Short- and long-term drought stress on photosystem II (PSII) and oxidative stress were studied in Arabidopsis thaliana. Under drought stress, chlorophyll (Chl) content, Chl fluorescence, relative water content and oxygen evolution capacity gradually decreased, and the thylakoid structure was gradually damaged. Short-term drought stress caused a rapid disassembly of the light-harvesting complex II (LHCII). However, PSII dimers kept stable under the short-term drought stress and significantly decreased only after 15 days of drought stress. Immunoblotting analysis of the thylakoid membrane proteins showed that most of the photosystem proteins decreased after the stress, especially for Lhcb5, Lhcb6 and PsbQ proteins. However, surprisingly, PsbS significantly increased after the long-term drought stress, which is consistent with the substantially increased non-photochemical quenching (NPQ) after the stress. Our results suggest that the PSII-LHCII supercomplexes and LHCII assemblies play an important role in preventing photo-damages to PSII under drought stress.

Wheat stripe rust (Puccinia striiformis f. sp. tritici, Pst), is one of the most serious diseases of wheat (Triticum aestivum L.) worldwide. To gain a better understanding of the protective mechanism against stripe rust at the adult plant stage, the differences in photosystem II and antioxidant enzymatic systems between susceptible and resistant wheat in response to stripe rust disease (P. striiformis) were investigated. We found that chlorophyll fluorescence and the activities of the antioxidant enzymes were higher in resistant wheat than in susceptible wheat after stripe rust infection. Compared with the susceptible wheat, the resistant wheat accumulated a higher level of D1 protein and a lower level of reactive oxygen species after infection. Furthermore, our results demonstrate that D1 and light-harvesting complex II (LHCII) phosphorylation are involved in the resistance to stripe rust in wheat. The CP29 protein was phosphorylated under stripe rust infection, like its phosphorylation in other monocots under environmental stresses. More extensive damages occur on the thylakoid membranes in the susceptible wheat compared with the resistant wheat. The findings provide evidence that thylakoid protein phosphorylation and antioxidant enzyme systems play important roles in plant responses and defense to biotic stress.

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Reversible phosphorylation of proteins and the assembly of thylakoid complexes are the important protective mechanism against environmental stresses in plants. This research was aimed to investigate the different responses of the antioxidant defense system and photosystem II (PSII) to osmotic stress between drought-resistant and drought-susceptible wheat cultivars. Results showed that the decrease in PSII photochemistry and six enzyme activities was observed in drought-susceptible wheat compared with drought-resistant wheat under osmotic stress. In addition, a lower accumulation of reactive oxygen species (ROS) and cell death were found in the resistant wheat compared with the susceptible wheat under osmotic stress. Western blot analysis revealed that osmotic stress led to a remarkable decline in the steady state level of D1 protein in drought-susceptible wheat. However, the CP29 protein was strongly phosphorylated in drought-resistant wheat compared with the susceptible wheat under osmotic stress. Our results also showed that drought-resistant wheat presented higher phosphorylated levels of the light-harvesting complex II (LHCII), D1, and D2 proteins and a more rapid dephosphorylated rate than drought-susceptible wheat under osmotic stress. Furthermore, the PSII-LHCII supercomplexes and LHCII trimers were more rapidly disassembled in drought-susceptible wheat than the drought-resistant wheat under osmotic stress. These findings provide that reversible phosphorylation of thylakoid membrane proteins and assembly of thylakoid membrane complexes play important roles in plant adaptation to environmental stresses.

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To investigate the evolutionary differences of wheat with different ploidy levels and octoploid Triticale, photosynthetic capacity, and antioxidant defenses system were compared within and between diploid, tetraploid and hexaploid wheat, and octoploid Triticale seedlings. The results showed that seed germination rate, chlorophyll content, and photochemical activity of photosystems, and the activities of antioxidative enzymes in hexaploid wheat and octoploid Triticale were significantly higher than in diploid and tetraploid wheat. Compared to other two wheat species and octoploid Triticale, hexaploid wheat presented lower levels of reactive oxygen species (ROS). Furthermore, we found that the levels of photosystem II reaction center protein D1, light-harvesting complex II b4 (CP29), and D subunit of photosystem I (PsaD) in diploid wheat were significantly lower compared with hexaploid wheat and octoploid Triticale. Taken together, we concluded that hexaploid wheat and octoploid Triticale have higher photosynthetic capacities and better antioxidant systems. These findings indicate that different ploidy levels of chromosome probably play an important regulatory role in photosystems and antioxidative systems of plants.

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