Pan-brassinosteroid signaling revealed by functional analysis of NILR1 in land plants

PHB3 interacts with BRI1 and BAK1 to mediate brassinosteroid signal transduction in Arabidopsis and tomato

Brassinosteroids (BRs) are plant hormones that are essential in plant growth and development. BRASSINOSTEROID-INSENSITIVE 1 (BRI1) and BRI1 ASSOCIATED RECEPTOR KINASE 1 (BAK1), which are located on the plasma membrane, function as co-receptors that accept and transmit BR signals. PROHIBITIN 3 (PHB3) was identified in both BRI1 and BAK1 complexes by affinity purification and LC-MS/MS analysis. Biochemical data showed that BRI1/BAK1 interacted with PHB3 in vitro and in vivo. BRI1/BAK1 phosphorylated PHB3 in vitro. When the Thr-80 amino acid in PHB3 was mutated to Ala, the mutant protein was not phosphorylated by BRI1 and the mutant protein interaction with BRI1 was abolished in the yeast two-hybrid assay. BAK1 did not phosphorylate the mutant protein PHB3T54A . The loss-of-function phb3 mutant showed a weaker BR signal than the wild-type. Genetic analyses revealed that PHB3 is a BRI1/BAK1 downstream substrate that participates in BR signalling. PHB3 has five homozygous in tomato, and we named the closest to AtPHB3 as SlPHB3.1. Biochemical data showed that SlBRI1/SlSERK3A/SlSERK3B interacted with SlPHB3.1 and SlPHB3.3. The CRISPR-Cas9 method generated slphb3.1 mutant led to a BR signal stunted relatively in tomatoes. PHB3 is a new component of the BR signal pathway in both Arabidopsis and tomato.

An update on evolutionary, structural, and functional studies of receptor-like kinases in plants

All living organisms must develop mechanisms to cope with and adapt to new environments. The transition of plants from aquatic to terrestrial environment provided new opportunities for them to exploit additional resources but made them vulnerable to harsh and ever-changing conditions. As such, the transmembrane receptor-like kinases (RLKs) have been extensively duplicated and expanded in land plants, increasing the number of RLKs in the advanced angiosperms, thus becoming one of the largest protein families in eukaryotes. The basic structure of the RLKs consists of a variable extracellular domain (ECD), a transmembrane domain (TM), and a conserved kinase domain (KD). Their variable ECDs can perceive various kinds of ligands that activate the conserved KD through a series of auto- and trans-phosphorylation events, allowing the KDs to keep the conserved kinase activities as a molecular switch that stabilizes their intracellular signaling cascades, possibly maintaining cellular homeostasis as their advantages in different environmental conditions. The RLK signaling mechanisms may require a coreceptor and other interactors, which ultimately leads to the control of various functions of growth and development, fertilization, and immunity. Therefore, the identification of new signaling mechanisms might offer a unique insight into the regulatory mechanism of RLKs in plant development and adaptations. Here, we give an overview update of recent advances in RLKs and their signaling mechanisms.

NILR1 perceives a nematode ascaroside triggering immune signaling and resistance

Plants use pattern recognition receptors (PRRs) to perceive conserved molecular patterns derived from pathogens and pests, thereby activating a sequential set of rapid cellular immune responses, including activation of mitogen-activated protein kinases (MAPKs) and Ca2+-dependent protein kinases (CDPKs), transcriptional reprogramming (particularly the induction of defense-related genes), ion fluxes, and production of reactive oxygen species.1 Plant PRRs belong to the multi-membered protein families of receptor-like kinases (RLKs) or receptor-like proteins (RLPs). RLKs consist of a ligand-binding ectodomain, a single-pass transmembrane domain, and an intracellular kinase domain, while RLPs possess the same functional domains, except for the intracellular kinase domain.2 The most abundant nematode ascaroside, Ascr18, is a nematode-associated molecular pattern (NAMP) that induces immune signaling and enhances resistance to pathogens and pests in various plant species.3 In this study, we found that the Arabidopsis NEMATODE-INDUCED LRR-RLK1 (NILR1) protein4 physically interacts with the Ascr18 elicitor, as indicated by a specific direct interaction between NILR1 and Ascr18, and NILR1 is genetically required for Ascr18-triggered immune signaling and resistance to both bacterium and nematode, as manifested by the abolishment of these immune responses in the nilr1 mutant. These results suggest that NILR1 is the immune receptor of the nematode NAMP Ascr18, mediating Ascr18-triggered immune signaling and resistance to pathogens and pests.

The receptor-like kinase EMS1 and BRI1 coordinately regulate stamen elongation via the transcription factors BES1/BZR1 in Arabidopsis

Plants possess a large family of receptor kinase proteins to mediate cell-to-cell and cell-to-environment communication, and these regulations are essential for plant growth and development as well as resistance to biotic or abiotic stresses. EMS1 is a receptor kinase which involved in tapetum cell fate determination during anther development, while brassinosteroid (BR) receptor, BRI1, controls most aspects of plant growth and development. Although EMS1 and BRI1 are known to regulate independent biological processes, they interact with identical components of the downstream signaling pathways. However, the biological processes other than the tapetum development controlled by the EMS1 signal are not clear. Here, we report that EMS1 signaling-related mutants exhibited an insufficient stamen elongation phenotype, similar to BR signaling mutants. Transgenic expression of BRI1 restored the short filament phenotype of ems1. Conversely, co-expression of EMS1 and TPD1 also restored the short filaments of BRI1 mutants, bri1. Genetic experiments confirmed that EMS1 and BRI1 regulate filament elongation through their downstream transcription factors BES1/BZR1. Molecular analysis suggested that the decrease in BR signaling output in filaments of the ems1 mutant caused deficient filament development. Moreover, in vitro and in vivo experiments proved BES1 interacts with filament-specific transcription factor MYB21. Together, we found that the two receptor-like kinases (RLKs) EMS1 and BRI1 are cooperatively involved in the regulation of filament elongation via the transcription factors BES1/BZR1. These results indicated that the biological processes regulated by EMS1 and BRI1 in plants are both independent and interactive, which provides us with insights into multidimensional molecular control of the RLK pathway.

Two Conserved Amino Acids Characterized in the Island Domain Are Essential for the Biological Functions of Brassinolide Receptors

Brassinosteroids (BRs) play important roles in plant growth and development, and BR perception is the pivotal process required to trigger BR signaling. In angiosperms, BR insensitive 1 (BRI1) is the essential BR receptor, because its mutants exhibit an extremely dwarf phenotype in Arabidopsis. Two other BR receptors, BRI1-like 1 (BRL1) and BRI1-like 3 (BRL3), are shown to be not indispensable. All BR receptors require an island domain (ID) responsible for BR perception. However, the biological functional significance of residues in the ID remains unknown. Based on the crystal structure and sequence alignments analysis of BR receptors, we identified two residues 597 and 599 of AtBRI1 that were highly conserved within a BR receptor but diversified among different BR receptors. Both of these residues are tyrosine in BRI1, while BRL1/BRL3 fixes two phenylalanines. The experimental findings revealed that, except BRI1Y597F and BRI1Y599F, substitutions of residues 597 and 599 with the remaining 18 amino acids differently impaired BR signaling and, surprisingly, BRI1Y599F showed a weaker phenotype than BRI1Y599 did, implying that these residues were the key sites to differentiate BR receptors from a non-BR receptor, and the essential BR receptor BRI1 from BRL1/3, which possibly results from positive selection via gain of function during evolution.

BES1/BZR1 Family Transcription Factors Regulate Plant Development via Brassinosteroid-Dependent and Independent Pathways

The BES1/BZR1 family is a plant-specific small group of transcription factors possessing a non-canonical bHLH domain. Genetic and biochemical analyses within the last two decades have demonstrated that members of this family are key transcription factors in regulating the expression of brassinosteroid (BR) response genes. Several recent genetic and evolutionary studies, however, have clearly indicated that the BES1/BZR1 family transcription factors also function in regulating several aspects of plant development via BR-independent pathways, suggesting they are not BR specific. In this review, we summarize our current understanding of this family of transcription factors, the mechanisms regulating their activities, DNA binding motifs, and target genes. We selectively discuss a number of their biological functions via BR-dependent and particularly independent pathways, which were recently revealed by loss-of-function genetic analyses. We also highlight a few possible future directions.