Abstract
Photorespiration coupled with CO2 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 (Oryza sativa L.) including transgenic plants with decreased and increased Rubisco content (RBCS-antisense and RBCS-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 CO2 assimilation (A) were significantly decreased by the PEG treatment, whereas the photorespiration activity estimated from the rates of electron transport in photosystem II (PSII) and A were not affected. The maximal quantum efficiency of PSII (F v/F m) and the maximal activity of PSI (P m) were also not affected. In RBCS-antisense plants, A and the estimated photorespiration activity were considerably lower than those in wild-type plants in the presence or absence of the PEG treatment. P m and both F v/F m and P m decreased in the 16% PEG-treated and 20% PEG-treated RBCS-antisense plants, respectively. Thus, the decrease in Rubisco content led to the photoinhibition of PSI and PSII, indicating the importance of photorespiration coupled with CO2 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 RBCS-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 RBCS-antisense plants. In addition, there were no large differences in the parameters measured between wild-type and RBCS-sense plants, as overproduction of Rubisco did not increase photorespiration activity.
Highlights
Drought stress is one of the most harmful environmental stresses on plant productivity
These results clearly indicate that photorespiration coupled with CO2 assimilation plays a crucial role in the protection of photosystem I (PSI) from photoinhibition under moderate osmotic stress conditions
It is shown that antisense suppression of Rubisco content led to decreases in energy consumption by photorespiration coupled with CO2 assimilation under PEGinduced osmotic stress in rice plants, leading to the photoinhibition of PSI and photosystem II (PSII)
Summary
Drought stress is one of the most harmful environmental stresses on plant productivity. It has been observed that PSI suffered from photoinhibition under severe drought stress, whereas PSII was not largely affected in some tropical tree species (Huang et al, 2013). Similar phenomena were observed when rice plants were subjected PEG-induced osmotic stress, which are widely used to mimic drought stress (Wada et al, 2019). These results show that PSI is more sensitive to drought or osmotic stress than PSII. PSI photoinhibition would be harmful under such stress conditions
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