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)