Cellular responses to naphthoquinones: juglone as a case study

The effects of juglone (JG) on the endogenous growth, growth in the presence of either indoleacetic acid (IAA) or fusicoccin (FC) and on proton extrusion were studied in maize coleoptile segments. In addition, membrane potential changes were also determined at chosen JG concentrations. It was found that JG, when added to the incubation medium, inhibited endogenous growth as well as growth in the presence of either IAA or FC. Simultaneous measurements of growth and external pH indicated that inhibition of either IAA-induced growth or proton extrusion by JG was a linear function of JG concentration. Addition of JG to the control medium caused depolarization of the membrane potential (E-m), value of which was dependent on JG concentration and time after its administration. Hyperpolarization of E-m induced by IAA was suppressed in the presence of JG. It was also found that for coleoptile segments initially preincubated with JG, although subsequently removed, addition of IAA was not effective in the stimulation of growth and medium acidification. Taken together, these results suggest that the mechanism by which JG inhibits the IAA-induced growth of maize coleoptile segments involves inhibition of PM H+-ATPase activity.