Photosystem biosynthesis

The present work illustrates in some detail the result of a unique mutation, one that impaired the biosynthesis of Chi b and resulted in a truncated Chi antenna size for the photosystems [Tanaka et al. 1998]. In the present Chi Mess mutant, PSII contained 93 Chi a molecules and PSI contained 246 Chi a molecules (Table 3). These antenna sizes are significantly larger than the PSII-core and PSl-core antennae, suggesting that Chi b may not be absolutely essential for the assembly of all Chi a-b light-harvesting complexes (see also [Ghirardi et al. 1986]). This was especially true for PSI which, in the Chi Mess mutant, had a Chi antenna size almost as large as that of the control (Table 3). It may be concluded that the Chi Mless mutation can be overcome by a nearly quantitative substitution of Chi b with Chi a in the Chi antenna of PSI, and by a partial substitution by Chi a in the antenna of PSII. 

The mode of their action consists in the acetolactate synthase (ALS) inhibition and, as a result, in the inhibition of the branched-chain amino acid biosynthesis from acetolactate (90MI1, 92MI7, 98MI4). Results of QSAR studies were published (03MI3). Another approach was used to analyze a model of inhibition of photosystem II (96MI1). 

Rhee KH, Morris EP, Zheleva D, Hankamer B, Kiihlbrandt W and Barber J (1997) Two dimensional structure of plant Photosystem II at 8 A resolution. Nature 389 522-526 Rock CD and Zeevaart JAD (1991) The aba mutant of Arabidopsis thaliana is impaired in epoxy-carotenoid biosynthesis. Proc Natl Acad Sci USA 88 7496-7499 Rock CD, Bowlby NR, Hoffmann-Benning S and Zeevaart JAD (1992) The aba mutant of Arabidopsis thaliana (L) Heynh. has reduced chlorophyll fluorescence yields and reduced thylakoid stacking. Plant Physiol 100 1796-1801 Romer S, Humbeck K and Senger H (1990) Relationship between biosynthesis of carotenoids and increasing complexity of Photosystem I in mutant C-6D of Scenedesmus obliquus. Planta 182 216-222... 

Masuda T, Polle JEW and Melis A (2002) Biosynthesis and distribution of chlorophyll among the photosystems during recovery of the green alga Dunaliella salina from irradiance stress. Plant Physiol. 128 603-614... 

A number of other herbicides interfere with photosynthesis in specific ways. Amitrole inhibits biosynthesis of chlorophyll and carotenoids. The affected plants present a bleached appearance before they die because of the loss of their characteristic pigments. Another herbicide, atrazine, inhibits the oxidation of water to hydrogen ion and oxygen. Still other herbicides interfere with electron transfer in the two photosystems. In photosystem II, diuron inhibits electron transfer to plastoquinone, whereas bigyridylium herbicides accept electrons by competing with the electron acceptors in photosystem I. The inhibitors active in photosystem I include diquat and paraquat. The latter substance attained some notoriety when it was used to interfere with an... 

Substituted pyridazinone herbicides directly inhibit photosystem II, chloroplast pigment biosynthesis, and membrane lipid biosynthesis. Depending on the substitution, pyridazinones can specifically inhibit the synthesis of linolenic acid in galacto-lipids and phospholipids preferentially alter the fatty acid composition of monogalactosyl diglycerides compared with digalactosyl diglycerides and cause a build-up of saturated fatty acids in the chloroplast membranes. 

In conclusion, our present study shows that repair of photosystem II after strong illumination and photoinhibition does not only Involve biosynthesis and insertion of Dl-protein. It also involves religation of manganese, assembly of several integral and extrinsic photosystem II subunits, and lateral migration between the two thylakoid regions. 

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