RALF, a 5-kDa ubiquitous polypeptide in plants, arrests root growth and development

A 5-kDa polypeptide was isolated from tobacco leaves that induced a rapid a lkalinization of the culture medium of tobacco suspension-cultured cells an d a concomitant activation of an intracellular mitogen-activated protein ki nase. An N-terminal sequence was obtained, and a cDNA coding for the 49-aa polypeptide was isolated from a tobacco cDNA library. The cDNA encoded a pr eproprotein of 115 amino acids that contained the polypeptide at its C term inus. A search among known expressed sequence tags revealed that genes enco ding Rapid ALkalinization Factor (RALF) preproproteins were present in vari ous tissues and organs from 16 species of plants representing 9 families. A tomato homolog of the polypeptide was synthesized and, when supplied to ge rminating tomato and Arabidopsis seeds, it caused an arrest of root growth and development. Although its specific role in growth has not been establis hed, the polypeptide joins the ranks of the increasing number of polypeptid e hormones that are known to regulate plant stress, growth, and development .