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.
Highlights
Several physiological processes related to growth, development, defence, and reproduction are coordinated by intercellular communication
Rapid alkalinization factor (RALF) is a peptide signal found throughout the plant kingdom that can exhibit either ubiquitous or tissue-specific expression patterns
Three lines with different levels of AtRALF1 transcripts were selected for the evaluation of root growth, hypocotyl elongation, and lateral root formation
Summary
Several physiological processes related to growth, development, defence, and reproduction are coordinated by intercellular communication. We showed that silencing of the AtRALF1 gene in Arabidopsis plants increased cell elongation and the formation of lateral roots, whereas AtRALF1 overexpression showed the opposite effects. Reduction of AtRALF1 transcripts in irAtRALF1 plants increases root length, lateral root number, hypocotyl elongation, and cell length
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