Protective mechanism chloroplast

Thylakoid packing ratios ewed a significant loss of actual thylakoid surface area following exposure to hi irradianoe (Table 2). Presumably the potential efficiency with vhich photons mi t be captured hy the palisade tissue would also decrease, thus acting as one mechanism to protect the chloroplast from photo-damaged membranes. 

Krause, G.H. Behrend, U. (1986). A pH-dependent chlorophyll fluorescence quenching indicating a mechanism of protection against photoinhibition of chloroplasts. FEBS Letters, 200, 298-302. 

However, cells of Chlamydomonas treated with ethidium bromide are able later to regenerate their chloroplast DNA. This result has been interpreted to mean that there may be one or a few "master copies" of chloroplast DNA in specially protected locations. The result should also be considered in relationship to the following observation. Although nuclear and organelle DNA molecules replicate at different times in the cell cycle, constant proportions of the organelle and nuclear DNA tend to be maintained. Thus, there must be some kind of control mechanism leading to a coupling of DNA replication in nuclei, mitochondria, and chloroplasts.465... 

While the flavonoids suppress oxygen uptake in isolated mitochondria and oxygen evolution from chloroplasts, there has been too little work to establish these organelle effects as the only mechanisms of action. Flavonoids are known to protect membrane lipids against destructive reactions and, based on current evidence, these compounds do not readily fit the model of Figure 11.2. The flavonol rutin did not show an effect on soybean seedling water relations.64 It is... 

Tricolorin A (46) and G (47) are prototype members of this class of amphiphilic glycoconjugates (24). They constitute the allelochemical principles of Ipomoea tricolor Cav., a plant used in traditional agriculture in Mexico as a cover crop to protect sugar cane against invasive weeds. Their molecular mechanism of action likely involves the inhibition of the FT-ATPase of the plasma membrane, an enzyme that plays a crucial role in plant cell physiology. Moreover, 46 acts as a natural uncoupler of photophosphorylation in spinach chloroplasts. This compound also displays general cytotoxicity against several... 

Isolated lettuce chloroplasts could epoxidize zeaxanthin in the presence of reduced pyridine nucleotides and oxygen and the process was stimulated by bovine serum albumin (which protected the epoxidase system from inhibition by fatty acids). Detailed study led to the conclusion that the epoxidase was an external monoxygenase and that the violaxanthin cycle (of which epoxidation of zeaxanthin is a part) was a trans-membrane system wherein de-epoxidation took place on the loculus side and epoxidation on the stroma side of the membrane. This arrangement requires migration of the carotenoids of the violaxanthin cycle across the membrane in a type of shuttle. The possible role of this cycle in some regulatory mechanism of photosynthesis at the membrane level was also discussed. 

The nuclear gene is translated in the cytoplasm, and the protein is then transported to the chloroplast, protected by a chaperonin, via targeting mechanisms (see Section 12.6 and the Biochemical Connections box on page xxx). Special targeting sequences are used to direct various nuclear products to the appropriate chloroplast location, using reactions that require ATP hydrolysis. The chaperonin aids in formation of the final, active complex. 

We are currently pursuing our investigation into the heat-shock protection against heat-induced damage to the water-oxidizing complex of PSII. Since several heat-shock proteins are imported into chlorplasts, and since the most heat-labile component of the chloroplast is the watersplitting complex, we feel we have developed a system in which to address the mechanism of heat-shock protein protection of the photosynthetic apparatus. 

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