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IN BRIEF

Aquaporins and Chloroplast Membrane Permeability

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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 of soybean root nodules, 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, H₂O₂, NH₃/NH₄⁺, 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 plasma membrane 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 CO₂ using RNA interference (RNAi) to suppress Nt AQP1. Permeability tests performed on isolated plasma membrane or chloroplast membrane vesicles showed that plasma membrane water permeability of AQP1-RNAi plants was reduced to approximately half that of the wild type, but chloroplast water membrane permeability remained essentially the same. By contrast, chloroplast membrane CO₂ permeability of AQP1-RNAi plants was roughly 10% of the wild type, whereas plasma membranes exhibited little difference in CO₂ permeability (see figure). The tests further indicated that the plasma membrane is 5 times more permeable to CO₂ than the chloroplast membrane. Finally, the authors show that photosynthetic rates were reduced by 15% in the AQP1-RNAi plants. Uehlein et al. conclude that CO₂ resistance of the inner chloroplast membrane contributes more than previously estimated to total leaf internal resistance to CO₂ transport and that Nt AQP1 localized to the inner chloroplast membrane contributes significantly to chloroplast CO₂ permeability in tobacco.

View larger version (10K): [in this window] [in a new window] [as a PowerPoint slide]   CO2 permeability (PCO2) of chloroplast envelope and plasma membranes isolated from wild-type and AQP1-RNAi tobacco plants.

www.plantcell.org/cgi/doi/10.1105/tpc.108.200311

REFERENCES

Agre, P. (2004). Aquaporin water channels (Nobel lecture). Angew. Chem. Int. Ed. 43: 4278–4290.[CrossRef]

Kaldenhoff, R., Bertl, A., Otto, B., Moshelon, M., and Uehlein, N. (2007). Characterization of plant aquaporins. Methods Enzymol. 428: 505–531.[CrossRef][ISI][Medline]

Maurel, C. (2007). Plant aquaporins: Novel functions and regulation properties. FEBS Lett. 581: 2227–2236.[CrossRef][ISI][Medline]

Uehlein, N., Otto, B., Hanson, D.T., Fischer, M., McDowell, N., and Kaldenhoff, R. (2008). Function of Nicotiana tabacum aquaporins as chloroplast gas pores challenges the concept of membrane CO₂ permeability. Plant Cell 20: 648–657.[Abstract/Free Full Text]

Related articles in Plant Cell:

Function of Nicotiana tabacum Aquaporins as Chloroplast Gas Pores Challenges the Concept of Membrane CO₂ Permeability

Norbert Uehlein, Beate Otto, David T. Hanson, Matthias Fischer, Nate McDowell, and Ralf Kaldenhoff
Plant Cell 2008 20: 648-657. [Abstract] [Full Text]  

This Article Full Text (PDF) All Versions of this Article: 20/3/499    most recent tpc.108.200311v1 Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Related articles in Plant Cell Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Eckardt, N. A. PubMed PubMed Citation Articles by Eckardt, N. A. Agricola Articles by Eckardt, N. A.