Poster: Tropisms

Abs # 678: Circumnutation of rice coleoptiles: its relationship with light and gravity

Presenter:	Yoshihara, Takeshi , tyoshiha@sci.osaka-cu.ac.jp
Authors	Yoshihara, Takeshi  (A)   Iino, Moritoshi  (A)
Affiliations:	(A): Botanical Gardens, Graduate School of Science, Osaka City University

The coleoptile of dark-grown rice (Oryza sativa L., cv. Nihonmasari) was found to show regular and large circumnutation. The mean period and amplitude were 170 min and 21 degrees, respectively. Both clockwise and counterclockwise movements were observed, but individual seedlings nutated only in one direction. The circumnutation was inhibited by a pulse of red light, being probably mediated by phytochrome. Both very-low-fluence and low-fluence responses were found to participate in the response. These results indicate that the coleoptile nutates while growing in dark soil environment and stops nutating as its tip emerges from the soil surface. Investigation of the effects of angular displacement (gravi-stimulation) revealed that the phase of circumnutation is synchronized after gravitropic curvature, that the direction of rotation can be reversed, depending on the timing of gravi-stimulation, and that circumnutation is abolished by rotation on a horizontal clinostat. The coleoptiles treated with a red-light pulse and subsequently submerged in water grew at a high rate but showed no circumnutation; upon brief gravi-stimulation (e.g., 5 min), these coleoptiles reinitiated circumnutation, and the amplitude of reinitiated circumnutation depended on the extent of gravi-stimulation. These results altogether indicate that graviperception plays a central role in circumnutation. The coleoptile of an agravitropic rice mutant, lazy, was found to show no circumnutation. On the other hand, the mutant coleoptile showed gravitropism with only partially impaired gravitropic responsiveness. The results suggest that circumnutation is not a consequence of gravitropism but is more directly linked to the graviperception mechanism that is shared between gravitropism and circumnutation.