NATRIURETIC PEPTIDE-HORMONES PROMOTE RADIAL A ATER MOVEMENTS FROM THEXYLEM OF TRADESCANTIA SHOOTS

Immunological evidence suggests that plants, like vertebrates, contain natriuretic peptides (NPs) and that rat atrial NP (rANP) binds specif ically to plant membranes and promotes concentration and conformation- dependent stomatal opening. Stomatal opening and specific increases in cGMP levels were also observed in response to immunoreactive plant NP (irPNP). Here we report that both 1 mu M rANP and irPNP (100 ng total protein/100 mu L) significantly increase radial water movements out o f the xylem of shoots of Tradescantia multiflora. Enhanced radial wate r movements are also observed in response to the cell permeant cGMP an alogue 8-Br-cGMP (100 nM). The water channel inhibitor mercuric chlori de (HgCl2) sig-nificantly inhibits radial water movements at concentra tions of 50 mu M, while the presence of 10 mu M 2-hydroxyethylmercapto ethanol (ME) prevents the inhibitory effect of the mercurial. The guan ylate cyclase inhibitor LY 83583 at a concentration of 20 mu M and sod ium azide (NaN3) at concentrations of greater than or equal to 1 mu M both also reduce radial water movements. We therefore conclude that th e regulation of radial water movement out of the xylem involves modula tion of cGMP levels, water channels and respiration-dependent processe s. In addition, we propose that NPs have a critical role to play in ra dial water movements out of the xylem and speculate that as in vertebr ates, NP effects might, at least in part, be mediated via the regulati on of guanylate cyclases and water channels.