Evidence that carotenoids are required for the accumulation of a functional photosystem II, but not photosystem I in the cotyledons of mustard seedlings

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Abstract
If Norflurazon-treated mustard (Sinapis alba L.) seedlings are grown in low-fluence-rate white light, accumulation of carotenoids is completely inhibited, while levels of chlorophyll (Chl) a and b are comparable to those of control seedlings. Measurements of fluorescence yield and oxygen evolution indicate that carotenoid-free, green cotyledons are unable to perform leephotosynthesis in vivo. When thylakoid membranes were prepared and electron transport was measured in vitro, only PSI but not PSII activity was detected. Solubilization of the photosystems from thylakoid membranes and separation by sucrose-gradient centrifugation confirmed that PSII is absent in carotenoid-free seedlings, while PSI is present. Western blot analysis for representative proteins of the four photosynthetic complexes showed that subunits 1 and 2 of PSI, the Rieske-iron sulfur-protein, the α-subunit of the CF1 moiety of the ATP-synthase complex, cytochrome b559 and the lumenal 33-kDa protein of the water-splitting apparatus of PSII are present in comparable amounts in Norflurazon-treated and control plants, while the amounts of Chl-binding proteins of PSII (the major light-harvesting Chl-a/b-binding protein of the antenna complex and the 51- and 44-kDa Chl-a-binding proteins) and two components of the PSII reaction center, (the D1 and D2 protein) are substantially reduced. The data indicate that accumulation of PSII polypeptides is either not inhibited or not completely inhibited in carotenoid-free mustard seedlings, but that assembly of a functional PSII complex does not occur. If Norflurazon-treated seedlings are transferred to water, lutein accumulates rapidly and reaches about 80% of the level detectable in control plants, while the level of other carotenoids is still less than 1%. The accumulation kinetics for lutein are similar to the kinetics for the appearance of PSII activity. This indicates that the availibility of lutein rather than that of other carotenoids might be rate-limiting for the appearance of PSII activity.

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