Drought-induced changes in photosystems and light-harvesting complexes organization in thylakoid membranes of <i>Pisum sativum</i>

The organization of thylakoid supercomplexes are important for the efficiency of photosynthesis; however, the organization of the photoprotective pigments in supercomplexes of thylakoids in Pisum sativum under drought stress have not previously been studied. Here, we examined the photoprotective pigments, protein-pigment interactions, and macromolecular structural organization from sucrose density gradient (SDG) fractions. Solubilized thylakoid membranes were separated from SDG, in which four fractions were obtained: light-harvesting complexes (LHC)II monomer (F1), LHCII trimer (F2), photosystem (PS)II core (F3), and PSI-LHCI (F4). Circular dichroism data revealed that LHCII trimer complexes marginally changed under drought stress. In addition, significant alterations were observed in PSI-LHCI complexes compared with the PSII complexes. Under drought stress, lutein and β-carotene levels increased in the PSII core, suggesting a protective function of these pigments against drought stress. In contrast, xanthophylls, lutein, and β-carotene concentrations were reduced in PSI-LHCI, suggesting that the reduction of these pigments and of the pigment-protein complexes is not important in drought stress. Further, zeaxanthin was enhanced in LHCII trimeric complexes, which induced non-photochemical quenching due to the dissipation of excess energy absorbed by chlorophylls through Chlorophyll-Carotenoid interactions. Additionally, under drought stress, carotenoid levels were significantly enhanced in the PSII core, while lutein levels increased in PSII-LHCII complexes. The levels of photoprotective pigments are in agreement with the data obtained from the differential expression of genes involved in the production of carotenoids. Furthermore, zeaxanthin-dependent genes and proteins accumulated under drought stress, as shown by real-time PCR and western blot data, suggesting that violaxanthin is converted to zeaxanthin in drought stress. Taken together, we show that the presence of zeaxanthin and the differential expression of lutein and violaxanthin probably lead to remedial structural changes in thylakoid supercomplexes.