PARENT SESSION
Posters P6B Photosynthetic acclimation: Mechanisms and gene expression. Abstracts (531-578)

Possible involvement of heterotrimeric G protein in phytochrome A signal transduction for induction of photosynthesis gene expression in etiolated rice seedlings. Noritoshi Inagaki^*,1, Yukiko Fujisawa², Takeshi Yoshizumi³, Satoru Tokutomi⁴, Zhao Li³, Naoki Yamamoto⁵, Yukimoto Iwasaki², Minami Matsui³, ¹ Photosynthesis Laboratory, Tsukuba, Japan² Department of Bioscience, Matsuoka-cho, Japan³ Genomic Sciences Center, Yokohama, Japan⁴ Research Institute for Advanced Science and Technology, Sakai, Japan⁵ Department of Biology, Tokyo, Japan

ABSTRACT- During germination of angiosperms, development of photosynthetic apparatus is tightly regulated by light. Therefore, plants have sensitive and sophisticated photoreceptors to detect environmental light conditions. Phytochrome is the most intensively studied photoreceptor to perceive red (R) and far-red (FR) light and acting as a key receptor to induce photomorphogenesis of the seedlings. In this study, we evaluated the physiological roles of heterotrimeric G protein in the phytochrome signal transduction pathway in rice plants (Oryza sativa L.). Because, a series of microinjection assays [Neuhaus et al. (1993) Cell 73: 937-] revealed that this sort of G protein is likely to act as a signaling component in the pathway for induction of several photosynthesis genes by light. According to the context, we examined expression levels of typical photosynthesis genes, such as LHCB and RBCS, in etiolated seedlings of the rice dwarf1 (d1) mutant under various light conditions. This mutant is known to have a mutation in the gene coding for the subunit of the heterotrimeric G protein (G). Induction of these photosynthesis genes were repressed in the mutant exposed to continuous and pulsed FR light irradiation, despite a fact that photoactive phytochromes could be detected in the mutant seedlings. The most probable interpretation is that the mutant is deficient in a signaling molecule for phytochrome A (phyA) mediated responses. Transgenic rice seedlings expressing the constitutively active G protein showed hypersensitivity to a FR light pulse in the photosynthesis gene expression. These results indicated that G acts as a positive regulator in the phyA signal transduction in terms of the photosynthesis gene expression in rice etiolated seedlings. The conclusion is not only consistent with the previous interpretation obtained from the microinjection assays, but also supplying the first and direct evidence of it.

KEY WORDS: phytochrome signal transduction, heterotrimeric G protein, gene expression, rice (Oryza sativa L.)