Cell signaling peptide that may regulate plant stress, growth, and development. Mediates a rapid alkalinization of extracellular space by mediating a transient increase in the cytoplasmic Ca(2+) concentration leading to a calcium-dependent signaling events through a cell surface receptor and a concomitant activation of some intracellular mitogen-activated protein kinases. Mostly active in roots. Prevents plant growth (e.g. root and leaf length). Suppresses cell elongation of the primary root by activating the cell surface receptor FER and triggering phosphorylation of AHA2 and subsequent extracellular alkalinization. Interacts with FER and promotes its phosphorylation and subsequent activation. Expressed in roots and stems. First detected in the root hair zone of seedlings, mostly in the vascular bundles, cortex, and endodermis. Later observed in hypocotyls and veins of cotyledons. More efficient to mediate cytoplasmic Ca(2+) accumulation when oxidized, but inactive when reduced. Proteolytically cleaved, probably by S1P, a subtilisin-like serine protease (subtilase). Longer roots. Belongs to the plant rapid alkalinization factor (RALF) family.