title: The tomato brassinosteroid receptor BRI1 increases binding of systemin to tobacco plasma membranes, but is not involved in systemin signaling.
abstract: The tomato wound signal systemin is perceived by a specific high-affinity, saturable, and reversible cell surface receptor. This receptor was identified as the receptor-like kinase SR160, which turned out to be identical to the brassinosteroid receptor BRI1. Recently, it has been shown that the tomato bri1 null mutant cu3 is as sensitive to systemin as wild type plants. Here we explored these contradictory findings by studying the responses of tobacco plants (Nicotiana tabacum) to systemin. A fluorescently-labeled systemin analog bound specifically to plasma membranes of tobacco suspension-cultured cells that expressed the tomato BRI1-FLAG transgene, but not to wild type tobacco cells. On the other hand, signaling responses to systemin, such as activation of mitogen-activated protein kinases and medium alkalinization, were neither increased in BRI1-FLAG-overexpressing tobacco cells nor decreased in BRI1-silenced cells as compared to levels in untransformed control cells. Furthermore, in transgenic tobacco plants BRI1-FLAG became phosphorylated on threonine residues in response to brassinolide application, but not in response to systemin. When BRI1 transcript levels were reduced by virus-induced gene silencing in tomato plants, the silenced plants displayed a phenotype characteristic of bri1 mutants. However, their response to overexpression of the Prosystemin transgene was the same as in control plants. Taken together, our data suggest that BRI1 can function as a systemin binding protein, but that binding of the ligand does not transduce the signal into the cell. This unusual behavior and the nature of the elusive systemin receptor will be discussed.
authors.affiliation: Department of Biological Sciences, University of South Carolina, Columbia, SC 29208, USA.
authors.forename: Robert
authors.initials: R
authors.lastname: Malinowski
authors.forename: Rebecca
authors.initials: R
authors.lastname: Higgins
authors.forename: Yuan
authors.initials: Y
authors.lastname: Luo
authors.forename: Laverne
authors.initials: L
authors.lastname: Piper
authors.forename: Azka
authors.initials: A
authors.lastname: Nazir
authors.forename: Vikramjit S
authors.initials: VS
authors.lastname: Bajwa
authors.forename: Steven D
authors.initials: SD
authors.lastname: Clouse
authors.forename: Paul R
authors.initials: PR
authors.lastname: Thompson
authors.forename: Johannes W
authors.initials: JW
authors.lastname: Stratmann
mesh_terms: D000595:Amino Acid Sequence; D060406:Brassinosteroids; D002462:Cell Membrane / Q000378:metabolism*; D002478:Cells, Cultured; D002777:Cholestanols / Q000378:metabolism / Q000494:pharmacology; D005456:Fluorescent Dyes / Q000737:chemistry; D020868:Gene Silencing; D015151:Immunoblotting; D047468:Immunoprecipitation; D018551:Solanum lycopersicum / Q000235:genetics / Q000378:metabolism; D008856:Microscopy, Fluorescence; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D030821:Plants, Genetically Modified; D011485:Protein Binding; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction; D013260:Steroids, Heterocyclic / Q000378:metabolism / Q000494:pharmacology; D013912:Threonine / Q000378:metabolism; D014026:Nicotiana / Q000166:cytology / Q000235:genetics / Q000378:metabolism
chemical_list: D060406:Brassinosteroids; D002777:Cholestanols; D005456:Fluorescent Dyes; D010455:Peptides; D010940:Plant Proteins; D013260:Steroids, Heterocyclic; D013912:Threonine; C073704:systemin; D011494:Protein Kinases; C023623:brassinolide