Plants need a highly responsive regulatory system to keep photosynthetic light reactions in balance with the needs and restrictions of the downstream metabolism. This mechanism optimises plant growth under naturally fluctuating light conditions. In this opinion article, we present a model addressing the biological role of the light intensity-controlled phosphorylation of light-harvesting complex II (LHCII) proteins and its relation with the non-photochemical quenching of excitation energy (NPQ). We overturn a long held view of the possible role of 'state transitions'. Instead, we discuss the interplay between LHCII protein phosphorylation and NPQ, a mechanism that is crucial for regulating excitation energy distribution to the two photosystems (PSII and PSI) and balancing the intersystem electron flow despite constant fluctuations in light intensity.
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