High light stress inducing photoinhibition and protein degradation of photosystem I in Brassica rapa

Abstract

PSI is one of the two photosynthetic reaction centers in the chloroplast of higher plants. It functions as a plastocyanin:ferredoxin oxidoreductase that catalyzes light-dependent electron transfer from reduced plastocyanin to oxidized ferredoxin. In order to understand the functional flexibility (i.e. stability, turnover and adaptation to stress) of PSI, a series of light stress conditions were introduced to induce PSI photoinhibition in Brassica rapa plants. Thylakoid membranes were subsequently isolated, and PSI photochemical activity and protein degradation were examined. The results demonstrated that high light stress (600 μmol photon m −2 s −1 versus 100 μmol photon m −2 s −1) induced PSI photoinhibition, by up to 35% decrease of PSI activity within 2–4 h. In addition, we found a recovery response of PSI to the high light stress, which brought the decreased PSI activity to a much higher level within 20–24 h, equivalent to that grown in constant high light conditions. However, this recovery process of PSI was eliminated and more severe PSI photoinhibition was induced by a combination of high light with drought and/or high temperature (35 °C). Western blotting analysis further showed that light-induced reduction of PSI activity was concomitant with degradation of PsaA and PsaB, and loss of PsaC in the thylakoid membranes. These results indicate that PSI photoinhibition may occur in most crop plants in a growth season since its reaction center proteins are the very early targets of photooxidative stresses.