The coleoptile chloroplast: Distinctdistribution of xanthophyll cycle pigments, andenrichment in Photosystem II

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• 作者：Zhu ; Jianxin ; Gó ; mez ; Stephen M. ; Mawson ; Bruce T. ; Jin ; Xiangqun ; Zeiger ; Eduardo
• 关键词：coleoptile chloroplast ; electrontransport ; fluorescence emission ; pigment– ; proteincomplexes ; xanthophyll cycle ; zeaxanthin
• 刊名：Photosynthesis Research
• 出版年：1997
• 出版时间：1997
• 年：1997
• 卷：51
• 期：2
• 页码：137-147
• 全文大小：630 KB

文摘

Recent studies have shown that coleoptile chloroplastsoperate the xanthophyll cycle, and that theirzeaxanthin concentration co-varies with theirsensitivity to blue light. The present studycharacterized the distribution of photosyntheticpigments in thylakoid pigment–protein complexes fromdark-adapted and light-treated coleoptile andmesophyll chloroplasts, the low temperaturefluorescence emission spectra, and the rates of PS Iand PS II electron transport in both types ofchloroplasts from 5-day-old corn seedlings. Pigmentswere extracted from isolated PS I holocomplex, LHC IIbtrimeric and LHC II monomeric complexes and analyzedby HPLC. Chlorophyll distribution in coleoptilethylakoids showed 31% of the total collected Chl inPS I and 65% in the light harvesting complexes ofPS II. In mesophyll thylakoids, the values were 44%and 54%, respectively. Mesophyll and coleoptile PS Iholocomplexes differed in their Chl t a/Chl tb ratios (8.1 and 6.1, respectively) andβ-carotene content. In contrast, mesophyll andcoleoptile LHC IIb trimers and LHC II monomers hadsimilar Chl t a/Chl t b ratios and β-carotene content. The threeanalyzed pigment–proteincomplexes from dark-adapted coleoptile chloroplastscontained zeaxanthin, whereas there was no detectablezeaxanthin in the complexes from dark-adaptedmesophyll chloroplasts. In both chloroplast types,zeaxanthin and antheraxanthin increased markedly inthe three pigment–protein complexes upon illumination,while violaxanthin decreased. In mesophyll thylakoids,zeaxanthin distribution as a percentage of thexanthophyll cycle pool was: LHC II monomers $>$ LHCIIb trimers $>$ PS I holocomplex, and in coleoptilethylakoids, it was: LHC IIb trimers $>$ LHC IImonomers = PS I holocomplex. Low temperature (77 K)fluorescence emission spectra showed that the 686 nmemission of coleoptile chloroplasts was approximately50% larger than that of mesophyll chloroplasts whennormalized at 734 nm. The pigment and fluorescenceanalysis data suggest that there is relatively morePS II per PS I and more LHC I per CC I in coleoptilechloroplasts than in mesophyll chloroplasts.Measurements of t in vitro uncoupledphotosynthetic electron transport showed approximately60% higher rates of electron flow through PS II incoleoptile chloroplasts than in mesophyllchloroplasts. Electron transport rates through PS Iwere similar in both chloroplast types. Thus, whencompared to mesophyll chloroplasts, coleoptilechloroplasts have a distinct PS I pigment composition,a distinct chlorophyll distribution between PS I andPS II, a distinct zeaxanthin percentage distributionamong thylakoid pigment–protein complexes, a higherPS II-related fluorescence emission, and higher PS IIelectron transport capacity. These characteristics maybe associated with a sensory transducing role ofcoleoptile chloroplasts.