THE COLEOPTILE CHLOROPLAST - DISTINCT DISTRIBUTION OF XANTHOPHYLL CYCLE PIGMENTS, AND ENRICHMENT IN PHOTOSYSTEM-II

Recent studies have shown that coleoptile chloroplasts operate the xan thophyll cycle, and that their zeaxanthin concentration co-varies with their sensitivity to blue light. The present study characterized the distribution of photosynthetic pigments in thylakoid pigment-protein c omplexes from dark-adapted and light-treated coleoptile and mesophyll chloroplasts, the low temperature fluorescence emission spectra, and t he rates of PS I and PS II electron transport in both types of chlorop lasts from 5-day-old corn seedlings. Pigments were extracted from isol ated PS I holocomplex, LHC IIb trimeric and LHC II monomeric complexes and analyzed by HPLC. Chlorophyll distribution in coleoptile thylakoi ds showed 31% of the total collected Chi in PS I and 65% in the light harvesting complexes of PS II. In mesophyll thylakoids, the values wer e 44% and 54%, respectively. Mesophyll and coleoptile PS I holocomplex es differed in their Chi a/Ch1 b ratios (8.1 and 6.1, respectively) an d beta-carotene content. In contrast, mesophyll and coleoptile LHC nb trimers and LHC II monomers had similar Chi a/Ch1 b ratios and beta-ca rotene content. The three analyzed pigment-protein complexes from dark -adapted coleoptile chloroplasts contained zeaxanthin, whereas there w as no detectable zeaxanthin in the complexes from dark-adapted mesophy ll chloroplasts. In both chloroplast types, zeaxanthin and antheraxant hin increased markedly in the three pigment-protein complexes upon ill umination, while violaxanthin decreased. In mesophyll thylakoids, zeax anthin distribution as a percentage of the xanthophyll cycle pool was: LHC II monomers > LHC IIb trimers > PS I holocomplex, and in coleopti le thylakoids, it was: LHC IIb trimers > LHC II monomers = PS I holoco mplex. Low temperature (77 K) fluorescence emission spectra showed tha t the 686 nm emission of coleoptile chloroplasts was approximately 50% larger than that of mesophyll chloroplasts when normalized at 734 nm. The pigment and fluorescence analysis data suggest that there is rela tively more PS II per PS I and more LHC I per CC I in coleoptile chlor oplasts than in mesophyll chloroplasts. Measurements of in vitro uncou pled photosynthetic electron transport showed approximately 60% higher rates of electron flow through PS II in coleoptile chloroplasts than in mesophyll chloroplasts. Electron transport rates through PS I were similar in both chloroplast types. Thus, when compared to mesophyll ch loroplasts, coleoptile chloroplasts have a distinct PS I pigment compo sition, a distinct chlorophyll distribution between PS I and PS II, a distinct zeaxanthin percentage distribution among thylakoid pigment-pr otein complexes, a higher PS II-related fluorescence emission, and hig her PS II electron transport capacity. These characteristics may be as sociated with a sensory transducing role of coleoptile chloroplasts.