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 (By similarity). Regulates negatively brassinolide (BL)-mediated signaling pathway (e.g. BL-induced hypocotyl elongation and branching limitation) (PubMed:19473327). In very young seedlings, confined to the tips of the cotyledons. Later expressed in the cotyledon vasculature and petiole. Present in the tip of emerging leaves. In mature leaves, observed in the vasculature. Repressed by brassinolide (BL) treatment. Proteolytically cleaved, probably by SBT6.1 (S1P), a subtilisin-like serine protease (subtilase). Belongs to the plant rapid alkalinization factor (RALF) family.