Increased Stacking Capacity by Modified Thylakoid Surfaces

Abstract

Most chloroplasts in higher plants have a differentiated thylakoidal system into zones with a potential for membrane adhesion forming grana stacks and zones of single unappressed membranes which lack this adhesion potential and form end-membranes, margins of grana and stroma lamellae. This structural differentiation is also accompanied by differences in function: Photosystem 2 (PS 2) together with most of the light-harvesting complex (LHC) is contained mainly in the appressed grana while Photosystem 1 (PS 1) is predominantly in the non-appressed membranes (Andersson, Haehnel 1982) together with the ATP synthetase (CF₀ and CF₁) (Faludi-Dániel et al. 1983). Although this differentiation is a very common ultrastructural feature, the nature and the control of membrane adherence is not understood completely. A well established hypothesis developed by Barber (1980) points to the importance of the low surface charge density as a prerequisite for membrane assembly. In the granum region negative charges originating from the occasionally occurring PS 1 particles are screened by the abundant LHC plus PS 2 particles and/or neutralized by cations. In the stroma thylakoids, however, negative surface density could not be sufficiently reduced by cation treatment to achieve a complete assembly (Barber et al. 1980). Here we show that EDTA washing of stroma thylakoids can unmask such regions of low surface charge density and facilitate the adherence of single membranes.