Sammendrag
Photosystem II (PSII) is susceptible to photodamage (also known as photoinhibition), induced by both visible and ultraviolet light. Under environmental stress PSII damage and repair come at a high cost affecting the overall photosynthetic activity, growth, and biomass productivity of natural photosynthesis. However, PSII complexes that undergo de novo assembly during biogenesis of the thylakoid membrane may be even more susceptible to photodamage than the mature PSII complexes because of the functional inefficiency of PSII during this process. In the present study, employing plastids (etiochloroplasts and chloroplasts) isolated from barley at various times during de-etiolation, we investigated the photoinhibitory effects at various de novo assembly stages of PSII during chloroplast development. Both membrane-inlet mass spectrometry (MIMS) and O2-polarographic data under various light conditions show that the PSII complexes in etiochloroplasts are much more susceptible to photoinhibitory degradation than the PSII of mature chloroplasts. Furthermore, photoinhibitory effects in etiochloroplasts were found to be inversely proportional to the duration of de-etiolation. Interestingly, the significant decrease of photoinhibitory effects clearly correlates with the first observation of progressive thylakoid stacking monitored by electron microscopy. This, together with our recent findings (Shevela et al., 2016), suggests that the buildup of the complete linear electron transfer chain plus establishment of stacked grana membranes that takes place at later stages of chloroplast development, reduce photodamage of the PSII complexes. Thus, self-organization of the thylakoid membrane into stacked grana might be an evolutionary invention in order to minimize the rate of photoinhibition.
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