Effects of water stress on photochemical function and protein metabolism of photosystem II in wheat leaves

The effects of osmotic dehydration in wheat leaves ( Triticum aestivum L. cv. Longchun No. 10) on the photochemical function and protein metabolism of PSII were studied with isolated thylakoid and PSII membranes. The results indicated that PSII was rather resistant to water stress as mild water deficit in leaves did nut significantly affect its activity. However, extreme stress conditions such as 40% decrease in relative water content (RWC) or 1.8 MPa in water potential (Ψ) caused ca 50% reduction in O₂ evolution and ca 25% inhibition of DCIP (2.6-dichlorophenol indophenol) photoreduction of PSII. In addition, it was found that the inhibited DCIP photoreduction of PSII could not be reversed by DPC (2.2-diphenylcarbazide), a typical electron donor to PSII, suggesting that water stress did not affect electron donation to PSII. Urea-SDS-PAGE and western blot analysis showed that the steady slate levels of major PSII proteins, including the D1 and D2 proteins in the PSII reaction center, declined on a chlorophyll basis with increasing water stress, possibly as a result of increased degradation. In vitro translation experiments and quantitative analysis of chloroplast RNAs indicated that the potential synthesis of chloroplast proteins from their mRNAs was impaired by water stress. From the results it is concluded that the effects of water stress on PSII protein metabolism, especially on the reaction center proteins, may account for the damage to PSII photochemistry.     

A Psb27 homologue in Arabidopsis thaliana is required for efficient repair of photodamaged photosystem II

Psb27 has been identified as a lumenal protein associated with photosystem II (PSII). To gain insight into the function of Psb27, we isolated a mutant Arabidopsis plant with a loss of psb27 function. The quantity of PSII complexes and electron transfer within PSII remained largely unaffected in the psb27 mutant. Our results also showed that under high-light-illumination, PSII activity and the content of the PSII reaction center protein D1 decreased more significantly in the psb27 mutant than in wild-type (WT) plant. Treatment of leaves with a chloroplast protein synthesis inhibitor resulted in similar light-induced PSII inactivation levels and D1 protein degradation rates in the WT and psb27 mutant plants. Recovery of PSII activity after photoinhibition was delayed in the psb27 mutant, suggesting that Psb27 is required for efficient recovery of the photodamaged PSII complex. Overall, these results demonstrated that Psb27 in Arabidopsis is not essential for oxygenic photosynthesis and PSII formation. Instead, our results provide evidence for the involvement of this lumenal protein in the recovery process of PSII. Hua Chen and Dongyuan Zhang contribute equally to this work.