Shedding light on photosynthesis

All plants use a form of photosynthesis to produce energy, though not all rely exclusively on it. In higher plants, capturing light takes place in specialized compartments called thylakoids. These are found in cell organelles called chloroplasts, which are the equivalent of a power station for the plant. Despite being well-defined from a biochemical perspective, photosynthesis is still a mystery when we consider what happens at the level of the cell. Collaborating in a study published in Plant Cell, EPFL scientists have used a range of microscopy and visualization techniques to understand how the thylakoids behave to capture light.

Photosynthesis involves the conversion of light into chemical energy for the plant. When light hits a plant’s leaves, the cells of that leaf capture light in organelles called chloroplasts. Chloroplasts contain chlorophyll, the light-absorbing pigment of plants that also gives them their green color. Chloroplasts also contain the membrane systems called thylakoids, which are the lipid membranes comprising embedded functional pigment-protein complexes. The largest photosynthetic pigment-protein antenna complex, known as light-harvesting complex II (LHCII), binds roughly half of the chlorophyll pool in the biosphere. The spatial organization of the thylakoid membranes is of particular interest as it is tightly related to the regulatory processes of photosynthesis.

Andrzej Kulik from Giovanni Dietler’s group at EPFL, collaborating with Wiesław Gruszecki at the Maria Curie-Sklodowska University and with researchers at the University,of Warsaw compared LHCII-membrane complexes isolated from spinach leaves. The difference lay in the amount of light the complexes had received: One group came from leaves adapted to the dark and the other from leaves previously exposed to high-intensity light. Using X-ray diffraction, infrared imaging microscopy, confocal laser scanning microscopy, and transmission electron microscopy, the researchers found that the dark-adapted LHCII-membranes complexes assembled into rivet-like stacks of bilayers (like a typical chloroplast membranes), while the pre-illuminated complexes formed 3-D forms that were considerably less structured.

The authors conclude that the formation of bilayer, rivet-like structures is crucial in determining how the thylakoid membrane structures itself in response to light exposure. Depending on how much light they receive, the membranes can either stack up on each other or unstack in order to better utilize the energy captured.