The tobacco aquaporin NtAQP1 is a membrane CO2 pore with physiological functions

Abstract

Aquaporins, found in virtually all living organisms, are membrane-intrinsic proteins that form water-permeable complexes. The mammalian aquaporin AQP1 has also shown CO2 permeability when expressed heterologously in Xenopus oocytes, although whether this is a biochemical curiosity or of physiological significance is a matter of debate. Here we report that, in the same expression system, a CO2 permeability comparable to that of the human AQP1 is observed for the tobacco plasma membrane aquaporin NtAQP1. NtAQP1 facilitates CO2 membrane transport in the homologous plant system at the cellular level, and has a significant function in photosynthesis and in stomatal opening. NtAQP1 overexpression heightens membrane permeability for CO2 and water, and increases leaf growth. The results indicate that NtAQP1-related CO2 permeability is of physiological importance under conditions where the CO2 gradient across a membrane is small, as is the case between the atmosphere and the inside of a plant cell.

Barley plasma membrane intrinsic proteins (PIP Aquaporins) as water and CO2 transporters

“…The same phenomenon was observed in tobacco plants, that is, the CO 2 assimilation rate increased in leaves over-expressing AtPIP1;2 (PIP1b) [1], or NtAQP1 [28]. Recently, we also observed increases in CO 2 assimilation in tobacco leaves over-expressing ice plant (Mesembryanthemum crystallinum) aquaporins (unpublished data).…”

“…This was the first evidence of a direct relation between aquaporins and g i (Fig.1). Although Uehlein et al showed that membrane permeability to CO 2 increased in Xenopus oocytes expressing the tobacco aquaporin NtAQP1 [28], they did not perform the measurement of g i . Recently, Flexas et al confirmed an increase in g i of leaves of transgenic plants over-expressing NtAQP1 [7].…”

“…In the plant genome, more than 30 aquaporin genes have been detected [11,21]. Some of them specifically transport water molecules, but others can mediate other low molecular weight compounds such as CO 2 [7,8,28], silicon [19], boron [24], ammonia [9], and H 2 O 2 [2].…”

Assessment of the requirement for aquaporins in the thylakoid membrane of plant chloroplasts to sustain photosynthetic water oxidation

“…A dual location of some PIPs and TIPs would not be surprising taking into consideration the case of AQP1 from Nicotiana tabacum, which localized to the plasma membrane and also to the chloroplast inner envelope [33,34]. Interestingly, the substrate specificity was found to be location-dependent since only the plasma membrane AQP1 transported water, whereas the envelope AQP1 facilitated diffusion of CO 2 , although the requirement for CO 2 aquaporin in biological membranes is still controversial.…”

Role of aquaporins in cell proliferation: What else beyond water permeability?

“…12,[38][39][40] In addition, several metabolically important small neutral solutes and ions have been identified as AQP substrates, namely hydrogen peroxide (H 2 O 2 ). 13,17,41,42 carbon dioxide, 43,44 nitric oxide, 45,46 ammonia, 47,48 metalloids such as arsenite and antimonite, 49-51 selenite, 52 boric acid, 53 and silicic acid. 51 AQP6 with low intrinsic water permeability may function primarily as an anion transporter.…”