Light-stress-related changes in photosystem I (PS I) were analyzed in photoautotrophic cultured cells of Marchantia polymorpha L. High light treatment (30 h; 1300 mol photons · m )2 · s )1 ) reduced the PS I-mediated electron-transport rate by more than 50% and the photochemical efficiency of photosystem II (PS II) by about 35%. In photoinhibited cells, 76% of the PS II centers remained closed in low light, which is in agreement with a preferential impairment of PS I. Our data indicate that excessive linear electron transport is a cause of the loss in PS I activity. Two PS I forms could be isolated by sucrose-gradient ultracentrifugation of mildly solubilized thylakoid membranes. After high-light treatment one of these forms, which showed a larger light-harvesting complex (LHC) I antenna and a specific association of LHC IIb, was enriched. The effect could be suppressed by blockage of linear electron transport. It is suggested that PS I inactivation and state transitions caused the change in PS I organisation.Abbreviations: Chl = chlorophyll; DCMU = 3-(3,4-dichlorphenyl)-1,1-dimethylurea; Fo, Fs, F′m = basal, steady-state (light) and maximum chlorophyll fluorescence yield; LHC = light-harvesting complex; PFD = photon flux density; PS I u , PS I l = upper and lower PS I-containing bands after ultracentrifugation Correspondence
The minor chlorophyll a/b-binding (CAB) proteins of the liverwort Marchantia polymorpha L. were investigated in order to compare the antenna organization and the light-acclimation potential in lower and higher plants. Homologues to the minor CAB proteins CP24, CP26 and CP29 were identified by the following criteria: enrichment in photosystem II preparations, immunological cross-reactivities, spectroscopic properties and protein-fragment amino acid sequences. The high violaxanthin content of the minor CAB proteins in M. polymorpha indicates that their role in protection from high light is comparable in lower and higher plants. Considerably more-alkaline isoelectric points are found for the minor CAB proteins of M. polymorpha than for their higher-plant counterparts. This might be due to a higher content of basic amino acids. While the N-terminal sequence of angiosperm CP29 contains a threonine that becomes phosphorylated during cold stress, this amino acid is substituted by valine in M. polymorpha. Therefore, the regulatory properties of this protein could differ in lower and higher plants.
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