Control of the photosynthetic electron transport by PQ diffusion microdomains in thylakoids of higher plants

We investigate the role of plastoquinone (PQ) diffusion in the control of t he photosynthetic electron transport. A control analysis reveals an unexpec ted flux control of the whole chain electron transport by photosystem (PS) II. The contribution of PSII to the flux control of whole chain electron tr ansport was high in stacked thylakoids (control coefficient, CJ(PSII) = 0.8 5), but decreased after destacking (CJ(PSII) = 0.25). From an 'electron sto rage' experiment, we conclude that in stacked thylakoids only about 50 to 6 0% of photoreducable PQ is involved in the light-saturated linear electron transport. No redox equilibration throughout the membrane between fixed red ox groups at PSII and cytochrome (cyt) bf complexes, and the diffusable car rier PQ is achieved. The data support the PQ diffusion microdomain concept by Lavergne ct al. [J. Lavergne, J.-P. Bouchaud, P. Joliot, Biochim. Biophy s. Acta 1101 (1992) 13-22], but we come to different conclusions about size , structure and size distribution of domains. From an analysis of cyt bb re duction, as a function of PSII inhibition, we conclude that in stacked thyl akoids about 70% of PSII is located in small domains, where only 1 to 2 PSI I share a local pool of a few PQ molecules. Thirty percent of PSII is locat ed in larger domains. No small domains were found in destacked thylakoids. We present a structural model assuming a hierarchy of specific, strong and weak interactions between PSII core, light harvesting complexes (LHC) II an d cyt bf: Peripheral LHCII's may serve to connect PSII-LHCII supercomplexes to a flexible protein network, by which small closed lipid diffusion compa rtments are formed. Within each domain, PQ moves rapidly and shuttles elect rons between PSII and cyt bf complexes in the close vicinity. At the same t ime, long range diffusion is slow. We conclude, that in high light, cyt bf complexes located in distant stromal lamellae (20 to 30%) are not involved in the linear electron transport. (C) 2000 Elsevier Science B.V. All rights reserved.