Darkness at Noon


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Work of art: A portion of a plant cell

The woman in the image has a gorgeous mane of hair that partly wraps her naked torso. This is not a newly discovered Modigliani. The “woman” is in fact a portion of a plant cell magnified more than twenty thousand times. (Above, top) In a project that won first prize in the Beauty of Science competition held recently in the Weizmann Institute’s Feinberg Graduate School, Onie Tsabari, a student in the lab of Prof. Ziv Reich of the Biological Chemistry Department, turned this image into a work of art. (Above, bottom)

Now in a new study published in the Proceedings of the National Academy of Sciences, USA, Reich and his colleagues from Washington State University have revealed why the “hair” in that image is so lush. It is in fact a network of tiny flattened vesicles called thylakoids which reside in the plant’s chloroplast and contain the machinery that carries out the primary steps of photosynthesis – the process by which plants, algae and certain bacteria use sunlight to produce chemical energy and, along the way, almost all of Earth’s oxygen. The new study – selected for the “Faculty of 1000” website of important articles on biology and medicine – has shown that the water-filled space within the thylakoids, called the lumen, nearly doubles in width when the leaves are exposed to light.

The expansion of the lumen facilitates photosynthesis by increasing the space available for the diffusion of a protein that transports electrons. These electrons are eventually used to convert carbon dioxide into sugars, and their flow is essential for the formation of ATP – the major energy molecule of living cells. The enlarging of the lumen also promotes the repair of damaged components of the photosynthetic apparatus – by facilitating their dismantling and shuttling of the components.

When light is replaced by darkness, the lumen shrinks, thus restricting the movement of proteins. This shrinkage is probably intended to prevent damage to the photosynthetic machinery at dawn or during rapid increases in light intensity (due to sudden changes in cloud cover, for instance). The gradually expanding lumen ensures that the rate of electron transport doesn’t rise too suddenly – such a rise could damage a key protein in the electron transport chain.
Prof. Ziv Reich
These findings overturn a long-standing belief that the thylakoid lumen contracts under light, which stemmed from the limitations of the research methods available in the early 1970s. The new study was performed with advanced techniques: A plant leaf was examined by cryo-electron microscopy after being rapidly vitrified at very high pressure in liquid nitrogen and cut up into ultra-thin slices, each only a few tens of nanometers (billionths of a meter) thick. The team, headed by the Institute’s Prof. Reich and Washington State University’s Prof. Helmut Kirchnoff included Washington State’s Chris Hall, Magnus Wood and Dr. Miroslava Hirbstova, and Weizmann’s Onie Tsabari, Dr. Reinat Nevo and Dr. Dana Charuvi of the Biological Chemistry Department, as well as Dr. Eyal Shimoni of the Electron Microscopy Unit.

In addition to revealing the beauty inside a common plant cell, the new findings provide crucial insights into the regulation of photosynthesis and into the ways by which plants adapt to changes in light conditions that occur throughout the day.
Prof. Ziv Reich’s research is supported by the Carolito Stiftung.