Aquaporins and Chloroplast Membrane Permeability

Aquaporins are intrinsic membrane proteins known to facilitate membrane water transport and are found in most living organisms (Agre, 2004). They comprise a large family in plants: Arabidopsis, rice, and maize each have ;35 different aquaporins. These initially were categorized into four subfamilies, largely based on apparent patterns of localization: plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), Nodulin26-like intrinsic membrane proteins, present in the peribacteroid membrane ofsoybean rootnodules, and small basic intrinsic proteins, found in the endoplasmic reticulum. However, this classification is somewhat misleading, as PIP aquaporins have been identified in organellar membranes and some TIP aquaporins in the plasma membrane (reviewed in Kaldenhoff et al., 2007; Maurel, 2007). Recent work has suggested that, in addition to water movement, aquaporins might transport other physiologically important molecules across membranes, including CO2 ,H 2O2 ,N H 3/NH4 1 , boron, and silicon, and therefore may be involved in a number of fundamental processes in plants, such as nutrient acquistion, photosynthesis, and stress responses. However, measuring transport via aquaporin channels directly and teasing apart a direct role in transport of these molecules and possible indirect affects associated with water transport has proven to be exceptionally difficult. Uehlein et al. (pages 648–657) show that the aquaporin Nt AQP1 is localized to the inner chloroplast membrane and the plasma membrane in tobacco and present evidence that it facilitates transport of CO2 into the chloroplast and impacts CO2 fixation and photosynthesis. Subcellular localization of Nt AQP1 to the inner chloroplast membrane and plasmamembrane was determined by immunblot analysis, electron microscopy using immungold labeling, and fluorescence microscopy using a green fluorescent protein fusion. The authors investigated AQP1 function in membrane permeability to water and CO2using RNA interference