Chapter 7 - RALF Peptides

Abstract

This chapter focuses on Rapid Alkalinization Factor (RALF) peptides. RALF peptides are composed of 49–52 amino acids. RALF peptides were discovered in leaf extracts of tobacco during a search for signaling peptides and are ubiquitous in plants. When added to suspension-cultured plant cells at low nM concentrations, the peptides cause a rapid activation of a MAP kinase and the blockage of a membrane-bound ATP-dependent proton pump that result in the rapid alkalinization of the culture medium. When added to the growth medium of germinating seeds, RALF causes an immediate arrest of root growth and elongation that can be reversed by transferring the seeds to a medium lacking the peptide. RALF peptides are cleaved from larger precursor proteins that are synthesized through the Golgi and secretory pathway and sequestered in the apoplast. RALF precursor genes are members of gene families that exhibit tissue-specific expression in roots, stems, leaves, and flowers. RALF interacts strongly and reversibly with a cell surface receptor complex composed of 120 kD and 25 kD RALF-binding proteins. In Arabidopsis roots, an endogenous RALF gene is expressed at high levels in slowly dividing cells and at low levels in rapidly dividing cells, suggesting that RALF peptides may play important tissue-specific roles in regulating cell division and cell expansion during growth and development.