Plant defensins are small, highly stable, cysteine-rich antimicrobial proteins 
that are thought to constitute an important component of plant defense against 
fungal pathogens. There are a number of such defensins expressed in various 
plant tissues with differing antifungal activity and spectrum. Relatively little 
is known about the modes of action and biological roles of these proteins. Our 
previous work on a virally encoded fungal toxin, KP4, from Ustilago maydis and 
subsequently with the plant defensin, MsDef1, from Medicago sativa demonstrated 
that some of these proteins specifically blocked calcium channels in both fungi 
and animals. The results presented here demonstrate that KP4 and three plant 
defensins, MsDef1, MtDef2, and RsAFP2, all inhibit root growth in germinating 
Arabidopsis seeds at low micromolar concentrations. We have previously 
demonstrated that a fusion protein composed of Rab GTPase (RabA4b) and enhanced 
yellow fluorescent protein (EYFP) is dependent upon calcium gradients for 
localization to the tips of the growing root hairs in Arabidopsis thaliana. 
Using this tip-localized fusion protein, we demonstrate that all four proteins 
rapidly depolarize the growing root hair and block growth in a reversible 
manner. This inhibitory activity on root and root hair is not directly 
correlated with the antifungal activity of these proteins and suggests that 
plants apparently express targets for these antifungal proteins. The data 
presented here suggest that plant defensins may have roles in regulating plant 
growth and development.