Understanding the mechanistic basis of balanced excitation energy distribution
between photosystem II and photosystem I (PSII and PSI) requires detailed investigation
of the thylakoid light-harvesting system composed of energetically connected
LHCII trimers. The exact mechanisms controlling the excitation energy distribution
remain elusive, but reversible phosphorylation is known to be one important component.
Here, we addressed the role of grana margins in regulation of excitation
energy distribution, as these thylakoid domains host all the complexes of photosynthetic
light reactions with dynamic response to environmental cues. First, the effect
of detergents for the thylakoid membrane connectivity is explained. We show that
a specific interaction between the separate LHCII trimers as well as between the
LHCII trimers and the PSII and PSI–LHCI complexes is a prerequisite for energetically
connected and functional thylakoid membrane. Second, we demonstrate that
the optimization of light reactions under changing light conditions takes place in
energetically connected LHCII lake and is attained by lateral rearrangements of the
PSII–LHCII and PSI–LHCI–LHCII complexes depending especially on the phosphorylation
status of the LHCII protein isoform LHCB2.