Brassinosteroids regulate plant growth and development through a protein complex that includes the leucine-rich repeat receptor-like protein kinase (LRR-RLK) brassinosteroid-insensitive 1 (BRI1). Activation tagging was used to identify a dominant genetic suppressor of bri1, bak1-1D (bri1-associated receptor kinase 1-1Dominant), which encodes an LRR-RLK, distinct from BRI1. Overexpression of BAK1 results in elongated organ phenotypes, while a null allele of BAK1 displays a semidwarfed phenotype and has reduced sensitivity to brassinosteroids (BRs). BAK1 is a serine/threonine protein kinase, and BRI1 and BAK1 interact in vitro and in vivo. Expression of a dominant-negative mutant allele of BAK1 causes a severe dwarf phenotype, resembling the phenotype of null bri1 alleles. These results indicate BAK1 is a component of BR signaling.

Pep1 is a 23-amino acid peptide that enhances resistance to a root pathogen, Pythium irregulare. Pep1 and its homologs (Pep2 to Pep7) are endogenous amplifiers of innate immunity of Arabidopsis thaliana that induce the transcription of defense-related genes and bind to PEPR1, a plasma membrane leucine-rich repeat (LRR) receptor kinase. Here, we identify a plasma membrane LRR receptor kinase, designated PEPR2, that has 76% amino acid similarity to PEPR1, and we characterize its role in the perception of Pep peptides and defense responses. Both PEPR1 and PEPR2 were transcriptionally induced by wounding, treatment with methyl jasmonate, Pep peptides, and pathogen-associated molecular patterns. The effects of Pep1 application on defense-related gene induction and enhancement of resistance to Pseudomonas syringae pv tomato DC3000 were partially reduced in single mutants of PEPR1 and PEPR2 and abolished completely in double mutants. Photoaffinity labeling and binding assays using transgenic tobacco (Nicotiana tabacum) cells expressing PEPR1 and PEPR2 clearly demonstrated that PEPR1 is a receptor for Pep1-6 and that PEPR2 is a receptor for Pep1 and Pep2. Our analysis demonstrates differential binding affinities of two receptors with a family of peptide ligands and the corresponding physiological effects of the specific receptor-ligand interactions. Therefore, we demonstrate that, through perception of Peps, PEPR1 and PEPR2 contribute to defense responses in Arabidopsis.

Seven dwarf mutants resembling brassinosteroid (BR)-biosynthetic dwarfs were isolated that did not respond significantly to the application of exogenous BRs. Genetic and molecular analyses revealed that these were novel alleles of BRI1 (BrassinosteroidInsensitive 1), which encodes a receptor kinase that may act as a receptor for BRs or be involved in downstream signaling. The results of morphological and molecular analyses indicated that these represent a range of alleles from weak to null. The endogenous BRs were examined from 5-week-old plants of a null allele (bri1-4) and two weak alleles (bri1-5 and bri1-6). Previous analysis of endogenous BRs in several BR-biosynthetic dwarf mutants revealed that active BRs are deficient in these mutants. However, bri1-4 plants accumulated very high levels of brassinolide, castasterone, and typhasterol (57-, 128-, and 33-fold higher, respectively, than those of wild-type plants). Weaker alleles (bri1-5 and bri1-6) also accumulated considerable levels of brassinolide, castasterone, and typhasterol, but less than the null allele (bri1-4). The levels of 6-deoxoBRs in bri1 mutants were comparable to that of wild type. The accumulation of biologically active BRs may result from the inability to utilize these active BRs, the inability to regulate BR biosynthesis in bri1 mutants, or both. Therefore, BRI1 is required for the homeostasis of endogenous BR levels.Based on their wide distribution in the plant kingdom, their diverse physiological effects at nanomolar levels, and the discovery of mutants deficient in their biosynthesis, it is now widely accepted that brassinosteroids (BRs) are important hormones that regulate growth and development

Our results provide evidence for a new regulatory mechanism for innate immune receptors with BIR2 acting as a negative regulator of PAMP-triggered immunity by limiting BAK1-receptor complex formation in the absence of ligands.

CorrectionsBIOCHEMISTRY. For the article ''Arg-302 facilitates deprotonation of Glu-325 in the transport mechanism of the lactose permease from Escherichia coli'' by Miklós Sahin-Tóth and H. Ronald Kaback, which appeared in number 11, May 22, 2001, of Proc. Natl. Acad. Sci. USA (98, 6068 -6073; First Published May 15, 2001; 10.1073͞pnas.111139698), Fig. 1 was printed incorrectly due to a printer's error. The correct figure and its legend appear below.www.pnas.org͞cgi͞doi͞10.1073͞pnas.151252598 Fig. 1.Model for H ϩ translocation during lactose͞H ϩ symport via lac permease. For clarity, 6 of the 12 helices that compose the permease are shown. The gray area designates the low dielectric environment of the lipid bilayer. (A) In the ground-state conformation, the relevant H ϩ is shared by His-322 (helix X) and Glu-269 (helix VIII), whereas Arg-302 (helix IX) is charge-paired with Glu-325 (helix X). In this conformation, lac permease binds substrate with high affinity at the outer surface (S o ). Glu-126 (helix IV) and Arg-144 (helix V) are charge-paired and represent the major components of the substrate-binding site. Also shown is the charge-pair between Asp-240 (helix VII) and Lys-319 (helix X), which are not essential for the mechanism. (B) Substrate binding induces a conformational change that disrupts the E269͞H322 and R302͞E325 charge-pairs and leads to the transfer of the H ϩ to Glu-325, now stabilized by the low dielectric environment. At the same time, the substrate-binding site becomes exposed to the inner surface of the membrane (S i ). After substrate dissociation, Glu-325 deprotonates at the inside surface (because of the rejuxtaposition of Glu-325 with Arg-302) as the permease relaxes back to the ground-state conformation.