Unveiling the mechanisms of acclimation to fluctuating light conditions

Penzler, Jan-Ferdinand (2023): Unveiling the mechanisms of acclimation to fluctuating light conditions: insights from suppressor mutants of pgr5. Dissertation, LMU München: Fakultät für Biologie

Abstract

Plants, algae and cyanobacteria can produce chemical energy in the form of organic compounds by using light in a process called photosynthesis. In a natural environment, light intensity changes from low to high in a matter of seconds (s) during the day. These rapid changes create a challenging environment that requires specific acclimation and protection mechanisms for the plant to survive. One of these mechanisms is cyclic elec-tron flow (CEF), mediated by PGR5, in which electrons are transferred from PSI back to the plastoquinone (PQ) pool. Indeed, plants without active PGR5-mediated CEF have a lethal phenotype under fluctuating light (FL) conditions. This alternative photo-synthetic electron pathway to the linear electron flow (LEF) allows an additional trans-fer of protons from the stroma to the lumen and a release of electrons from PSI, main-taining a proper ATP/NADPH ratio and protecting PSI from over-reduction. In plants, the key protein for this electron pathway is PGR5, but its molecular mechanism of ac-tion and how the electrons are transferred from PSI to the PQ pool is still unclear. To elucidate this mechanism and to identify additional components of light acclimation, a suppressor screen was performed in pgr5 under FL, using random mutagenesis. Several suppressor mutants were obtained of the lethal phenotype of pgr5 under FL. In this the-sis, four of these suppressors are described in more detail, including the mechanisms by which they allow acclimation to FL in the absence of PGR5. The most efficient way to suppress the lethal FL phenotype of pgr5 was to downregulate LEF, in particular by reducing PSII activity. In addition, the abundance and activity of FBPase was identi-fied as a possible key regulatory point to allow pgr5 survival under FL. Causative muta-tions were identified in two genes encoding proteins of completely unknown function. One of them, in the suppressor pgr5 S261, is the affected protein S261, which is in-volved in the assembly of the Cyt b6f complex, as recently described. In the second part of this work, the phenotype of mutants with different levels of PGR5 was studied in detail, including new pgr5 mutants generated by CRISPR/Cas9, pgr5-Cas#1 and pgr5-Cas#2. By analysing photosynthetic performance, proteomic composi-tion and growth under high light (HL) conditions, differences between the lines were observed. Interestingly, the original pgr5-1 mutant showed significant differences from the new pgr5-Cas lines, most likely caused by an SNP in the CGL20a gene, which is present in pgr5-1 but not in pgr5-Cas. Furthermore, it was found that mutants lacking both PGRL1 and PGR5 behaved differently under HL than mutants lacking only PGR5, suggesting an additional function of PGRL1 in the absence of PGR5.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 570 Biowissenschaften, Biologie
• Fakultäten: Fakultät für Biologie
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 25. Oktober 2023
• 1. Bericht­erstatter:in: Leister, Dario