AUTHOR=Wada Shinya , Miyake Chikahiro , Makino Amane , Suzuki Yuji 

TITLE=Photorespiration Coupled With CO2 Assimilation Protects Photosystem I From Photoinhibition Under Moderate Poly(Ethylene Glycol)-Induced Osmotic Stress in Rice

JOURNAL=Frontiers in Plant Science

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2020.01121

DOI=10.3389/fpls.2020.01121

ISSN=1664-462X

ABSTRACT=<p>Photorespiration coupled with CO<sub>2</sub> assimilation is thought to act as a defense system against photoinhibition caused by osmotic stress. In the present study, we examined whether such a mechanism is operative for the protection of photosystem I (PSI) in rice (<italic>Oryza sativa</italic> L.) including transgenic plants with decreased and increased Rubisco content (<italic>RBCS</italic>-antisense and <italic>RBCS</italic>-sense plants, respectively). All plants were hydroponically grown and moderate osmotic stress was imposed using hydroponic culture solutions containing poly(ethylene glycol) (PEG) at 16% or 20% (w/v) for 2 d. In wild-type plants, the rates of CO<sub>2</sub> assimilation (<italic>A</italic>) were significantly decreased by the PEG treatment, whereas the photorespiration activity estimated from the rates of electron transport in photosystem II (PSII) and <italic>A</italic> were not affected. The maximal quantum efficiency of PSII (<italic>F</italic><sub>v</sub>/<italic>F</italic><sub>m</sub>) and the maximal activity of PSI (<italic>P</italic><sub>m</sub>) were also not affected. In <italic>RBCS</italic>-antisense plants, <italic>A</italic> and the estimated photorespiration activity were considerably lower than those in wild-type plants in the presence or absence of the PEG treatment. <italic>P</italic><sub>m</sub> and both <italic>F</italic><sub>v</sub>/<italic>F</italic><sub>m</sub> and <italic>P</italic><sub>m</sub> decreased in the 16% PEG-treated and 20% PEG-treated <italic>RBCS</italic>-antisense plants, respectively. Thus, the decrease in Rubisco content led to the photoinhibition of PSI and PSII, indicating the importance of photorespiration coupled with CO<sub>2</sub> assimilation for the protection of PSI from moderate PEG-induced osmotic stress. It was also shown that PSI was more sensitive to osmotic stress than PSII. In the PEG-treated wild-type and <italic>RBCS</italic>-antisense plants, osmotic-stress responses of the photosynthetic electron transport reactions upstream of PSI led to the oxidation of P700, which is thought to prevent PSI from over-reduction. Although such a defense system operated, it was not sufficient for the protection of PSI in <italic>RBCS</italic>-antisense plants. In addition, there were no large differences in the parameters measured between wild-type and <italic>RBCS</italic>-sense plants, as overproduction of Rubisco did not increase photorespiration activity.</p>