Unavoidable side reactions of photosynthetic energy conversion can damage the water-splitting photosystem II (PSII) holocomplex embedded in the thylakoid membrane system inside chloroplasts. Plant survival is crucially dependent on an efficient molecular repair of damaged PSII realized by a multistep repair cycle. The PSII repair cycle requires a brisk lateral protein traffic between stacked grana thylakoids and unstacked stroma lamellae that is challenged by the tight stacking and low protein mobility in grana. We demonstrated that high light stress induced two main structural changes that work synergistically to improve the accessibility between damaged PSII in grana and its repair machinery in stroma lamellae: lateral shrinkage of grana diameter and increased protein mobility in grana thylakoids. It follows that high light stress triggers an architectural switch of the thylakoid network that is advantageous for swift protein repair. Studies of the thylakoid kinase mutant stn8 and the double mutant stn7/8 demonstrate the central role of protein phosphorylation for the structural alterations. These findings are based on the elaboration of mathematical tools for analyzing confocal laser-scanning microscopic images to study changes in the sophisticated thylakoid architecture in intact protoplasts.confocal microscopy | macromolecular crowding | photosynthesis P hotosynthetic transformation of sunlight into metabolic energy equivalents is a dangerous venture, because toxic side products of the primary photochemical processes can lead to uncontrolled damage. Harmful photosynthetic side reactions cannot be avoided completely and become a serious problem under stress (e.g., high light stress). The main target of photoinhibition (PI) is the D1 subunit of the water-splitting photosystem II (PSII) (1, 2). The D1 subunit is buried in the massive (1,400 kDa) PSII holocomplex that is organized as a dimer and binds between two and four trimeric light-harvesting complex IIs (LHCIIs) (3, 4). Estimates predict that without repair of damaged PSII, the efficiency for photosynthetic energy conversion would drop below 5% (5). Consequently, plants would not survive in a highly dynamic and competitive natural environment. Plants address this challenge through the evolutionary invention of one of the fastest and most efficient molecular repair mechanisms in nature, the PSII repair cycle (5-7).The PSII repair cycle consist of a series of events, including phosphorylation/ dephosphorylation of PSII subunits, holocomplex disassembly/reassembly, and D1 degradation/de novo synthesis (8-10). All of these reactions are harbored in or at the thylakoid membrane system inside the chloroplast. The complex folding of the thylakoid membrane leads to a structural differentiation into stacked grana regions interconnected by unstacked stroma lamellae (11)(12)(13). This structural heterogeneity is accompanied by, and partly driven by, differential protein distributions. PSII and LHCII are concentrated in grana stacks, photosystem I (PSI) and the ATPas...

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This paper presents the results of a study to develop improved design guidance for high- to low-speed transition zones on rural highways. The primary steps of the research included a literature review and state-of-practice review on speed reduction treatments used in transition zones and observational field studies of several transition zone treatments in the United States. The principal findings from the observation field studies are that roundabouts and transverse pavement markings increase the rate of compliance of vehicles traveling at or below the speed limit at the end of a transition zone by 15% and 20%, respectively, as compared with no treatment. Roundabouts increase the rate of compliance of vehicles traveling within 5 mph (8 km/h) of the speed limit at the end of a transition zone by 11%, compared with no treatment. The findings support previous research that indicates the need to provide additional measures through the community to maintain speed reduction downstream of the transition zone through the community. This paper also addresses several issues to be considered in the design of high- to low-speed transition zones, including definitions and site characteristics of the geographical limits or boundaries of the transition zone study area; a methodology for assessing whether a high- to low-speed transition zone has speed limit compliance or safety issues to support the need for, and the selection of, an appropriate treatment to address the issues; and guiding principles and design concepts to be considered in the design of a transition zone.