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 ofBRI1 (Brassinosteroid-Insensitive 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.

Auxins are growth regulators involved in virtually all aspects of plant development. However, little is known about how plants synthesize these essential compounds. We propose that the level of indole-3-acetic acid is regulated by the flux of indole-3-acetaldoxime through a cytochrome P450, CYP83B1, to the glucosinolate pathway. A T-DNA insertion in the CYP83B1 gene leads to plants with a phenotype that suggests severe auxin overproduction, whereas CYP83B1 overexpression leads to loss of apical dominance typical of auxin deficit. CYP83B1 N-hydroxylates indole-3-acetaldoxime to the corresponding aci-nitro compound, 1-aci-nitro-2-indolyl-ethane, with a K(m) of 3 microM and a turnover number of 53 min(-1). The aci-nitro compound formed reacts non-enzymatically with thiol compounds to produce an N-alkyl-thiohydroximate adduct, the committed precursor of glucosinolates. Thus, indole-3-acetaldoxime is the metabolic branch point between the primary auxin indole-3-acetic acid and indole glucosinolate biosynthesis in Arabidopsis.

Since the isolation and characterization of dwarf1-1 (dwf1-1) from a T-DNA insertion mutant population, phenotypically similar mutants, including deetiolated2 (det2),constitutive photomorphogenesis and dwarfism(cpd), brassinosteroid insensitive1 (bri1), and dwf4, have been reported to be defective in either the biosynthesis or the perception of brassinosteroids. We present further characterization ofdwf1-1 and additional dwf1 alleles. Feeding tests with brassinosteroid-biosynthetic intermediates revealed that dwf1 can be rescued by 22α-hydroxycampesterol and downstream intermediates in the brassinosteroid pathway. Analysis of the endogenous levels of brassinosteroid intermediates showed that 24-methylenecholesterol in dwf1 accumulates to 12 times the level of the wild type, whereas the level of campesterol is greatly diminished, indicating that the defective step is in C-24 reduction. Furthermore, the deduced amino acid sequence of DWF1 shows significant similarity to a flavin adenine dinucleotide-binding domain conserved in various oxidoreductases, suggesting an enzymatic role for DWF1. In support of this, 7 of 10 dwf1 mutations directly affected the flavin adenine dinucleotide-binding domain. Our molecular characterization of dwf1 alleles, together with our biochemical data, suggest that the biosynthetic defect indwf1 results in reduced synthesis of bioactive brassinosteroids, causing dwarfism.