PARENT SESSION
Posters P1B Photo-oxidative stress, photoinhibition. Abstracts (394-443)

Impairment of energy utilization for the Calvin cycle triggers inhibition of PSII repair cycle. Shunichi Takahashi^*,1, Norio Murata¹, ¹ National Institute for Basic Biology, Myodaiji, Okazaki, 444-8585, Japan

ABSTRACT- Impairment of energy utilization for the Calvin cycle enhances the extent of photoinactivation of PSII (photoinhibition). To understand its molecular mechanism, we investigated effects of impairment of the Calvin cycle on the photoinactivation of PSII in Chlamydomonas reinhardtii cells. When wild-type cells were exposed to light, glycolaldehyde (GA), which inhibits the Calvin cycle at the site of phosphoribulokinase, enhanced the extent of photoinactivation of PSII. The effect of GA on the photoinactivation was very similar to that of chloramphenicol, which inhibits protein synthesis in chloroplasts. Photoinactivation of PSII in a mutant of Rubisco large subunit missense (10-6C) was faster than that in wild-type cells. However, neither GA nor chloramphenicol further accelerated photoinactivation of PSII in 10-6C cells. After photoinactivation treatment, PSII activity in wild-type cells gradually recovered under low light and reached a level close to the initial in four hours. However, no significant recovery was observed in the presence of GA or in 10-6C cells. Impairment of the Calvin cycle did not influence degradation of photodamaged D1 protein or the level of psbA mRNA during illumination of light. However, light-dependent synthesis of D1 protein de novo in chloroplasts was significantly inhibited. These results suggest that impairment of the Calvin cycle inhibits protein synthesis-dependent repair of photodamaged PSII (PSII repair cycle) and it results in the enhancement of extent of photoinactivation of PSII. Based on these observations, we discuss the photoinactivation mechanism caused by environmental stress that inhibits the Calvin cycle.

KEY WORDS: photoinhibition , photosystem II, photoinactivation , Calvin cycle