Abstract
PSI photoinhibition is usually avoided through P700 oxidation. Without this protective mechanism, excess light represents a potentially lethal threat to plants. PGR5 is suggested to be a major component of cyclic electron transport around PSI and is important for P700 oxidation in angiosperms. The known Arabidopsis PGR5 deficient mutant, pgr5-1, is incapable of P700 oxidation regulation and has been used in numerous photosynthetic studies. However, here it was revealed that pgr5-1 was a double mutant with exaggerated PSI photoinhibition. pgr5-1 significantly reduced growth compared to the newly isolated PGR5 deficient mutant, pgr5hope1. The introduction of PGR5 into pgr5-1 restored P700 oxidation regulation, but remained a pale-green phenotype, indicating that pgr5-1 had additional mutations. Both pgr5-1 and pgr5hope1 tended to cause PSI photoinhibition by excess light, but pgr5-1 exhibited an enhanced reduction in PSI activity. Introducing AT2G17240, a candidate gene for the second mutation into pgr5-1 restored the pale-green phenotype and partially restored PSI activity. Furthermore, a deficient mutant of PGRL1 complexing with PGR5 significantly reduced PSI activity in the double-deficient mutant with AT2G17240. From these results, we concluded that AT2G17240, named PSI photoprotection 1 (PTP1), played a role in PSI photoprotection, especially in PGR5/PGRL1 deficient mutants.
Highlights
Photosynthesis consists of two steps: the electron transport reaction and the carbon fixing reactions
We previously performed screening for Arabidopsis mutants in ∆pH formation across thylakoid membrane by monitoring chlorophyll fluorescence under hypoxic conditions [34]. hope1 was isolated as a high chlorophyll fluorescent mutant in hypoxic conditions (Supplementary Figure S1A,B) and was identified a mutation in proton gradient regulation 5 (PGR5) gene by genome mapping
The growth of pgr5-1 was severely affected by light intensity, the growth declines of pgr5-1 under constant and natural light conditions were consistent with [32,42]
Summary
Photosynthesis consists of two steps: the electron transport reaction and the carbon fixing reactions. The electron transport reaction converts light energy absorbed in chloroplast thylakoid membrane to chemical energy (NADPH and ATP), while the subsequent carbon fixing reaction (Calvin–Benson cycle) consumes NADPH and ATP to fix CO2. These reactions are regarded as an electron source-sink relationship. The electron transport reaction consists of photophysical and biochemical processes, while the carbon fixing reaction is biochemical; the impacts of environmental stresses (such as strong light, temperature, drought, etc.) are expected to be different between these reactions, despite the activities of the two reactions being tightly linked [1]. When the electron transport chain is full of electrons, O2 can be reduced into the superoxide radical (O2−) on the components with the lowest redox potential, in other words, the acceptor side of PSI [2]
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