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Function of Nicotiana tabacum Aquaporins as Chloroplast Gas Pores Challenges the Concept of Membrane CO₂ Permeability^[W]

^a Department of Applied Plant Sciences, Institute of Botany, Darmstadt University of Technology, D-64287 Darmstadt, Germany
^b Department of Biology, University of New Mexico, Albuquerque, New Mexico 87131-1091
^c Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87544

¹ Address correspondence to kaldenhoff{at}bio.tu-darmstadt.de.

Photosynthesis is often limited by the rate of CO₂ diffusion from the atmosphere to the chloroplast. The primary resistances for CO₂ diffusion are thought to be at the stomata and at photosynthesizing cells via a combination resulting from resistances of aqueous solution as well as the plasma membrane and both outer and inner chloroplast membranes. In contrast with stomatal resistance, the resistance of biological membranes to gas transport is not widely recognized as a limiting factor for metabolic function. We show that the tobacco (Nicotiana tabacum) plasma membrane and inner chloroplast membranes contain the aquaporin Nt AQP1. RNA interference–mediated decreases in Nt AQP1 expression lowered the CO₂ permeability of the inner chloroplast membrane. In vivo data show that the reduced amount of Nt AQP1 caused a 20% change in CO₂ conductance within leaves. Our discovery of CO₂ aquaporin function in the chloroplast membrane opens new opportunities for mechanistic examination of leaf internal CO₂ conductance regulation.

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