Natriuretic peptides - A new class of plant hormone?

Recent immunological and functional evidence suggests the presence of a biologically active natriuretic peptide hormone (NP) system in plants. The evidence includes specific binding of rat atrial NP [rANP (99-126)] to isolated plant membranes and the promotion of stomatal opening that is concentration and conformation dependent. The native circular molecule is active whereas the linearized molecule shows no biological activity. Stomatal opening mediated by rANP (99-126) is inhibited by LY 83583, an antagonist of guanylate cyclase, while 8-Br-cGMP, a cell permeant cyclic guanosine-3'-5'-monophosphate (cGMP) analogue, mimics rANP (99-126) effects. Most importantly, isolation and immunoaffinity purification of biologically active plant NP (irPNP) from Hedera helix has been achieved and immunoaffinity purified peptide has been shown to induce rapid and specific increases in cGMP levels in Zea mays root stele. Furthermore, rANP (99-126), irPNP and cGMP promote radial water movements from the xylem of Tradescantia multiflora and these NP-induced and cGMP-dependent increases are prevented by a water channel inhibitor. Taken together, the data are consistent with the presence of a biologically active NP system that, as in vertebrates, signals via cGMP dependent pathways. The evidence suggests that the NP system has a role in maintaining water and salt homeostasis in plants.