A zeaxanthin-independent nonphotochemical quenching mechanism localized in the photosystem II core complex

Abstract

Illumination of dark-adapted barley plants with low light transiently induced a large nonphotochemical quenching of chlorophyll fluorescence. This reaction was identified as a form of high-energy-state quenching. Its appearance was not accompanied by zeaxanthin synthesis but was associated with a reversible inactivation of a fraction of photosystem II (PSII) centers. Both the fluorescence quenching and PSII inactivation relaxed in parallel with the activation of the Calvin cycle. We interpret the induction of this phenomenon as due to the generation of a quenched state in the PSII core complex. This reaction is probably caused by the transient overacidification of the thylakoid lumen, whereas its dissipation results from the relaxation of both the pH gradient across the thylakoid membrane and redox pressure upon activation of carbon fixation. At saturating light intensities, inactivation of PSII was still observed at the onset of illumination, although its recovery did not result in dissipation of high-energy quenching, which presents typical characteristics of an antenna-associated quenching at steady state. Reaction-center quenching seems therefore to be a common transient feature during illumination, being replaced by other phenomena (photochemical or antenna quenching and photoinhibition), depending on the balance between light and carbon fixation fluxes

Nonphotochemical Quenching of Chlorophyll Fluorescence inChlamydomonas reinhardtii

“…While the wild type displayed multiphasic kinetics, with slow components developing in the time range of minutes, the onset of NPQ was essentially monophasic in the npq1 lor1 mutant. This behavior is consistent with our previous observations in plants (13) that the onset of reaction center-based quenching precedes generation of antenna-based quenching, owing to This compound was added to cells 5 min prior to illumination. In low-temperature experiments, cells were dark adapted at room temperature and then loaded into a cuvette that was prerefrigerated in an ice/water bath for 20 min.…”

“…When the amount of antheraxanthin plus zeaxanthin synthesized during illumination was tested in the RubisCO mutant, it turned out to be well below that observed in illuminated plants. While only 0.7 mol per 100 mol of chlorophyll was synthesized in the case of Chlamydomonas (Table 1), more than 5 mol per 100 mol of chlorophyll was produced in plants in the same conditions (13).…”

“…However, there are also striking differences. While the transient inactivation of PSII centers disappears upon prolonged illumination in plants (13), this quenching is maintained at steady state in Chlamydomonas. This indicates that both reaction center and antenna nonphotochemical quenching processes coexist in the alga under steady-state conditions and, thus, that the transition from reaction center to antenna quenching is less efficient in Chlamydomonas than in plants (see below).…”

“…However, in vitro at least, this term also encompasses acidic pHinduced processes in the PSII reaction center, specifically following the release of a Ca 2+ ion associated with the watersplitting complex (e.g., ref 12). We recently presented evidence suggesting a mechanistic link between these two forms of quenching in vivo, with a reaction center process occurring at the onset of quenching and then "migrating" to the antenna as the process develops (13).…”

Fluorescence as a Tool to Understand Changes in Photosynthetic Electron Flow Regulation

“…Quenching based in the PSII reaction center (as opposed to LHC antenna) has also been observed in higher plants (Bukhov et al 2001;Stroch et al 2004) and green microalgae (Finazzi et al 2004), and possibly in diatoms (Eisenstadt et al 2008). Appearance of reaction centre quenching depends on the balance between light and carbon fixation fluxes (Finazzi et al 2004) along with a clear temperature influence (Kornyeyev et al 2004).…”