Poster: Late and Moved Abstracts

Abs # 1417: Understanding the chilling-induced cytoplasmic acidification in a chilling-sensitive plant

Presenter:	Kawamura, Yukio , ykawa@wam.umd.edu
Authors	Kawamura, Yukio  (A) (…A)  Sze, Heven  (A) (…A)
Affiliations:	(A): University of Maryland

In chilling-sensitive plants, chilling rapidly results in cytoplasmic acidification (Yoshida et al. 1999); however, the mechanism of acidification is unclear. To reveal this mechanism, vacuolar vesicles were isolated from hypocotyls of control mung bean (Vigna radiata L.) or after chilling plants at 0 C for 1 d. H⁺-pumping activities by vacuolar H⁺-ATPase (V-ATPase) or H⁺-PPase (V-PPase) and the passive H⁺-permeability of the membrane were monitored by fluorescence intensity changes of acridine orange. In control plants, V-ATPase could only form a small delta pH in vitro below 10 C, and the ratio of H⁺-efflux to H⁺-influx increased as temperature declined. In contrast, V-PPase generated a delta pH even at 0 C, and the ratio of H⁺-efflux to H⁺-influx was relatively constant. After 1 day of chilling, vacuolar ATP hydrolysis activity decreased, while V-PPase activity did not. However, the initial rate of H⁺-pumping by both pumps and the steady state delta pH decreased after 1 d of chilling. The passive permeability to H⁺ after delta pH formation was hardly changed by chilling. These results suggest that cold-induced cytoplasmic acidification is caused by two events: 1) V-ATPase is more sensitive to cold than PPase and fails to form significant delta pH at temperature below 10 C; and 2) After chilling in vivo , PPi hydrolysis is uncoupled from H⁺-pumping, thus reducing delta pH formation. To our knowledge, this is the first study to demonstrate quantitative relationships of active H⁺-influx and passive H⁺-efflux in vacuolar membranes caused by low temperature or after plants were chilled for 1 d. (Supported by Japan Society PS Postdoctoral Fellowships for Research Abroad).