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Abstract
Photosynthesis is the process that harnesses, converts and stores light energy in the form of chemical energy in bonds of organic compounds. Oxygenic photosynthetic organisms (i.e., plants, algae and cyanobacteria) employ an efficient apparatus to split water and transport electrons to high-energy electron acceptors. The photosynthetic system must be finely balanced between energy harvesting and energy utilisation, in order to limit generation of dangerous compounds that can damage the integrity of cells. Insight into how the photosynthetic components are protected, regulated, damaged, and repaired during changing environmental conditions is crucial for improving photosynthetic efficiency in crop species. Photosystem I (PSI) is an integral component of the photosynthetic system located at the juncture between energy-harnessing and energy consumption through metabolism. Although the main site of photoinhibition is the photosystem II (PSII), PSI is also known to be inactivated by photosynthetic energy imbalance, with slower reactivation compared to PSII; however, several outstanding questions remain about the mechanisms of damage and repair, and about the impact of PSI photoinhibition on signalling and metabolism. In this review, we address the knowns and unknowns about PSI activity, inhibition, protection, and repair in plants. We also discuss the role of PSI in retrograde signalling pathways and highlight putative signals triggered by the functional status of the PSI pool.
Highlights
Photosynthesis, the primary source of oxygen and organic compounds, is vital for life on Earth
Unlike decades of extensive research on the damage and repair of photosystem II (PSII) (e.g., Aro et al, 1993; Zavafer and Mancilla, 2021), outstanding questions relating to damage and repair of Photosystem I (PSI), the other lightharnessing reaction centre protein complex of the thylakoid membrane, have been less well-studied
Absorbance measurements of P700, the special chlorophylls at the PSI reaction centre, are commonly used to assess PSI quantum yield and electron transport reactions involving PSI (Klughammer and Schreiber, 2008, 2016; Schreiber and Klughammer, 2016), which has improved the understanding of factors regulating PSI activity and/or inactivation
Summary
Photosynthesis, the primary source of oxygen and organic compounds, is vital for life on Earth. Some studies have demonstrated the negative effects of PSI photoinhibition on CO2 fixation, and sugar and starch accumulation, which is attributed to decreased electron transport by a partly inactive PSI pool, and a subsequent decrease in reduced NADPH to power the CBB cycle (Zivcak et al, 2015; Gollan et al, 2017; Lima-Melo et al, 2019a,b).
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