title: Arabidopsis thaliana RALF1 opposes brassinosteroid effects on root cell elongation and lateral root formation.
abstract: Rapid alkalinization factor (RALF) is a peptide signal that plays a basic role in cell biology and most likely regulates cell expansion. In this study, transgenic Arabidopsis thaliana lines with high and low levels of AtRALF1 transcripts were used to investigate this peptide's mechanism of action. Overexpression of the root-specific isoform AtRALF1 resulted in reduced cell size. Conversely, AtRALF1 silencing increased root length by increasing the size of root cells. AtRALF1-silenced plants also showed an increase in the number of lateral roots, whereas AtRALF1 overexpression produced the opposite effect. In addition, four AtRALF1-inducible genes were identified: two genes encoding proline-rich proteins (AtPRP1 and AtPRP3), one encoding a hydroxyproline-rich glycoprotein (AtHRPG2), and one encoding a xyloglucan endotransglucosylase (TCH4). These genes were expressed in roots and involved in cell-wall rearrangement, and their induction was concentration dependent. Furthermore, AtRALF1-overexpressing plants were less sensitive to exogenous brassinolide (BL); upon BL treatment, the plants showed no increase in root length and a compromised increase in hypocotyl elongation. In addition, the treatment had no effect on the number of emerged lateral roots. AtRALF1 also induces two brassinosteroid (BR)-downregulated genes involved in the BR biosynthetic pathway: the cytochrome P450 monooxygenases CONSTITUTIVE PHOTOMORPHISM AND DWARFISM (CPD) and DWARF4 (DWF4). Simultaneous treatment with both AtRALF1 and BL caused a reduction in AtRALF1-inducible gene expression levels, suggesting that these signals may compete for components shared by both pathways. Taken together, these results indicate an opposing effect of AtRALF1 and BL, and suggest that RALF's mechanism of action could be to interfere with the BR signalling pathway.
authors.affiliation: Laboratório de Bioquímica de Proteínas, Departamento de Ciências Biológicas, Escola Superior de Agricultura Luiz de Queiroz, Universidade de São Paulo (ESALQ/USP), Piracicaba, SP, 13418-900, Brazil.
authors.forename: Tábata
authors.initials: T
authors.lastname: Bergonci
authors.forename: Bianca
authors.initials: B
authors.lastname: Ribeiro
authors.forename: Paulo H O
authors.initials: PH
authors.lastname: Ceciliato
authors.forename: Juan Carlos
authors.initials: JC
authors.lastname: Guerrero-Abad
authors.forename: Marcio C
authors.initials: MC
authors.lastname: Silva-Filho
authors.forename: Daniel S
authors.initials: DS
authors.lastname: Moura
mesh_terms: D017360:Arabidopsis / Q000166:cytology / Q000235:genetics* / Q000254:growth & development* / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000235:genetics* / Q000378:metabolism*; D060406:Brassinosteroids / Q000378:metabolism; D018506:Gene Expression Regulation, Plant; D020868:Gene Silencing; D036361:Peptide Hormones / Q000235:genetics* / Q000378:metabolism*; D018517:Plant Roots / Q000166:cytology / Q000235:genetics* / Q000254:growth & development* / Q000378:metabolism; D016133:Polymerase Chain Reaction; D013260:Steroids, Heterocyclic / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; D060406:Brassinosteroids; D036361:Peptide Hormones; C529880:RALF1 protein, Arabidopsis; D013260:Steroids, Heterocyclic; C023623:brassinolide
keywords: Root development; brassinolide; peptide hormone.