Shedding Light on Photoprotection

Figure 1: Professor Harry Y. Yamamoto standing next to a high-performance liquid chromatography apparatus, which is used to separate and quantify biomolecules (July 2003). Photo courtesy of Jennifer Henry.

Figure 2: Excess light promotes the production of antheraxanthin and zeaxanthin pigments from a parent compound, violaxanthin. Under low light, antheraxanthin and zeaxanthin are converted back to violaxanthin by the enzyme zeaxanthin epoxidase (ZE). Light gives rise to the acidic pH at which the violaxanthin de-epoxidase (VDE) reaction generates antheraxanthin and zeaxanthin. (Courtesy of H. Y. Yamamoto)

Figure 3: False-color image of a tobacco leaf tip. The blue and green regions, which have been exposed to continuous excess light, are actively photosynthesizing. Excess light energy is being released from the blue and green regions by non-photosynthetic quenching (NPQ) that is mediated by antheraxanthin and zeaxanthin. A strip of aluminum foil was removed from the leaf tip shortly before the image was captured. The tissue once covered by the foil is now red and yellow, indicating release of excess energy through fluorescence. As the dark-exposed band of tissue absorbs light, antheraxanthin and zeaxanthin will be formed, NPQ will be activated, and fluorescence will diminish. (Photo courtesy of H. Y. Yamamoto)

Professor Harry Yoshimi Yamamoto (Department of Molecular Biosciences and Bioengineering, see Figure 1) was recently honored by the American Society of Plant Biologists for his contributions to plant science. In July 2003, Prof. Yamamoto received the Charles Reid Barnes Life Membership Award for more than forty years of meritorious research on how plants respond to light. The work he first published in 1962 and continues today addresses how plants balance their need for light against the damage that light energy can cause. Prof. Yamamoto laid the foundation for a new field of inquiry, photoprotection research, which probes a fundamental challenge that plants face daily: if you can’t escape the sun, how do you avoid getting burned?

During the second half of the twentieth century, many aspects of how plants respond to light were described in previously unattainable detail. One such discovery has held Harry Yamamoto’s attention for more than 40 years. As a doctoral candidate at the University of California at Davis, Prof. Yamamoto published a novel finding:in the chloroplasts that green plant cells use to convert light into sugar, light induces changes in membrane composition. This process, now known as the xanthophyll cycle, involves reversible changes in the concentrations of three yellow membrane pigments (xanthophylls, see Figure 2). During his long career in the University of Hawaii at Manoa’s College of Tropical Agriculture and Human Resources (CTAHR), Prof. Yamamoto has engaged in research relating to tropical crops including macadamia nut, papaya, and acerola, but he has also continued his investigation of the xanthophyll cycle, seeking to uncover its purpose and significance. His work reveals details of an important mechanism by which plants protect themselves from excess light.

A plant is unable to control the amount of light to which it is exposed. Although light is necessary for photosynthesis, too much light can harm the molecular machinery that makes photosynthesis possible. Whereas an animal might react to intense sunlight by moving to a patch of shade, a plant adjusts its internal chemistry to avoid injury. If these physiological adjustments fail, the excess energy can cause destructive, unregulated chemical reactions that damage chloroplast components and impair photosynthesis.To survive high light, plants utilize non-destructive pathways that release the excess energy as light (fluorescence) or heat. The xanthophyll cycle, which is present in all higher plants, contributes to non-photosynthetic quenching (NPQ), the regulated release of excess light energy as heat (see Figure 3).

Prof. Yamamoto grew up in Honolulu and attended UH-Manoa, receiving a B.S. in Medical Technology in 1955. Funding opportunities led him to the University of Illinois at Urbana-Champaign, where he completed an M.S. in Food Technology. His doctoral studies, in the lab of Clinton O. Chichester at U.C. Davis, took place at a time when the work of Soviet scientists was available in English translation and brought him into contact with a paper by D. I. Sapozhnikov. The paper described reversible changes in leaf concentrations of violaxanthin in response to light. Support from a National Science Foundation pre-doctoral fellowship allowed Yamomoto to pursue his interest in the Sapozhnikov paper and perform the research that established the existence of a xanthophyll cycle. Upon completing a Biochemistry doctorate in 1962, he returned to Honolulu and joined the CTAHR faculty as a member of the Food Science and Technology Department. In his years with the College, Prof. Yamamoto has produced an important body of elegant, creative research and has served as acting director of research, associate dean of agricultural research, and chair of the former Department of Plant Molecular Physiology. Although retired, he continues to conduct research on a part-time basis.

Posted on September 5, 2003