Proteomic and Physiological Studies Provide Insight into Photosynthetic Response of Rice (Oryza sativa L.) Seedlings...

The mechanisms whereby how photosynthesis is regulated and maintained under conditions of microgravity remain incompletely understood. Herein, we took a combination of proteomic and physiological approaches to examine the response of rice (Oryza sativa L.) seedlings to spaceflight conditions. Our results show that both PSI fluorescence emission peak and P700 absorbance amplitude are severely decreased in spaceflight seedlings under microgravity. This is consistent with an observed significant reduction in PSI efficiency (ϕI ). To further analyze global changes of protein profiles under microgravity, isobaric tags for relative and absolute quantization (iTRAQ) labeling technology were deployed. Four hundred fifty-four differentially expressed proteins were identified by comparison of spaceflight and ground control. Of proteins relevant to photosynthesis, 34 were downregulated and 4 were upregulated. The significantly downregulated ones are essential components of PSI, NDH and the Cytb6 f complex. This downregulation of PSI proteins and/or protein structure changes may cause the overall reduction in PSI activity. Intriguingly, although abundance of some PSII proteins was altered under microgravity, no significant changes in PSII activity were detected. Taken together, our results suggest that PSI, rather than PSII being usually much more sensitive to environmental stresses, is more susceptible to spaceflight conditions in rice seedlings.