2. Redox regulation in the stroma
In chloroplasts, thiol-disulfide redox regulation is essential for both the stroma and lumen, but redox regulation in the stroma has been more extensively investigated than in the lumen (Buchanan, Kalberer & Arnon 1967; Wolosiuk & Buchanan 1977; Meyer, Belin, Delorme-Hinoux, Reichheld & Riondet 2012; Buchanan, Holmgren, Jacquot & Scheibe 2012; Balsera, Uberegui, Schürmann & Buchanan 2014; Buchanan 2016a). However, in order to fully understand lumenal redox regulation, we need to have a good understanding of the stroma side as well. This is because the stroma and lumen are interconnected, and the redox status of the stroma can affect the redox status of the lumen.
During oxygenic photosynthesis, electrons are transported from photosystem I (PSI) to ferredoxin (Fd) in the stroma. Fd carries electrons between donor and acceptor pairs. In the stroma, the electrons from Fd along with ATP and NADPH derived from electron and proton transport reactions respectively are utilized to fix carbon dioxide through a series of enzymes that constitute the Calvin-Benson-Bassham (CBB) cycle. Redox regulation in the stroma is also essential for modulating the activity of various enzymes of the CBB as well as controlling numerous other metabolic pathways. Hence, the redox regulation system has been long considered an important means to synchronize photosynthetic reactions to respond to both light availability and quality (Pearcy, Krall & Sassenrath-Cole 2004; Kaiser et al. 2015). Indeed, several enzymes are known to be regulated by light-dependent reductive activation, for example, the gamma subunit of chloroplast ATP synthase (CF1-γ) and several key enzymes in the CBB cycle such as fructose-1,6 bisphosphatase (FBPase), sedoheptulose -1,7 bisphosphatase (SBPase), Phosphoribulokinase (PRK), glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and rubisco activase (Rca) (Micheletet al. 2013).
The following subsections will briefly discuss two current stroma reduction systems as well as a recently discovered new oxidation system. For more detailed information on these systems, the reader is encouraged to the following relevant papers (Nikkanen & Rintamäki 2014; Yoshida, Hara & Hisabori 2015; Cejudo et al.2019; Cejudo, González & Pérez-Ruiz 2021; Gurrieri, Fermani, Zaffagnini, Sparla & Trost 2021)