title: BAK1 and BKK1 regulate brassinosteroid-dependent growth and brassinosteroid-independent cell-death pathways.
abstract: Brassinosteroids (BRs) are phytosteroid hormones controlling various physiological processes critical for normal growth and development. BRs are perceived by a protein complex containing two transmembrane receptor kinases, BRASSINOSTEROID INSENSITIVE 1 (BRI1) and BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) [1-3]. BRI1 null mutants exhibit a dwarfed stature with epinastic leaves, delayed senescence, reduced male fertility, and altered light responses. BAK1 null mutants, however, only show a subtle phenotype, suggesting that functionally redundant proteins might be present in the Arabidopsis genome. Here we report that BAK1-LIKE 1 (BKK1) functions redundantly with BAK1 in regulating BR signaling. Surprisingly, rather than the expected bri1-like phenotype, bak1 bkk1 double mutants exhibit a seedling-lethality phenotype due to constitutive defense-gene expression, callose deposition, reactive oxygen species (ROS) accumulation, and spontaneous cell death even under sterile growing conditions. Our detailed analyses demonstrate that BAK1 and BKK1 have dual physiological roles: positively regulating a BR-dependent plant growth pathway, and negatively regulating a BR-independent cell-death pathway. Both BR signaling and developmentally controlled cell death are critical to optimal plant growth and development, but the mechanisms regulating early events in these pathways are poorly understood. This study provides novel insights into the initiation and crosstalk of the two signaling cascades.
authors.affiliation: Department of Botany and Microbiology, University of Oklahoma, Norman, OK 73019, USA.
authors.forename: Kai
authors.initials: K
authors.lastname: He
authors.forename: Xiaoping
authors.initials: X
authors.lastname: Gou
authors.forename: Tong
authors.initials: T
authors.lastname: Yuan
authors.forename: Honghui
authors.initials: H
authors.lastname: Lin
authors.forename: Tadao
authors.initials: T
authors.lastname: Asami
authors.forename: Shigeo
authors.initials: S
authors.lastname: Yoshida
authors.forename: Scott D
authors.initials: SD
authors.lastname: Russell
authors.forename: Jia
authors.initials: J
authors.lastname: Li
mesh_terms: D017360:Arabidopsis / Q000254:growth & development* / Q000378:metabolism / Q000502:physiology; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D016923:Cell Death / Q000502:physiology; D010840:Phytosterols / Q000378:metabolism*; D010937:Plant Growth Regulators / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology*
chemical_list: D029681:Arabidopsis Proteins; D010840:Phytosterols; D010937:Plant Growth Regulators; C463660:BAK1 protein, Arabidopsis; D017346:Protein Serine-Threonine Kinases