in the KD of BRI1 are functionally replaceable by that of GSO1 although the in vitro kinase activities vary after replacements, suggesting their functional robustness and mutational plasticity with diverse kinase activity. Interestingly, S1 contains αC-β4 loop as an allosteric hotspot and S2 includes kinase activation loop, proposedly regulating kinase activities. Further analysis reveals that this specific function requires β4 and β5 in addition to αC-β4 loop in S1. We, therefore, suggest that BRI1 specifies its kinase function through an allosteric regulation of these two subdomains to control its distinct biological functions, providing a new insight into the kinase evolution.

Barley "uzu" and Wheat "uzu-like" Brassinosteroid Receptor BRI1 Kinase Domain Variations Modify Phosphorylation Activity In Vitro

Brassinosteroid insensitive1 (BRI1), a leucine-rich repeat receptor kinase, is responsible for the perception of the brassinosteroid (BR) phytohormone in plants. While recent evidence has implicated a naturally occurring Hordeum vulgare V. (barley) HvBRI1 kinase domain (KD) variant (H857R; "uzu" variation) in increased fungal disease resistance, the impact of the variation on receptor function and thus the mechanism by which disease resistance might be imparted remain enigmatic. Here, the functional implications of the uzu variation as well as the effects of newly identified naturally occurring Triticum aestivum L. (wheat) TaBRI1-KD variants are investigated. Recombinantly produced KDs of wild-type (WT) and uzu HvBRI1 were assessed for phosphorylation activity in vitro, yielding WT K_M and V_MAX values similar to those of other reports, but the uzu variation delayed saturation and reduced turnover levels. In silico modeling of the H857R variation showed it to be surface-exposed and distal from the catalytic site. Further evaluation of three naturally occurring wheat TaBRI1 variants, A907T, A970V, and G1019R (barley numbering) identified in the A, B, and D subgenomic genes, respectively, highlighted a significant loss of activity for A907T. A907T is located on the same surface as the H857R variation and a negative regulatory phosphorylation site (T982) in Arabidopsis thaliana BRI1. A fourth variation, T1031A (barley numbering), unique to both subgenomic A proteins and localized to the BKI1 binding site, also decreased activity. The outcomes are discussed with respect to the predicted structural contexts of the variations and their implications with respect to mechanisms of action.

The Evolutionarily Conserved Serine Residues in BRI1 LRR Motifs Are Critical for Protein Secretion

As a well-studied leucine-rich-repeat receptor-like kinases (LRR-RLKs) in Arabidopsis (Arabidopsis thaliana), BRI1 functions as a cell surface receptor for sensing the smallest ligand molecule identified thus far. The weak allele bri1-9 (S662F) harbors a mutation at the conserved serine (Ser*) residue among 25 LRRs, which leads to the protein retention in the ER. However, very little is known about the importance of these residues. Through site-directed mutagenesis and a phenotypic complementation test, we examined the effects of these conserved serine residues (S*-chain) on protein secretion and functions. The results showed that the replacements of these serine residues significantly changed the sub-localization of BRI1-GFPs to the ER and that rigid space constraints, as well as the requirement of successive inner polar contacts, affect these sites. In addition, the continuous presence of Ser* is mainly disrupted at the LRR-island domain interface, and the changes of these four nonserine residues to serine greatly decreased the protein ability to complement bri1-301 compact phenotype and the BR signaling activation. The sequence alignment revealed that other known LRR-RLK also harbors the S*-chain and the non-Ser* residues at the ligand-binding region along the S*-chain, which confirms the evolutionary significance of residues at these sites in plant LRR-RLKs.

Brassinosteroids: Multidimensional Regulators of Plant Growth, Development, and Stress Responses

Brassinosteroids (BRs) are a group of polyhydroxylated plant steroid hormones that are crucial for many aspects of a plant's life. BRs were originally characterized for their function in cell elongation, but it is becoming clear that they play major roles in plant growth, development, and responses to several stresses such as extreme temperatures and drought. A BR signaling pathway from cell surface receptors to central transcription factors has been well characterized. Here, we summarize recent progress toward understanding the BR pathway, including BR perception and the molecular mechanisms of BR signaling. Next, we discuss the roles of BRs in development and stress responses. Finally, we show how knowledge of the BR pathway is being applied to manipulate the growth and stress responses of crops. These studies highlight the complex regulation of BR signaling, multiple points of crosstalk between BRs and other hormones or stress responses, and the finely tuned spatiotemporal regulation of BR signaling.

A Mutant Allele Uncouples the Brassinosteroid-Dependent and Independent Functions of BRASSINOSTEROID INSENSITIVE 1

Plants depend on various cell surface receptors to integrate extracellular signals with developmental programs. One of the best-studied 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.

The brassinosteroid receptor kinase, BRI1, plays a role in seed germination and the release of dormancy by cold stratification

Seed dormancy is a critical mechanism that delays germination until environmental conditions are favorable for growth. Plant hormones gibberellin (GA) and abscisic acid (ABA) have long been recognized as key players in regulating dormancy and germination. Recent data have increased interest in brassinosteroid (BR) hormones that promote germination by activating GA downstream genes and inactivating ABA signaling. Exposure of imbibed seeds to low temperature (cold stratification) is widely used to release seed dormancy and to improve germination frequency. However, the mechanism by which cold stratification overcomes the inhibitory role of ABA is not completely understood. In the present study, we show delayed germination of seeds of the BR insensitive mutant, bri1-5, that was largely reversed by treatment with fluridone, an inhibitor of ABA biosynthesis. In addition, the bri1-5 seeds were markedly less sensitive to the cold stratification release of dormancy. These results suggest that BR locates upstream of ABA signaling and downstream of cold stratification signaling in dormancy and germination pathways. Consistent with this notion, BR biosynthetic genes, DWF4 and DET2, were upregulated by cold stratification. The transcripts of the GA biosynthesis gene, GA3ox1, and cold responsive genes, CBF1 and CBF2, increased in response to cold stratification in wild type seeds but not in bri1-5 seeds. Conversely, transgenic seeds overexpressing BRI1 germinated more rapidly than wild type in the absence of cold stratification. Thus, we propose that BR signaling plays a previously unrecognized role in the cold stratification pathway for seed dormancy and germination.

Less Conserved LRRs Is Important for BRI1 Folding

Brassinosteroid insensitive 1 (BRI1) is a multidomain plant leucine-rich repeat receptor-like kinase (LRR-RLK), belongs to the LRR X subfamily. BRI1 perceives plant hormone brassinosteroids (BRs) through its extracellular domain that constitutes of LRRs interrupted by a 70 amino acid residue island domain (ID), which activates the kinase domain (KD) in its intracellular domain to trigger BR response. Thus, the KD and the ID of BRI1 are highly conserved and greatly contribute to BR functions. In fact, most bri1 mutants are clustered in or surrounded around the ID and the KD. However, the role of the less conserved LRR domains, particularly the first few LRRs after the signal peptide, is elusive. Here, we report the identification of a loss-of-function mutant bri1-235 that carries a mutation in the less conserved fourth LRR of BRI1 extracellular domain in Arabidopsis. This mutant had a base alteration from C to T, resulting in an amino acid substitution from serine to phenylalanine at the 156th position of BRI1. Compared with the wild-type plants, bri1-235 exhibited round leaves, prolonged life span, shorter stature, and approximately normal fertility under light conditions. The bri1-235 mutant was less sensitive to exogenous brassinolide under normal conditions. Importantly, both wild-type BRI1 expression and a sbi1 mutant that activates BRI1 rescued bri1-235 and resembled the wild type. Furthermore, bri1-235 protein was localized in endoplasmic reticulum rather than plasma membrane, suggestive of a cause for reducing BR sensitive in bri1-235. Taken together, our findings provide an insight into the role of the less conserved LRRs of BRI1, shedding light on the role of LRRs in a variety of LRR-RLKs that control numerous processes of plant growth, development, and stress response.

Identification of critical cysteine sites in brassinosteroid-insensitive 1 and novel signaling regulators using a transient expression system

The plant hormones brassinosteroids (BRs) modulate plant growth and development. Cysteine (Cys) residues located in the extracellular domain of a protein are of importance for protein structure by forming disulfide bonds. To date, the systematic study of the functional significance of Cys residues in BR-insensitive 1 (BRI1) is still lacking. We used brassinolide-induced exogenous bri1-EMS-Suppressor 1 (BES1) dephosphorylation in Arabidopsis thaliana protoplasts as a readout, took advantage of the dramatic decrease of BRI1 protein levels during protoplast isolation, and of the strong phosphorylation of BES1 by BR-insensitive 2 (BIN2) in protoplasts, and developed a protoplast transient system to identify critical Cys sites in BRI1. Using this system, we identified a set of critical Cys sites in BRI1, as substitution of these Cys residues with alanine residues greatly compromised the function of BRI1. Moreover, we identified two negative regulators of BR signaling, pattern-triggered immunity compromised RLCK1 (PCRK1) and PCRK2, that were previously known to positively regulate innate immunity signaling. This work not only provides insight into the functional importance of critical Cys residues in stabilizing the superhelical conformation of BRI1-leucine-rich-repeat, but also reveals that PCRK1/2 can inversely modulate BR and plant immune signaling pathways.

A Temperature-Sensitive Misfolded bri1-301 Receptor Requires Its Kinase Activity to Promote Growth

BRASSINOSTEROID-INSENSITIVE1 (BRI1) is a leucine-rich-repeat receptor-like kinase that functions as the cell surface receptor for brassinosteroids (BRs). Previous studies showed that BRI1 requires its kinase activity to transduce the extracellular BR signal into the nucleus. Among the many reported mutant bri1 alleles, bri1-301 is unique, as its glycine-989-to-isoleucine mutation completely inhibits its kinase activity in vitro but only gives rise to a weak dwarf phenotype compared with strong or null bri1 alleles, raising the question of whether kinase activity is essential for the biological function of BRI1. Here, we show that the Arabidopsis (Arabidopsis thaliana) bri1-301 mutant receptor exhibits weak BR-triggered phosphorylation in vivo and absolutely requires its kinase activity for the limited growth that occurs in the bri1-301 mutant. We also show that bri1-301 is a temperature-sensitive misfolded protein that is rapidly degraded in the endoplasmic reticulum and at the plasma membrane by yet unknown mechanisms. A temperature increase from 22°C to 29°C reduced the protein stability and biochemical activity of bri1-301, likely due to temperature-enhanced protein misfolding. The bri1-301 protein could be used as a model to study the degradation machinery for misfolded membrane proteins with cytosolic structural lesions and the plasma membrane-associated protein quality-control mechanism.

Thermal-Enhanced bri1-301 Instability Reveals a Plasma Membrane Protein Quality Control System in Plants

Brassinosteroids (BRs) are essential phytohormones mainly perceived by a single-pass transmembrane receptor-like protein kinase (RLK), BRASSINOSTEROID INSENSITIVE 1 (BRI1). bri1-5 and bri1-9, two distinct mutants with point mutations in the extracellular domain of BRI1, show weak defective phenotypes. Previous studies indicated that bri1-5 and bri1-9 mutated proteins can be recognized and eliminated via an endoplasmic reticulum quality control (ERQC) mechanism. Most of these two proteins, therefore, cannot reach their destination, plasma membrane. Here, we report our functional characterization of bri1-301, another BRI1 mutant protein with an amino acid substitution in the cytoplasmic kinase domain. bri1-301 is a partially functional BR receptor with significantly decreased protein abundance. Interestingly, protein stability and subcellular localization of bri1-301 are temperature-sensitive. At 22°C, an optimal temperature for indoor Arabidopsis growth, bri1-301 shows a weak defective phenotype. At a lower temperature condition such as 18°C, bri1-301 exhibits subtle morphological defects. At a higher temperature condition such as 28°C, on the other hand, bri1-301 displays an extremely severe phenotype reminiscent to that of a null bri1 mutant due to greatly increased bri1-301 internalization and degradation. Our detailed analyses suggest that bri1-301 stability is controlled by ERQC and plasma membrane quality control (PMQC) systems. Since PMQC has not been well studied in plants, bri1-301 can be used as a model mutant for future genetic dissection of this critical process.