Plant and Cell Physiology Advance Access originally published online on April 1, 2008
Plant and Cell Physiology 2008 49(5):801-813; doi:10.1093/pcp/pcn054
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Drought Stress Alters Water Relations and Expression of PIP-Type Aquaporin Genes in Nicotiana tabacum Plants
1Department of Biology, The University of Isfahan, Isfahan, 81746-73441, Iran
2Research Institute for Bioresources, Okayama University, Kurashiki, 710-0046 Japan
*Corresponding author: E-mail, kmaki{at}rib.okayama-u.ac.jp; Fax, +81-086-434-1249.
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Abstract |
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Plasma membrane intrinsic proteins (PIPs), a type of aquaporins, mediate water transport in many plant species. In this study, we investigated the relationship between the functions of PIP-type water channels and water relations of tobacco plants (Nicotiana tabacum cv. Samsun) under drought stress. Drought stress treatments have led to reductions in the stomatal conductance, transpiration, water potential and turgor pressure in leaves, and also the sap flow rate and osmotic hydraulic conductance in roots. In contrast, leaf osmotic pressure was increased in response to drought stress. Interestingly, the accumulation of NtPIP1;1 and NtPIP2;1 transcripts was significantly decreased, but only that of the NtAQP1 transcript was increased under drought stress. Functional analysis using Xenopus laevis oocytes revealed that NtPIP2;1 shows marked water transport activity, but the activities of NtAQP1 and NtPIP1;1 are weak or almost negligible, respectively, when expressed alone. However, co-expression of NtPIP1;1 with NtPIP2;1 significantly enhanced water transport activity compared with that of NtPIP1;1- or NtPIP2;1-expressing oocytes, suggesting that these two aquaporins may function as a water channel, forming a heterotetramer. Heteromerization of NtPIP1;1 and NtPIP2;1 was also suggested by co-expression analyses of NtPIP1;1–GFP (green fluorescent protein) and NtPIP2;1 in Xenopus oocytes. Re-watering treatments recovered water relation parameters and the accumulation of the three NtPIP transcripts to levels similar to control conditions. These results suggest that NtPIP1;1 and NtPIP2;1 play an important role in water transport in roots, and that expression of NtPIP1;1 and NtPIP2;1 is down-regulated in order to reduce osmotic hydraulic conductance in the roots of tobacco plants under drought stress.
Keywords: Aquaporin - Drought stress - Nicotiana tabacum - Water channel - Water relations
Abbreviations:
E, transpiration rate; GFP, green fluoresent protein; Gs, stomatal conductance; Jv, sap flow rate; Lo, osmotic root hydraulic conductance; MBS, modified Barth's solution; MIPs, major intrinsic proteins; NIP, nodulin-like intrinsic protein; P, turgor pressure; PBS, phosphate-buffered saline; PEG, polyethylene glycol; Pf, osmotic water permeability coefficient; PIP, plasma membrane intrinsic protein; RT–PCR, reverse transcription–PCR; SIP, small basic intrinsic protein; 
w, leaf water potential; 
, leaf osmotic pressure.
(Received February 12, 2008; Accepted March 26, 2008)
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