Relationship between membrane potential and ATP level in Xanthomonas campestris pv. malvacearum infected cotton cotyledons

Abstract

The transmembrane potential (Em) in cotton cotyledons (Gossypium hirsutum) was measured during the development of bacterial blight caused by Xanthomonas campestris pv. malvacearum. Em was recorded by intracellular glass microelectrodes 48 h after inoculation with 107 cells ml−1 of bacteria into the intercellular space. The Em of the healthy tissue was −170 mV in light and dark at K0+ = 1 mM. The Em of bacteria-inoculated tissue was −176 mV in the light and −95 mV, the apparent diffusion potential (ED), in the dark. In the light Em of the diseased tissue, in contrast to the control, became very sensitive to CN−, FCCP (p-trifluoromethoxy carbonyl cyanide phenylhydrazone) and DCMU (3-(3′,4′-dichlorophenyl)-1,1-dimethylurea) and was rapidly lowered to the value of ED.In contrast to Em, ATP level and respiration rate were slightly higher in diseased than in healthy tissue. The ATP level remained unchanged in the dark. Inhibition of bacterial growth by rifampicin 28 h after inoculation had no effect on either the ATP level or respiration of the host tissue. Also photosynthetic O2 evolution was primarily unaffected in diseased tissue.Fusicoccin (FC) hyperpolarized Em by 30 mV in the light in healthy and diseased cotyledons. But there was no FC effect on Em in diseased tissue in the dark. The results suggest either pathogen-induced changes in the energy sink or an uncoupling of the electrogenic proton pump in the plasmalemma from energy supply in the dark. This is due possibly to a pathogen-induced disturbance of plant intracellular pH regulation.