Abstract
Cytb559, β-carotene (Car), and redox-active accessory chlorophyll (ChlZ) work as secondary electron donors to donate an electron to P680+ in PSII at low temperatures. Although this secondary electron donation plays an important role in photoprotection of PSII, its electron-transfer pathway remains unclarified. In this study, we have investigated the oxidation pathway of ChlZ by means of FTIR spectroscopy. Two Car molecules in PSII membranes of spinach were selectively bleached by illumination at 250 K under an oxidizing condition. Even in this Car-bleached PSII, the FTIR signal of ChlZ+/ChlZ at 1713/1687 cm-1 was developed by illumination at cryogenic temperatures. The kinetics of the ChlZ+ formation upon continuous illumination was mostly unchanged at 80 K, whereas the formation rate was appreciably reduced at 210 K. This result indicates that the oxidation pathway of ChlZ without involving Car is dominant at 80 K, while the pathway via Car becomes significant at higher temperatures.
