Journal of Experimental Botany, Vol. 53, No. 374, pp. 1581-1591,
July 1, 2002
© 2002 Oxford University Press
Kinetic characteristics of chloroplast glucose transport
Received 23 November 2001; Accepted 13 March 2002
1 Department of Biology, University of Dayton, Dayton, OH 454692320, USA
1 To whom correspondence should be addressed. Fax: +1 937 775 3320. E-mail: jerome.servaites{at}wright.edu
Influx of labelled D-glucose into isolated spinach (Spinacia oleracea L. cv. Melody hybrid) chloroplasts was initially rapid followed by a period of slower influx. The stroma glucose concentration attained equilibrium rapidly with low external glucose concentrations and the two were linearly proportional. The period of slower influx resulted from conversion of glucose to acidic products that remained trapped in the chloroplast. As the external glucose concentration increased, the stroma glucose concentration increased less and less, attaining a maximal concentration of 72 mol m3. The maintenance of an equilibrium stroma glucose concentration lower than that in the external medium is evidence that plastid glucose efflux involves secondary active transport. The equilibrium stroma glucose concentration increased in response to light and protonophoric uncouplers. It is proposed that glucose efflux is coupled with a proton and the stroma glucose concentration equilibrates in response to the proton gradient across the membrane. To determine if glucose is a significant product of starch mobilization, chloroplasts were isolated from spinach leaves labelled with 14CO2 during the preceding light period. Chloroplasts degraded starch at the same rate as the intact leaf. Glucose, maltose, and isomaltose were the principal labelled products that appeared in the medium during starch mobilization. The glucose concentration in the chloroplast was 2 mol m3, which is similar to the measured Km for zero trans efflux. The data support the role of the glucose translocator as an important component in the pathway for sucrose synthesis at night.
Key words: Key words: Glucose translocator, maltose translocator, starch mobilization, stroma space.
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