Introduction to the Symposium‐in‐Print on Photosynthesis

Photochemistry in nature drives an incredible range of signaling pathways. Blue-light receptors, based on flavin chromophores, are found in virtually all major groups of organisms save for animals. Phytochromes abound in green plants, algae, fungi and some bacteria. Retinylidene proteins are essential for animal vision, but also play important signaling roles in certain bacteria. Somewhere waiting to be identified is a UVB receptor that modulates development in plants. However, the photochemistry of photosynthesis surpasses in magnitude that of all other photobiological processes in organisms combined. The retinylidene-based photoreceptors in some bacteria play a minuscule role in photosynthesis, but chlorophyll-driven photosynthesis predominates and is the focus of this Symposium-in-Print. The fixed carbon that it provides is essential to the life of virtually all other living organisms. Indeed, the most abundant protein on earth is the plant protein ribulose bisphosphate carboxylase, the key enzyme in capturing carbon dioxide and entering it into the Calvin cycle in plants and driving the wheel that spins off fixed carbon as carbohydrate. Ever since Joseph Priestley demonstrated that plants could ‘‘purify’’ air that had been ‘‘vitiated’’ by a candle or a mouse, plant biologists have worked to uncover and understand the complex mechanisms of this remarkable system for energy capture and transformation. Thus it comes as no surprise that we find people applying the most sophisticated modern techniques of molecular biology, biochemistry, genetics (both forward and reverse), cell biology, structural biology both at the organelle and molecular level, and spectroscopy. Each of these techniques is essential for one or another of the investigations that follow and sometimes several techniques play essential roles. In the papers that follow, the range and sophistication of the methodologies is truly impressive as are the consequent results. The papers fall into several groups. Of the 14, four deal with different aspects of protection of the photosynthetic apparatus from damage. As excess photoexcited chlorophyll can generate reactive oxygen species, it is not surprising that mechanisms to quench the photoexcited chlorophyll harmlessly and to repair any incurred damage have evolved. Thus Cai et al. (1) demonstrate that the cyclophilin ROC4 is an important component in repairing damage to Photosystem II incurred at high light intensity. It has been known for some time that excess acidification of the thylakoid lumen was a signal in higher plants to turn on the quenching mechanism and the protein PsbS serves as the sensor to initiate the process. Bonente et al. (2) find, however, that despite the commonality of photosynthetic mechanisms over a wide range of photosynthetic organisms, there seems not to be a PsbS homolog either in Chlamydomonas reinhardtii, a model unicellular alga used widely in photosynthetic studies, or in several other algae, even though these algae clearly are capable of quenching under high light conditions (nonphotochemical quenching or NPQ). Other mechanisms for detecting excess protons in the thylakoid lumen or some other consequence of excess light must have developed in the course of evolution. For a long time it was thought that cyanobacteria did not possess the NPQ mechanism well established in higher plants for protection against high light damage. Bailey and Grossman (3) review recent findings that demonstrate that cyanobacteria do indeed carry out nonphotochemical quenching, but use a different mechanism. A carotenoid binding protein in the phycobilisome core is likely a major player. Finally, Kitajima (4) reviews the role of two chloroplast-located peroxidases in scavenging potentially damaging hydrogen peroxide, and how the peroxide itself can damage these enzymes. Iron plays essential roles in photosynthesis. Iron is required in the iron–sulfur clusters of ferredoxin and in all of the photosynthetic cytochromes. However, it is possible to have too much iron. Excess iron can be toxic, a response attributed to the generation of reactive oxygen species. Long and Merchant (5) show that under excess iron and high light, several iron-assimilation components are selectively downregulated. Evidently light levels and iron nutrition interact in some fashion to optimize photosynthesis. Copper is another essential element in photosynthesis with its important role in the plastocyanin component of the electron transport pathway. Allen et al. (6) investigated the role of two enzymes involved in chlorophyll biosynthesis, components of the cyclase that closes the fifth ring found in all chlorophylls. One of these accumulates in copper-deficient conditions and the other predominates under copper-replete conditions. Both enzymes are found in both the chloroplast envelope and the thylakoid membranes. Baniulis et al. (7) then discuss the role of the cytochrome b6f in view of its known crystal structure and its interaction with the bc1 complex and review existing questions related to the shuttling of the various components of the electron transport pathway in the electron-transfer circuit in the light of cyclic and noncyclic electron transfer. Rounding out the studies on metal-containing components in photosynthesis is the work by Hanke and Hase (8). Using RNAi *Corresponding authors email: clin@mcdb.ucla. edu (Chentao Lin) and briggs@stanford.edu (Winslow Briggs) 2008TheAuthors. JournalCompilation.TheAmericanSociety ofPhotobiology 0031-8655/08 Photochemistry and Photobiology, 2008, 84: 1300–1301