An Arabidopsis thaliana transcript ( AtPNP-A) encoding an immunoreactant plant natriuretic peptide (irPNP) analog was identified and isolated. The encoded protein shows similarity to CjBAp12, a functionally undefined protein from citrus that is induced in response to blight infection. CjBAp12 shows significant sequence identity to domains found in the cell wall loosening expansins but has tested negative for cell wall loosening activity. We have thus undertaken to establish the evolutionary and functional relationships of irPNP-like molecules within the superfamily of expansins, pollen allergens, and distantly related molecules such as endoglucanases. We show that irPNP-like molecules are related to expansins and fall in two groups; one includes CjBAp12 and the other AtPNP-A. Members of both groups share distinct sequence motifs (K[VI]VD and [LM]SxxAFxxI) but do not contain the tryptophan and tyrosine rich C-terminal putative polysaccharide-binding domain typical of expansins or bacterial cellulases and hemicellulases. We argue that both irPNP-like molecules and expansin have evolved from primitive/ancestral glucanase-like molecules that hydrolysed the cell wall. Importantly, we have previously demonstrated that irPNPs act on protoplasts, that is plant cells without cell walls as well as microsomes, indicating that these novel proteins specifically interact with the plasma membrane. It follows that the cell wall cannot be an obligatory substrate for irPNPs. Thus, both irPNP function and domain structure point to these molecules having a systemic role in H2O and solute homeostasis.