Regulation of the photosynthetic apparatus under fluctuating growth light

Abstract

Safe and efficient conversion of solar energy to metabolic energy by plants is based on tightly inter-regulated transfer of excitation energy, electrons and protons in the photosynthetic machinery according to the availability of light energy, as well as the needs and restrictions of metabolism itself. Plants have mechanisms to enhance the capture of energy when light is limited for growth and development. Also, when energy is in excess, the photosynthetic machinery slows down the electron transfer reactions in order to prevent the production of reactive oxygen species and the consequent damage of the photosynthetic machinery. In this opinion paper, we present a partially hypothetical scheme describing how the photosynthetic machinery controls the flow of energy and electrons in order to enable the maintenance of photosynthetic activity in nature under continual fluctuations in white light intensity. We discuss the roles of light-harvesting II protein phosphorylation, thermal dissipation of excess energy and the control of electron transfer by cytochrome b(6)f, and the role of dynamically regulated turnover of photosystem II in the maintenance of the photosynthetic machinery. We present a new hypothesis suggesting that most of the regulation in the thylakoid membrane occurs in order to prevent oxidative damage of photosystem I.

Figure 1.

Model of integrative photoprotection of the electron transfer chain upon changing intensity of white light. Experiments with three ‘regulatory mutants of thylakoid function’, stn7, pgr5 and npq4, have enabled the formulation of the model and are included in the model, in addition to WT. (a) The electron transfer chain under constant low and moderate light intensity in WT and different mutants. (b) The situation upon sudden increase in light intensity and (c) upon prolonged high light stress with PSII photoinhibition. For details, see the text.